ARCHITECTURE OF A MORROWAN (PENNSYLVANIAN) FLUVIAL SANDSTONE, IMPLICATIONS FOR EXPLORATION IN THE ARKOMA BASIN OF NORTHERN ARKANSAS

The Bloyd Formation is prominently exposed in the Early Pennsylvanian succession of northwest Arkansas. Composed dominantly of marine shale and limestone, the formation contains an extensive non-marine sandstone member here referred to as the middle Bloyd sandstone. Deposition occurred in a major braided fluvial system during lowstand conditions and formed a braid plain that was more than 95 km in in width from west to east. Transport direction was south and southwestward across the Arkoma shelf toward the incipient Arkoma basin. Later during basin formation, middle Bloyd strata were displaced into the subsurface of the Arkoma basin along east-trending faults. The middle Bloyd sandstone is composed of fine to medium grained sand with metamorphic rock fragments of phyllite and schist forming 5 to 10 percent of the framework grains. Well-rounded granule to pebble size grains of polycrystalline quartz with abundant fluid inclusions are abundant. The unit is intensely cross-stratified with tabular sets up to three meters in thickness. The middle Bloyd ranges from three meters to over 35 meters in thickness across the outcrop belt in the Boston Mountains. Intervals in excess of 25 meters are characterized by multiple sets of nested cross strata. These intervals are elongate in a north-south direction and thin laterally to the east and west. They pass into intervals with small-scale cross strata rarely interbedded with shale. The elongate bodies are recognized in the subsurface of the northern Arkoma basin and reflect positions of braid strands within a migrating fluvial system