Delaware Shelf Valley: Estuary Retreat Path, Not Drowned River Valley

摘要

The transverse shelf valleys of the North American Atlantic Shelf have generally been interpreted as relict fluvial landforms. While they were indeed initiated as such, they subsequently became estuary retreat paths, in which constructional as well as erosional topography has been formed in a shallow marine environment subsequent to transgression. The Delaware Shelf Valley is an especially informative example of such an estuary retreat path, in that the shelf valley has not been decoupled from the generative estuary mouth by the late Pleistocene reduction in the rate of sea-level rise, as many other estuary mouths have been. The transverse bar blocking the mouth of Delaware Bay is breached on the south side by a well-developed, en echelon set of ebb-dominated and flood-dominated channels. The seaward-trending flood channel, a response to the tidal regime of the bay mouth, is continuous with the Delaware Shelf Valley. This feature is only approximately superimposed on the subsurface fluvial channel and is more precisely termed a flood-channel retreat path. North of the conjugate set of ebb and flood channels, Delaware Bay mouth is blocked by a large shoal which is the depocenter for the littoral drift system of the New Jersey coastal compartment. It may be traced seaward into a shelf–transverse high on the north flank of the flood-channel retreat track, which is thus an estuary shoal-retreat massif. This set of paired morphologic retreat elements terminates in a large mid-shelf high which is interpreted as a mid-shelf delta. However, comparison with the modern Apalachicola Delta suggests that, as it was transgressed, the delta had superimposed upon it a cape shoal-retreat massif, deposited by the littoral drift convergence of a retreating cuspate foreland. The tide-generated ridges and swales of the estuary-mouth shoal are normal to shore. As the shoal is traced seaward into the shoal-retreat massif, ridge-and-swale orientations rotate until they trend across the massif, parallel to shore. This change in orientation suggests that as the shoreline has retreated, the initially tidal topography has been remade in response to southwest-trending storm currents. Such a compensating response to a change in regime is the definitive characteristic of an equilibrium system. Thus, while a relict component may be observed in the topographic pattern, the descriptor "relict" is not particularly appropriate for the sand body itself.