NEW GEOLOGIC MAPPING OF OLIGOCENE AND MIOCENE VOLCANIC AND VOLCANICLASTIC ROCKS IN THE PALEO-STANISLAUS RIVER CANYON, SIERRA NEVADA (CALIFORNIA)

Detailed geologic mapping of an 8 by 4.5 km area in the Sierra Nevada near Sonora Pass, combined with geochemical work and previous geochronological work, reveals extreme lateral complexity of Oligocene and Miocene volcanic and volcaniclastic rocks deposited into a paleocanyon that experienced re-incision events. We demonstrate this lateral variability by describing two stratigraphic sections in the map area, from bottom to top:

Dardanelles Cone: Miocene Relief Peak Formation, consisting of subequal amounts of andesite volcanic-clast debris flow and fluvial deposits and minor andesite block-and-ash tuffs; 10.4 – 9.3 Ma Stanislaus Group high-K volcanic rocks including trachyandesite lava flows of the Table Mountain Latite, the Tollhouse Flat Member of the Eureka Valley Tuff (EVT), an olivine basalt lava flow, our newly-defined Lava Flow Member of the EVT, which is trachydacite (Koerner et al., 2009), and a basaltic-trachyandesite lava flow of the Dardanelles Formation.

The Dardanelles: Oligocene ignimbrites of the Valley Springs Formation; Relief Peak Formation olivine basaltic lava flows in a very thick section (up to about 200 m); discontinuous lenses of andesite volcaniclastic rocks Relief Peak Formation; Table Mountain Latite; Tollhouse Flat Member of the Eureka Valley Tuff, locally eroded and redeposited as a boulder conglomerate with lesser clasts of olivine basalt; and up to 60 m of trachydacite Lava Flow Member of EVT.

This study is the first to recognize the trachydacite Lava Flow Member of EVT on The Dardanelles, where it was previously considered to be Dardanelles Formation, and also the Tollhouse Flat Member had not been recognized there. Furthermore, Dardanelles Formation had not previously been recognized on Dardanelles Cone. Paleocurrent studies combined with detailed mapping of erosional surfaces will be used to understand the paleomorphologic evolution of the paleocanyon system.

Koerner, A., Busby, C., Putirka, K., and Pluhar, C., 2009, New evidence for alternating effusive and explosive eruptions from the type section of the Stanislaus Group in the “Cataract” paleocanyon, central Sierra Nevada (CA): in, Gary Ernst, editor, The Rise and Fall of the Nevadaplano, International Geology Reviews, v. 51, nos. 9-11, p. 962-985.