CORRELATION AND ANALYSIS OF PALEOCHANNELS IN THE SIERRA NEVADA, CALIFORNIA, AS DISPLAYED BY THE STANISLAUS GROUP; TOWARDS UPDATING LATE CENOZOIC UPLIFT ESTIMATES

The proposed Neogene uplift of the Sierra Nevada has been debated amongst geologists for several decades. Although multiple lines of evidence indicate substantial uplift of the Sierran crest over the last ten million years, new isotopic and paleobotanical methods have called into question the validity of these calculations. This study focuses on the discontinuous units of the Miocene Stanislaus Group: Table Mountain Formation lavas, Dardenelles Formation lavas, and Eureka Valley Tuff lavas and tuffs. These units are significant because they have been used as a tilt indicator to calculate the uplift of the Sierra Nevada crest since their emplacement, 9.0-10.2 million years ago.

This study involves detailed field research to increase the quantity of primary data, which will be used to develop and verify a digitally-derived model of the paleodrainage systems of the ancient Sierra Nevada. The field localities, Dorrington, Pikes Peak, Rancheria Mountain, Sonora Pass, and Three Chimneys, are of significant importance because they may represent the few remaining outcrops of previously unmapped paleodrainage channels. To test this hypothesis, geochemical and paleomagnetic data were used to correlate the localities and thereby reconstruct the paleochannels.

Outcrops near Dorrington contain biotite, display eutaxitic textures and are characterized by a reverse polarity indistinguishable from the Tollhouse Flat Member of the Eureka Valley Tuff. Although previously mapped as Table Mountain Latite, we reclassify these units Tollhouse Flat Member and eliminate the hypothesis that the units were once part of a potentially lava-back-flooded tributary of the paleo-Stanislaus River. Columnar jointed Pikes Peak lavas record normal polarity, characteristic of most flows within the Table Mountain Formation. Research conducted on Rancheria Mountain verifies the presence of Stanislaus Group lavas and tuffs previously mapped by Huber et al. (1989). This project tests Huber’s (1990) hypothesis that lavas and tuffs flowed southwest from the Buckeye Pass area in paleo-creeks of the paleo-Tuolumne drainage basin, but have been almost completely eroded away since emplacement. We tentatively correlate at least one lava flow found near Rancheria Mountain to outcrops documented by Carlson (2012) in the Bridgeport area.