A Sequence Stratigraphic Model for Shelf Sand Ridge Genesis and Evolution

John W. Snedden, Ronald D. Kreisa

The stepwise genesis and evolution of modern shelf sand ridges is investigated through comparison of sand bodies in four separate study sites on the New Jersey Atlantic shelf, ranging in depth from less than 4 m (shoreface-attached ridge) to over 45 m (detached ridge). Study of these areas suggests that a ridge may initially originate from an ebb-tidal delta, while the adjacent swale is cut by an obliquely migrating tidal-inlet channel. Once the inlet closes, migration ceases and the ridge becomes attached to the barrier shoreface as an oblique-trending sand body. With continued sealevel rise, the ridge becomes detached. This model for ridge genesis was established by earlier workers who noted geographic associations between ridges and present and former inlet positions. he stratigraphy of the nearshore ridges confirms this genetic scenario.

Ridges located further offshore show obvious differences with the nearshore ridges, variations which may be explained as a function of post-detachment evolution. Under the prevailing wave and current regime, ridges experience progressive changes in microfaunal content, texture, bathymetric profile, and sand volume. Ridges on the New Jersey shelf are often associated with bathymetric scarps, former Holocene shoreline positions, suggesting that they originated and evolved at various points during the latest 10 ky transgression. However, these ridges are not relict features. The active nature of the modern ridge sands is evident in the surficial bedforms, textural trends, and relative youth in terms of subsurface radiocarbon age.

Understanding the evolutionary pathway followed by these large (often >1 × 6 km) modern sand bodies provides a series of testable predictions to made for analogous ancient reservoirs. We expect these types of sand bodies to be common and important components of transgressive systems tracts in low subsidence, microtidal, storm-dominated shelf settings.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995