| 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.
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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.
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| 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 |