Fluvial Processes as Interpreted from Ancient and Recent Fluvial Deposits
Detailed outcrop studies of fluvial deposits in the Missourian of Oklahoma 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 trough cross-bedded zone related to sand wave and/or dune transport; (2) a current laminated zone deposited in low amplitude sand waves or on a plane bed; (3) a fine-grained symmetrical ripple and/or micro-trough cross-laminated zone containing sediment formerly transported in suspension; and (4) a laminated clay and fine-grained sand zone deposited from suspension (often 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 suggest an upward decrease in energy. The mean and maximum grain size both decrease upwards, and the sediment is progressively more poorly sorted upwards. These changes are directly related to variation in the type 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.
Factor analysis of characteristics of the grain size distributions provided four strongly defined classes of curves, each related to a different sedimentary structure. Analysis of the shape of the size curves provided additional insight into the mechanics of sedimentation, and the significance of the commonly developed positive skewness, and bimodality.