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Detailed outcrop studies of fluvial deposits in the Missourian of Oklahoma and the Atokan of the Arkoma basin have revealed a systematic vertical variation in grain size, sedimentary structures, bedding characteristics, and morphology of sedimentary units. The frequency with which this "ideal" sequence is developed suggests a common process in the deposition of many of these fluvial sandstones. The "ideal" vertical sequence is as follows: (1) a lowermost festoon cross-bedded zone related to sand wave and (or) "dune" transport; (2) a parallel laminated zone deposited in low amplitude sand waves or in a plane bed; (3) a fine-grained symmetrical ripple zone containing sediment formerly transported in suspension; and (4) a laminated clay and fine-grained sand zone deposited f om suspension (commonly outside of the fluvial channel). This sequence has been found in both Recent and ancient fluvial deposits.
Flume and river studies have demonstrated that specific sedimentary structures are directly related to sediment transport and the dynamics of open channel flow. Sedimentary structures in the ancient sandstones of the study area were related to bedforms described from Recent channels. Furthermore, additional information on the morphology of the fluvial channels was obtained from an analysis of the thickness and distribution of specific zones.
The grain-size distribution of the fluvial sands of the study area suggests an upward decrease in energy. Both the mean and maximum grain size decrease upward, and the sediment is progressively more poorly sorted upward. These changes are directly related to variation in the type and size of the sedimentary structures. Discontinuities resulting from successive floods were identified by abrupt changes in grain size and (or) in the sequence of sedimentary structures.
The "ideal" vertical sequence, as given is commonly encountered in the field. This suggests some common underlying cause. This study illustrates that use of the flow regime concept, together with an understanding of river hydraulics and channel behavior, can provide a better insight into fluvial sedimentation.
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