OF-03-21 Geologic Map of the Electra Lake 7.5-Minute Quadrangle, La Plata County, Colorado


Describes the geologic setting, structural geology, thermal springs, and mining history of this 7.5-minute quad located north of Durango. Includes geologic setting, unit descriptions, structural geology, prospects and mines, correlation of map units, oblique view and accompanying cross-sections. 1:24,000. 29-page booklet accompanies the map. Digital ZIP download. OF-03-21D

From the Author’s Notes:

The Electra Lake 7.5-minute quadrangle includes about 60 square miles of chiefly mountainous terrain within the northern 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° 30′ 00″ N and the north margin is at latitude 37° 37′ 30″ N. The southern margin of the quadrangle lies about 20 miles north of the city of Durango. The quadrangle lies near the transition of the east-central part of the Colorado Plateau physiographic province and western edge of the Southern Rocky Mountains (Fenneman, 1931). The terrain in the mapping area varies from an elevation of about 7,000 ft above sea level along the Animas River to the rugged surrounding mountainous terrain, where the elevation is more than 10,000 ft above sea level. The Animas River, a major south-flowing river that drains much of the southwestern San Juan Mountains, flows across the quadrangle from north to south along the eastern margin. Nearly 3,000 ft of rock record that spans about 2 billion years of geologic time is exposed in the quadrangle.

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 to 1,400 million-year-old Proterozoic crystalline rocks mantled by south-dipping strata of Paleozoic to Mesozoic sedimentary rock units (Figs. 2 and 4). Phanerozoic strata on the south flank of the dome define the northern margin of the San Juan Basin.

The oldest rocks in the map area are exposed in the precipitous walls of the Animas River canyon. Proterozoic rocks in the Electra Lake quadrangle include 1,800-million-year-old metamorphosed volcanic arc rocks of the Irving Formation and 1,780- to 1,760-million-year-old metamorphosed intermediate to mafic plutonic rocks of the Twilight Gneiss (Gonzales, 1997). The Irving Formation and Twilight Gneiss underwent amphibolite facies metamorphism and multiple phases of deformation prior to emplacement of plutonic igneous rocks of the 1,700- million-year-old Bakers Bridge Granite, 1,400-million- year-old Eolus Granite and Electra Lake Gabbro, and swarms of Proterozoic mafic dikes whose absolute age is uncertain. The Irving Formation, Twilight Gneiss, Bakers Bridge Granite, Eolus Granite, and Electra Lake Gabbro form the eroded basement on which Paleozoic sedimentary rocks 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.

The geologic framework of the Electra Lake quadrangle is the product of a long and complex history of metamorphic and igneous events, deformation, sedimentation, uplift, and erosion. The general sequence of events that is preserved in the rock record of the mapping area are as follows:

(1) Formation, metamorphism, and deformation of volcanic and intrusive rocks in a volcanic arc system between 1,800 and 1,750 million years ago. Intrusion of granitic and minor gabbroic rocks into the older metamorphic complex between 1,700 and 1,400 million years ago.

(2) A major gap in the rock record for about the next 900 million years because of erosion or nondeposition. Mafic dikes that cut the Irving Formation, Twilight Gneiss, Bakers Bridge Granite, and Eolus Granite may have been emplaced during this period of time.

(3) Deposition of a thick succession of marine and deltaic carbonate and clastic rocks with minor local uplift and erosion between about 550 and 320 million years ago.

(4) Tectonic uplift in the Late Pennsylvanian that produced a northwest-southeast trending belt of mountains referred to as the ancestral Rocky Mountains.

(5) Erosion of the uplifted region by streams and rivers and deposition of this material as “redbed” clastic sedimentary rocks about 300 million years ago. There are younger units exposed in the region but they are not exposed within the map area.

(6) Renewed tectonic uplift caused by compressional forces during the Laramide orogeny producing the San Juan uplift. This event was accompanied by intrusion of ore-bearing igneous rocks in the La Plata Mountains about 75 to 65 million years before present.

(7) Quaternary glaciation, alluviation, and other surficial processes that formed the present day landscape.