Abstract

The stratigraphy of the Frontier Sandstone Member of the Mancos Shale (Upper Cretaceous) was studied on the south flank of the eastern Uinta Mountains, Utah and Colorado. The Frontier Sandstone Member (about 275 feet thick) is divided into five depositional units, (A through E, which are identified on outcrop by their stratigraphic position, lithology, and inorganic and organic structures. These depositional units are distinct, local stages of a full cycle of deltaic sedimentation in the thesis area.

Heavy mineral analysis of the sandstone can be used to delineate fluvial and marine sandstones by determining their zircon: Tourmaline ratios. With zircon (Sp. gr. 4.7) and tourmaline (Sp. gr. 3.1 to 3.3) having widely different specific gravities their relative abundances can be used as sensitive indicators of depositional environments. This grain size sorting of zircon and tourmaline is related to the particular energy regime which existed at the time of deposition. Ratios greater than 1:1 are representative of high energy fluvial sandstones, while ratios less than 1:1 are representative of lower energy marine sandstones. Further subdivision of the marine environment can be divided between littoral and shallow neritic (Barrier Island, offshore bar, and shoreface environments) and deeper neritic with the sandstones of the littoral and shallow neritic having abundant heavy minerals and with the sandstones of the deeper neritic having rare heavy minerals.