DETAILED MAPPING REVEALS COMPLETE STRATIGRAPHIC SECTION THROUGH 10 – 9 MA HIGH-K VOLCANIC ROCKS OF THE SONORA PASS REGION, SIERRA NEVADA (CA)

High-K volcanic rocks of the central Sierra Nevada were first recognized in 1898 by Ransome, who described latite lava flows (Table Mountain Latite, TML) overlain by biotite-augite-latites, in turn overlain by the Dardanelle flow. The quartz latites were later recognized as welded and unwelded ignimbrites by Slemmons (1966), who referred to them as the Eureka Valley Member of the Stanislaus Formation, grouping all the high-K volcanic rocks into the Stanislaus Formation. Although Slemmons (1966) reported that latite lava flows existed within the Eureka Valley Member, their location was not identified, and no later reports were made of them. Noble et al. (1974) raised the Eureka Valley Member to formational status (Eureka Valley Tuff, EVT), dividing it into three members, all composed of ignimbrite (from bottom to top): the Tollhouse Flat, By-Day, and Upper members. For the past 34 years, the stratigraphy of the Stanislaus Group has been portrayed as a section of ignimbrites (EVT members) sandwiched between latite lava flows above and below (TML and Dardanelles Formation). However, the reference section proposed in 1974 (Noble et al.) has no Dardanelles Formation, making its stratigraphic position uncertain relative to the EVT members. The stratigraphy of these high-K volcanic rocks is complicated by the fact that numerous erosional unconformities separate the different units, which are preserved in paleochannels that once crossed the Sierra Nevada.

Our new 1:6,000 mapping and detailed measured section in the Bald Peak-Red Peak area of the Sonora Pass region demonstrates the following stratigraphy (from base to top): (1) Table Mountain Latite with phenocrystic clinopyroxene and skeletal plagioclase, and groundmass olivine (25 m); (2) Tollhouse Flat Member (EVT) with abundant phenocrystic biotite (13 m); (3) a single lava flow with phenocrystic olivine (35 m), (4) By-Day member (EVT) which lacks phenocrystic biotite (20 m); (5) unwelded Upper Member (8 m); and (6) a single very thick (60 m) aphyric lava flow that we assign to the Dardanelles Formation.

Our work provides the first confirmation that the Dardanelles Formation exists as a unit that overlies the EVT. Our documentation of a lava flow within the EVT shows that effusive volcanism alternated with explosive volcanism, rather than merely preceding and following it.