## GeMS validation of OF-23-02_Ignacio.gdb

File written by GeMS_ValidateDatabase.py, version of 5/15/2024
Thu Dec 5 09:36:17 2024
Runtime parameters
Database path: C:\Data\Pangaea\CGS\2023Ignacio\OF-23-02_Ignacio\OF-23-02_Ignacio_Publication\OF-23-03_Ignacio_Database\OF-23-02_Ignacio.gdb
Output directory: C:\Data\Pangaea\CGS\2023Ignacio\OF-23-02_Ignacio
Metadata file: C:\Data\Pangaea\CGS\2023Ignacio\OF-23-02_Ignacio\OF-23-02_Ignacio_Publication\OF-23-02_Ignacio-metadata_EAP.xml
Check embedded metadata: False
Skip topology check: False
Refresh GeoMaterialDict: True
Delete extra rows in Glossary and DataSources: False
Compact GDB: True
This database is LEVEL 3 COMPLIANT.
The database-level FGDC metadata are formally correct although the metadata record should be reviewed to verify that it is meaningful.
This file should be accompanied by OF-23-02_Ignacio.gdb-ValidationErrors.html and OF-23-02_Ignacio-metadata_EAP.xml_errors.txt in the same directory.
If this database will be submitted to the NGMDB, it also needs to be accompanied by a reviewed Geologic Names report that includes identification of any suggested modifications to Geolex. Use the Geologic Names Check tool to generate that report or provide other documentation of a review.
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### Contents

Compliance Criteria
Warnings
Content not specified in GeMS schema
Occurrence of MapUnits in DMU, feature datasets, or geopackage tables
Contents of Nonspatial Tables
AgeDates
DataSources
DescriptionOfMapUnits
Glossary
MiscellaneousMapInformation
OSL
PollenAge
Database Inventory

### Compliance Criteria

#### LEVEL 1

Criteria for a LEVEL 1 GeMS database are:

- No overlaps or internal gaps in map-unit polygon layer
- Contacts and faults in single feature class
- Map-unit polygon boundaries are covered by contacts and faults lines

Databases with a variety of schema may meet these criteria. This script cannot confirm LEVEL 1 compliance.

#### LEVEL 2--MINIMALLY COMPLIANT

A LEVEL 2 GeMS database is accompanied by a peer-reviewed Geologic Names report, including identification of suggested modifications to Geolex, and meets the following criteria:

2.1 Has required elements: nonspatial tables DataSources, DescriptionOfMapUnits, GeoMaterialDict; feature dataset GeologicMap with feature classes ContactsAndFaults and MapUnitPolys | PASS
2.2 Required fields within required elements are present and correctly defined | PASS
2.3 All MapUnitPolys and ContactsAndFaults based feature classes obey Level 2 topology rules: no internal gaps or overlaps in MapUnitPolys, boundaries of MapUnitPolys are covered by ContactsAndFaults | PASS
2.4 All map units in MapUnitPolys have entries in DescriptionOfMapUnits table | PASS
2.5 No duplicate MapUnit values in DescriptionOfMapUnit table | PASS
2.6 Certain field values within required elements have entries in Glossary table | PASS
2.7 No duplicate Term values in Glossary table | PASS
2.8 All xxxSourceID values in required elements have entries in DataSources table | PASS
2.9 No duplicate DataSources_ID values in DataSources table | PASS

#### LEVEL 3--FULLY COMPLIANT

A LEVEL 3 GeMS database meets these additional criteria:

3.1 Table and field definitions beyond Level 2 conform to GeMS schema | PASS
3.2 All MapUnitPolys and ContactsAndFaults based feature classes obey Level 3 topology rules: No ContactsAndFaults overlaps, self-overlaps, or self-intersections. | PASS
3.3 No missing required values | PASS
3.4 No missing terms in Glossary | PASS
3.5 No unnecessary terms in Glossary | PASS
3.6 No missing sources in DataSources | PASS
3.7 No unnecessary sources in DataSources | PASS
3.8 No map units without entries in DescriptionOfMapUnits | PASS
3.9 No unnecessary map units in DescriptionOfMapUnits | PASS
3.10 HierarchyKey values in DescriptionOfMapUnits are unique and well formed | PASS
3.11 All values of GeoMaterial are defined in GeoMaterialDict. GeoMaterialDict is as specified in the GeMS standard | PASS
3.12 No duplicate _ID values | PASS
3.13 No zero-length, whitespace-only, or bad null values | PASS

### Warnings

There are 45 warnings

### Content not specified in GeMS schema

Some of the extensions to the GeMS schema identified here may be necessary to capture geologic content and are entirely appropriate. Please document these extensions in metadata for the database, any accompanying README file, and (if applicable) any transmittal letter that accompanies the dataset. Other extensions may be intermediate datasets, fields, or files that should be deleted before distribution of the database.

#### Fields

GenericPoints, StationID

#### Tables

GeoContactsAnno
Line
CorrelationOfMapUnits
AgeDates
CrossSectionA
CSAMapUnitPolysAnno
PollenAge
OSL
MapUnitPolysAnno
GenericPtsAnno
OrientationPointsAnno
GenericPoints
GeologicLinesAnno
CMUMapUnitPolysAnno
CMULineAnno
CSADHTrace
CSAPoints
CSAWellsAnno
CSAGeologicLinesAnno

### Occurrence of MapUnit in DMU, feature datasets, or geopackage tables

MapUnit | DescriptionOfMapUnits | GeologicMap | CrossSectionA | CorrelationOfMapUnits
--- | --- | --- | --- | ---
Kme | X | -- | X | X
Qa4 | X | X | -- | X
Qf | X | X | -- | X
Qac | X | X | -- | X
Jm | X | -- | X | X
Qa | X | X | -- | X
PEn | X | X | X | X
Kpc | X | -- | X | X
Qa1 | X | X | -- | X
Kdb | X | -- | X | X
Qra | X | X | -- | X
Kl | X | -- | X | X
Km | X | -- | X | X
Kpl | X | -- | X | X
Kch | X | -- | X | X
PEsju | X | X | X | X
af | X | X | -- | X
Kkl | X | -- | X | X
Qa2 | X | X | -- | X
Kf | X | -- | X | X
Qe | X | X | -- | X
Qg | X | X | -- | X
PEsn | X | X | X | X
PEsjr | X | X | X | X
Qls | X | X | -- | X
Qgl | X | X | -- | X
Qgmm | X | X | -- | X

### Contents of Nonspatial Tables

#### AgeDates

OBJECTID | FieldNumber | UTMEasting | UTMNorthing | Latitude | Longitude | Nc | nd | YoungestDate | YoungestPopulation | nd1 | MSWD | principal | mode2 | mode3 | mode4
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
1 | IG22-DZ03 | 263651.0 | 4108913.0 | 37.096741 | -107.659441 | 127 | 106 | 67.2 ± 3.5 Ma | 70.7 ± 1.7 Ma | 5 | 0.58 | 1718.9 Ma | 1411.9 Ma | 145.5 Ma | 2152.5 Ma
2 | IG22-DZ09 | 258378.0 | 4107569.0 | 37.083297 | -107.718282 | 137 | 121 | 68.4 ± 5.5 Ma | 75.3 ± 1.4 Ma | 5 | 0.57 | 160.1 Ma | 1693.3 Ma | 82.8 Ma | 1382 Ma
3 | IG22-DZ10 | 265989.0 | 4109975.0 | 37.107082 | -107.633946 | 132 | 123 | 68.6 ± 3 Ma | 71.6 ± 1.5 Ma | 5 | 0.27 | 1679.8 Ma | 157.2 Ma | 1420.7 Ma | 74.4 Ma

#### DataSources

OBJECTID | Source | Notes | URL | DataSources_ID
--- | --- | --- | --- | ---
7 | OSL analysis | None | https://geosciences.artsandsciences.baylor.edu/about-us/facilities/geoluminescence-dating-research-lab | BAYLOR1
9 | Detrital zircon analysis | None | https://www.boisestate.edu/earth-isotope/analytical-capabilities/la-icpms-u-pb-geochronology/ | BOISE1
3 | This study | None | None | DAS1
5 | Online dictionary | None | https://dictionary.com/ | DICT1
2 | Energy and Carbon Management Commision | None | htps://ecmc.state.co.us | ECMC1
1 | Federal Geographic Data Committee [prepared for the Federal Geographic Data Committee by the U.S. Geological Survey], 2006, FGDC Digital Cartographic Standard for Geologic Map Symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC-STD-013-2006, 290 p., 2 plates. | None | https://ngmdb.usgs.gov/fgdc_gds/geolsymstd.php | FGDC-STD-013-2006
6 | GeMS standard | None | https://ngmdb.usgs.gov/Info/standards/GeMS/ | GeMS1
4 | Online geologic dictionary | None | https://glossary.americangeosciences.org | GEODICT1
8 | Pollen analysis | None | https://pursuit.unimelb.edu.au/topics/pollen | MELBOURNE

