FACIES ANALYSIS OF THE UPPER PENNSYLVANIAN-LOWER PERMIAN SYSTEM OF WESTERN NEBRASKA

Depositional patterns in the carbonate-dominated Pennsylvanian-Permian Alliance Basin (northern Denver Basin) of western Nebraska are not well established due to poor surface exposure. Examination of petroleum drillcores and wireline logs allow new insight into facies distributions and depositional environments in the Nebraska Panhandle. Preliminary analyses show facies ranging from skeletal and oolitic grainstones to dolomudstone and nodular anhydrite. These units record a range of depositional conditions, from open to restricted and evaporative marine environments. Although facies are arranged in a cyclic fashion, cycle character differs from the classic transgressive-regressive cyclothems typical of other nearby areas of the US Midcontinent. Key differences include (1) the absence of black fissile “core” shales that record deeper water conditions, and (2) the presence of restricted evaporite facies, namely dolomitized mudstone, microbial laminite, and bedded anhydrite. These differences imply a shallow, arid depositional basin that at times became isolated. We propose that sections of the Transcontinental Arch, which bounds the southern part of the study area, at times served as a physical barrier to open marine conditions. By improving the constraints on the stratigraphy and sedimentology of western Nebraska, this research will contribute to our understanding of the range of environments recorded in Pennsylvanian-Permian strata of the US Midcontinent.