Abstract

We develop a model to help explain the wide variety of vertical stratification sequences in shallow-marine sandstones. This model, embodied in a set of vertical-sequence cartoons, predicts or accounts for vertical stratification sequences in beds of free sandstone deposited from a full range of decelerating combined flows, as well as from purely unidirectional and oscillatory flows. According to the model, vertical stratification sequences in storm-deposited sandstone beds are the outcome of the time history of the bed configuration as deposition proceeds. The idealized sequences are based on the initial velocities and subsequent deceleration histories of the unidirectional and oscillatory flow components. Twenty qualitatively different sequences starting with plane bed are generated, and an additional 26 sequences result from omitting one or more of the lowermost intervals in succession. The model thus accounts for 46 distinct kinds of sequences. Five major simplifying assumptions are built into the model: bidirectional flows, linear deceleration of both unidirectional and oscillatory flow components, time-independent deposition rate, time-independent sediment size, and equilibrium bed configurations. Owing to these assumptions, the model is likely to be applicable to only a small subset of vertical stratification sequences observed in shallow marine sandstones. As more information on combined-flow bed configurations becomes available, these assumptions could be relaxed to provide a more refined and complete representation of vertical stratification sequences. Since the model generates vertical sequences formed from any kind of decelerating flow, the Bouma sequence for deposits from purely unidirectional turbidity currents and also sequences resulting from decelerating purely oscillatory flows (similar in appearance to many proposed generalized tempestite sequences) are end-member cases in the model. The model is also extended slightly to explain the geometry of the uppermost bedding surfaces of storm-generated sandstones by consideration of disequilibrium effects.

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