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.

Abstract Modeling of early Paleozoic passive margins in the Cordilleran and Appalachian orogens indicates that factors controlling growth of early Paleozoic passive-margin carbonate platforms were thermally controlled subsidence, time-dependent flexure of the lithosphere, and at least two orders of eustatic sea-level changes. Initiation of the carbonate platforms in Middle Cambrian time followed a marked reduction in supply of Lower Cambrian coarse siliciclastic material to the passive margins. Two-dimensional modeling of palinspastically restored cross sections implies that the reduction in relief of onshore sediment sources resulted mainly from increased time-dependent flexural rigidity and extension of the area of subsidence into the craton. Continued increase in rigidity and bending of the craton edge, combined with a long-term eustatic sea-level rise, further reduced the supply of siliciclastic material to the carbonate platforms, resulting in a progressive cratonward shift of the siliciclastic shoreline and cratonward expansion of the carbonate platforms. Additional evidence of eustatic controls on growth of the platforms is obtained from one-dimensional analyses of post-rift subsidence of the platforms. The effects of sediment loading and lithification are removed from cumulative subsidence curves, producing reduced cumulative curves, designated R1 curves. The first-order form of the R1 curves is exponential, matching closely the form of theoretical curves calculated from cooling plate models for passive margins. After subtracting best-fit model cooling curves from the R1 curves, the residual curves, designated R2 curves, contain evidence of two orders of "events" superimposed on the thermally controlled subsidence of the margins. One event is the long-term rise and fall of sea level observed in the two-dimensional modeling. The long-term event coincides temporally with the Sauk transgression-regression on the craton. The other consists of repeating short-term sea-level changes with wave lengths of 2 to 6 Ma. The short-term sea-level events have similar timing in the southern Canadian Rockies, in the Great Basin, and in the Virginia-Tennessee Appalachians, suggesting a eustatic control. These inferred eustatic events appear to have exerted a major influence on the lithologic framework of the carbonate platforms. The long-term eustatic fall in Late Cambrian and Ordovician time augmented the reduction in rate of net subsidence of the platforms resulting from decay of the thermal anomaly. The much slower subsidence probably was the principal cause of the marked expansion in Late Cambrian and Ordovician time of carbonate shoal facies within the platforms. The short-term eustatic events produced distinct cycles composed of fine-grained shaley material in their lower halves and coarser grained shoal facies in their upper halves. Apparently, each short-term sea-level rise reduced the rate of carbonate production sufficiently to allow widespread deposition of subtidal facies with large amounts of interbedded siliciclastic mud. During each short-term fall, rates of carbonate production increased and led to expansion of shoal facies across the platforms.

Abstract It was little more than a decade ago (Sheridan, 1974) that it was realized that the sedimentary thickness at the U.S. Atlantic margin was in excess of 10 km, 2 to 3 times the previous estimates of basement depth. In the years that followed, multichannel seismic (MCS) reflection profile data first became available (Schlee and others, 1976), the first deep offshore well was drilled (COST B-2 in March 1976), and industry and academic scientists rapidly increased their knowledge of the margin. Concurrent with the increase in data, the first models of continental margin subsidence (Sleep, 1971; Falvey, 1974; McKenzie, 1978) were developed, and techniques for studying basin subsidence were introduced (Watts and Ryan, 1976; Van Hinte, 1978; Steckler and Watts, 1978). As a result, the U.S. Atlantic margin was one of the first to be subjected to quantitative subsidence analysis, and our present view of passive margin development has been greatly influenced by research at this margin. Thus, it is appropriate that we now assess what has been learned and what information may be obtained from future subsidence studies of this margin.