Morphology and distribution of Miocene slope incisions off New Jersey: Are they diagnostic of sequence boundaries?

Abstract

High-resolution multichannel seismic reflection profiles confirm that middle–late Miocene continental slope canyons off New Jersey are rare, in contrast to their prevalence on the slope today. Localized canyon incision occurred in two physiographic settings bounded by declivity breaks: steeper clinoform fronts and more shallowly dipping regions seaward of clinoform toes. Canyons incising Miocene clinoform fronts are U and V shaped, relatively small, closely spaced, and localized along strike. They apparently developed independently of known fluvial inputs, although rivers probably discharged sediment near paleo-shelf edges (clinoform breakpoints) during Miocene relative sea-level lowstands. However, not all clinoform fronts are incised; nonchannelized downslope sediment transport also appears to have been a common process. V-shaped canyons are the most common type recognized seaward of clinoform toes; occasional amphitheater-shaped failures also occur. Rare canyons with partly aggradational banks may reflect either overbank and/or levee or contour-current deposition. Many Miocene canyons definitely incise sequence boundaries; they may all do so. This implies that canyons formed during relative sea-level lowstands, in accord with the classic sequence stratigraphic model of siliciclastic depositional systems. However, depth of incision on clinoform fronts decreases downslope, whereas seaward of toes it decreases upslope; development of canyon systems in the two settings is not linked by a single set of processes. Therefore, the presence or absence of canyon incision on this margin must be dictated not only by fluctuations in sea level, but by subtle changes in local conditions or regime variables. These include the efficiency of downslope sediment transport, rate of sediment supply, grain size, and possible slope collapse related to fluid escape.