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.

End_Page 381------------------------