The Origin and Internal Characteristics of a Modern Mid-Shelf Ridge Sand from a Microtidal Setting: Evidence from Vibracores and High-Resolution Seismic, Atlantic Shelf, New Jersey

Ronald D. Kreisa, J. W. Snedden, R. W. Tillman, W. J. Schweller, S. J. Culver

A modern ridge-sand has been studied as part of a larger project examining the origin of Atlantic Shelf ridge sands. The subject ridge is farthest offshore (50 km) and the deepest (45 m water depth) of four sites studied. The ridge is asymmetrically steeper offshore, 5m tall and 2km across. Internally the sand body comprises an upward-coarsening lithosome divisible into two facies. A lower fine-grained (200-250 microns) sand is present only beneath the landward portion of the ridge and is 4-5 thousand years old (kyBP). Overlying this at a sharp contact is a lower medium-grained sand (250 microns) that coarsens to upper medium-grained (400 microns) graveliferous sand at the crest of the ridge and comprises upper portion of the ridge, including the more steeply dipping seaw rd flank. It dates at 1-2 kyBP. Associated foraminifera, bedding structures, and comparison to local current patterns demonstrate that the ridge is an active bathymetric feature.

The ridge sands overlie back-barrier facies (10-14kyBP) at a ravinement surface. These facies contain rooted lagoonal muds, intertidal macro- and microfauna and tidal channel sands. This interval is about 5m thick. Interpreted tidal inlet facies comprise a lenticular-shaped interval that has carved into the lagoonal facies and is truncated by the ravinement surface. Graveliferous coarse-grained sands underlie the back-barrier deposits and are interpreted as >20kyBP valley fill deposits. We believe the ridge complex formed at the shoreline 10kyBP during post-Pleistocene transgression and its present geometry and characteristics to be a function of later reworking under mid-shelf conditions.

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