#### DescriptionOfMapUnits

OBJECTID | MapUnit | Name | FullName | Age | Description | HierarchyKey | ParagraphStyle | Label | Symbol | AreaFillRGB | AreaFillPatternDescription | DescriptionSourceID | GeoMaterial | GeoMaterialConfidence | DescriptionOfMapUnits_ID
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
44 | None | DESCRIPTION OF MAP UNITS | DESCRIPTION OF MAP UNITS | None | The geologic descriptions and mapped contacts in this report are based on field observations in accessible areas. However mechanical leveling of irrigated agricultural fields obscured some geologic features. For land without access, contacts are based on observations from other accessible areas, publicly available aerial imagery, lidar digital elevation data, and hillshade models. Particle-size classes for sandy to clayey sediments are based on the Udden-Wentworth scale (Carpenter and Keene, 2023, p. 161), except for USDA soil textural terms that are used for the near-surface parts of certain Quaternary units (Pannell and others, 1988; Carpenter and Keene, 2023, p. 461). Descriptive phrases for textural mixtures are based on the Folk classification (Folk, 1974). Color names and Munsell codes were obtained for dry sediments with the aid of Munsell and Globe soil color books (Munsell Color, 1975; Visual Color Systems, 2012) or from soil-series data (Pannell and others, 1988). Descriptions of soil calcium carbonate (CaCO3) stage morphology are after Machette (1985). Divisions of geologic time follow Cohen and others (2023) and the U. S. Geological Survey (2018). | 00 | DMUHeading1 | None | None | None | None | DAS1 | None | None | DMU37
1 | None | SURFICIAL DEPOSITS | SURFICIAL DEPOSITS | None | None | 01 | DMUHeading2 | None | None | None | None | DAS1 | None | None | DMU01
2 | None | HUMAN-MADE DEPOSITS | HUMAN-MADE DEPOSITS | None | None | 01-01 | DMUHeading3 | None | None | None | None | DAS1 | None | None | DMU02
3 | af | Artificial fill | Artificial fill | Uppermost Holocene | Unit includes fill material placed during the construction of well pads, road embankments, water catchments, dams, buildings, and residences. Fill consists of locally derived materials and (or) externally sourced materials such as waste rock, demolition concrete, gravel, and topsoil. These materials are usually unsorted and may be as much as 12 m (40 ft) thick. Some fills are loose while others are placed to engineering recommendations. Overlying structures may settle if fill is loose or inadequately compacted. Some areas of artificial fill in the town of Ignacio may be subject to infrequent flooding along the Los Pinos River (Colorado Water Conservation Board, 2024). | 01-01-01 | DMUUnit1 | af | af | 255-255-255 | None | DAS1 | "Made" or human-engineered land | High | DMU03
4 | None | ALLUVIAL AND COLLUVIAL DEPOSITS | ALLUVIAL AND COLLUVIAL DEPOSITS | None | These unconsolidated sedimentary deposits consist of clay, silt, sand, and clasts ranging in size from very fine pebbles (granules) to rare boulders up to 1 m (3 ft) in diameter. Alluvium was deposited in low-lying areas along the Los Pinos River and smaller tributary streams, on floodplains, and on debris-fan units or alluvial fans. Sediment size and bedding vary with factors including flow velocity and sediment source. Distinctive glaciofluvial, sandy gravel composed of well-rounded, pebble- to boulder-sized, igneous and metamorphic rocks from the San Juan Mountains underlies terraces of the Los Pinos River (see provenance in descriptions below). Some of these clasts have been reworked into younger deposits of alluvium and colluvium. Bedrock strata yield mostly clay and silt, but also sand and rock fragments from pebble- to boulder-size. Finer-grained deposits accumulated along ephemeral tributary streams as well as during overbank flow on the floodplain of the Los Pinos River. Observed thicknesses of alluvium range from very thin (less than 30 cm or 1 ft) to approximately 9 m (30 ft). Colluvium was deposited on slopes or at the base of slopes mainly by gravity-influenced processes such as creep, sheetwash, and rockfall that mobilized materials from higher on the slopes. Most colluvial deposits are relatively small and vary in thickness so colluvium was mapped mainly along with adjacent alluvium in a combined unit Qac. Most young deposits are on or near valley floors, but older deposits cap benches, hills, or uplands as topography is inverted because later erosion and stream incision has lowered the adjacent ground surface of the watershed. | 01-02 | DMUHeading3 | None | None | None | None | DAS1 | None | None | DMU04
5 | Qa | Alluvium | Alluvium | Holocene and Upper Pliestocene? | Light brownish-gray (10YR 6/2), grayish-brown (10YR 5/2), and reddish-gray (5YR 5/2) sediment composed of stratified, moderately sorted, unconsolidated sand, silt, clay and rarely up to 10% pebbles. Upper 1.5 m (5 ft) of unit typically classified as silty clay loam and sandy clay (Pannell and others, 1988). Unit is composed almost entirely of sediment eroded locally from nearby bedrock units and deposited in stream channels and on floodplains of tributaries of the Los Pinos River and, near the northwestern corner of the quadrangle, to the Florida River. Deposits accumulated on slopes as much as 5° to 7°. Thickness varies; three water-well logs document thicknesses of 3.0 to 3.7 m (10 to 12 ft) (Sebol and others, 2023; Colorado Division of Water Resources, 2024) but locally thickness may be as much as 6 m (20 ft). Incised channels vary in depth below adjacent alluvium or fan (unit Qf) surfaces from <0.3 m (<1 ft) in some higher-elevation parts of the deposits to as much as 5.2 m (17 ft) locally along larger streams. Holocene and Upper Pleistocene(?) age is estimated based on the unit’s height above and proximity to modern drainages (Johnstone and others, 2023). Surface disturbance resulting from agricultural practices is common. Low-lying areas are locally prone to periodic flooding and possibly over-bank sediment deposition. Unit is not a suitable source of sand and gravel. Clayey deposits have high shrink-swell potential (Pannell and others, 1988) and may be susceptible to hydrocompaction (White and Greenman, 2008). | 01-02-01 | DMUUnit1 | Qa | Qa | 255-255-190 | None | DAS1 | Alluvial sediment | High | DMU05
8 | Qac | Alluvium and colluvium, undivided | Alluvium and colluvium, undivided | Holocene and Upper Pleistocene? | Undifferentiated alluvium and colluvium consisting of light brownish-gray (10YR 6/2), grayish-brown (10YR 5/2), brown (7.5YR 5/2; 5/3), light brown (7.5YR 6/3), and reddish-gray (5YR 5/2) sediment composed of unconsolidated sand, silt, clay and rarely up to 10% matrix-support pebbles and cobbles. Upper 1.5 m (5 ft) of unit typically classified as silt loam, silty clay loam, clay loam, sandy clay, and clay (Pannell and others, 1988). Deposits on the Mesa Mountains are sandier than those in the lowlands and contain pebble- and cobble-sized fragments of San Jose Formation sandstone as well as reworked clasts from unit Qgmm. Unit was mapped mainly based on its topographic expression on lower hillsides and valley floors where individual deposits cannot be differentiated. These deposits accumulated as stream-channel, floodplain, fan, sheetwash, debris flows, and sediment transported by gravitational process such as creep. The uppermost deposits locally contain very weakly-developed soils with Bw and minor Bk horizons (Pannell and others, 1988), while deeper deposits may contain buried A horizons. Thicknesses as much as 6 m (20 ft) are common in water wells (Sebol and others, 2023; Colorado Division of Water Resources, 2024) but thickness locally exceeds 8.2 m (27 ft). Depths of incised channels range from <0.3 m (<1 ft) in higher-elevation parts of the deposits to locally as much as 3.7 m (12 ft) in the lowlands and 8.2 m (27 ft) in the Mesa Mountains. The unit’s age is estimated from its proximity to and height above modern drainages and its position in the landscape. Surface disturbance resulting from agricultural practices is common. Low-lying areas are locally prone to periodic flooding and possible sediment deposition. Unit is not a suitable source of sand and gravel. Clayey deposits may have high shrink-swell potential (Pannell and others, 1988) and may be susceptible to hydrocompaction (White and Greenman, 2008). | 01-02-02 | DMUUnit1 | Qac | Qac | 255-255-175 | None | DAS1 | Clastic sediment | Medium | DMU06
9 | None | Alluvium of the Los Pinos River | Alluvium of the Los Pinos River | None | River alluvium is present in modern stream channel (Qra) and in terrace deposits, the latter numbered from youngest (Qa1) to oldest (Qa4) based on the terminology of Gonzales and others (2008). Alluvium is commonly brown, dark gray, gray or yellowish-brown. It consists mainly of poorly sorted, clast-supported, weakly stratified, subrounded to well-rounded large pebble- to cobble-dominated gravel with subordinate amounts of finer pebbles and rare boulders up to 1 m (3 ft) in diameter in a matrix composed of sand, silt, or silty clay. The gravel may locally contain lenses of moderately to poorly sorted sand and silt that vary from brown to light olive-brown and gray. Beneath terrace surfaces, these units contain mostly hard metamorphic and igneous clasts of San Juan Mountains provenance and are resistant to erosion. The most common clasts are various quartzites. Distinctive and diagnostic to the Los Pinos drainage basin are prominent cobbles and boulders composed of silicic Vallecito metaconglomerate. Other clast types, in very roughly decreasing abundance, are porphyritic rhyolite, andesite, granite, granodiorite, gabbro, amphibolite, felsic gneiss, schist, limestone, siltstone, sandstone, greenstone, and vein quartz (Scott and Moore, 2007; Gonzales and others, 2008). Included in these units are overlying mantles of unmapped fine-grained deposits such as alluvial overbank flood deposits, loess, and local fan alluvium. Terrace gravel alluvium commonly occurs in treads that overlie relatively flat, river-cut bedrock surfaces (straths) and is often overlain by younger loess. Gravelly alluvium has been mined for sand and gravel in nearby quadrangles but not in the Ignacio quadrangle because of buildings, roads, infrastructure facilities, and riparian parklands within or near the town of Ignacio that is located in the northeast corner of the map area. | 01-02-05 | DMUUnit1 | None | None | None | None | DAS1 | None | None | DMU07
6 | Qra | Alluvium of the Los Pinos River | Alluvium of the Los Pinos River | Holocene | Unit includes the modern stream channel, floodplain, and low terrace deposits of the Los Pinos River that are generally less than 1.5 m (5 ft) above the modern river channel. Bar-and-swale topography occurs locally on the unit's surface. Clasts may be derived mainly from the reworking of older river alluvium, but may also be transported as bedload from farther upstream in the San Juan Mountains. Correlative units in the Bayfield quadrangle are unit Qal (Moore and Scott, 1995) and units Qa and Qsa (Gonzales and others, 2008). Several water well logs record thicknesses of about 4.3 to 6.7 m (14 to 22 ft) (Sebol and others, 2023; Colorado Division of Water Resources, 2024). Unit Qra is hydrologically connected to the river (Sebol and others, 2023) and is within zone A or AE of the 1% (100-year) regulatory floodplain (FEMA, 2024a,b; Colorado Water Conservation Board, 2024). | 01-02-05-01 | DMUUnit1 | Qra | Qra | 255-246-132 | 230-0-169 ESRI 24k geology 602 gravel (94.5 scale) | DAS1 | Alluvial sediment, mostly coarse-grained | High | DMU08
10 | Qa1 | Alluvium one | Alluvium one | Holocene and Upper Pleistocene? | Unit forms the first well-defined terrace above the Los Pinos River. Unit Qa1 is distinguished from lower unit Qra by a smoother tread surface and slightly higher elevation above the modern river channel, as much as 3.7 m (12.1 ft). Thin, up to 1.5 m (5 ft) of discontinuous mantles of unmapped Holocene alluvial and floodplain deposits are locally included. This unit underlies areas around the Tribal Fairgrounds and the Post Office in Ignacio. Correlative units on the Bayfield quadrangle are Qtg1 (Moore and Scott, 1995) and Qt1 (Gonzales and others, 2008). Unit Qa1 may be hydrologically connected to the river during stream flood flows (Sebol and others, 2023) and most of its surface may be within zone AE of the 1% (100-year) regulatory floodplain (FEMA, 2024a,b; Colorado Water Conservation Board, 2024). The unit’s estimated age (Johnstone and others, 2023) is based on its low height above the modern channel. | 01-02-05-02 | DMUUnit2 | Qa1 | Qa1 | 255-246-132 | 0-112-255 ESRI 24k geology 602 gravel (94.5 scale) | DAS1 | Alluvial sediment, mostly coarse-grained | High | DMU09
11 | Qa2 | Alluvium two | Alluvium two | Upper Pleistocene | Unit forms the next-higher terrace above the Los Pinos River and includes unmapped deposits of sheetwash and colluvium in hillside wedges at the foot of the slope that rise upslope toward the terrace surface underlain by unit Qa4 in the map area. Unit Qa2’s height above the modern river channel is about 4.3 to 5.8 m (14 to 19 ft) but is difficult to determine accurately because much of the town of Ignacio is built on unit Qa2 and terrace scarps are obscured or obliterated. Correlative units on the Bayfield quadrangle are Qtg1 (Moore and Scott, 1995), later called Qt2 (Gonzales and others, 2008). Outcrops and one water well log indicate thicknesses from 2.0 to 3.7 m (6.5 to 12 ft) (Sebol and others, 2023; Colorado Division of Water Resources, 2024), but the unit may be thicker locally. A few alluvial terraces along Rock Creek, in the northeast part of the quadrangle, are also mapped as unit Qa2. The age of this unit (Richmond, 1965; Johnstone and others, 2023) is estimated from its height above the river, which is roughly comparable to the height of unit Qa3 along the Florida River where an optically stimulated luminescence age estimate of 27,090 ± 1715 yrs was obtained (Lindsey and Gillam, in prep.). A higher alluvium along the Los Pinos River, intermediate between Qa2 and Qa4, was likely removed by fluvial erosion and was not identified in the Ignacio quadrangle. However, this unit Qa3 farther north in the nearby Bayfield quadrangle, is as much as 15 m (50 ft) above the Los Pinos River (Gonzales and others, 2008). | 01-02-05-03 | DMUUnit2 | Qa2 | Qa2 | 255-246-132 | 255-170-0 ESRI 24k geology 602 gravel (94.5 scale) | DAS1 | Alluvial sediment, mostly coarse-grained | High | DMU10
13 | Qa4 | Alluvium four | Alluvium four | Upper Pleistocene | Unit forms the highest terrace alluvium along the Los Pinos River in the Ignacio quadrangle, although higher terraces deposits are locally present farther upstream (Moore and Scott, 1995; Gonzales and others 2008; Rogers, in prep.), as well as east and downstream from the Ignacio quadrangle (Barnes, 1953; Condon, 1990). Unit Qa4 forms the relatively flat to gently sloping mesa west of the town of Ignacio where the elementary and middle schools are located, as well as another mesa south of town where the cemetery is located. Gravel in this unit is lithologically and texturally similar to that of units Qa1 and Qa2, but the bottoms of cobbles in unit Qa4 have calcium carbonate rinds 0.5 mm to 1.0 mm thick that effervesce when treated with dilute hydrochloric acid. The deposits locally contain up to 1 meter (3.3 ft) thick Bt and Bk (carbonate Stage II) soil horizons (Pannell and others, 1988) that are very poorly exposed. Included in unit Qa4 are overlying, poorly exposed, fine-grained deposits that are typically reddish brown (5YR 5/4) to yellowish red (5YR 5/6). These deposits of varying thickness consist of unconsolidated silt, clay, and sand that are generally classified as loam or silty clay loam in the upper 1.5 m (5 ft) of the deposits (Pannell and others, 1988). Most of the fine-grained deposits are probably loess, but locally reworked as sheetwash or low-gradient alluvial fan deposits. The highest levels of the gravelly deposits that underlie the fine-grained deposits are approximately 25.9 to 29.0 m (85 to 95 ft) above the modern river channel. However, a break in slope along the east edge of this terrace remnant suggests that the top of the alluvial gravel is roughly 22.9 to 24.4 m (75 to 80 ft) above the modern channel. Five water well logs record thicknesses of 0.6 to 6.7 m (2 to 22 ft) for the fine-grained deposits and 2.4 to 7.6 m (8 to 25 ft) for the underlying gravel, with total thickness about 6.7 to 9.1 m (22 to 30 ft) (Sebol and others, 2023; Colorado Division of Water Resources, 2024). The gravel-bedrock contact is as much as 21 m (70 ft) above the modern Pinos River channel. The correlative unit on the Bayfield quadrangle is the upper level of unit Qtg2 (Moore and Scott, 1995), later called unit Qt4 (Gonzales and others, 2008). The age of unit Qa4 in the Ignacio quadrangle, and of its correlatives along the Pine River, was previously estimated as Middle Pleistocene (Richmond, 1965; Johnstone and others, 2023). However, an optically stimulated luminescence age, 34,085 ± 1,605 years before present (Plate 2, Table 3), was obtained from a correlative deposit of the Los Pinos River downstream in the adjacent Tiffany quadrangle, revising the unit’s age to Upper Pleistocene. The dated sample was collected from a gravel quarry in the southeast quarter-quarter of sec. 16 T. 32 N., R. 7 W. This sample is located off this map area approximately 1.6 km (1 mile) east of the southeastern corner of the Ignacio quadrangle. Unit Qa4 along the Pine River could correlate either to unit Qa3 along the Animas and Florida rivers on the Loma Linda quadrangle (Lindsey and Gillam, in prep.), or it could be intermediate in age between units Qa3 and Qa4 on that quadrangle. Unit Qa4 may be a gravel resource if the fine-grained overburden is removed, but most of the unit is within or adjacent to the city limits of Ignacio. | 01-02-05-04 | DMUUnit2 | Qa4 | Qa4 | 255-246-132 | 0-255-255 ESRI 24k geology 602 gravel (94.5 scale) | DAS1 | Alluvial sediment, mostly coarse-grained | Medium | DMU11
17 | Qgl | Gravel lag | Gravel lag | Holocene to Middle Pleistocene? | Locally thin, up to 1.5 m (5 ft) thick deposits consisting of lag cobbles in a fine-grained matrix. Clasts are typically matrix-supported, and consist mostly of hard, rounded, weathering-resistant rock types. Local sandstone and mudstone commonly comprise up to 5% of the clasts. These originally alluvial and (or) colluvial deposits have been reworked mainly from older, topographically higher gravelly deposits such as old river alluvium that is no longer preserved in the area, older gravels on the Mesa Mountains (unit Qgmm), and old gravels (unit Qg) north of the Mesa Mountains. Deposits commonly consist of a cobbly layer that contains rare boulders but typically lacks granules, pebbles, sand, and certain clast types that have either been disintegrated by weathering processes or have been selectively removed during transport while the original source deposits were being eroded and redeposited. The present matrix usually consists of loess, loess-like material, and (or) sediment derived from nearby bedrock. Deposits commonly occur on ridge tops and flanks or on slopes where they protect underlying bedrock from erosion. The unit’s estimated Holocene to Middle(?) Pleistocene age is based on its relationship to old gravels (unit Qg), the presence of secondary calcium carbonate coatings up to 0.5 to 1.0 mm thick on the bottoms of some clasts, and the likelihood that some of the unit was reworked during Upper Pleistocene and Holocene time. | 01-02-06 | DMUUnit1 | Qgl | Qgl | 254-249-194 | 76-230-0 10% stipple | DAS1 | Sand and gravel of unspecified origin | Medium | DMU12
16 | Qg | Older gravel | Older gravel | Lower upper and Middle Pleistocene? | Cobble-rich gravel in a matrix composed of unconsolidated sand, silt, and clay. Clasts are mainly subangular to well-rounded, hard igneous and metamorphic rocks that resemble those in gravel lag (unit Qgl) and old gravel on Mesa Mountains (unit Qgmm), but unit Qg also includes very small amounts of locally sourced sedimentary rocks. The matrix consists of sediment derived from sedimentary rocks and probably loess-like sediment. These deposits cap low ridges that radiate out from the Mesa Mountains and are roughly parallel to the trends of adjacent streams with drainage basins limited to the flank of the Mesa Mountains. This pattern suggests that most of the gravel was initially eroded from sources on or near the mountain top by mass wasting and (or) alluvial-fan processes that also mobilized locally sourced materials. Alluvial-fan facets rise toward the mountain front but are no longer connected to the channel mouth or fan apex, which suggests subsequent erosion and southward retreat of the mountain front after deposition of the unit. In the unit, pebble-sized igneous and metamorphic clasts are less common than larger clasts of similar composition. Once deposited, the gravel armored the channel floors overlying relatively soft bedrock. Subsequent erosion and topographic inversion formed gravel-capped ridges between more recent lower-stream tributary deposits (units Qa and Qac). Unit Qg ranges in thickness from 0.6 to 2.0 m (2 to 6.5 ft). A lower Upper to Middle(?) Pleistocene age is inferred based on the height of unit Qg above adjacent drainages (as much as 45 m or 150 ft) and discontinuous carbonate coatings as much as 1 mm thick on the bottoms of some clasts. Some deposits near the northwestern corner of the quadrangle slope toward Middle Pleistocene terrace deposits of the Florida River in the adjacent Loma Linda quadrangle (Lindsey and Gillam, in prep.). The Upper Pleistocene age is based on the likelihood that some of deposits in unit Qg may have been reworked during Upper Pleistocene time by continued alluvial and (or) colluvial processes. | 01-02-07 | DMUUnit1 | Qg | Qg | 254-249-194 | 178-178-178 10% stipple | DAS1 | Alluvial sediment, mostly coarse-grained | Medium | DMU13
