UNKPAPA SANDSTONE (JURASSIC), BLACK HILLS, SOUTH DAKOTA: AN EOLIAN FACIES OF THE MORRISON FORMATION

Abstract

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 theUnkpapa include 1 large scale high angle cross beds tangential at the base 2 thestratigraphic 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 theUpper Jurassic and Lower Cretaceous sequence along the east flank of theBlack Hills has been interpreted as follows As theSundance Sea withdrew from the region the climate became arid to semi arid 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 theaforementioned low relief topography These dune sands comprise theUnkpapa Sandstone which overlies the Sundance Formation in the southeastern part of the Black Hills area In the area of eolian sand dune accumulation thelower 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 theMorrison Formation filled the low areas betweenthe 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 theMorrison 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