OF-02-01 Geologic Map of the Hermosa Quadrangle, La Plata County, Colorado

Describes the geologic setting and structural geology of this 7.5 minute quad located north of Durango in Plata County. Includes geologic setting, unit descriptions, structural geology, correlation of map units, cross-sections and an oblique view . (1:24,000). A 28-page booklet accompanies map. Digital PDF download. OF-02-01D

From the Author’s Notes:

The Hermosa 7.5-minute quadrangle includes about 60 sq mi of chiefly mountainous terrain within the central part of La Plata County in southwestern Colorado. The west and east boundaries of the quadrangle lie at longitudes of 107° 52′ 30″ W and 107° 45′ 00″ W, respectively. The south margin of the map is at latitude 37°22’30” N and the north margin is at latitude 37°30’00” N. The southern margin of the quadrangle lies about 8 mi north of the town of Durango. The quadrangle lies on the transition of the east-central part of the Colorado Plateau physiographic province and western edge of the Southern Rocky Mountains. The terrain in the mapping area rises from an elevation of about 6,500 ft above sea level along the Animas River to the rugged surrounding mountainous terrain, where the elevation is more than 9,500 ft above sea level. The Animas River, a major south-flowing river that drains much of the southwestern San Juan Mountains, bisects the quadrangle along a north and south line. Nearly 3,000 ft of rock record is exposed in the walls of Animas River canyon.

The mountains within the map area form part of the southern flank of the Laramide San Juan Uplift, which has a core of 1,800–1,400 million year old Proterozoic crystalline rocks that are mantled by south-dipping strata of Paleozoic to Mesozoic sedimentary rock units. The oldest rocks in the map area are exposed on either side of the Animas River canyon at the north edge of the quadrangle. These Proterozoic rocks include 1,800 million year old metamorphosed volcanic arc rocks of the Irving Formation, which are intruded by plutonic igneous rocks of the 1,700-million year old Bakers Bridge Granite and 1,400 million year old Eolus Granite. The Irving Formation, Bakers Bridge Granite, and Eolus Granite form the foundation for sedimentary rocks that were deposited in marine to continental environments. The Animas River and its tributaries, assisted by intense glacial erosion, have carved the deep canyons and steep ridges in the map area, producing the spectacular landscape seen today.