15 | Qgmm | Gravel on Mesa Mountains | Gravel on Mesa Mountains | Lower Pleistocene? | Rounded, typically clast-supported gravel with a matrix of reddish-gray-brown (5YR 5/2.5), reddish-brown (5YR 5/4) or light brown (7.5YR 6/3) sediment composed of unconsolidated sand, silt, and clay. Informally named unit (Richmond, 1965; Scott and Moore, 2007) consists of alluvium and colluvium that locally mantle the highest ridges and underlie stream terraces in the Mesa Mountains (e.g. Pantea, 1989; Gillam, 1998). Some gravel deposits are overlain by loess and thinner unmapped gravel deposits are common. Clasts are mostly cobbles and pebbles, but also include rare boulders up to 1 m (3 ft) in diameter. These clasts are composed mainly of igneous and metamorphic rocks of San Juan Mountains provenance, as well as smaller amounts of locally sourced sedimentary rocks. Except for the local rocks, clasts resemble rock types deposited by the Animas, Florida, and Los Pinos rivers. Unit Qgmm is poorly exposed except for one deposit in the Bondad Hill quadrangle (NE 1/4 sec. 35, T. 33 N., R. 9 W.), where it consists of clast-supported, stratified, gravelly alluvium as much as 4.9 m (16 ft) thick that contains lenses of silty clay (Pantea, 1989; Scott and Moore, 2007; McCalpin and Gillam, in prep.). Comparable thicknesses, fluvial-cut scarps, and paleochannels suggest that poorly exposed, southward-sloping deposits likely contain similar alluvium. One such deposit (SE ¼ sec. 17, T. 32 N., R. 8 W.), up to 4 m (13 ft) thick, has been mined for gravel while others range from 0.3 to 2.1 m (1 to 7 ft) thick, likely reworked into younger colluvium or alluvium. Near the southwest corner of the quadrangle (sec. 21, T. 32 N., R. 8 W.), reworked alluvium forms two terraces, 67 m (220 ft) and 91 m (300 ft) below the mined deposit, indicating at least three cycles of alluviation followed by stream incision. A lower inset deposit, 55 m (180 ft) above the canyon floor, aligns in height with Qg deposits in the northern part of the quadrangle. While the clasts originated from the San Juan Mountains, it is uncertain whether Qgmm represents ancient river alluvium or material reworked from older, eroded deposits. Cobbles of Vallecito Conglomerate, likely from the Los Pinos River, are found in the eastern Mesa Mountains, while clasts further west reflect Animas and Florida River sources. The varying heights of deposits suggest different ages, with many likely dating to the Lower Pleistocene, their elevations: ~275 m (900 ft) above the Los Pinos River and ~425 m (1,400 ft) above the Animas River. An earlier Upper Pliocene age proposal (Scott and Moore, 2007), was revised to Lower Pleistocene. Although one deposit was briefly mined, unit Qgmm is not economically viable for sand and gravel due to remoteness, weathered clasts, and a clayey matrix. | 01-02-08 | DMUUnit1 | Qgmm | Qgmm | 254-249-194 | 197-0-255 10% stipple | DAS1 | Alluvial sediment, mostly coarse-grained | High | DMU14
18 | None | EOLIAN DEPOSITS | EOLIAN DEPOSITS | None | None | 01-03 | DMUHeading3 | None | None | None | None | DAS1 | None | None | DMU15
19 | Qe | Loess | Loess | Holocene to Middle? Pleistocene | Reddish-brown (5YR 5/4) and yellowish-red (5YR 5/6) sediment that consists of unconsolidated silt, fine sand, and clay. Upper 1.5 m (5 ft) of unit typically classified as clay loam, silty clay loam, and loam (Pannell and others, 1988). Deposits up to 4.6 m (15 ft) thick were observed on relatively flat to gently sloping surfaces such as those formed on some deposits of old gravel (unit Qg) and older gravel on Mesa Mountains (unit Qgmm). Unit Qe locally includes unmapped deposits of sheetwash alluvium. Many deposits of unit Qe are too thin and (or) too discontinuously mantle other surficial units to be mapped individually. Loess deposits locally contain moderately to strongly developed Bt and Bk soil horizons (Pannell and others, 1988) that are poorly exposed. Elsewhere in the region, loess deposits thinner than about 2 m (6 ft) have been dated from approximately 50 to 2 ka in age, but basal parts of thicker deposits could be older (Reheis and others, 2017; Price and others, 1988). Along the Animas and Florida rivers, loess and reworked loess as much as 10.6 m (34.8 ft) thick, containing several buried paleosols, overlie alluvial terraces of probable Middle Pleistocene age (Gillam, 1998; Lindsey and Gillam, in prep.). An optically stimulated luminescence age estimate of 220,180 ± 17,520 yr was obtained from one deposit (Lindsey and Gillam, in prep.), suggesting that some basal loess in the Ignacio quadrangle may be as old as Middle Pleistocene. Unit locally has moderate shrink-swell potential (Pannell and others, 1988). | 01-03-01 | DMUUnit1 | Qe | Qe | 255-255-115 | None | DAS1 | Loess | High | DMU16
20 | None | MUD-FLOW, FAN, AND MASS-WASTING DEPOSITS | MUD-FLOW, FAN, AND MASS-WASTING DEPOSITS | None | None | 01-04 | DMUHeading3 | None | None | None | None | DAS1 | None | None | DMU17
21 | Qf | Debris-fan deposits | Debris-fan deposits | Holocene and Upper Pleistocene | Unit consists of light brownish-gray (10YR 6/2), brown (7.5YR 5/4) and pinkish-gray (5YR 6/2) sediment composed of unconsolidated clay, silt, sand, and locally small amounts of pebbles and cobbles, and is weakly stratified. The upper 1.5 m (5 ft) of unit are typically classified as silty clay loam, clay loam, and clay (Pannell and others, 1988). Unit was deposited at the base of the Mesa Mountains front mainly in broad fan-shaped deposits that were identified from lidar-derived 1-meter elevation contours. Some proximal deposits contain matrix-supported, angular clasts composed of sandstone or reworked rounded igneous and metamorphic clasts. Varying colors and clast types reflect distinctive, localized sediment sources such as purplish brown mudstones of the San Jose Formation or brownish loess and gravelly deposits. Deposits of unit Qf accumulated as stacked mud flows initiated by concentrated runoff events from adjacent, steep slopes composed mainly of erodible shale and mudstone. Downslope, as slope gradients flatten, these deposits transition to form alluvium and colluvium (units Qa and Qac). Not all deposits of unit Qf have a fan-shaped geometry, especially the older ones that have been buried or reworked. Ten water well logs indicate thicknesses ranging from about 4.3 to 30.5 m (14 to 100 ft) (Sebol and others, 2023; Colorado Division of Water Resources, 2024). Depths of incised channels vary from <0.3 m (<1 ft) in some higher-elevation deposits in the proximal parts of the fans, to as much as 4.3 m (14 ft) locally along larger streams. Deposits of unit Qf may be prone to hydrocompaction settlement (White and Greenman, 2008) and piping. They may also have expansive clay and high shrink-swell potential (Pannell and others, 1988). | 01-04-01 | DMUUnit1 | Qf | Qf | 255-255-190 | 115-178-255 ESRI 24k geology 602 gravel (47.25 scale) | DAS1 | Debris flows, landslides, and other localized mass-movement sediment | High | DMU18
22 | Qls | Landslide deposits | Landslide deposits | Holocene to Middle? Pleistocene | Unit is composed of sheared and chaotically mixed masses of disturbed mudstone, shale, clay, silt, sand, and sandstone, produced by gravitational failure and downslope movement on relatively steep slopes. Unit locally contains transported bedrock blocks. Landslides have formed along roughly half of the length of the Mesa Mountains front, where sandstone of the upper part of the Ditch Canyon Member of the San Jose Formation (unit PEsju) at the mountain crest overlies shale-dominated, landslide-prone Regina Member (unit PEsjr). In addition, higher elevations and northern to northeastern aspects likely promote higher seasonal soil moisture along the mountain front. Sandstone blocks of the Ditch Canyon Member (upper part) contained within the landslide deposit indicate that most of these blocks originated either at the crest or as colluvial debris farther downslope. Landslide deposits have not been differentiated by age or by movement type as defined by Varnes (1978). Younger landslide deposits commonly have fresh morphologic features; typically, some of the youngest deposits have fresh scarps, ruptured ground surfaces, and disturbed and sloughed vegetation. Older landslide deposits are more eroded, have muted morphologic features, and their vegetative cover has regrown. Landslide deposits may be as much as 50 m (165 ft) thick. Along with the soft underlying bedrock, loose disturbed landslide deposits are sources for Quaternary mudflow deposits (unit Qf) as well as other deposits (units Qac and Qa). The abundance of landslides and landslide deposits along the northern slopes of the Mesa Mountains indicates this area has been prone to mass wasting, both historic and prehistoric. Many of the older landslides may have occurred during colder and wetter climatic conditions when glaciers were present in the San Juan Mountains. Site-specific geological hazard evaluation, slope-stability analyses, and appropriate engineering considerations are recommended prior to construction, road building, water works, and placement of heavy loads on landslide deposits and geologically unstable slopes. Unit Qls may be susceptible to instability and mass-movement processes. | 01-04-02 | DMUUnit1 | Qls | Qls | 255-231-97 | None | DAS1 | Debris flows, landslides, and other localized mass-movement sediment | High | DMU19
23 | None | SEDIMENTARY BEDROCK GEOLOGY | SEDIMENTARY BEDROCK GEOLOGY | None | None | 02 | DMUHeading2 | None | None | None | None | DAS1 | None | None | DMU20
24 | None | San Jose Formation | San Jose Formation | Upper Paleocene and Lower Eocene? | The San Jose Formation consists of four members listed in ascending order: Cuba Mesa Member (coarse, locally conglomeratic, sandstone), lower part of the Ditch Canyon Member (sandstones and shales), Regina Member (variegated shale), and upper part of the Ditch Canyon Member (sandstone and shale). Sandstone in the San Jose Formation is arkosic compared to the older andesitic sandstone of the Animas Formation (Reeside, 1924). The weakly cemented San Jose Formation consist of fluvial sandstone and shale floodplain and channel deposits. Resistant sandstone beds are typically buff to rusty-yellow in color. Easily eroded shales are variegated white, purple, green, and red. In Colorado, the San Jose Formation contains the Tiffanian North American Land Mammal Ages (NALMA) zones Ti4 and Ti5 (Sloan, 1987). A recent palynological determination (see Table 2, Plate 2) suggests that the San Jose contains pollen zones (P5) of Nichols and Ott (1978) which corresponds to Tiffanian age (Upper Paleocene). This unit includes the following mapped members. | 02-01 | DMUUnit1 | :sj | PEsj | None | 168-168-0, 0.4 pt, 45°, 5 pt separation | DAS1 | Sandstone and mudstone | High | DMU21
25 | PEsju | Ditch Canyon Member, upper part | Ditch Canyon Member, upper part | Upper Paleocene and Lower Eocene? | The upper part of the Ditch Canyon Member is an informally named unit (Smith, 1992) that is the highest stratigraphic member of the San Jose Formation observed on the Ignacio quadrangle. The unit is a light-gray to buff-colored, medium- to fine-grained, cross-bedded sandstone. Distinctively cliff-forming sandstone beds thicker than 3 m (10 ft) overlie the erosion-prone underlying shales along the north rim of the Mesa Mountains. On the northernmost part of the Mesa Mountains, there are two distinct resistive sandstone ledges in the unit. Further south in La Boca Canyon, there are as many as five identifiable resistive sandstone ledges. In the mapped area, the basal contact of the Ditch Canyon Member (upper part) was mapped progressively lower in the section toward the south at the base of the lowest traceable sandstone overlying the main body of Regina Member shale. The thickness of the Ditch Canyon Member (upper part) thins northward from 100 m (330 ft) in La Boca Canyon (near southeast corner of map area) to 10 m (33 ft) along the northernmost rim of the Mesa Mountains. One detrital zircon sample was collected from the southeast quarter of sec. 21, T. 33 N., R. 8 W. (Plate 2, Table 1). Sample IG22-DZ09 was collected from a gray-tan sandstone outcrop and yielded a youngest age population of 75.3 ± 1.4 Ma (n = 5; Mean Square Weighted Deviate (MSWD) = 0.57; youngest single grain of 68.4 ± 5.5 Ma) indicating a recycling of zircons from Upper Cretaceous strata. A fossil pollen sample (IG22-PL01) from a hillside trench in the southwest quarter of sec. 35, T. 33 N., R. 8 W. was interpreted to be zone P5 of Tiffanian age (Plate 2, Table 2), but more study of paleontology and palynology is needed. | 02-01-01 | DMUUnit2 | :sjud | PEsju | 137-90-68 35% transparency | None | DAS1 | Sandstone and mudstone | High | DMU22
26 | PEsjr | Regina Member | Regina Member | Upper Paleocene | Unit is variegated shale and mudstone with light-gray, maroon red, white, lavender, and brown colors that give the unit a banded appearance where exposed in hill slopes of the Mesa Mountains. The unit forms colorful badlands where exposed. Sandstone beds are very rare. This shale unit interfingers with the contemporaneous and overlying Ditch Canyon Member (upper part) sandstones, as seen near La Boca Canyon. The total thickness of the Regina Member in the Ignacio quadrangle is 223 m (670 ft) but it thins to the south as a sandier facies of the Ditch Canyon Member is more prevalant. The basal contact of the Regina Member is identified at the top of the first occurrence of a regionally continuous sandstone, when present, in the underlying undivided lower San Jose and Nacimiento Formations. The upper contact of the Regina Member is the base of the lowest traceable sandstone overlying the main sequence of banded shale strata. Good exposures of the Regina Member on the northern slope of the Mesa Mountains are discontinuous due to the presence of landslide deposits. | 02-01-02 | DMUUnit2 | :sjr | PEsjr | 236-166-138 | None | DAS1 | Mostly mudstone | High | DMU23
41 | PEsn | San Jose and Nacimiento Formations, undivided | San Jose and Nacimiento Formations, undivided | Upper Paleocene | Unit includes the undivided San Jose Formation Ditch Canyon Member (lower part), Cuba Mesa Member, and the uppermost portion of the underlying Nacimiento Formation, listed in order from top to bottom. These were mapped as an undifferentiated unit in the Ignacio quadrangle due to the difficulty in determining formation and member boundaries in outcrops. The lower part of the Ditch Canyon Member is an informally named unit (Smith, 1992) that is a sequence of buff-colored, arkosic, fluvial-channel sandstone benches interspersed with grayish to green slope-forming mudstones. The sandstone benches are similar in lithology and sedimentary character to that of the underlying Cuba Mesa Member, but lack the lenses of very coarse, conglomeratic facies that are diagnostic of the Cuba Mesa Member. No fossils have been observed or reported in the Ditch Canyon Member (lower part) (Smith, 1992). Further detailed paleontological investigation is needed. The Cuba Mesa Member is the basal unit of the San Jose Formation and consists of a sequence of buff to rusty-yellow, coarse-grained, sandstone that overlies gray shale sequences of the underlying Nacimiento Formation. The Cuba Mesa Member is typically 1.6 meters (5 ft) thick. Diagnostic of the Cuba Mesa Member sandstone interval is the unique coarseness of arkosic grains compared to sandstone strata in other members of the San Jose Formation. Although sand grain size ranges from fine to very coarse, there are local granule laminae less than 2.5 cm thick (1 inch) and lenses of coarse sand as much as 1.6 m (5 ft) thick (Barnes, 1953). Crossbedding occurs in laminated beds. The Cuba Mesa Member is recognized where it forms resistant cliffs in the adjacent Bondad Hill quadrangle (McCalpin and Gillam, in prep.). At the western boundary of the Ignacio quadrangle, the Cuba Mesa Member is not exposed. The Nacimiento Formation is a nonresistant greenish-gray shale with local beds more than 6 m (20 ft) thick composed of fine-grained, well-sorted sandstone. The Animas Formation mapped to the north in the Gem Village quadrangle (Rogers, in prep.) and Nacimiento Formation mapped in this area are coeval (Fassett and Hinds, 1971; Cather and others, 2019), gradational to each other, and locally interfinger. Nacimiento strata in the Ignacio quadrangle are included in this undifferentiated unit due to poor exposures where the unit is weathered or present in gently sloping areas that typically are locally mantled by Quaternary deposits. Two detrital zircon samples were collected from sandstone within the undivided Lower San Jose and Upper Nacimiento Formations (Plate 2, Table 1). Sample IG22-DZ03 was taken from the southeast quarter of sec. 24, T. 33 N., R. 8 W., from a roadcut through tan sandstone along La Plata County Road 318. This sample yielded a youngest-age population of 70.7 ± 1.7 Ma (n = 5; MSWD = 0.58) and a youngest single grain of 67.2 ± 3.5 Ma. Sample IG22-DZ10 is from the southeast quarter of sec. 17, T. 33 N., R. 7 W. from a tan sandstone along a Colorado Highway 172 roadcut. It yielded a youngest-age population of 71.6 ± 1.5 Ma (n = 5; MSWD = 0.27; youngest single grain of 68.6 ± 3 Ma). These two detrital zircon analyses indicate recycling of zircons from Upper Cretaceous strata. | 02-02 | DMUUnit1 | :sn | PEsn | 221-199-177 | None | None | Sandstone and mudstone | High | DMU24
27 | PEn | Nacimiento Formation | Nacimiento Formations | Paleocene | Unit is composed of greenish-gray shale and local beds composed of fine-grained, well-sorted sandstone. Shown in cross section only. | 02-03 | DMUUnit1 | :n | PEn | 205-137-102 | None | DAS1 | Mostly mudstone | High | DMU25
30 | Kkl | Kirtland Formation | Kirtland Formation | Upper Cretaceous | Unit is composed of shale and sandstone deposited in a non-marine environment. Shown in cross section only. | 02-04 | DMUUnit1 | Kkl | Kkl | 187-217-117 | None | DAS1 | Sandstone and mudstone | High | DMU26
31 | Kf | Fruitland Formation | Fruitland Formation | Upper cretaceous | The unit is composed of shale, discontinuous non-marine sandstone, and is a significant coal-bearing unit. Shown in cross section only. | 02-05 | DMUUnit1 | Kf | Kf | 104-150-104 | None | DAS1 | Sandstone and mudstone | High | DMU27
32 | Kpc | Pictured Cliffs Sandstone | Pictured Cliffs Sandstone | Upper Cretaceous | Unit is composed of primarily marine light-gray sandstone interbedded with dark-gray shale. Shown in cross section only. | 02-06 | DMUUnit1 | Kpc | Kpc | 163-255-115 | None | DAS1 | Mostly sandstone | High | DMU28
33 | Kl | Lewis Shale | Lewis Shale | Upper Cretaceous | Unit is composed of dark-gray marine claystone and shale and contains the Huerfanito Bentonite bed, a significant regional marker bed in oil and gas geophysical logs. Shown in cross section only. | 02-07 | DMUUnit1 | Kl | Kl | 168-168-0 | None | DAS1 | Mudstone | High | DMU29
34 | None | Mesaverde Group | Mesaverde Group | Upper Cretaceous | Stratigraphic group composed of the Cliff House Sandstone, Menefee Formation and Point Lookout Sandstone. | 02-08 | DMUHeading3 | None | None | None | None | DAS1 | None | None | DMU30
35 | Kch | Cliff House Sandstone | Cliff House Sandstone | Upper Cretaceous | Marine unit composed of sandstone and minor shale. Shown in cross section only. | 02-08-01 | DMUUnit2 | Kch | Kch | 176-204-137 | 197-0-255, 0.5 pt, 135°, 5 pt separation | DAS1 | Mostly sandstone | High | DMU31
36 | Kme | Menefee Formation | Menefee Formation | Upper Cretaceous | Non-marine unit composed of fluvial sandstone, overbank shales, and coal beds. Shown in cross section only. | 02-08-02 | DMUUnit2 | Kme | Kme | 176-204-137 | 255-255-0, 0.5 pt, 0°, 5 pt separation, dotted 1.5 1.5 | DAS1 | Coal and lignite | Medium | DMU32
37 | Kpl | Point Lookout Sandstone | Point Lookout Sandstone | Upper Cretaceous | Marine unit composed of sandstone and minor shale. Shown in cross section only. | 02-08-03 | DMUUnit2 | Kpl | Kpl | 100-200-175 | None | DAS1 | Mostly sandstone | High | DMU33
38 | Km | Mancos Shale | Mancos Shale | Upper Cretaceous | Marine unit composed of dark-gray shale. Shown in cross section only. | 02-09 | DMUUnit1 | Km | Km | 176-204-137 | None | DAS1 | Mudstone | High | DMU34
39 | Kdb | Dakota Formation and Burro Canyon Formation, undivided | Dakota Formation and Burro Canyon Formation, undivided | Upper and Lower Cretaceous | This unit is composed of sandstone interbedded with siltstone and shale. The deposits are predominantely non-marine, however, the upper Dakota Formation sandstone beds are marine, showing evidence of near-shore influence. Shown in cross section only. | 02-10 | DMUUnit1 | Kdb | Kdb | 178-178-102 | None | DAS1 | Mostly sandstone | High | DMU35
40 | Jm | Morrison Formation | Morrison Formation | Upper Jurassic | Non-marine unit composed of varicolored mudstone and sandstone. Shown in cross section only. | 02-11 | DMUUnit1 | Jm | Jm | 132-205-205 | None | DAS1 | Sandstone and mudstone | High | DMU36

