| Description |
Preconceived ideas concerning desertic environments, coupled with alack of any recognized modern analog, have hampered an objective reconstruction of the surficial environment under which the Upper Triassic- Lower Jurassic cross-bedded sandstones of the western United States were deposited. The lateral equivalents of the Nugget Sandstone are not conclusively known, although the unit is commonly referred to as the northern extension of the Navajo Sandstone. However, paleocurrent analysis and lithologic aspects indicate that the Nugget (?) Sandstone of the south flank of the Uinta Mountains may not be identical to the type Nugget Sandstone in Wyoming and may correlate, at least in part, with the Wingate Sandstone. Sedimentary structures in the Nugget (?) Sandstone include medium-scale cross-bedding, planar beds, and multiple parallel truncation bedding planes. The cross- bedding indicates that the dunes were relatively small, low, and probably mound-shaped. The cross-beds also have an alternating grain size lamination which might be a type of eolian varving. Ripples on the lee sides of the dunes are high index and subparallel to dip. Dunal material, also displays parting lineation parallel to the ripples. The planar beds always lie directly above and parallel to a multiple parallel truncation bedding plane.These beds are interpreted as sebkha deposits formed by-wind-blown sand accretion to a damp surface. These beds are essentially the product of a rising ground water table. Multiple parallel truncation bedding planes reflect deflation down to the water table. Two, possibly three, vertebrate trails, two (?) scorpionid trails, and two burrows were discovered on the planar beds. The high number of individuals but low number of species represented indicates few available ecological niches and, probably, a transient population. No body fossils of any kind were found. The trackways are always impressed into a fine-grained lamina, raising the possibility that the animals went to the sebkha in the evening when the air is generally still and when finer particles could have settled out of the air. The source of the sand may have been stream-trans ported material from surrounding highlands. A mixed terrane could have supplied the required quartz, as few inclusions remain in such small grain sizes to earmark a specific provenance. Winds may have been derived from a stable high pressure cell centered over western seas. Warm, orographic winds descending the western highlands may have contributed to the aridity and windswept nature of the Nugget (?) environment. The Nugget (?) Sandstone represents the end product of a climatic change which began in early Ghinle time. A warm, humid, exoreic system became drier and endoreic into late Ghinle time and, finally, became areic in Nugget (?) time. This change is best explained in terms of local orographic and subsiding basin effects. The area near Heber was basically a low- relief deflation basin (P'ang Kiang Hollow), relatively close to the source area, with alternating low mound or transverse dunes and interdune sebkhas. |
