Genesis and Evolution of a Mid-shelf, Storm-Built Sand Ridge, New Jersey Continental Shelf, USA
Sand ridges form discrete parts of a modern surficial veneer on many continental shelves worldwide yet are seldom cited as analogs for isolated marine sand bodies found in outcrop or the subsurface. One reason may be that a comprehensive explanation for the evolution of modern mid-shelf sand ridges has proven elusive. New information, most notably high-resolution seismic data collected on modern sand ridges in adjacent areas by other workers, has led to this reevaluation of a largely unpublished but extensive vibracore dataset from a mid-shelf sand ridge off New Jersey (USA). The presence of dipping reflections within other ridges is particularly notable on new high-resolution seismic data, but has yet to be confirmed in these other ridges by extensive coring. Reexamining the original well-preserved vibracore peels from the mid-shelf sand ridge sampled in the 1980’s reveals that these internal reflections could be associated with burrowed, low-density mud beds. New cross -sections constructed with 20 vibracores suggest that the dipping internal reflections are accretionary surfaces associated with episodic ridge migration to the east and south. Radiocarbon dating indicates that the mid-shelf sand ridge has migrated episodically over the last 5 ky and movement probably continues today during major storms. This documented migration continues well after transgressive ravinement (approximately 9-10 ka), contradicting existing views that mid-shelf ridges become moribund sometime after detachment from the shoreline. The morphology, migration history, and internal sedimentary character suggest that the ridges are acting much like primary bedforms, although these sand bodies are much larger in scale (5 to 8 m height or thickness) than typical subaqueous dunes observed on modern storm-dominated shelves. Our study suggests that mid-shelf sand ridges are likely to be preserved and we would expect these sand bodies to occur most often where paleogeographic reconstructions indicate wide, storm-dominated shelves with limited sediment supply. Detailed sedimentological descriptions, observed textural patterns, sand body map geometry and other information provide sufficient information to permit identification of analogous ancient sandstone packages in the outcrop or subsurface.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California