#### Glossary

OBJECTID | Term | Definition | DefinitionSourceID | Glossary_ID
--- | --- | --- | --- | ---
19 | 1 SD | A statistic used as a measure of the dispersion or variation in a distribution or set of data, equal to the square root of the arithmetic mean of the squares of the deviations from the arithmetic mean. | DICT1 | GLO19
12 | Age | the length of time during which a being or thing has existed; length of life or existence to the time spoken of or referred to | DICT1 | GLO01
9 | Anticline | A fold in rock strata with a convex upward shape. The rocks in the core of an anticline are the oldest. Identity and existence certain, location accurate | GEODICT1 | GLO02
26 | Bedding | Bedding refers to a measurement convention used to describe the orientation, or attitude, of a planar geologic feature. A feature's strike is the azimuth of an imagined horizontal line across the plane, and its dip is the angle of inclination measured downward from horizontal. | GEODICT1 | GLO26
23 | Bentonite | Soft clay or greasy claystone composed largely of smectite formed by the chemical alteration of glassy volcanic ash in contact with water. It often contains accessory crystal grains that were originally phenocrysts in the parent rock. The rock commonly has the ability to absorb large quantities of water accompanied by a large increase in volume that can result in a thixotropic gel. The term "taylorite" was used by Knight (1898), after the owner of a quarry near Rock Springs in the Wyoming territory (see Taylor, 1897). It was later renamed "bentonite" after the Benton Formation (formerly Fort Benton Formation) in eastern Wyoming. Syn: volcanic clay; soap clay; mineral soap; amargosite. Cf: fuller's earth. (b) A commercial term applied to clay deposits (especially bentonite) containing smectite as the essential mineral. This clay presents a very large total surface area, swells in water, and is used chiefly to thicken oil-well drilling mud. (c) Any clay composed dominantly of a smectite clay mineral whose physical properties are dictated by this mineral (Grim and Güven, 1978, p.1). | GEODICT1 | GLO23
15 | Bentonite bed | Soft clay or greasy claystone composed largely of smectite formed by the chemical alteration of glassy volcanic ash in contact with water. It often contains accessory crystal grains that were originally phenocrysts in the parent rock. The rock commonly has the ability to absorb large quantities of water accompanied by a large increase in volume that can result in a thixotropic gel. The term "taylorite" was used by Knight (1898), after the owner of a quarry near Rock Springs in the Wyoming territory (see Taylor, 1897). It was later renamed "bentonite" after the Benton Formation (formerly Fort Benton Formation) in eastern Wyoming. Syn: volcanic clay; soap clay; mineral soap; amargosite. Cf: fuller's earth. (b) A commercial term applied to clay deposits (especially bentonite) containing smectite as the essential mineral. This clay presents a very large total surface area, swells in water, and is used chiefly to thicken oil-well drilling mud. (c) Any clay composed dominantly of a smectite clay mineral whose physical properties are dictated by this mineral (Grim and Güven, 1978, p.1). | GEODICT1 | GLO15
4 | boundary | A line that marks the limits of an area | DICT1 | GLO03
1 | certain | Identity of a feature can be determined using relevant observations and scientific judgment; therefore, one can be reasonably confident in the credibility of this interpretation. | FGDC-STD-013-2006 | GLO04
3 | contact | A geological contact is a boundary which separates one rock body from another. A contact can be formed during deposition, by the intrusion of magma, or through faulting or other deformation of rock beds that brings distinct rock bodies into contact. | GEODICT1 | GLO05
11 | Deposit type | Groupings for surficial units in the CMU | GEODICT1 | GLO06
7 | DMUHeading1 | GeMS hierarchy formatting term | GeMS1 | GLO07
8 | DMUHeading2 | GeMS hierarchy formatting term | GeMS1 | GLO08
13 | DMUHeading3 | GeMS hierarchy formatting term | GeMS1 | GLO13
5 | DMUUnit1 | GeMS hierarchy formatting term | GeMS1 | GLO09
6 | DMUUnit2 | GeMS hierarchy formatting term | GeMS1 | GLO10
18 | DZ | A mineral: ZrSiO4 . It occurs in tetragonal prisms, has various colors and is a common accessory mineral in siliceous igneous rocks, crystalline limestones, schists, and gneisses, in sedimentary rocks derived therefrom, and in beach and river placer deposits. It is the chief ore of zirconium, and is used as a refractory; when cut and polished, the colorless varieties provide exceptionally brilliant gemstones. Syn: zirconite; hyacinth ; jacinth. (b) A group name for tetragonal minerals with the general formula A SiO4, where A = Zr, Hg, Th, or U. Pertaining to or formed from detritus; said esp. of rocks, minerals, and sediments. The term may indicate a source outside the depositional basin (Krynine, 1948, p.133) or a source within it. | GEODICT1 | GLO18
25 | ELEV_FT | A hatch mark shown on the edges of geologic cross sections to denote the elevation in feet | DICT1 | GLO25
22 | ELEV_M | A hatch mark shown on the edges of geologic cross sections to denote the elevation in meters | DICT1 | GLO22
28 | High | unusual or considerable in degree, power, intensity, etc. | DICT1 | GLO28
10 | Line of Section | A graphic representation of the intersection of the geological bodies in the subsurface with a vertical plane of a certain orientation showing relationships between rock units. | GEODICT1 | GLO11
16 | Ma | mega annum or millions of years | GEODICT1 | GLO16
27 | mean square weighted deviation (MSWD) | statistical error measured in age date analysis | GEODICT1 | GLO27
29 | Medium | something intermediate in nature or degree | DICT1 | GLO29
24 | O&G well | An oil well is a boring in the Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released as associated petroleum gas along with the oil. A well that is designed to produce only gas may be termed a gas well. | GEODICT1 | GLO24
17 | OSL | Optically-Stimulated Luminescence is a late Quaternary dating technique used to date the last time quartz sediment was exposed to light. As sediment is transported by wind, water, or ice, it is exposed to sunlight and zeroed of any previous luminescence signal. | GEODICT1 | GLO17
20 | Pollen | The several-celled microgametophyte of seed plants, enclosed in the microspore wall. Fossil pollen consists entirely of the microspore wall or exine, from which the microgametophyte itself was removed during or before lithification. The term "pollen" is a collective plural noun, and it is incorrect to say "a pollen". See also: pollen grain. | GEODICT1 | GLO20
30 | questionable | Identity of a feature cannot be determined using relevant observations and scientific judgment; therefore, one cannot be reasonably confident in the credibility of this interpretation. For example, IdentityConfidence = questionable is appropriate when a geologist reasons "I can see some kind of planar feature that separates map units in this outcrop, but I cannot be certain if it is a contact or a fault." | FGDC-STD-013-2006 | GLO30
14 | surface | The topographic profile of the cross section | DICT1 | GLO14
21 | yr | the time taken by the earth to make one revolution around the sun | DICT1 | GLO21

