Unkpapa Sandstone (Jurassic), Black Mills, South Dakota: An Eolian Facies of the Morrison Formation
George J. Szigeti, James E. Fox, 1981. "Unkpapa Sandstone (Jurassic), Black Mills, South Dakota: An Eolian Facies of the Morrison Formation", Recent and Ancient Nonmarine Depositional Environments: Models for Exploration, Frank G. Ethridge, Romeo M. Flores
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The Unkpapa Sandstone in the Black Hills of South Dakota is interpreted to be an eolian facies of the Morrison Formation based on the stratigraphic framework and lithologic features. Criteria considered compatible with an eolian environment for the Unkpapa include: (1) large scale, high angle cross-beds, tangential at the base; (2) the stratigraphic position within the terrestrial Morrison Formation; (3) the lack of fossils and abundant bioturbation; (4) textural parameters including size, sorting, grain size distribution and quartz grain surface textures; and (5) composition.
The paleogeographic history of the Upper Jurassic and Lower Cretaceous sequence along the east flank of the Black Hills has been interpreted as follows: As the Sundance Sea withdrew from the region, the climate became arid to semiarid. Lacustrine and fluvial depositional systems were established on low-relief topography on which sediment of the Morrison Formation was deposited. At this time, Sundance sandstone, in an area of northwestern Nebraska, was being eroded and fine sand was being supplied to an eolian dune field which extended northward into the aforementioned low relief topography. These dune sands comprise the Unkpapa Sandstone, which overlies the Sundance Formation in the southeastern part of the Black Hills area. In the area of eolian sand dune accumulation, the lower part of the Morrison Formation consists of interdune silt and clay containing dinosaur fossils, gastroliths, ostracodes, charophytes and gastropods. As a more humid climate developed during the Late Jurassic, the dunes were stabilized and low energy fluvial floodplain sediments of the Morrison Formation filled the low areas between the dunes. The type of dunes could not be ascertained due to scarcity of good exposures. High energy fluvial systems of the Lakota Formation then buried the Unkpapa and Morrison beds with coarser clastic sediment.
The Unkpapa Sandstone is an eolian facies of the Morrison Formation. According to the definition of the Morrison Formation, which includes all nonmarine sedimentary rocks stratigraphically above the marine Jurassic sequence and below the basal Cretaceous beds, the Unkpapa Sandstone should be designated as a member of the Morrison Formation.
The high permeability of the Unkpapa Sandstone and its association with possible source rocks make it a potential host for deposits of uranium or petroleum.