OF-01-04 Geologic Map of the Basin Mountain Quadrangle, La Plata County, Colorado


Describes the geologic setting and mineral resource potential of this 7.5-minute quad located immediately southwest of the town of Durango. Includes the geologic setting, fracture data, descriptions of map units, corresponding cross-sections and an oblique view. 1 color plate (1:24,000). A 42-page booklet accompanies the map. Digital PDF download. OF-01-04D

From the Author’s Notes:

The Basin Mountain 7.5-minute quadrangle is located in La Plata County in southwestern Colorado, immediately southwest of the town of Durango. It is in the east-central part of the Colorado Plateau physiographic province. The Animas River, a major south-flowing river that drains much of the southwestern San Juan Mountains, runs along the eastern margin of the quadrangle. Ridges Basin, Basin Mountain, and Bridge Timber Mountain are prominent landforms within the quadrangle. A dam site associated with the proposed U.S. Bureau of Reclamation’s Animas-La Plata project is at the downstream end of Ridges Basin. Bridge Timber Mountain, on the western edge of the quadrangle, lies along the drainage divide between the Animas and La Plata Rivers. Elevations within the map area range from about 6,140 to 8,366 ft above sea level.

The map area is bisected by the northeast-trending Hogback monocline, a major, down-to-the-southeast, Laramide-age structure that borders the northern and northwestern sides of the San Juan basin. Basin Mountain is a prominent landform associated with the Hogback monocline, and Bridge Timber Mountain, which is capped by late Tertiary fluvial deposits, lies astride the monocline. Rocks that crop out along the Hogback monocline dip south and southeast as much as 34° in the quadrangle. A subtle flexure in the axis of the monocline between Bridge Timber Mountain and Basin Mountain coincides with West Gap and East Gap. Barnes and others (1954) described this flexure as a shallow syncline. Southwest of West Gap, rocks generally strike about N30°E, whereas northeast of East Gap they have an average strike of about N70°E for a short distance before returning to a N30°E that continues northeastward beyond the
quadrangle boundary.

The Four Corners platform is northwest of the Hogback monocline. This broad, northeast-trending structural bench extends from northwestern New Mexico into southwestern Colorado and is underlain by relatively flat-lying sedimentary rocks. The northwestern one-third of the quadrangle is on the Four Corners platform, where rocks generally dip less than 10° south, southeast, or southwest.