STRATIGRAPHIC REVISION OF THE GLEN CANYON SANDSTONE, SOUTH FLANK OF UINTA MOUNTAINS, UTAH

Along the southern flank of the Uinta Mountains recent field work has suggested the need for a revision in the Triassic/Jurassic stratigraphy. The Glen Canyon Sandstone includes a lower group of strata that are not entirely eolian, but include significant fluvial and marginal marine beds. These lower beds may be Triassic in age and their relationship with the underlying Chinle Formation is unclear. To aid in understanding the Glen Canyon and Chinle formations along the south flank of the Uintas, geologic mapping (sponsored by EDMAP) of the Donkey Flat quadrangle was completed at the 1:24,000 scale, and several detailed stratigraphic sections have been measured through these formations. The lower part of the Glen Canyon Sandstone is characterized by ripple laminated sand-rich beds, often bioturbated and mottled from organic detritus. Interbedded with these sands are green, red, and purple mudstones that resemble the mudstones in the underlying Chinle Formation. Also included in this interval are some high-angle, trough cross-bedded, clean quartz sandstones that are similar to the overlying upper Glen Canyon Sandstone (Navajo Sandstone equivalent beds). At the base of the lower Glen Canyon beds, we find in all of our sections a pebble conglomerate that appears to contain mostly intraclast rip-ups but may contain some extrabasinal chert fragments. The top of lower Glen Canyon is generally characterized by a purple mudstone interval in sharp contact with massive quartz-rich sand of the upper Glen Canyon Sandstone. However, the lowermost massive sands of the upper Glen Canyon are still not completely eolian in nature, but contain significant layers of ripple laminated sandstone.