Subsurface Upper Cretaceous Stratigraphy of Southwestern Arkansas: ABSTRACT

ABSTRACT The subsurface Gulf Series of southwestern Arkansas is characterized by uniformity in lithologic features, a minimum of facies variations except on a local scale, general eastward thinning of units toward a shoreline area, and features typical of neritic, eulittoral to sublittoral, and alluvial to prodeltaic environments. The Woodbine Group of southwestern Arkansas consists of the Lewisville Formation of fluvial to littoral varicolored shale and sandstone and the overlying Eagle Ford Formation of dark gray to black littoral to sublittoral shale. The Woodbine is unconformable on Comanchean rocks throughout the area studied. The Austin Group in this area is represented by the Tokio Formation of littoral to sublittoral ashy sandstone and shales and the Brownstown Formation of more offshore calcareous, glauconitic shale and sandstone. The Taylor Group here consists of the Ozan Formation of slightly calcareous shale and glauconitic sandstone with a basal glauconitic sandstone (Buckrange Sand Lentil) followed above by the neritic Annona Chalk, Marlbrook Marl, and Saratoga Chalk. The Ozan Formation tends to thin southwestward in contrast to the underlying Gulf deposits which generally thicken in that direction. The chalk formations of the Taylor Group mark the onset of large numbers of Senonian planktonic coccoliths and foraminifers in this region. In northeastern Texas the Austin Chalk (Tokio equivalent) represents an earlier Senonian invasion of calcareous planktonic organisms. In this region the Austin and Taylor planktonics are associated with typical benthonic fossils and require a neritic environment. The Navarro Group of southwestern Arkansas consists of the Nacatoch Sand and Arkadelphia Marl. The Nacatoch is neritic, littoral, and sublittoral glauconitic sandstone and shale whereas the Arkadelphia is neritic calcareous shale and marl, in part glauconitic. The base of the Paleocene marine Midway Group is distinct throughout the area studied but there is little or no evidence of an unconformable relation with the Gulf Series. There is some evidence of a paleontologic break at the contact, and it probably should be termed a paraconformity.