FLUVIO-ESTUARINE TRANSITION IN A MIDDLE CARBONIFEROUS SANDSTONE UNIT, MORROWAN OF NORTHWEST ARKANSAS

The transition from a braided fluvial facies to an estuarine facies strongly dominated by tidal processes is preserved within a prominent but unnamed sandstone unit in the Bloyd Formation (Morrowan, Pennsylvanian) of northwest Arkansas. The unit is packaged stratigraphically between two members of the Bloyd; the underlying Brentwood Limestone Member and the overlying Dye Shale Member. Both are marine. The sandstone unit is exposed in an outcrop belt that extends over 100 km. from east to west and 75 km. from north to south. To the north, the unit is truncated by the modern erosion surface. It passes southward into open marine facies in the subsurface of the Arkoma basin and interfingers to the east with coastal plain shale and siltstone deposits of the non-marine Woolsey Member. The stratigraphic relationships of its eastern terminus are unknown. The sandstone ranges in thickness from a western minimum of 1 meter to over 40 meters in the south-central part of the outcrop belt. Directional features in the fluvial facies indicate that sediment transport was generally southward toward an open marine shelf with depositional strike oriented east-west. The fluvial facies occupies the northern two-thirds of the outcrop belt. The facies is characterized by vertical successions of genetic packages each reflecting deposition in a braid channel from initial high discharge conditions through waning and ultimate cessation of flow. Genetic successions include an erosion surface succeeded by a quartz pebble conglomerate, one or more sets of tabular cross-beds, an interval of ripple laminations and thin clay drapes on ripple bed forms. The fluvial facies grades south into a tide-dominated estuarine facies. The transition occurs in the southern third of the outcrop belt where the sandstone unit exceeds 30 meters in thickness and is expressed as intervals of tidal rhythmites ranging to 7 meters in thickness interbedded with intervals composed of multiple sets of tabular cross-beds up to 2 meters in thickness. Individual rhythmites range from 3 to 6 cm. in thickness. They are composed of planar and climbing ripple laminated beds of sand separated by 1 to 3 mm thick layers of clay and organic material. The couplets are sand-dominated; flaser and wavy bedding occur but are not common.