A STROLL ALONG A NEOPROTEROZOIC SHORELINE, UP GILBERT PEAK, KINGS PEAK QUADRANGLE, UINTA MOUNTAINS, UTAH

A continuous section of mid-Neoproterozoic lower and middle Uinta Mountain Group is exposed in the Kings Peak Quadrangle, Utah on the northeast ridge of Gilbert Peak. It contains the formations of Red Castle, Dead Horse Pass, and Mount Agassiz. Three sedimentary facies associations are recognized: (1) The fluvial and tidal facies is characterized by couples of a) medium to coarse, moderately sorted, arkosic fluvial arenite containing trough cross bedding and b) interbedded tidal (flaser bedded and herringbone cross-bedded) green and red shale and fine sandstone. Shaley halves of the sandstone-shale couples thin upwards through the section, from 15 meters to a few centimeters thick, and are laterally continuous. (2) The marine facies is a fine to medium, moderately to well sorted quartz arenite with planar cross bedding interbedded with shale and siltstone, with shale beds approximately 10-30 meters thick. This facies shows progradational and retrogradational geometries. (3) The arkosic marine facies is a very coarse, unsorted arkosic arenite with parallel laminations and pebble lags.

The upper Red Castle is composed of aggradational fluvial and tidal facies (a highstand systems tract), overlain by an erosional stratigraphic sequence boundary across which paleocurrents and lithologies change abruptly. The formations of Dead Horse Pass and Mount Agassiz contain the two marine facies, which compose a retrogradational-progradational cycle punctuated by four higher order sequence boundaries. The arkosic facies is interpreted to represent near shore lowstand deposits of minimally reworked fluvial sediments. This facies likely indicates a significant lowstand with a near return to fluvial conditions similar to those controlling deposition of Red Castle. Low diversity acritarchs from shales suggest a restricted marine environment.