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Abstract:

The thickest and most extensive Upper Cretaceous coals of the western interior of the United States occur at the top of, and landward of, shoreface and delta front platforms that are stacked vertically. An explanation for this observation was sought through numerical models derived from the interactions of the three fundamental processes that control stratal geometries and lithofacies distributions. These are eustatic fluctuations, tectonic movement, and quantity of sediment delivered to or produced in a sedimentary basin.

The models show that the fundamental building block of marine-shelf, to coastal-plain stratigraphic sequences is the progradational event, expressed in vertical profile as a shallowing-upward succession of facies. The shallowest facies at the top of one event is capped abruptly by the deepest facies at the base of the subsequent event. This facies asymmetry is modeled by sinusoidal sea-level oscillations superimposed on a constant rate of tectonic subsidence; disharmonic variations in either sea level or tectonic movement are unnecessary to produce this asymmetry.

The models simulate a hierarchical stacking of progradational events that display three geometric patterns: seaward-stepping, landward-stepping, and vertical stacking. The models show that the thickest and most extensive coals accumulate when accommodation space in the lower, potentially coal-bearing portion of the coastal plain is near maximum, and when the rate of sea-level change is balanced by the rate of sediment supplied by progradational events. These factors result in vertical aggradation of coastal-plain facies tracts and vertical stacking of the progradational events.

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