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Abstract

Observations of modern and ancient sedimentary basins indicate that the shoreface and the depositional shelf break (DSB) can range from coincident to more than 100 km apart. Most conceptual and numerical models of sequence formation do not adequately separate these two features. The model used here incorporates independent calculation of the position of both the shoreface and the DSB; however, at present the lack of an adequate understanding of long-term shoreline response to sea level and other environmental change is a serious limitation to our understanding of the genesis of continental margin stratigraphy.

When independent movements of the shoreface and the depositional shelf break were incorporated into the model, both ravinement surfaces and regressive surfaces of erosion developed in the simulations. This response attests to the importance of distinguishing these two features and to the important role of the physiographic break at the shoreface. In model results, the shoreface and DSB do not always respond similarly to sea level fluctuations. The relationship between shoreface and DSB movements differs depending upon whether they are geographically separated. As a result, defining sequence boundaries and systems tracts can be difficult. The extent of the transgressive systems tract, in particular, is a problem because progradation of the DSB begins partway through the shoreline transgression. This mismatch between the shoreface and DSB predicted by the model has not previously been noted. Fluvial erosion of the coastal plain develops progressively as the shoreface advances, indicating a progressive development of the sequence boundary. Onlap onto the front of the previous regressive shorefaces occurs primarily during the transgressive systems tract. Systems tracts therefore should be based on stratigraphic and lithological distinctions in the rock record, and not be tied to an interpretive model.

To accurately calculate vertical motions and resultant stratigraphy for high-frequency eustatic fluctuations, isostatic adjustment and erosion to an equilibrium profile are modeled as time-dependent processes. These models show that the form of sequences changes with the frequency of eustatic fluctuations. Similarly, the erosion rate has a major influence on stratal relationships. The rate of isostatic compensation can alter whether a sea level cycle generates a sharp-based or gradational-based shoreface. The type of base at prograding shoreface successions can change within sequences. Thus, the sequence boundary should be placed at the top of regressive shoreface packages. Continued isostatic or compactional subsidence following the deposition of depositional delta lobes can explain the formation of flooding surfaces and parasequences.

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