An Experimental Study of the Influence of Subaqueous Shelf Angles on Coastal Plain and Shelf Deposits
L.J. Wood, F.G. Ethridge, S.A. Schumm, 1993. "An Experimental Study of the Influence of Subaqueous Shelf Angles on Coastal Plain and Shelf Deposits", Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, Paul Weimer, Henry Posamentier
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Research in the past decade has failed to properly assess the influence of subaqueous shelf angle on coastal plain and shelf systems along passive margins. This chapter reports on a series of experiments performed in a 7-m-long, 4.5-m-wide, 0.9-m-deep, nonrecirculating flume to examine the effects that differing shelf angles have on deposition and erosion on the coastal plain and shelf areas of a basin.
Results of these experiments indicate that the angle of the shelf affects (1) rates of stream incision, sinuosity, and width-to-depth ratios, (2) sedimentation rates and sedimentologic character of deposits, (3) rates of knickpoint migration and timing of stream capture, and (4) the depositional thickness and preservation potential of deposits formed during transgression.
Base-level fall over a steeply dipping shelf results in deep, narrow, straight incised valleys on the shelf, as well as thick shelf and shelf-margin deltas that are dominantly derived from the cannibalized shelf and shelf margin. Deposition rates are high with a low ratio of coarse- to fine-grained sediment. Multiple fluvial incisions on the shelf are rapidly abandoned for a single incised valley. During subsequent base-level rise, deposits are well preserved because they are relatively thick and are submerged relatively quickly below wave base.
In contrast, base-level fall over a gently dipping shelf results in shallow, wide, more sinuous incised valleys on the shelf. Thinner shelf and shelf-margin deltas form and contain a higher percentage of hinterland sediment. Deposition rates are low with a high ratio of coarse- to fine-grained sediment. Multiple fluvial incisions on the shelf are active for a longer period of time. During subsequent base-level rise, deposits have a low potential for preservation due to their thin nature and the slower rate at which water deepening occurs over the shelf.
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Applying depositional sequence stratigraphic concepts to the interpretation of siliciclastic depositional systems is becoming an increasingly important tool in petroleum geology. After a succession of breakthroughs during the 1970s and 1980s, sequence stratigraphic concepts now have entered a phase of intense application and documentation, especially with regard to successful implementation in the field of petroleum geology. Workers have applied these concepts to a variety of databases, ranging from outcrop to cores to electric logs and to multifold seismic data. Clearly, sequence stratigraphic concepts embody–not a rigid model or template–but rather a way of looking at geology. This volume has two purposes: to compile some recent applications of siliciclastic sequence stratigraphic concepts, and to present new studies focused on refining conceptual models. This memoir grew out of a 1991 symposium, "Variations in Depositional Systems Within a Sequence Stratigraphic Framework: Applications to Exploration," organized by the authors at the AAPG annual meeting at Dallas. Robert Loucks and Rick Sarg have edited a companion volume, also published by AAPG,“Entitled Carbonate Sequence Stratigraphy: New Developments and Applications.”