Red Hills (Tuolumne County) - Geology

Geology

The Red Hills Management Area is located within the western tectonic block of the Sierra Nevada metamorphic belt. Within this block are Upper Jurassic volcanic and sedimentary rocks of island arc derivation which were highly deformed and weakly metamorphosed during the Nevada orogeny. These rocks were then intruded by plutons associated with the emplacement of the Sierra Nevada batholith, east of the metamorphic belt.

The Management Area includes much of the Tuolumne ultramafic complex, one of the largest exposures of serpentine rocks in the Sierra Nevada metamorphic belt. Nearly the entire area is underlain by dunite, a variety of peridotite consisting of dark green olivine and minor chromite, which has been partly or entirely serpentinized to antigorite magnesite-magnetite. The dunite intruded into andesite pillow breccias and flows of the Peñon Blanco volcanic formation, which crops out near the northeast and southern boundaries of the Management Area. The intrusion of ultramafic material occurred along crustal weaknesses associated with the Bear Mountain fault, a northwestward trending thrust fault that dips steeply to the northeast, and has over 10,000 feet of vertical displacement. The fault zone is on the southwestern boundary of the Management Area, and is believed to have once penetrated the earth's crust down to the mantle, a probable source of the peridotite.

Other geologic features of the area include northeastward trending dikes that consists of massive, dark brown, hornblende-plagioclase diorite which intruded into the dunite bedrock. A swarm of several dikes occurs in the southeast portion of the Management Area. Some of the dikes exceed one mile in length. Amphibolite and chert tectonic blocks also occur in the dunite near the area's southern and western boundaries.

Tertiary and Pleistocene alluvium consisting of sand, gravel, pebbles, and cobbles has accumulated in Six Bit and Poor Man's Gulches in the northern and eastern portions of the Management Area.