#### MiscellaneousMapInformation

OBJECTID | MapProperty | MapPropertyValue | MiscellaneousMapInformation_ID
--- | --- | --- | ---
1 | TITLE | GEOLOGIC MAP OF THE IGNACIO QUADRANGLE, LA PLATA COUNTY, COLORADO | MMI01
2 | AUTHOR | NATHAN T. ROGERS AND MARY L. GILLAM | MMI02
3 | DOI | https://doi.org/10.58783/cgs.of2302.xomi4718 | MMI03
4 | YEAR | 2024 | MMI04
5 | STATEMAP AGREEMENT NUMBER | This mapping project was funded jointly by the Colorado Geological Survey and the U.S. Geological Survey through the National Cooperative Mapping Program under STATEMAP agreement G22AC00302. | MMI05
6 | INTRODUCTION | The Ignacio quadrangle lies in La Plata County, Colorado. The town of Ignacio is in the northeast corner of the quadrangle. Land ownership within the quadrangle is approximately 57.7% Southern Ute Indian Tribe, 41.8% private, and 0.44% state, county, or town (La Plata County Assessor’s Office, 2024). Four main physiographic areas in the quadrangle are: the low-lying Los Pinos River valley near the northeastern corner of the quadrangle; La Boca Canyon near the southeastern corner; the high Mesa Mountains (as locally known), mostly in the southern half; and more subdued mid-elevation drainage swales and low ridges. Several major drainage divides occur in the map area. The highest point on the quadrangle is at 2284.3 m (7495 ft) in the Mesa Mountains along the western boundary of the quadrangle. The highest point of the Mesa Mountains is 55.2 m (181 ft) higher one mile west on the Bondad Hill quadrangle, coinciding with the stratigraphically highest and youngest bedrock strata in the area. The lowest elevation on the quadrangle is at 1953.1 m (6304 ft) where La Boca Canyon leaves its southeast boundary as it drains toward the Los Pinos River. Most seasonal streams on the Mesa Mountains are south-flowing tributaries to the San Juan River, whereas those in the northeastern portion of the quadrangle flow easterly to the Los Pinos River. An unnamed seasonal stream near the northwestern corner of the quadrangle flows northwesterly toward the Florida River. Precipitation varies across the mapped area with the higher Mesa Mountains receiving more snowfall and having seasonally longer snow retention than the 260 m (853 ft) lower lowlands in the town of Ignacio. The average annual precipitation at the town of Ignacio is 34.3 cm (13.5 inches), with the lowest amount in June (1 cm or 0.4 inches), and the highest amount in August (5 cm or 1.97 inches) (Colorado Climate Center, 2024). Snow may fall from November through May, with an annual average of 0.86 m (33.8 inches) and the highest amount in February (0.25 meters or 9.8 inches). | MMI06
8 | STRUCTURE | Most strata in the Ignacio quadrangle dip about 2° to the south, like other strata in the central part of the San Juan Basin. However, moderate dips are recorded along the Hogback monocline which forms the northern basin margin 21 km (13 miles) to the north. The Ignacio-Bondad anticline is the main structural feature in the quadrangle and is unique in the central part of the basin (Fassett and Boyce, 2005, p. 113). The location of the Ignacio-Bondad anticline for this map was based on its subsurface expression at the base of the Dakota Formation (Anderson, 1995; Steven and others, 1974) where it has more than 30.5 m (100 ft) of closure in the Dakota Group (Bowman, 1978, p. 132-133). Surface dips and closure are less in outcrops of the San Jose Formation. Such upward-decreasing dips (Reeside, 1924) from older to younger units indicate that the structure was likely active during or between deposition of the San Jose and Nacimiento formations. Earlier mappers and the present authors have had difficulty in accurately measuring low surface dips that are consistent with the axial trend of the anticline as determined from subsurface data, probably because of the lenticular nature of fluvial sandstones in the San Jose Formation (Barnes, 1953). | MMI07
9 | GEOLOGIC HAZARDS | The major hazard in the Ignacio quadrangle is the risk of landslides along the northern flank of the Mesa Mountains, as indicated by the presence of recent and historic landslide deposits. Steep areas along the northern flank are also subject to rockfall and debris-flow flooding. The siting of oil and gas well pads, roadways, pipelines, and other human-made features along these steep, unstable shale slopes should be based on consideration of these hazards. Debris flows and thick alluvial mud fans could plug current drainage channels, cause channel avulsion, and bury and damage or destroy roads, structures, driveways, exposed pipelines, and culverts. Low-lying areas along the Los Pinos River may be subject to flooding (FEMA, 2024a,b; Colorado Water Conservation Board, 2024). In addition, occasional heavy rains have caused flash flooding along ephemeral streams in the area, sometimes locally depositing alluvium on and near county roads. Many of the bedrock and surficial units are easily erodible and have moderate to high shrink-swell potential (Pannell and others, 1988; Noe and others, 2014). Also, poorly consolidated low-density surficial units may be hydrocompactive, and locally prone to settling, forming piping voids, and collapsing when wetted (White and Greenman, 2008). In general, surficial deposits are not well exposed without trenching or drilling. Detailed investigation for site planning and construction projects is typically needed on a site-by-site basis. | MMI08
10 | MINERAL RESOURCES | Deposits of sand and gravel are present mainly along the Los Pinos River and beneath its alluvial terraces. They also occur in a few small areas on top of the Mesa Mountains in unit Qgmm. One such deposit (SW ¼ sec. 17, T. 32 N., R. 8 W.) has been used as an aggregate source for construction of roadways and gas facilities in the Mesa Mountains. Elsewhere on the Ignacio quadrangle, sand and gravel are unlikely to be extracted commercially as aggregate, either because the deposits are thin, or because they underlie developed areas in and near the town of Ignacio. However, there is some potential for limited use by local landowners as borrow material or fill. | MMI09
11 | GROUNDWATER RESOURCES | There are 409 records of constructed water wells in the Ignacio quadrangle as of January 2024 (Colorado Division of Water Resources, 2024), although records may not exist for some of the older wells. The majority of recorded wells (269) are for industrial use, of which 35 in the Mesa Mountains have depths from 790 to 1,675 m (2,600 to 5,500 ft). Fewer wells (132) are for other uses such as commercial, domestic, household, or stock watering. These wells are all in lower-elevation areas of the quadrangle. Almost all of these wells produce groundwater from bedrock, which is mostly shale, but seven produce from alluvium near the Los Pinos River valley or on river terraces. Depths for these wells range from 3.6 to 146.6 m (12 to 481 ft). The occurrence of water varies, but commonly shallow water is encountered at depths of 7.6 to 30.5 m (25 to 100 ft) and more water is present an additional 9 to 30 m (30 to 100 ft) lower. Static water levels in wells, after the water level has stabilized, range from 6 to 95 m (20 to 310 ft). Numerous springs indicate that there is a perched water table near the base of unit Qa4 that is likely recharged by seasonal runoff and irrigation water applied to the terrace top. This water table would likely drop and spring discharges could decline if irrigation decreases in the future. In this quadrangle, some ground-water locations have selenium concentrations that exceed the allowable maximum contaminant level (MCL) established by the Environmental Protection Agency (EPA) which is 0.05 mg/L (Butler and others, 1993, p. 1). Ground and surface waters have elevated concentrations of contaminants, especially in the northern portion of the Ignacio quadrangle. In U.S. Geological Survey studies (Butler and others, 1993), Rock Creek surface-water samples have concentrations for selenium (45 µg/l, p. 13), cadmium (4 µg/l, p. 54), and manganese (210 µg/l, p. 65) that are greater than the MCL. Surface water from the Oxford tract of the Gem Village quadrangle, adjoining the north edge of the Ignacio quadrangle, has selenium concentrations as high as 94 µg/l (Butler and others, 1993, p. 54). Groundwater in the Oxford tract has one of the highest selenium concentrations of 4,800 ug/L (Butler and others, 1993, p. 54), 96 times the allowable MCL for human consumption. Overlaying the maps of Brogden and others (1979, Plate 1), with selenium maximum contaminant level exceedance maps of Sebol and others (2023, Fig. D-9), and this geologic map suggests that the area of elevated selenium concentrations roughly coincides with the locations of the Nacimiento-San Jose contact and Ignacio-Bondad anticline. Further work is needed to confirm this potential geological control on selenium content in ground water because no wells in the Mesa Mountains were tested. | MMI10
12 | PETROLEUM RESOURCES1 | As of January 2024, there are 642 oil and gas wells in the Ignacio quadrangle: 512 producing gas, 57 plugged and abandoned, 46 shut-in, 15 drilled and abandoned, 5 temporarily abandoned, 5 permitted, and 2 injection wells (Colorado Energy and Carbon Management Office, 2024). Most are in the Ignacio-Blanco field, one of the largest gas fields in the United States (Coalson, 2014). Harr (1988), Fassett and Boyce (2005), and Fassett (2010) provide more information for the Colorado portion of the northern San Juan Basin. Historically, gas production was from the Morrison and Burro Canyon Formations and Dakota Group along the Ignacio-Bondad anticline. Early drilling followed surface mapping of the Ignacio-Bondad anticline. Development of gas from the Mesaverde Group in the 1950's, early 1960's, and late 1970's to early 1980's was primarily from the Point Lookout Sandstone. Currently the vast majority of gas in the area is being produced from the Fruitland-Pictured Cliffs gas “pool,” as shown by well counts and cumulative production. This includes gas from the basal Pictured Cliffs Sandstone, the overlying coals of the Fruitland Formation, and sandstones above the coals. Coalbed methane (CBM) is produced by pumping large amounts of water from the coal seams, thereby reducing water pressure and freeing gas molecules that had been adsorbed onto the coal-cleat surfaces to flow toward the well bore. Coalbed methane production began in the 1980’s and dominated activity for the following decades. More recently horizontal drilling into the coal beds and testing of Lewis and Mancos shales have occurred in the area. There are two deep test wells in the Ignacio quadrangle. The deepest oil and gas well in the entire San Juan Basin is in this quadrangle, the 1976 Amoco Jessie Hahn #11 (API: 05-067-06114). This well was drilled to a total depth of 4,420 m (14,503 ft) in SE ¼ SW ¼ sec. 15, T. 33 N., R. 8 W., penetrating 65.5 m (215 ft) into the Precambrian basement rocks. Only one other well also penetrated Precambrian basement in the Colorado portion of the San Juan basin. Another significant deep well is the 1955 Stanolind Oil & Gas Ute Indian “B” 6 well (API: 05-067-05416), which reached the Carboniferous Molas Formation at a depth of 4,001 m (13,127 ft) in SE ¼ SE ¼ sec. 17, T. 33 N., R. 7 W. This is the second deepest test well of only four in the Colorado portion of the San Juan Basin and the 11th deepest well in the entire San Juan Basin. These two wells, and three others that penetrate the Entrada Sandstone in the Ignacio quad, provide the best subsurface control in the quadrangle for strata older and deeper than those of the Dakota Group. The formations penetrated and logged below the Dakota in these wells include the Morrison, Entrada, Shinarump, Cutler, Hermosa, Molas Formations, and Precambrian rock. The units below the Morrison Formation are not shown in the map cross section. | MMI11
13 | REFERENCES 1 | Anderson, R.C., 1995, The oil and gas opportunities on Indian lands: Exploration policies and procedures (1995 edition): Golden, Colo., U.S. Department of the Interior, Bureau of Indian Affairs, Division of Energy and Mineral Resources, General Publication G-95-3, 149 p. Atwood, W.W., and Mather, K.F., 1932, Physiography and Quaternary geology of the San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 166, 171 p., https://doi.org/10.3133/pp166. Aubrey, W.M., Molenaar, C.M., and Baird, J.K., 1991, Geologic framework and stratigraphy of Cretaceous and Tertiary rocks of the Southern Ute Indian Reservation, southwestern Colorado: U.S. Geological Survey Professional Paper 1505-B, 13 p., https://doi.org/10.3133/pp1505bc. Baltz, E.H., 1967, Stratigraphy and regional tectonic implications of part of Upper Cretaceous and Tertiary rocks adjacent to the Cretaceous-Tertiary boundary, western San Juan basin, New Mexico: U.S. Geological Survey Professional Paper 552, 101 p., https://doi.org/10.3133/pp552. Barnes, H., 1953, Geology of the Ignacio area, Ignacio and Pagosa Springs quadrangles, La Plata and Archuleta Counties, Colorado: U.S. Geological Survey Oil and Gas Investigations Map OM 138, scale 1:63,360. https://doi.org/10.3133/om138. Barnes, H., Baltz, E.H., Jr., and Hayes, P.T., 1954, Geology and fuel resources of the Red Mesa area, La Plata and Montezuma Counties, Colorado: U.S. Geological Survey Oil and Gas Investigations Map OM-149, scale 1:62,500. https://doi.org/10.3133/om149. Bowman, K.C., 1978, Ignacio Blanco Dakota (gas), in Fassett, J.E., Thomaidis, N.D., Matheny, M.L, and Ullrich, R.A., eds., Oil and gas fields of the Four Corners area, v. 1: Durango, Colo., Four Corners Geological Society, p. 131-133. https://archives.datapages.com/data/fcgs/data/014a/014001/pdfs/131.pdf Brimhall, R.M., 1973, Ground water hydrology of the San Juan Basin, New Mexico, in Fassett, J.E., ed., Cretaceous and Tertiary rocks of the southern Colorado Plateau: Durango, Colo., Four Corners Geological Society Memoir, p. 197-207. https://archives.datapages.com/data/fcgs/data/011/011001/pdfs/197.pdf. Brogden, R.E., Hutchinson, E.C., and Hillier, D.E., 1979, Availability and quality of ground water, Southern Ute Indian Reservation, southwestern Colorado: U.S. Geological Survey Water Supply Paper 1576, 28 p., pl. 1, scale 1:250,000. https://doi.org/10.3133/wsp1576J. Butler, D.L., Krueger, R.P., Thompson, A.L., Formea, J.J, and Wickman, D.W., 1993, Reconnaissance investigation of water quality, bottom sediment, and biota associated with irrigation drainage in the Pine River project area, Southern Ute Indian Reservation, southwestern Colorado and northwestern New Mexico, 1988-89: U.S. Geological Survey Water-Resources Investigations Report 92-4188, 105 p. https://doi.org/10.3133/wri924188. Butler, R.F., Gingerich, P.D. and Lindsay, E.H., 1981, Magnetic polarity stratigraphy and biostratigraphy of Paleocene and lower Eocene continental deposits, Clarksfork Basin, Wyoming: Journal of Geology v. 89, p. 299-316. https://doi.org/10.1086/628593. Butler, R.F., and Lindsay, E.H.., 1985, Minerology and magnetic minerals and revised magnetic polarity stratigraphy of continental sediments, San Juan Basin, New Mexico: Journal of Geology, v. 94, p. 535-554. https://doi.org/10.1086/628979. Carpenter M.B., and Keane C.M., eds., 2023, The geoscience handbook, AGI data sheets (5th ed.): Alexandria, Virginia, American Geosciences Institute, 494 p., ISBN: 978-0-913312-47-6. Cather, S.M., Heizler, M.T., and Williamson, T.E., 2019, Laramide fluvial evolution of the San Juan Basin, New Mexico and Colorado: Paleocurrent and detrital sanidine age constraints from the Paleocene Nacimiento and Animas formations: Geosphere, v. 15, no. 5, p. 1641–1664. https://doi.org/10.1130/GES02072.1. Coalson, E.B., 2014, Ignacio Blanco Field, in Rogers, J.P., Milne, J.J., Cumella, S.P., Dubois, D.P., and Lillis, P.G., eds., Oil & gas fields of Colorado: Denver, Rocky Mountain Association of Geologists, Digital Publication, p. 142-149. Cohen, K.M., Finney, S.C., Gibbard, P.L. and Fan, J.-X., 2023, IUGS chronostratigraphic chart, 2023-09: International Union of Geological Sciences, International Commission on Stratigraphy. http://www.stratigraphy.org/ICSchart/ChronostratChart2023-06.pdf. Colorado Climate Center, 2024, Station Normals, Ignacio 6ESE CO US Station: Fort Collins, Colorado State University. https://climate.colostate.edu/station_normal.html?USC00054254. Colorado Division of Water Resources (DWR), 2024, DWR Well Permit Research Viewer. https://maps.dnrgis.state.co.us/dwr/Index.html?viewer=dwrwellpermit. Colorado Energy and Carbon Management Commission (ECMC), 2024, Colorado Oil and Gas Information System (COGIS). https://ecmc.state.co.us/data.html#/cogis. Colorado Water Conservation Board (CWCB), 2024, Colorado Hazard Mapping and Risk Map Portal, https://coloradohazardmapping.com/map. Colton, R.B., Holligan, J.A., and Anderson, L.W., 1975, Preliminary map of landslide deposits, Durango 1-degree by 2-degree quadrangle, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map 703, scale 1:250,000. https://doi.org/10.3133/mf703. Condon, S.M., 1990, Geologic and structure contour map of the Southern Ute Indian Reservation and adjacent areas, southwest Colorado and northwest New Mexico: U.S. Geological Survey Miscellaneous Investigations Map I-1958, scale 1:100,000. https://doi.org/10.3133/i1958. Condon, S.M., 1992, Geologic Framework of pre-Cretaceous rocks in the Southern Ute Indian Reservation and adjacent areas, southwestern Colorado and northwestern New Mexico: U.S. Geological Survey Professional Paper 1505-A, 66 p., 1 plate, scale 1:100.000. https://doi.org/10.3133/pp1505A. Craigg, S.D., 2001, Geologic framework of the San Juan structural basin of New Mexico, Colorado, Arizona, and Utah, with emphasis on Triassic through Tertiary rocks: U.S. Geological Survey Professional Paper 1420, 70 p., scale 1:500,000. https://doi.org/10.3133/pp1420. Dane, C.H., 1946, Stratigraphic relations of Eocene, Paleocene, and Latest Cretaceous formations of eastern side of San Juian Basin, New Mexico: U.S. Geological Survey Oil and Gas Investigations Preliminary Chart 24. https://doi.org/10.3133/oc24. Fassett, J.E., 1974, Cretaceous and Tertiary rocks of the eastern San Juan Basin, New Mexico and Colorado, in Siemers, C.T., Woodward, L.A., and Callender, J.F., eds., Ghost Ranch: New Mexico Geological Society, 25th Field Conference Guidebook, p. 225-230. https://doi.org/10.56577/FFC-25. Fassett, J.E., 2010, Oil and gas resources of the San Juan Basin, in Fassett, J.E., Zeigler, K.E., and Lueth, V., eds., Geology of the Four Corners country, New Mexico and Colorado: New Mexico Geological Society, 61st Field Conference Guidebook, p. 181-196. https://doi.org/10.56577/ffc-61. Fassett, J.E., and Boyce, B.C., 2005, Fractured-sandstone reservoirs, San Juan Basin, New Mexico and Colorado: Stratigraphic traps, not basin-centered gas deposits – with an overview of Fruitland Formation coal-bed methane, in Bishop, M.G., Cumella, S.P., Robinson, J.W., and Silverman, M.R., eds., Gas in low permeability reservoirs of the Rocky Mountain Region: Denver, Rocky Mountain Association of Geologists, 2005 Guidebook [on CD ROM], p. 109-185. Fassett J.E., and Hinds, J.S., 1971, Geology and fuel resources of the Fruitland Formation and Kirtland Shale of the San Juan Basin, New Mexico and Colorado: U.S. Geological Survey Professional Paper 676, 76 p., https://doi.org/10.3133/pp676. Federal Emergency Management Agency (FEMA), 2024a, Flood insurance rate map 08067C0975, La Plata County, Colorado: National Flood Insurance Program, https://map1.msc.fema.gov/firm?id=08067C0975G. [Map shows regulatory floodplain only for private lands.] | MMI12
15 | OF NUMBER | OF-23-02 | MMI13
16 | ACKNOWLEDGMENTS | The authors thank La Plata County, the Southern Ute Indian Tribe, and private landowners who allowed access to their property. Detrital zircon analyses were performed by Darin Shwartz of Boise State University. Fossil pollen was identified by Vera Korasidis at the University of Melbourne, Australia. Steve Forman, of the Geoluminescence Dating Laboratory at Baylor University, processed the OSL age. Caitlin Bernier of Pangaea Geospatial compiled the GIS files and generated the map plates. The map was reviewed by Ralph Shroba, Jonathan White, and Matt Morgan of the Colorado Geological Survey. | MMI14
17 | 1PALEOCENE1 (?) AND LOWER EOCENE (?) STRATA | Paleogene units that crop out in and near the Ignacio quadrangle include the Paleocene-age Animas and Nacimiento Formations as well as the San Jose Formation, which is mostly Lower Eocene, but may include some Paleocene deposits at its base. Only the Nacimiento and San Jose formations are locally exposed in the Ignacio quadrangle. Stratigraphic relations among these formations are complex and vary locally within the San Juan Basin. In some places their contacts are unconformable, but in others they are gradational or interfinger. This complexity and the rarity of good exposures in low-relief areas have led some previous mappers either to group formations in and near the Ignacio quadrangle or to assign the same rocks to different formations, sometimes with arbitrary changes in nomenclature along river valleys or at state lines (e.g. Fassett and Hinds, 1971; Steven and others, 1974; Condon, 1990; and Craigg, 2001). The Colorado Geological Survey has identified and mapped formation contacts here and on adjacent quadrangles (see Geological Mapping section), but more studies may be needed for verification. Fassett and Hinds (1971, p. 34), suggested that the andesitic Animas Formation grades southward into non-andesitic Nacimiento Formation, the oldest formation exposed in the Ignacio quadrangle, along an interfingering boundary. Interpretation of the gradational boundary between the Animas Formation and Nacimiento Formation is problematic, and any resolution would require further regional studies. However, the Animas Formation does not appear to crop out in the Ignacio quadrangle. The use of the name Animas Formation is mainly confined to Colorado, although outcrop exposures of the Animas Formation have been correlated along the Animas River southward as far south as Dulce, New Mexico (Fassett, 1974). Subsurface correlation of the Animas has been challenging due to difficulty in interpreting the interfingering relationship of the Animas Formation and Nacimiento Formation on early well logs (Fassett and Hinds, 1971, p. 33). Advancements in subsurface wireline logging may allow for reevaluation of this subsurface formation contact. The Nacimiento Formation contains the lower Puerco and overlying Torrejonian faunal zones in New Mexico (Baltz, 1967), which could help to resolve formation assignments, but these zones have yet to be recognized in Colorado. Also, parts of both formations are similar in outcrop appearance, for example, varicolored badlands of Puercan-aged sediments in New Mexico resemble much younger beds in the badlands of the Regina Member of the San Jose Formation in the map area. Relevant lithologic or paleontological data is needed to assist in distinguishing these units. Flora of the Animas Formation could be reevaluated from existing published sites (Knowlton, in Reeside, 1924) and possibly new ones. | MMI15
