HR: 0830h
AN: V51G-0354    [PDF]
TI: Tertiary Volcanic Stratigraphy and Structure of the Sonora Pass Region, Central Sierra Nevada, California
AU: * Busby, C J
EM: busby@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Rood, D
EM: dylan@crustal.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Wagner, D
EM: davlwagner@comcast.net
AF: California Geological Survey, 801 K St. MS 12-31, Sacramento, CA 95814 United States
AB: Mapping north and west of Sonora Pass provides information about the paleogeography and structure of the Sierra Nevada-Basin Range transition in Oligocene to Miocene time. The stratigraphy includes: (1) Late Oligocene-Early Miocene (?) Valley Springs Formation, composed of at least 5-6 petrographically distinctive silicic ignimbrites, overlain with erosional unconformity by (2) Early to Middle Miocene (?) "Relief Peak Formation", here referred to as Mehrten Formation, composed of hornblende andesite volcanic debris flow deposits, overlain with erosional uncomformity by channelized fluvial deposits, in turn overlain with erosional unconformity by block and ash flow tuffs, overlain with erosional unconformity by (3) the Middle or Late Miocene (?) Stanislaus Formation, composed of (a) latite lava flows ("Table Mountain Latite") and interbedded volcanic debris flow and fluvial deposits, overlain with erosional unconformity by (b) the Eureka Valley Tuff (EVT), also of latitic composition, in turn overlain with erosional unconformity by (c) latite flows, volcaniclastic fluvial deposits, or a debris avalanche deposit. The entire section is cut by andesite plugs. The basal ignimbrites are distal facies with source calderas in Nevada. The Merhten Formation is dominated by primary volcanic deposits in the east. The "Table Mountain Latite" is coarsely porphyritic plagioclase- and augite-phyric lava flows and flow breccias; it is only 83 m thick, with two flow units, in the west, and thickens to 344 m, with 21 flow units, in the east, suggesting a source not far to the east of the crest. The EVT consists of two members, each of which grade upward from densely welded to nonwelded latite ignimbrite. Distinctive lapilli- to block-sized clasts in both members are a distinctive mixture of highly flattened and plastically-deformed obsidian and rigid, vesicular bombs cored by obsidian. The tuff was probably erupted from a caldera only 22 km to the east. The Miocene strata generally thicken from west to east (over a map distance of 25 km) but locally thicken dramatically into paleocanyons cut in Mesozoic granitic rocks. A paleocanyon along the present-day Sierran crest trends NNW-SSE from Stanislaus Peak to Sonora Peak; it is sited along NNW-SSE, steeply-dipping faults in the Merhten Formation and filled by undeformed Stanislaus Formation latite flows. Similar relations occur along what is probably the same fault 5 km to the SSE at Sardine Falls. To the west of the crest, a pair of paleocanyons trending ENE-WSW occur at the Dardenelles in the north and the Red Peak-Bald Peak area in the south. This pair of paleocanyons is cut by a series of steeply-dipping NNW-SSE trending faults that include (from west to east): 1) a down-to-the-west fault between the Dardenelles and Dardenelles Cone, along which the Merhten Formation is highly 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, also down to the west, 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 Peak fault, which drops the EVT down to the west approximately 200 m. Miocene strata of the Sonora Pass region record the onset of faulting both during and after deposition of the Merhten Formation, with continued faulting after eruption of the latite flows and tuffs. We speculate that the latites erupted during a phase of rapid extension in an arc otherwise dominated by andesitic volcanism.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting