Lithospheric flexure due to prograding sediment loads: implications for the origin of offlap/onlap patterns in sedimentary basins

Simple elastic plate models have been used to determine the stratigraphic patterns that result from prograding sediment loads. The predicted patterns, which include coastal offlap/onlap and downlap in a basinward direction, are generally similar to observations of stratal geometry from Cenozoic sequences of the U.S. Atlantic and Gulf Coast margins. Coastal offlap is a feature of all models in which the water depth and elastic thickness of the lithosphere, T e (which is a measure of the long‐term strength of the lithosphere), are held constant, and is caused by a seaward shift in the sediment load and its compensation as progradation proceeds. The coastal offlap pattern is reduced if sediments prograde into a subsiding basin, since subsidence causes an increase in the accommodation space and loading landward of a prograding wedge. The stratal geometry that results is complex, however, and depends on the sediment supply, the amount of subsidence, and T e . If the sediment supply to a subsiding basin proceeds in distinct ‘pulses’ (due, say, to different tectonic events in a source region) then it is possible to determine the relationship between stratal geometry and T e . Coastal offlap and downlap are features of most models where the lithosphere either has a constant T e slowly increases T e with time, or changes T e laterally; however, in the case where sediments prograde onto lithosphere that rapidly increases T e with rime, the offlap can be replaced by onlap. Lithospheric flexure due to prograding sediment loads is capable of producing a wide variety of stratal geometries and may therefore be an important factor to take into account when evaluating the relative role of tectonics and eustatic sea‐level changes in controlling the stratigraphic record.