19 | 3PALEOCENE3 (?) AND LOWER EOCENE (?) STRATA | the Mesa Mountains are part of the Ditch Canyon Member (upper part) used in this report. Baltz (1967, p. 56) and Smith (1992, p. 306) suggest that these sandstones may correlate to the medial sandstone of the Llaves Member of the San Jose Formation farther south in New Mexico. Sandstone of the Ditch Canyon Member (lower part), has not been definitively identified in the Ignacio quadrangle, as discussed below. The Regina Member, between the upper and lower parts of the Ditch Canyon Member, consists mostly of gray, reddish, and purplish variegated shales that are subject to landsliding. The Regina Member type section is about 96 km (60 mi.) to the southeast of this quadrangle in New Mexico (Baltz, 1967, p. 48). Total thickness of the Regina Member in the Ignacio quadrangle is about 223 m (670 ft) compared to 192 to 300 m (575 to 900 ft) in and near its type section (Baltz, 1967, p. 48). More easily accessible exposures of the Regina Member form colorful badlands along La Plata County Road 318 in the Bondad Hill quadrangle in sec. 27, T. 33 N., R. 9 W. San Jose Formation lithology, nomenclature and stratigraphic relations have been summarized by previous authors (Aubrey and others, 1991, p. B18-B20; Craigg, 2001, p. 54; and Fassett, 1974, p. 229). The variegated shales of the Nacimiento Formation are easily mistaken for the variegated shales of the overlying Regina Member of the San Jose Formation. The contact between the San Jose Formation and the underlying Nacimiento Formation is difficult to identify because of similar lithologies in both formations and poor, widely spaced outcrops in areas of low relief. Therefore, the contact between these two units has been poorly defined by previous mapping (e.g. Condon, 1990). The nature of the contact varies within the San Juan Basin. Along the Hogback monocline, northwest of the Ignacio quadrangle, Kirkham and Navarre (2001) mapped an angular unconformity at the base of the San Jose Formation, where the Nacimiento, Animas, and Kirtland Formations are missing due to erosion or non-deposition. Farther south in New Mexico, the contact appears to be conformable (Barnes and others, 1954; Stone and others 1983), but it is unconformable along the basin edges (Baltz, 1967; Lucas and others, 1981). The base of the Regina Member was placed above scattered sandstone beds in the central part of the Ignacio quadrangle. It is uncertain whether these sandstone beds are within the Ditch Canyon Member (lower part), Cuba Mesa Member, or Nacimiento Formation. Barnes (1953) was unable to trace his reference bed "a" (the top of the Cuba Mesa Member) into the Ignacio quadrangle. Smith (1992) concluded that the Cuba Mesa Member may be absent here. These sandstone beds contain a few mudstone intraclasts at their base, suggesting an erosional basal contact. However, these sandstone beds lack the coarse clasts indicative of the Cuba Mesa Member (Barnes, 1953). The Cuba Mesa Member, Ditch Canyon Member (lower part), and Nacimiento Formation are lumped here into an undifferentiated unit because they could not be confidently identified from their outcrop expression. More fieldwork and sample analysis are needed to determine the correct stratigraphic unit assignments for these beds. In the Ignacio quadrangle and adjacent Tiffany quadrangle to the east, the Paleocene and Lower Eocene(?) San Jose Formation may contain a more complete record of Paleocene and Eocene(?) strata than the better-studied and understood strata in New Mexico (Aubrey and others 1991, p. B20). The Tiffany quadrangle contains the “Tiffany beds” of Granger (1917), which are the type locality for the Tiffanian North American Land Mammal Assemblage (NALMA) faunal zone (Wood, 1941). These “Tiffany beds” appear to correlate with part of the Regina Member of the San Jose Formation in the Ignacio quadrangle. However, Tiffanian beds are not present to the south in New Mexico due to erosion associated with the basal San Jose Formation unconformity in that area (Baltz, 1967). Therefore, a more complete stratigraphic record of the Paleocene and Eocene may be recorded in the northern part of the San Juan Basin than in the southern part of the basin in New Mexico (Lucas and Ingerson 1981, p. 920; Simpson, 1948, p. 257-258). Simpson (1948, p. 257-258) made a similar statement that “in southern Colorado there was continuous deposition of sediments during the Upper Paleocene into Eocene and the Tiffany fauna [which he tentatively assigned to San Jose] are well known to be of Upper Paleocene age.” Little study of the Tiffanian has been done in Colorado since its type section was designated (Woodburne, 2006). | MMI16
23 | GEOLOGIC MAPPING | This mapping project was aided by a review of previous geologic maps that covered the Ignacio quadrangle at varying scales. These included published maps by Reeside (1924, 1:250,000 scale), Atwood and Mather (1932, 1:250,000 scale), Barnes (1953, 1:63,360 scale), Fassett and Hinds (1971, Plate 1, 1:380,160 scale), Steven and others (1974, 1:250,000 scale), Condon (1990, 1992, 1:100,000 scale) and Craigg (2001, 1:500,000 scale). A preliminary 1:250,000 scale landslide map by Colton and others (1975) also aided our mapping. Sand and gravel deposits were mapped by Pantea (1989, 1:24,000 scale). Geologic maps of adjacent or nearby quadrangles, previously published or in preparation by the Colorado Geological Survey at 1:24,000 scale include the following quadrangles: Basin Mountain (Kirkham and Navarre, 2001), Bayfield (Gonzales and others, 2008), Loma Linda (Lindsey and Gillam, in prep.), Bondad Hill (McCalpin and Gillam, in prep.), and Gem Village (Rogers, in prep.). Several geologic maps have been published for adjacent areas of the San Juan Basin in New Mexico, such as that by Manley and others (1987, 1:250,000 scale). | MMI17
24 | NOTES AND METHODS | The geologic descriptions and mapped contacts in this report are based on field observations in accessible areas. However mechanical leveling of irrigated agricultural fields obscured some geologic features. For land without access, contacts are based on observations from other accessible areas, publicly available aerial imagery, lidar digital elevation data, and hillshade models. Particle-size classes for sandy to clayey sediments are based on the Udden-Wentworth scale (Carpenter and Keene, 2023, p. 161), except for USDA soil textural terms that are used for the near-surface parts of certain Quaternary units (Pannell and others, 1988; Carpenter and Keene, 2023, p. 461). Descriptive phrases for textural mixtures are based on the Folk classification (Folk, 1974). Color names and Munsell codes were obtained for dry sediments with the aid of Munsell and Globe soil color books (Munsell Color, 1975; Visual Color Systems, 2012) or from soil-series data (Pannell and others, 1988). Descriptions of soil calcium carbonate (CaCO3) stage morphology are after Machette (1985). Divisions of geologic time follow Cohen and others (2023), and U. S. Geological Survey (2018). The methodologies utilized in the Optically Stimulated Luminescence (OSL) analysis (Table 3) of Steve Forman reference a variety of sources (Prescott and Hutton, 1994; Murray and Wintle, 2003; Wintle and Murray, 2006; Galbraith and Roberts, 2012; Liang and Forman, 2019). | MMI18
25 | REFERENCES 2 | Federal Emergency Management Agency (FEMA), 2024b, Flood insurance study, La Plata County, Colorado and unincorporated areas: Flood Insurance Study No. 08067CV001B, v. 3, 86 p., and flood profile plates. https://msc.fema.gov/portal/home. Folk, R.L., 1974, The petrology of sedimentary rocks: Austin, Tex., Hemphill Publishing Co., 182 p., http://hdl.handle.net/2152/22930. Galbraith, R.F., and Roberts, R.G., 2012, Statistical aspects of equivalent dose and error calculation and display in OSL dating – An overview and some recommendations: Quaternary Geochronology, v. 11, p. 1-27. https://doi.org/10.1016/j.quageo. Gillam, M.L. 1998, Late Cenozoic geology and soils of the lower Animas River valley, Colorado and New Mexico: Boulder, University of Colorado, Ph.D. dissertation, 477 p., 3 plates, scale 1:50,000. Gingerich, P. D., 1983, New Adapisoricidae, Pentacodontidae, and Hyopsodontidae (Mammalia, Insectivora and Condylarthra) from the late Paleocene of Wyoming and Colorado: Ann Arbor, University of Michigan Museum of Paleontology Contributions, v. 26, p. 227–255. https://deepblue.lib.umich.edu/handle/2027.42/48519. Gonzales, D.A., Potter, K.E., and Turner, D., 2008, Geologic map of the Bayfield quadrangle, La Plata County, Colorado: Colorado Geological Survey Open File Report 08-15, scale 1:24,000. https://coloradogeologicalsurvey.org/publications/geologic-map-bayfield-quadrangle-la-plata-colorado/. Gonzales, D.A., 2010, The enigmatic Late Cretaceous McDermott Formation, in Fassett, J.E., Zeigler, K.E., and Lueth, V., eds., Geology of the Four Corners country, New Mexico and Colorado: New Mexico Geologic Society Guidebook, 61st Conference Guidebook, p. 157-162. https://doi.org/10.56577/FFC-61.157. Gonzales, D.A., 2017, A review and revision of late Mesozoic to Cenozoic pluton chronology in the Rico Mountains, southwestern Colorado, in Karlstrom, K.E., Gonzales, D.A., Zimmerer, M.J., Heizler, M., and Ulmer-Scholle, D.S., eds., The geology of the Ouray-Silverton area: New Mexico Geological Society, 68th Field Conference Guidebook, p. 91-96. https://doi.org/10.56577/FFC-68. Granger, W., 1914, On the names of lower Eocene faunal horizons of Wyoming and New Mexico: Bulletin of the American Museum of Natural History, v. 33, p. 201-207. http://hdl.handle.net/2246/713. Harr, C.L., 1988, The Ignacio Blanco gas field, northern San Juan Basin, Colorado and New Mexico, in Fassett, J.E., ed., Geology and coalbed methane resources of the northern San Juan Basin, Colorado and New Mexico: Denver, Rocky Mountain Association of Geologists, p. 205-219. Johnstone, S.A., Berry, M.E., Hudson, A.M., Ruleman, C.A., Alexander, K.A., and Turner, K.J., 2023, Surficial geologic map database of the Durango 1-degree by 2-degree quadrangle, southern Colorado: Contributions to the National Geologic Map: U.S. Geological Survey data release, https://doi:10.5066/P9IE20T4. Karlstrom, K.E., Coblentz, D., Dueker, K., Ouimet, W., Kirby, E., van Wijk, J., Schmandt, B., Kelley, S., Lazear, G., Crossey, L.J., Crow, R., Aslan, A., Darling, A., Aster, R., MacCarthy, J., Hansen, S.M., Stachnik, J., Stockli, D.F., Garcia, R.V., Hoffman, M., McKeon, R., Feldman, J., Heizler, M., Donahue, M.S., Farmer, L., Shaw, C., Leonard, E., Chase, C., and Cole, R., 2011, Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response; toward a unified hypothesis: Lithosphere, v. 4, no. 1, p. 3-22. https://doi.org/10.1130/L150.1. Kirkham, R.M., and Navarre, A.K., 2001, Geologic map of the Basin Mountain quadrangle, La Plata County, Colorado: Colorado Geological Survey Open-File Report 01-4, scale 1:24,000. https://doi.org/10.58783/cgs.of0104.fgpn4242. | MMI19
26 | REFERENCES 3 | Knowlton, F.H., 1924, Flora of the Animas Formation: U.S. Geological Survey Professional Paper 134, p. 71-114. https://doi.org/10.3133/pp134. La Plata County Assessor’s Office, Downloadable GIS datasets. Available online: https://www.co.laplata.co.us/departments/gis___mapping/download_gis_datasets.php. Lanphere, M.A., Champion, D.E., Christiansen, R.L., Izett, G.A., and Obradovich, J.D., 2002, Revised ages for tuffs of the Yellowstone Plateau volcanic field—Assignment of the Huckleberry Ridge Tuff to a new geomagnetic polarity event: Geological Society of America Bulletin, v. 114, p. 559–568. https://doi.org/10.1130/0016-7606(2002)114<0559:RAFTOT>2.0.CO;2. Liang, P., and Forman, S.L., 2019, LDAC: An Excel-based program for luminescence equivalent dose and burial age calculations: Ancient TL, v. 37, no. 2, p. 21-40. ISSN 0735-1348. Lindsey, K.O., and Gillam, M.L., in prep., Geologic map of the Loma Linda quadrangle, La Plata County, Colorado: Colorado Geological Survey Open-File Report 22-05, scale 1:24,000. Lucas, S.G., and Ingersoll, R.V., 1981, Cenozoic continental deposits of New Mexico: An overview: Geological Society of America Bulletin, v. 92, no. 12. pt. I, p. 917-932. https://doi.org/10.1130/0016-7606(1981)92<917:CCDONM>2.0.CO;2. Lucas, S. G., Schoch, R. M. Manning, E., and Tsentas, C., 1981, The Eocene biostratigraphy of New Mexico: Geological Society of America Bulletin, v. 92, no. 10, pt. I, p. 951-967. https://doi.org/10.1130/0016-7606(1981)92<951:TEBONM>2.0.CO;2. Machette, M.N., 1985, Calcic soils of the southwestern United States: Geological Society of America Special Paper 2013, 21 p. https://www.nrc.gov/docs/ML0037/ML003747879.pdf. Manfrino, C., 1984, Stratigraphy and palynology of the Upper Lewis Shale, Pictured Cliffs Sandstone, and lower [part] Fruitland Formation (upper Cretaceous) near Durango, Colorado: Golden, Colorado School of Mines, M.S. thesis, 96 p. Manley, K., Scott, G.R., and Wobus, R.A., 1987, Geologic map of the Aztec 1-degree x 2-degree quadrangle, northwestern New Mexico and southern Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-1730, scale 1:250,000. https://doi.org/10.3133/i1730. Matthews, N.E., Vasquez, J.A., and Calvert, A.T., 2015, Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 49Ar/39Ar and U-Pb dating of sanidine and zircon crystals: Geochemistry, Geophysics, Geosystems, v. 16, p. 2508-2528, https://doi.org/10.1002/2015GC005881. McCalpin, J.P., and Gillam, M.L., in prep., Geologic map of the Bondad Hill quadrangle, La Plata County, Colorado: Colorado Geological Survey Open-File Report 22-03, scale 1:24,000. Moore, D.W. and Scott, G.R., 1995, Generalized surficial geologic map of the Bayfield quadrangle, La Plata County, Colorado: U.S. Geological Survey Open-File Report 95-266, scale 1:24,000. https://doi.org/10.3133/ofr95266. Munsell Color, 1975, Munsell soil color charts: Evanston, Illinois, Soiltest Inc., unpaginated. Murray, A.S., and Wintle, A.G., 2003, The single aliquot regenerative dose protocol – Potential for improvements in reliability: Radiation Measurements, v. 27, p. 377-381. https://doi.org/10.1016/S1350-4487(03)00053-2. Nichols, D.J., 2009, On the palynomorph-based biozones: Rocky Mountain Association of Geologists, The Mountain Geologist, v. 46, no. 3. p. 105-124. Nichols, D.J., and Ott, H.L., 1978, Biostratigraphy and evolution of the Momipites-- Caryapollenites lineage in the Early Tertiary in the Wind River Basin, Wyoming: Palynology, v. 2. p. 93-112. https://www.jstor.org/stable/40072719. Noe, D.C., Jochim, C.L., and Rogers, W.P., 2014, A guide to swelling soil for Colorado homebuyers and homeowners, second edition: Colorado Geological Survey Special Publication (SP) 43, 68 p. Pannell, J.P., Benton, B.A., Cencich, B.W., Deutsch, P.A., Fritch, J.L., and Neely, M.C., 1988, Soil survey of La Plata County area, Colorado: Natural Resources Conservation Service, 238 p., 23 maps, scale 1:24,000. http://soildatamart.nrcs.usda.gov/Manuscripts/CO669/0/laplata.pdf. Pantea, M.P., 1989, Preliminary evaluation of potential construction-grade sand and gravel deposits in the Long Mountain, Bondad Hill, and Ignacio quadrangles, Southern Ute Indian Reservation, Colorado: unpublished administrative report BIA-19-II-O, prepared by the U.S. Geological Survey for U.S. Bureau of Indian Affairs, 22 p., 3 plates, scale 1:24,000. Prescott, J.R., and Hutton, J.T., 1994, Cosmic ray contributions to dose rates for luminescence and ESR dating – Large depths and long-term time variations: Radiation Measurements, v. 23, p. 497-500. https://doi.org/10.1016/1350-4487(94)90086-8. Price, A.B., Nettleton, W.D., Bowman, G.A., and Clay, V.L., 1988, Selected properties, distribution, source, and age of eolian deposits and soils of southwest Colorado: Soil Science Society of America Journal, v. 52, p. 450–455. https://doi.org/10.2136/sssaj1988.03615995005200020027x. Raynolds, R.G., and Hagadorn, J.W., 2017, Colorado stratigraphy chart: Colorado Geological Survey and Denver Museum of Nature and Science, Colorado Geological Survey Map Series MS-53, https://doi.org/10.58783/cgs.ms53.cxwh3412. Reeside, J.B., 1924, Upper Cretaceous and Tertiary Formations of the western part of the San Juan basin, Colorado and New Mexico: U.S. Geological Survey Professional Paper 134, p. 1-70, Plate I, scale 1:250,000. https://doi.org/10.3133/pp134. Reheis, M.C., Goldstein, H.L., Reynolds, R.L., Forman, S.L., Mahan, S.A., and Carrara, P.E., 2017, Late Quaternary loess and soils on uplands in the Canyonlands and Mesa Verde areas, Utah and Colorado: Quaternary Research, v. 89, no. 3, p. 1-21, https://doi.org/10.1017/qua.2017.63. Richmond, G.M., 1965, Quaternary stratigraphy of the Durango area, San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 525-C, p. C137-C143. https://doi.org/10.3133/pp525C. | MMI20
27 | GEOLOGIC SETTING | The mapped area lies in the northern part of the San Juan Basin, north of the basin axis, in southern Colorado. This basin contains a variety of sedimentary deposits including marine, near shore, lagoonal, and continental deposits that have been the focus for exploration and development of coal-bed methane and other petroleum resources (see the Petroleum Resources section of this report). Sedimentary rock units in the San Juan Basin range in age from Upper Devonian to Lower Eocene (Raynolds and Hagadorn, 2017), and are underlain by older Precambrian basement rocks. The geologic history discussed here for the Ignacio quadrangle, and the sequence of strata shown on the cross-section, focus on sedimentary strata of Jurassic and younger overlying rocks. Structural mapping of the subsurface units helps to guide structural observations on the ground surface. Continental shale and sandstone of the Jurassic Morrison Formation have been penetrated by multiple deep wells in this quadrangle. Younger Cretaceous strata are economically more significant and are commonly exploited for fossil fuels. The oldest Cretaceous rocks consist of braided stream deposits of the Albian-age Burro Canyon Formation. Overlying this unit are the Cenomanian-age shoreface and deltaic sandstone of the Dakota Group. The Dakota Group generally accumulated during the initial transgression of the Western Interior Seaway (WIS). Continuing transgression and subsequent regression produced marine deposits of the Mancos Shale. Overlying lower strata of Campanian-age Mesaverde Group include the Point Lookout Sandstone, Menefee Formation, and Cliff House Sandstone. The youngest of these is the marine Point Lookout Sandstone which formed as the WIS receded. The overlying Menefee Formation consists of terrestrial deposits, which are overlain by transgressive marine sandstones of the Cliff House Sandstone as rising sea levels flooded the western margin of the Cretaceous shoreline of the WIS. The return of the seaway deposited marine sediments of the Lewis Shale. The Huerfanito Bentonite bed, within the upper part of the Lewis Shale, is a 75.76 Ma volcanic ash-fall horizon that is commonly used as a datum in subsurface well log correlations (Fassett and Boyce, 2005, p. 113). Due to its relative ease of identification and regional extent, this datum greatly aids in subsurface well log correlation and constrains the structural and stratal relations of subsurface rocks in the San Juan Basin. The final regression of the WIS formed the last marine-influenced strata in the San Juan Basin, the Campanian-age Pictured Cliffs Sandstone. Overlying terrestrial deposits of the Campanian-age Fruitland Formation contain numerous coal beds that are exploration targets for intensive coal-bed methane gas resources. The overlying Maastrichtian-age Kirtland Formation generally consists of lower shale units, the Farmington Sandstone Member, and upper shale units. The tectonic influence of the Laramide orogeny began impacting stratigraphic units in the San Juan Basin as early as 80 Ma (Cather and others, 2019, p. 1657), indicated by local thinning of the Cretaceous Pictured Cliffs Sandstone and thickening of the Lewis Shale. Both formations are beneath the surface in the Ignacio quadrangle but crop out along nearby basin margins. Continuing tectonism also affected younger strata in the San Juan Basin that crop out in this quadrangle. Recent work by Cather and others (2019) provides the most comprehensive understanding to date of the Paleogene lithostratigraphy and depositional history for the entire San Juan Basin. Their work demonstrates complex fluvial systems with changing sediment transport directions over time. Cather and others (2019) attributed this complex fluvial evolution mainly to varying amounts of basin subsidence that were driven chiefly by the weight of accumulating sediments. Corresponding local basin uplift is reflected in Cenozoic hiatuses in sedimentation, which led to erosion and disconformities. | MMI21
28 | 4PALEOCENE4 (?) AND LOWER EOCENE (?) STRATA | Unfortunately, the senior author was unable to find any recent published data for the Colorado portion of the San Juan Basin that constrains the age range of supposed Paleocene and Eocene deposits. The most recent publications for these deposits in the Colorado portion of the San Juan Basin includes: paleomagnetic polarity (Butler and others, 1981, p. 301-310; Sloan, 1987, p. 173, p.177; and Butler and Lindsey, 1985, p. 552), mammalian paleontology (Gingerich, 1983), paleobotany (Reeside, 1924), dinosaur paleontology (Simpson, 1950), palynology (Manfrino, 1984), and fossil bone discovery (Roland Brown, written communication cited by Dane, 1946). Palynological work, following the classification schema originally described by Nichols and Ott (1978) and more recently summarized by Nichols (2009), may provide the needed constraints for bracketing Paleogene mudstone deposits. For this study, one pollen sample from the Regina Member was correlated to pollen zone P5 of Tiffanian age (Plate 2, Table 2), but more studies on paleontology and palynology are needed. Sloan (1987) discussed correcting errors in past correlations based on earlier paleomagnetic measurements in the San Juan Basin. Sloan’s revisions to the work of Butler and others (1981), shifted the Mason quarry (site of the type Tiffanian NALMA) from the top of chron 27 to the middle part of chron 25, or 62.3-56.2 Ma, and essentially Upper Paleocene. The last significant paleontological and mammalian studies of these strata was undertaken during 1951 by scientists of the American Museum of Natural History “San Juan Basin Program” (Simpson 1955, cited in Simmons 1960, p. 67). The senior author has yet to relocate the 1951 AMNH “Colorado 2” site mentioned by Simpson (1955, correspondence cited by Simmons, 1960, p. 67) which may have been in or near the Ignacio quadrangle. Simpson's correspondence described a fossil specimen that Harley Barnes had discovered during his 1950’s geologic mapping for the U.S. Geological Survey. Simmons (1960, p. 30) assumed the Barnes fossil location is in SW 1/4, sec. 29, R 8 W, T 34 N., near the northwest part of the Durango-La Plata County Airport, about three kilometers (2 miles) north of the Ignacio quadrangle. During excavation, it was noted that parts of the well-articulated fossil were within both a sandstone bed and an underlying shale bed. The base of this mappable sandstone bed was previously proposed as boundary between the San Jose Formation and the underlying Nacimiento Formation (Simmon, 1960). However, the fossil's position in both beds indicates relatively continuous deposition and demonstrates the need for further reevaluation of the formation boundary. For mapping purposes, the authors have placed these sandstones at the boundary between the Cuba Mesa Member and the overlying Regina Member of the San Jose Formation. There is still conjecture as to whether these sandstones could be of the Ditch Canyon Member (lower part) of the San Jose Formation rather than the Cuba Mesa Member. If the sandstones are in the Ditch Canyon Member (lower part), then the underlying coarse and highly cross bedded Cuba Mesa Member is likely be at a lower stratigraphic level. Smith (1992) indicated that the Cuba Mesa Member pinches out towards the north and may not be present in this quadrangle. The description of these beds as hard sandstone and fine conglomerate (Simpson 1955, in Simmons, 1960, p. 67) suggests that they may be in the Cuba Mesa Member and not the Ditch Canyon Member (lower part). However, assignment of these sandstone beds to the Ditch Canyon Member (lower part) cannot been ruled out. Beds that could be part of three units (Cuba Mesa Member of the San Jose Formation, the Ditch Canyon Member (lower part), and Nacimiento Formation) were combined into one undifferentiated unit (unit PEsn) because of the uncertainty in surficial mapping. More field work to distinguish these sandstones is needed in the adjacent Bondad Hill and Loma Linda quadrangles where exposures are better. Possibly, relevant information may exist in archives of the American Museum of Natural History or the U.S. Geological Survey. Paleomagnetic data and mammalian faunas both suggest the accuracy of Simpson's statement (in Simmons, 1960, p. 67) that “It now seems highly probable that there is no Eocene in the Tiffany region and that all these beds are quite distinct from the more southern Eocene formation...”, implying that the beds could be Paleocene(?). Future palynological, geobotanical, and other geochronological sampling and analysis would help clarify the ages of these important Paleogene deposits in Southern Colorado. | MMI22
