The Depositional Environment of the Entrada Sandstone (Jurassic) from Beulah, Pueblo County, Colorado to the Cimarron River Canyon, Union County, New Mexico

The deposition of the Entrada Sandstone in southwest Colorado and northeastern New Mexico occurred during the Middle Callovian to Early Oxfordian (Jurassic). Deposition was greatly influenced by three factors: 1) oscillating shoreline of the western interior sea due to many minor transgressions and regressions; 2) the migration of sand from the highlands to the west and south; subsequently these sands were reworked and deposited on the eastern and northern margins of the sand sea; and, 3) the constant and variable winds of the region. Four lithofacies were deposited within this framework of controlling factors. Lithofacies A, a planar laminated, shaly siltstone was deposited in a shallow restricted marine environment. Lithofacies B, a planar tabular, small scale (1 meter thick), cross-stratified, sandstone represents the initial encroachment of the eolian dunes into the study area. Lithofacies C, the large scale (> I meter thick) cross-stratified, sandstone represents a later stage of dune development. The poorly laminated sandstone of Lithofacies D represents dune sands that were reworked and modified by the transgression of the interior sea. Comparison of the established Entrada Sandstone of the Cimarron River Canyon, Union County, New Mexico to the undifferentiated Morrison Formation, Entrada Formation, and Ralston Creek Formation strata of the Beulah, Colorado area has determined that the term Entrada Formation should no longer be used for the sedimentary units in the Beulah , Colorado area. The Entrada Formation sediments were deposited by eolian processes on the eastern flank of the Entrada erg and later reworked and modified by the transgressing Western Interior Sea. Rocks in the Beulah area were deposited in a near-shore, restricted marine environment and have been correlated to the Ralston Creek Formation. Stratigraphically Ralston Creek Formation possibly intertongue with the Entrada strata to the south between Farisita, Colorado and Beulah, Colorado.