Abstract

Middle Ordovician Simpson Group rocks were deposited in greenhouse conditions during a period of overall sea level rise and marine transgression following an extensive hiatus and development of an irregular karst topography on Lower Ordovician Ellenburger carbonates. Simpson Group production comes from three sandstone units in the middle of the group, each sourced from eroding cratonic highs and transported to the margin by eolian and/or fluvial processes. These sandstones, the Connell, Waddell, and McKee members, occur at the base of the Oil Creek, McLish, and Tulip Creek formations, respectively, from oldest to youngest.

Simpson sandstones represent lowstand and/or early transgressive deposits of three third-order sequences; non-reservoir shales and clay-rich carbonates, deposited during maximum flooding, and normal marine and restricted carbonates, deposited during high-stand, comprise the balance of the sequences. McKee Member facies, as observed in the C.C. Canon 3-10 core from Pecos County, Texas, are consistent with this general depositional setting. The cored interval contains inter-bedded and mixed siliciclastics and carbonates, not just sandstone as the nomenclature suggests. Siliciclastic facies include sandstones with a bimodal distribution of silt and medium-grained sand. Sand grains are subrounded to rounded and 200-400 μm in size. These rocks are locally burrowed and/or contain dolomite or calcite cement. Rounded, mature grains are consistent with distant sediment sources and eolian or fluvial transportation mechanisms, cementation supports a margin setting, and burrowing would be expected during relative sea level fall. Silty, fossiliferous wackestone, siltstone, and silty dolostone are common accessory facies.