29 | 2PALEOCENE2 (?) AND LOWER EOCENE (?) STRATA | There is a gradual lithologic change as the Animas Formation grades southward into the Nacimiento Formation, which contains no andesitic material in New Mexico (Fassett and Hinds, 1971, p. 34). The most representative outcrops of the Nacimiento Formation in and near the Ignacio quadrangle are along the Animas River, at a roadcut along State Highway 172 near the Durango-La Plata County Airport and along stream-cut bluffs in the Florida River valley. Outcrop exposures of the Regina Member of the San Jose Formation appear to have more reddish and purplish paleosols interbedded with gray shales, which creates a highly banded appearance in outcrop; in contrast, the Nacimiento Formation has a higher proportion of gray shales. The lack of sandstone beds along the Regina-Nacimiento stratigraphic boundary results in shale-on-shale contact that has not been confidently identified. Correlation of the basal and upper contacts is challenging, because the Nacimiento Formation in Colorado overlies and interfingers with the underlying Animas Formation (Aubrey and others 1991, p. B18). Sloan (1987, fig. 2, p. 167) proposed that two separate continental divides were present in the region affecting drainage directions over time. During the deposition of the Nacimiento Formation (during Puercan-Torrejonian time) there was a western divide, approximately mid-Utah, that served as the continental divide. Later, during the deposition of the San Jose (during the Tiffanian time) there was an eastern, mid-Colorado/mid-New Mexico, continental divide. The San Jose is the youngest formation in the San Juan Basin (Simpson, 1948; Simpson, 1950, p. 86), reaching 680 m (2,230 ft) in thickness (Brimhall, 1973, p. 198). The name San Jose Formation was first used by Simpson (1948, p. 258), and later Baltz (1967) subdivided it into separate facies in New Mexico. Barnes (1953) made excellent observations and correlations of Tertiary marker beds in the Colorado portion of the San Juan Basin that were substantiated by mapping done for this report. Although Baltz (1967) named facies of the San Jose Formation in New Mexico, the more recent member names of Smith (1992) have been used for the San Jose Formation in the Ignacio quadrangle. Presently, the subsurface and outcrop work of Smith (1992) seems to best represent the San Jose Formation's lithologic units in both the northern Colorado and adjacent New Mexico portions of the San Juan Basin. Smith (1992) divided the San Jose Formation into the Cuba Mesa Member, Regina Member, and informal lower and upper parts of the Ditch Canyon Members. He recognized that a tongue of the shale-dominated Regina Member projects into the middle of the Ditch Canyon Member in Colorado. The Cuba Mesa Member is the basal unit of the San Jose Formation. It has not been identified in the Ignacio quadrangle, but crops out in the adjacent Bondad Hill quadrangle (McCalpin and Gillam, in prep.) and is important for understanding complex relationships among formations in the northern part of the San Juan Basin. The San Jose Formation consists mainly of sandstone beds that are separated by thinner shales. Some sandstone beds of the Cuba Mesa Member are more coarse-grained than those in other members of the San Jose Formation. Some conglomeratic sandstone beds contain a few distinctive igneous and metamorphic cobbles as well as mudstone intraclasts. In the Bondad Hill quadrangle, Barnes (1953) noted pink feldspars and mapped reference bed “a”, which separates the Cuba Mesa Member from the overlying Ditch Canyon Member (lower part). In the subsurface, both members thin to the north, intertongue with beds of the overlying Regina Member and underlying Nacimiento Formation, and may pinch out along the flanks of the Ignacio-Bondad anticline (Smith, 1992, p. 301). Subsurface thinning of these lower San Jose Formation sandstone units at the axis of the Ignacio-Bondad anticline may indicate the initiation of structural uplift during early San Jose Formation deposition (early Paleocene time). Cather and others (2019, p. 1657) agreed with Smith (1992) that in the northern part of the San Juan Basin, mudstones of the upper Nacimiento Formation appear to intertongue with the basal San Jose Formation sandstone, indicating a conformable contact. However, non-deposition or removal due to an undocumented period of erosion cannot be ruled out. More study of existing and new paleontological data is needed to determine the cause of thinning of sandstone facies overlying the Ignacio-Bondad anticline. Data and observations from additional studies likely will help to clarify the deposition and more precise timing of intra-basin Laramide tectonism and uplift in the area during the Paleocene. Larry Smith (1992, p. 305) named the Ditch Canyon Member for exposures along Ditch Canyon in northern New Mexico. The type section and corresponding subsurface type log for the member are only 8 km (5 mi.) southwest of the Ignacio quadrangle and only 16 km (10 mi.) from upper Ditch Canyon Member outcrops in the Ignacio quadrangle. The Ditch Canyon Member consists mainly of sandstone that locally contains mudstone intraclasts, but notably lacks igneous and metamorphic clasts that are sometimes found in the underlying Cuba Mesa Member. This member consists of upper and lower parts that are separated by a tongue of Regina Member shale (Smith, 1992, Figure 8, p. 303). Reference beds “b” and “c” of Barnes (1953), mark the top of a “fairly persistent sandstone” that forms the northeastern rim of the Mesa Mountains in the Ignacio quadrangle. These sandstones and the overlying deposits that extend to the top of the | MMI23
30 | 1SURFICIAL DEPOSITS | The characteristics of surficial deposits reflect differing sediment sources, local topography and climate. River alluvium and loess are composed of materials that were transported to this area mainly from distant sources, whereas all other deposits were sourced locally, although some contain reworked materials such as durable, rounded igneous and metamorphic clasts recycled from older river alluvium. For most types of deposits, topographic form and (or) landscape position aided mapping. The Pleistocene Epoch was characterized chiefly by climatic episodes that were typically cooler and effectively wetter during glacial episodes or warmer and effectively drier during interglacial episodes. The latter were more like today’s post-glacial Holocene Epoch. During times of major cooling, glaciers formed in the San Juan Mountains and more moisture was available to transport alluvium and facilitate mass-movement such as landslide activity in the Ignacio area. Rough age estimates for the surficial units are based on several types of reasoning and could change if more numeric ages become available in the future. Unfortunately, no numeric dates were obtained for surficial deposits in the Ignacio quadrangle. One sample from Los Pinos River alluvium in the adjacent Tiffany quadrangle was dated (Table 3). Other age estimates are based on three relative dating methods: height above stream level (fluvial incision and amount of post-depositional erosion); degree of progressive, weathering-related changes in sediments and soil development; and possible correlations to dated deposits in nearby areas. In response to past and likely continuing regional uplift (Karlstrom and others, 2011; Gonzales, 2017), major rivers in the region are slowly deepening their valleys, and the rest of the landscape gradually lowers in response to stream incision. Therefore, the height of surficial deposits above adjacent drainages is a rough guide to their age, with the youngest deposits along modern drainages and older deposits at increasingly greater heights above stream level. For example, a local deposit of Lava Creek B volcanic ash (or tephra) overlies Los Pinos River alluvium on a terrace remnant in the Gem Village quadrangle (Scott and Moore, 2007; Rogers, in prep.). This ash has been dated elsewhere at approximately 630,000 to 640,000 years before present (Lanphere and others, 2002; Matthews and others, 2015) and is now roughly 90 m (295 ft) above the modern river channel, yielding a long-term-average river incision rate around 140 m (460 ft) per million years. Rates of stream incision and landscape lowering would have varied over shorter and longer time intervals, and from place to place. Nevertheless, such calculations help with age estimates for surficial deposits in the Ignacio quadrangle, such as gravel on the Mesa Mountains (unit Qgmm), which averages about 275 m (900 ft) above the Los Pinos River. Major rivers in this region, such as the Los Pinos, Animas, and Florida head in the San Juan Mountains but the locations of their drainage divides and courses in the San Juan Basin shifted during deposition of surficial deposits in the Ignacio quadrangle. Their alluvium, except for that in modern stream channels, accumulated chiefly as glaciofluvial outwash (Richmond, 1965; Gillam, 1998; Scott and Moore, 2007). Surficial deposits containing primary or reworked river gravels are very resistant to erosion because they contain mostly hard metamorphic and igneous clasts. These clasts armor and help to protect underlying bedrock units from erosion, a process that leads to topographic inversion (a situation in which alluvium that was deposited on valley floors now caps ridges). Episodic Pleistocene glacial and interglacial periods of deposition and incision have formed downward-stepping levels of glaciofluvial outwash terraces, including three in the map area. Within the Holocene modern river channel are floodplain and low terrace deposits (Qra) but much older river gravels have been eroded and recycled into stream alluvium and colluvium on the Mesa Mountains (unit Qgmm) and in lower areas north of the mountains (units Qg, Qgl, and others). Mapping in the Ignacio quadrangle yielded new perspectives on reworked river gravels and the evidence that they provide for potential ancient courses of major rivers in the region. The oldest surficial deposits in the quadrangle are gravels on the Mesa Mountains (unit Qgmm), the oldest of which are thought to be Lower Pleistocene based on their great heights above modern rivers. This unit (informally named by Richmond, 1965) has attracted interest because it could record very early river deposition and constrain the timing and rate of erosional ground lowering in the area between the Animas and the Los Pinos rivers. Initial small-scale mapping showed the gravel, then considered to be river alluvium, as a widespread blanket on broad interfluves (Atwood and Mather, 1932; Richmond, 1965), implying that the gravel might have accumulated on a peneplain. Later detailed studies found patches of alluvium and colluvium that are much less extensive (Pantea, 1989; Gillam, 1998; Scott and Moore, 2007; McCalpin and Gillam, in prep.). It seems that bedrock lithology controls the topography of the Mesa Mountains rather than proposed peneplain of Atwood and Mather (1932). | MMI24
31 | 3SURFICIAL DEPOSITS | alluvial matrix could have been locally rich in grains and clasts of fine-grained, erodible rocks which would likely weather to clay. In contrast, grains and clasts of soft sedimentary rocks are rare in gravelly alluvium of major rivers that head in the San Juan Mountains. Their alluvium typically has a light brownish-gray matrix composed mostly of quartz-rich, medium to coarse sand. Following deposition of unit Qgmm, sandstones in the upper Ditch Canyon Member of the San Jose Formation limited erosion of the present Mesa Mountains. However, their steep northern and eastern slopes probably retreated while surrounding areas were lowered by hundreds of meters. In response, widespread landslide deposits formed along the mountain slopes. Reworked igneous and metamorphic clasts are common in tributary alluvium and colluvium throughout the lowlands (in units Qg and Qgl and many small deposits too thin to be mapped), which may be as old as Middle Pleistocene. These deposits, which mantle most hills in the area, were informally called the Oxford gravel (Atwood and Mather, 1932). As local deposits, they are not useful for identifying glacially controlled depositional cycles along major rivers (Richmond, 1965) so the name has been abandoned. However, they record alluvial and colluvial processes that may, in places, have included the development of coalesced alluvial fans. Resistant clasts in the lowlands must have been reworked from much older alluvium that formerly existed at or above the present level of the Mesa Mountains (Scott and Moore, 2007). The youngest deposits, of probable Upper Pleistocene to Holocene age, formed along existing courses of local streams and the Los Pinos River. Those along local streams include debris fans sourced from the slopes of the Mesa Mountains (unit Qf), alluvium and colluvium (units Qa and Qac). Downward-stepping alluvial terrace and channel deposits of the Los Pinos River (units Qa4, Qa2, Qa1, and Qra) are Upper Pleistocene to Holocene. Loess (unit Qe) probably accumulated at slow rates on many surfaces undergoing little erosion. Although parts of some deposits in the Mesa Mountains may be as old as Middle Pleistocene, most deposits are probably Upper Pleistocene. Some loess was also reworked into the matrix of alluvium, colluvium and lag deposits. | MMI25
32 | REFERENCES 4 | Rogers, N.T., in prep., Geologic map of the Gem Village quadrangle, La Plata County, Colorado: Colorado Geological Survey Open-File Report 22-04, scale 1:24,000. Scott, G.R., and Moore, D.W., 2007, Pliocene and Quaternary deposits in the northern part of the San Juan Basin in southwestern Colorado and northwestern New Mexico: U.S. Geological Survey Scientific Investigations Report 2007-5006, 13 p. https://doi.org/10.3133/sir20075006. Sebol, L.A., Palkovic, M.J., Broes, L.D., and Barkmann, P.E., 2023, Geology and groundwater resources of La Plata County, Colorado: Colorado Geological Survey Open File Report 19-01, 66 p., 30 plates, GIS data, https://doi.org/10.58783/cgs.of1901.ayne6155. Simmons, E.L., 1960, The Paleocene Pantodonta: Transactions of the American Philosophical Society, New Series, v. 50, no., p. 67. https://www.jstor.org/stable/1005810. Simpson, G.G., 1948, The Eocene of the San Juan basin, New Mexico, Part 1: American Journal of Science, v. 246, p. 257-282. https://doi.org/10.2475/AJS.246.5.257. Simpson, G.G., 1950, Lower Tertiary formations and vertebrate faunas of the San Juan basin, in Kelley, V.C., Beaumont, E.C., and Silver, C., eds., San Juan Basin, New Mexico and Colorado: New Mexico Geological Society, 1st Field Conference Guidebook, p. 85-89. https://doi.org/10.56577/FFC-1.135. Simpson, 1955, in Simmons, E.L., 1960, The Paleocene Pantodonta: Transactions of the American Philosophical Society, New Series, v. 50, no. 6, p. 67. https://www.jstor.org/stable/1005810. Sloan R.E., 1987, Paleocene and latest Cretaceous mammal ages, biozones, magnetozones, rates of sedimentation, and evolution, in Fassett, J. E., and Rigby, J. K., Jr., eds., The Cretaceous–Tertiary boundary in the San Juan and Raton Basins, New Mexico and Colorado: Geological Society of America Special Paper 209, p. 165–200. https://doi.org/10.1130/SPE209-p165. Smith, L.N., 1992, Stratigraphy, sediment dispersal and paleogeography of the lower Eocene San Jose Formation, San Juan basin, New Mexico and Colorado, in Lucas, S.G., Kues, B.S., Williamson, T.E., and Hunt, A.P., eds., San Juan Basin IV: New Mexico Geological Society 43rd Field Conference Guidebook, p. 297-309. https://doi.org/10.56577/FFC-43.297. Steven, T.A., Lipman, P.W., Hail, W.J., Jr., Barker, Fred, and Luedke, R.G., 1974, Geologic map of the Durango quadrangle, southwestern Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-764, scale 1:250,000. https://doi.org/10.3133/i764. Stone, W.J., Lyford, F.P., Frenzel, P.F., Mizell, N.H., and Padgett, E.T., 1983, Hydrogeology and water resources of San Juan basin, New Mexico: New Mexico Bureau of Mines and Mineral Resources Hydrologic Report 6, 70 p., https://doi.org/10.58799/HR-6. U.S. Geological Survey (USGS), 2018, Divisions of geologic time—major chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2018-3054, 2 p., https://doi.org/10.3133/fs20183054. Varnes, D.J., 1978, Slope movement types and processes, in Schuster, R.L., and Krizek, R.J., eds., Landslides — Analysis and control: Washington, D.C., National Research Council, Transportation Research Board Special Report 176, p. 11–33. http://onlinepubs.trb.org/Onlinepubs/sr/sr176/176-002.pdf. Visual Color Systems, 2012, The Globe soil color book: A pocket guide for the identification of soil colors: Kingston, New York, Visual Color Systems, 41 p. White, J.L., and Greenman, C., 2008, Collapsible soils in Colorado: Colorado Geological Survey, Engineering Geology Report EG-14, 104 p., https://doi.org/10.58783/cgs.eg14.ttwf2291. Wintle, A.G., and Murray, A.S., 2006, A review of quarts optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols: Radiation Measurements, v. 41, p 369-391. https://doi.org/10.1016/j.radmeas.2005.11.001. Wood, H. E., II, Chaney, R. W., Clark, J., Colbert, E. H., Jepsen, G. L., Reeside, J. B., Jr., and Stock, C., 1941, Nomenclature and correlation of the North American continental Tertiary: Geological Society of America Bulletin, v. 52, p. 1–48. https://doi.org/10.1130/GSAB-52-1. Woodburne, M.O., 2006, Mammal ages: Stratigraphy, v. 3, p. 229–261. https://doi.org/10.29041/strat.03.4.01. | MMI26
33 | 2GEOLOGIC SETTING | Compositional signatures in sediments indicate different source rocks and regions in the neighboring evolving uplifts. In the northwestern part of the San Juan Basin, the Cretaceous age McDermott Formation and Paleocene age Animas Formation reflect a sudden and pronounced influx of coarse debris (Reeside, 1924). The older McDermott Formation represents debris flows with high portions of boulder and cobble-rich conglomerates with igneous intrusive clasts, in contrast to the finer grained stream and lacustrine deposits containing volcanic fragments of the Animas Formation (Gonzales, 2010). As the basin continued to subside and fill with sediments, the average size of sediment particles decreased. This was likely due to the recycling of earlier sedimentary units concurrently unroofed and transported from neighboring uplifts. A trend of recovering increasingly older recycled detrital zircons in progressively younger Paleogene sandstone deposits appears to support the unroofing and erosion of sedimentary strata in the uplands likely in the San Juan Uplift (Plate 2, Table 1). Surficial map units include unconsolidated river and side-stream alluvium, colluvium, alluvial fan, eolian, and landslide deposits that range in age from Lower Pleistocene(?) to Upper Holocene. Compared to the eroded sedimentary rocks that they overlie, these deposits are erosional remnants and very small in area and volume. They differ locally in mode of deposition and thickness over relatively short distances. Because of progressive erosion by rivers and streams, older deposits are found at progressively higher elevations. The oldest deposits are about 275 meters (900 ft) above the Los Pinos River, indicating significant regional erosion during a period whose length is estimated to be approximately 2 million years though the actual duration could be longer or shorter. | MMI27
34 | 2SURFICIAL | Two hypotheses for deposition of the oldest alluvium in this unit are possible. One is that a former course of the combined Animas and Florida rivers, and a former course of the Los Pinos River, both flowed from the north onto the Mesa Mountains in separate valleys and deposited river alluvium there. Another is that even older courses of these rivers once passed above the present level of the Mesa Mountains, either across or near them; then, during regional erosion, large tributary streams formed in the San Juan Basin north of the Mesa Mountains and flowed onto the present surface of the mountains, eroding older river alluvium within their drainage basins and depositing reworked river clasts in tributary alluvium on the mountains. Unfortunately, most alluvium in Qgmm is poorly exposed. Observed features that could support either scenario are thicknesses in the range of 4.0 to 4.9 m (13 to 16 ft), commonly clast-supported fabrics, locally well-developed imbrication, and subhorizontal beds with minor fine-grained lenses. Features that may be more likely to indicate river deposition are possible fluvial-cut scarps in one deposit, a paleochannel in the top of another, and boulder diameters rarely as large as 1 m (3 ft). Conversely, the reddish-gray, clayey matrix argues strongly for stream deposition. Local tributaries from farther north in the San Juan Basin could have flowed for long distances through mudstones and muddy sandstones, such as the reddish-gray Regina Member of the San Jose Formation and comparable parts of other formations. In that case, the _______________ | MMI28

