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Depositional systems on the margins of continents, both modern and ancient, have been related to sea level changes. Two simplified diagrams by Vail, Mitchum and Thompson (1977) illustrate highstand and lowstand sea level conditions and associated unconformities (Fig. 3.1). The highstand diagram represents a depositional system that might be observed in many coastal areas today or at times of highstand in the past. The four main components are the coastal plain, shelf, slope, and rise (or deep water basin). An unconformity related to coastal onlap during the rise of sea level to the highstand condition is shown. With a drop in sea level to the edge of the continental shelf (a lowstand condition), sediment bypasses the shelf and is deposited in deep water. The entire shelf is exposed to subaerial erosion and streams adjusting to the lower base level incise into the older shelf deposits. The depocenter shifts from the deltaic system under the highstand to submarine fans of the lowstand. Unconformities are present within the marine strata and also project into the coastal plain deposits. Many depositional environments are possible between these two end members of sea level conditions in ancient strata.

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