Kansas Geological Survey, Subsurface Geology 12, p. 19-18
J. A. Thorne1 and D. J. P. Swift2
1ARCO Oil and Gas Company
2Old Dominion University
The shelf surface can be viewed as a surface of dynamic equilibrium controlled by such variables as sea-level change, sediment input, oceanographic conditions, and basin subsidence which, taken together, define the sedimentary regime. Sediment accumulating on a continental margin, given sufficient time, builds up towards a steady-state topographic profile which is a function of sedimentary regime. The role of such a topographic profile in controlling the external geometry of sedimentary rocks depends on its type: 1) graded, 2) isostatic equilibrium, 3) equilibrium shoreline, 4) steady-state advective, and 5) steady-state diffusive.
Changes of sea level relative to the surface of the shelf and coastal plain have profound effects on the sedimentary regime, by controlling the ability of sediment to pass through the shoreline. Shoaling and breaking waves on oceanic coasts create a mean landward-directed bottom stress which tends to drive sand towards the shore, resulting in a "littoral energy fence." Sediment passes through this fence in one of two ways. During regressions or slow transgressions, river-mouth bypassing transports sediment to the shoreface via the flood-stage jet of a deltaic river mouth. It is then transported along the coast and offshore by coastal-storm currents. During transgressions, river mouths become sediment-trapping estuaries. The entire shoreface may become a direct sediment source as a consequence of erosional shoreface retreat (shoreface bypassing).
Shelf sedimentary regimes, operating through time in response to shifting process variables, create in this fashion the depositional sequences of the rock record. Three examples of regime sedimentation are illustrated. The first, from the modern, discusses the sedimentation regime of inshore areas of the Mississippi delta. It is shown that the topographic profile of these areas maintains its shape as it progrades. The form of this regime profile is uniquely determined by the Gulf of Mexico wave climate and the input of sediment by Mississippi river-mouth bypassing.
In the second example, the regime approach is used to integrate measurements of storm-deposited bed thicknesses into a sequence-stratigraphic interpretation of prograding-shoreface deposits of the Mesaverde Formation of the Big Horn basin, Wyoming.
The third example applies a simplification of regime sedimentation to the modeling and interpretation of large-scale sequence geometries as are typically observed on seismic profiles of passive continental margins. Regime models of this type serve as a basis for the depositional-sequence model of Vail (1987) and Haq et al. (1987), which are used to interpret the geohistory of sea level, subsidence, and sediment supply, on the basis of observed patterns of stratal geometries.
Haq, B. U., Hardenbol, J., and Vail, P. R., 1987, Chronology of fluctuating sea levels since the Triassic: Science, v. 235, p. 1,156-1,167
Vail, P. R., 1987, Seismic stratigraphy interpretation using sequence stratigraphy, part 1--seismic stratigraphy interpretation procedure; in, Atlas of Seismic Stratigraphy, A. W. Bally (ed.), v. 1: American Association of Petroleum Geologists, Studies in Geology 27, p. 1-10
Kansas Geological Survey
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Web version May 7, 2010. Original publication date 1989.