The Claiborne Group in southwest Arkansas
Although the middle Eocene age Claiborne Group of southwest Arkansas covers a large area, outcrops are sparse. The easily erodable Claiborne is composed of unconsolidated sand, silt, clay, and lignite. It has a maximum thickness of 1,500 ft (457 m) in Arkansas, generally strikes to the northeast, and dips an average 60 ft to the mi (11 m to the km) to the southeast. The exposures at Camden and Red River in southwestern Arkansas (Fig. 1) display features of a deltaic plain environment. The two exposures are 75 mi ( 120 km) apart and are probably of different parts of the Claiborne section. Currently, the Claiborne Group is interpreted as having accumulated in fluvial-deltaic system. Studies by Bernard and others (1970) have established a classification scheme of sedimentary environments within deltas. Texture, fabric, sedimentary features, geometry, and trends of sediments associated with deltas are controlled by geologic processes and the availability and influx of different sediment types. Based on these criteria, delta depositional environments have been divided into (1) delta plain with natural levees, distributary channels, interdistributary marshs, and flood basins; (2) inner fringe with rivermouth bars; (3) outer fringe; (4) prodelta; and (5) open marine (Figs. 2 and 3). These environments are discussed within the context of high sediment influx, low energy type of coastal delta (birdfoot, lobate) interpretation of the Claiborne Depositional system, as opposed to low sediment, high coastal energy deltas (arcuate, estuarine).