A stratigraphic motif observed in many foreland basins is the development of basinward tapering siliciclastic wedges characterized by various scales of depositional cycles. The Middle Devonian (Givetian) Mahantango Formation in the central Appalachian foreland basin is such an example. It consists of both small- and large-scale thickening- and coarsening-upward cycles; the small-scale cycles are typically less than 10 m thick whereas larger-scale cycles are generally a few tens of metres thick and commonly contain several of the smaller-scale cycles. Outcrop-based facies analyses indicate that the depositional cyclicity resulted from episodic progradation of a regionally straight, tide-dominated shoreline onto a storm-dominated, shallow marine shelf. The depositional model for this ancient shallow marine system consists of a vertical facies succession in which storm-dominated offshore marine mudstone and fine sandstone pass gradationally upward into storm-dominated nearshore marine shelf and shoreface sandstone overlain by, in proximal sections, tide-dominated shoreline sandstone, pebbly sandstone and mudstone. Transgressively reworked lag deposits cap most of the thickening- and coarsening-upward packets. In this model, coarse-grained rocks, rather than implying basinward shifts of facies, are a consanguineous part of the stacked shoaling cycles. Lateral facies relationships show that the dominance of storm- vs. tide-generated sedimentary features is simply a function of palaeogeographical position within the basin; proximal sections contain tidally influenced sedimentary features whereas more distal sections only display evidence for storm-influenced deposition. These results suggest caution when inferring palaeoceanographic conditions from sedimentological datasets that do not contain preserved examples of palaeoshorelines.
|Published - Aug 1996