2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 15
Presentation Time: 8:00 AM-12:00 PM


STANLEY, Richard G., U.S. Geol Survey, 345 Middlefield Rd., MS 969, Menlo Park, CA 94025 and VEDDER, John G., U.S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025, rstanley@usgs.gov

New field investigations, new photogeologic interpretations, and older mapping were used to make a digital (ARC/INFO), 1:24,000-scale geologic map of the San Rafael Mtn. 7.5-minute quadrangle. Prominent geologic features shown on the map include (1) Lower Cretaceous(?) and Upper Jurassic Franciscan Complex melange and serpentinite; (2) Lower Cretaceous(?) and Upper Jurassic Espada Formation consisting mainly of argillite and turbidite sandstone; (3) thick (>2,000 m) sequences of unnamed Upper Cretaceous sandstone, mudstone, and conglomerate deposited on deep-sea fans; (4) an unnamed, lower Miocene shallow-marine sandstone unit, 0-50 m thick, that rests in angular unconformity on Mesozoic rocks and gave a strontium isotopic age of about 20 Ma on molluscan shells; (5) Miocene Monterey Formation consisting of more than 700 m of siliceous, calcareous, and phosphatic shale and mudstone with interbeds of turbidite sandstone; (6) Miocene basalt and volcaniclastic rocks, and several diabase intrusions of presumed Miocene age; (7) at least 6 levels of Quaternary alluvium perched as high as 250 m above modern stream channels; (8) the apparent western termination of the Big Pine fault, North Branch, near a diabase dike that intrudes Upper Cretaceous strata; and (9) the laterally persistent Big Pine fault, South Branch, a north-dipping fault that separates moderately deformed Upper Cretaceous strata on the north from overturned, intensely folded Monterey Formation on the south.

Our mapping indicates that the Camuesa fault is older than formerly thought. Previous geologic maps portrayed the Camuesa fault as a relatively young, throughgoing, right-lateral strike-slip fault that separated upright Cretaceous and Miocene strata on the north from Franciscan rocks on the south. Detailed field studies show instead that (1) the Camuesa fault separates overturned Espada Formation on the north from Franciscan rocks on the south, and (2) the Camuesa fault appears to be truncated by an angular unconformity at the base of the Miocene sequence. These relationships suggest that no significant movement has occurred along the Camuesa fault for at least 20 million years and the fault is therefore an unlikely source of damaging earthquakes.