USGS EDMAP STUDY OF THE TERTIARY VOLCANIC STRATIGRAPHY AND STRUCTURE OF THE SONORA PASS REGION, CENTRAL SIERRA NEVADA, CALIFORNIA

Mapping north and west of Sonora Pass records faulting that was active during deposition of the Mehrten Formation, and continued during and after deposition of the Stanislaus Formation.

The stratigraphy includes 5 sequences, which are all bounded by erosional unconformities. From the base of the section, the sequences are: (1) Oligocene (?) Valley Springs Formation, composed of silicic ignimbrites, (2) dominantly andesite volcaniclastic Early to Middle Miocene (?) Mehrten Formation, composed of debris flow deposits, channelized fluvial deposits, block and ash flow tuffs, (3) dominantly latitic Late Miocene (?) Stanislaus Formation, composed of (3a) lava flows (“Table Mountain Latite”) and interbedded debris flow and fluvial deposits, (3b) Eureka Valley Tuff, (3c) lava flows, fluvial deposits, or a debris avalanche deposit. The entire section is cut by andesite plugs.

Over 25 km, the Miocene strata thicken westward, but locally thicken into paleocanyons cut in Mesozoic granitic rocks. A paleocanyon along the modern Sierran crest trends NNW-SSE from Stanislaus Pk to Sonora Pk; it is sited along NNW-SSE, steeply-dipping faults in the Merhten Formation and filled by undeformed Stanislaus Formation latite flows. The same relations occur 5 km to the SSE at Sardine Falls. West of the crest, two paleocanyons trending ENE-WSW occur at The Dardenelles in the north and the Bald Peak area in the south. These paleocanyons are cut by steeply-dipping NNW-SSE trending down-to-the-west faults that include (from west to east): 1) a fault between the Dardenelles and Dardenelles Cone, along which the Merhten Formation is deformed in a 250 m wide shear zone, and thickens from about 60 m to 200 m to the west across the fault, while overlying latite flows are only downdropped 60 m to the west, 2) the Bald Peak fault, where Merhten Formation thickens from 50 to 100 m to the west across the fault, and the base of the Table Mountain Latite is dropped 45 m to the west, and 3) the Red Pk fault, which drops the Eureka Valley Tuff ~200 m. A fourth fault with a more northwesterly trend displaces a paleocanyon 80 m down to the NE with ~1 km of dextral offset.

We speculate that the latites erupted during a phase of rapid extension in an arc otherwise dominated by andesitic volcanism. 40Ar/39Ar dating will constrain the ages of the stratigraphy and structure of the region.