REVISION OF THE STRATIGRAPHY FOR HIGH-POTASSIUM VOLCANIC ROCKS OF THE STANISLAUS GROUP IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

We present new evidence requiring revision of the stratigraphy of the high-K Stanislaus Group, central Sierra Nevada. This group is purported to have erupted from the Little Walker center and has been radio-isotopically dated as spanning 9.1-10.2 Ma.

Previous stratigraphic descriptions of Noble et al., 1974 show the high-K Stanislaus Group as containing three separate formations. The lowermost unit is the Table Mountain Latite consisting of at least 23 lava flows. Overlying this, the Eureka Valley Tuff (EVT) is composed of quartz latite ignimbrites with three members: the Tollhouse Flat member, the By-Day member, and the Upper member. The Dardanelles Latite lava flows were described as lying on top of all of these other units. Based on stratigraphic and magnetostratigraphic evidence, we propose that the Dardanelles latite actually erupted after the Tollhouse Flat member and before the By-Day member of the Eureka Valley Tuff.

Our stratigraphic and magnetostratigraphic work in the Sweetwater “Roadless” Area exhibits most of this stratigraphy in a composite section on either side of Burcham Creek. One normal-polarity flow forms the base of the section, with reversed-polarity latite ignimbrite on top of it. The handsample petrology, polarity and remanence direction all support interpretation of this unit as EVT, Tollhouse Flat member. Possibly reversed-polarity, but certainly normal-polarity latite lava overlies the Tollhouse Flat member, with normal polarity latite ignimbrite capping the section. The handsample petrology, paleomagnetic polarity, and remanence direction of the capping ignimbrite all support its interpretation as EVT By-Day member.

Recent geologic field mapping east of Disaster Peak in the Carson-Iceberg Wilderness provides support for these conclusions. The stratigraphy of this unfaulted, continuous section begins with Cretaceous granitic basement overlain in a buttress unconformity by Relief Peak andesitic volcanic-volcaniclastic rocks, which are in turn overlain by Table Mountain latite lava flows. Above this is the biotite-bearing EVT Tollhouse Flat member, which is covered by Dardanelles latite. Found in an erosional contact with the upper, Dardanelles, lava flows are two separate ignimbrites, which match field characteristics of the EVT By-Day and Upper members.