#### OSL

OBJECTID | Field_Number | Laboratory_Number | Map_Unit | Material | Grain_size__μm_ | UTM_Easting_a | UTM_Northing_a | Latitude | Longitude | Elevation__m_ | Depth_Below_Ground_Surface__m_ | Aliquots | Equivalent_Dose__De___Gy__c | Over_dispersion_____d | U__ppm__e | Th__ppm__e | K2O_____e | Rb__ppm_ | H20____ | Cosmic_dose_rate__mGray_yr__f | Dose_Ra_te__mGray_yr_ | SAR_OSL_age__yr__g
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
1 | IG21-OSL1/2.3 | 5527.0 | N/A (Qa4 equiv?) | fine to medium sand | 150-255 | 267896.0 | 4099202.0 | 37.083297 | -107.718282 | 1898.2 | 2.3 | 35/36 | 100.24 ± 3.71 | 22 ± 3 | 2.86 ± 0.01 | 8.66 ± 0.01 | 2.34 ± 0.01 | 92.1 ± 0.01 | 15 ± 3 | 0.234 ± 0.023 | 2.94 ± 0.09 | 34,085 ± 1605

#### PollenAge

OBJECTID | SampleNumber | UTMX | UTMY | Latitude | Longitude | Zone | Age
--- | --- | --- | --- | --- | --- | --- | ---
1 | IG22-PL01 | 260532 | 4104947 | 37.060235 | -107.693233 | P5 | Tiffanian (upper Paleocene)

### Database Inventory

This summary of database content is provided as a convenience to GIS analysts, reviewers, and others. It is not part of the GeMS compliance criteria.
AgeDates, nonspatial table, 3 rows
DataSources, nonspatial table, 9 rows
DescriptionOfMapUnits, nonspatial table, 37 rows
GeoMaterialDict, nonspatial table, 101 rows
Glossary, nonspatial table, 29 rows
MiscellaneousMapInformation, nonspatial table, 28 rows
OSL, nonspatial table, 1 rows
PollenAge, nonspatial table, 1 rows
GeologicMap, feature dataset
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DataSourcePolys, simple polygon feature class, 2 rows
GeologicLines, simple polyline feature class, 1 rows
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GenericPoints, simple point feature class, 15 rows
GeologicLinesAnno, annotation polygon feature class, 1 rows
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FossilPoints, simple point feature class, 1 rows
ContactsAndFaults, simple polyline feature class, 3472 rows
GeologicMap_topology, topology
CorrelationOfMapUnits, feature dataset
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CMUPoints, simple point feature class, 0 rows
CMUMapUnitPolysAnno, annotation polygon feature class, 30 rows
CMULineAnno, annotation polygon feature class, 25 rows
CrossSectionA, feature dataset
CSAMapUnitPolys, simple polygon feature class, 15 rows
CSAContactsAndFaults, simple polyline feature class, 50 rows
CSAGeologicLines, simple polyline feature class, 2 rows
CSADHTrace, simple polyline feature class, 13 rows
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CSAWellsAnno, annotation polygon feature class, 17 rows
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Line, simple polyline feature class, 10 rows
CSAMapUnitPolysAnno, annotation polygon feature class, 24 rows