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ABSTRACT: Effects of Base-Level Change on Coastal Plain and Shelf Systems--An Experimental Approach

John E. Koss, Frank G. Ethridge, Stanley A. Schumm

Assumptions concerning the effects of base-level changes proposed under the sequence stratigraphic framework need critical testing. Considerable controversy exists surrounding the impact of base-level changes on channel response, nature of valley-fill deposits, nature and distribution of sedimentary deposits, and distance and magnitude of influence upstream on slopes of different gradients. To test these concepts, a series of experiments was performed in the Rainfall Erosion Facility (a l0 × 15 m flume) at Colorado State University. Using different gradients for successive runs, a typical drainage basin-shelf-slope system was constructed. For each run, base-level was varied systematically by lowering and raising a body of water following a sinusoidal curve.

Base-level changes had little effect on the upper drainage basin. Also, a considerable lag existed between base-level fall below a shelf break and development of an integrated cross-shelf valley system. During this lag time, deposition occurred on the exposed shelf as a broad alluvial apron with a series of low relief distributary channels, whereas between six and twelve valleys developed at the shelf edge and grew by headward erosion at various rates. These valleys were not directly related to the main drainage basin, but ultimately one captured the flow from this drainage, allowing coarser potential reservoir sediments to be deposited on the slope or in submarine fans. During the subsequent base-level rise, this single, larger valley filled with coarse-grained sediments supplied dir ctly from the drainage basin whereas other valleys partially filled with finer suspended-load sediments.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990