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Epicontinental seaways, such as those that existed during the Jurassic through the Cretaceous in North America, do not have modern counterparts. Thus, exact analogs are not available for use as sedimentologic models of ancient epicontinental seaway depositional systems. As a result, well-documented modern shoreline features traditionally have been used as physical analogs to describe most paleodepositional features.
During the past decade, marine geologists have accumulated a sufficient amount of data concerning modern continental-shelf sedimentologic processes and resulting features to relate these to similar features recognized in epicontinental deposits. For example, studies along the Atlantic epicontinental margin of North America and the North Sea between Great Britain and Europe reveal the dynamics of sand movement in response to storm-generated and tidal currents. These processes interact with sediment-covered sea bottoms to form transverse and longitudinal bed forms. Transverse bed forms, which include sand waves and ripples, form in response to the opposing current couplets created by tidal circulation systems. Longitudinal bed forms, which include sand ribbons, longitudinal furrows, and large-scale sand ridges, are formed predominantly on storm-dominated sand-covered shelves. The mechanisms which create and maintain these features are poorly understood but sand ridges and related longitudinal features apparently may form in response to cellular flow structure in storm-generated currents. Similar processes probably operated within ancient epicontinental seaways.
Using present-day continental-shelf environments as partial analogs, we can construct process-response models to explain the development of some epicontinental paleodepositional systems rather than relying solely upon shoreline physical models. This method of model construction consists of an empirical, or data-gathering phase, followed by an interpretive phase. The objective is to match sedimentary sequences to process elements, and then combine these elements with information concerning the regional geologic setting to make paleoenvironmental interpretations. In this way, interpretations are based upon possible processes within particular geologic settings rather than the physical comparison of stratigraphic sections and well-documented present-day depositional systems. The process- esponse approach to sedimentary models frees us from the constraints imposed by the lack of well-documented sedimentologic studies of offshore depositional systems. This method of model building has been used to recognize and delineate offshore depositional systems in the Jurassic and Cretaceous epicontinental seaways of North America. It is postulated that currents generated by tides, storms, and regional circulatory mechanisms played major roles in creating and distributing continental-shelflike depositional systems within the offshore portions of these and other ancient epicontinental seaways.
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