Detailed investigation of sorted bedforms, or “rippled scour depressions,” within the Martha's Vineyard Coastal Observatory, Massachusetts

Abstract

We examine in detail the seafloor and cross-sectional morphology of sorted bedforms (i.e., “rippled scour depressions”) in the Martha's Vineyard Coastal Observatory (MVCO). Sorted bedforms are seen as alternating bands of coarse and fine sands oriented nearly perpendicular to the shoreline. The coarse sand zones (CSZs) of the sorted bedforms are tens to hundreds of meters wide, and extend up to several kilometers from the shoreface. Data considered here include time series of swath backscatter and bathymetry, high resolution chirp seismic reflection, and grain-size analyses from grab samples, vibracores and push cores. The sorted bedforms observed within the MVCO survey area exhibit a broad spectrum of bathymetric relief (from ∼10 cm to ∼3 m), grain-size contrast (from ∼250 to>2000 μm) and morphologic form (moats, steps, and dune forms). All forms observed display lateral asymmetry in both grain size and bathymetric expression. In general, grain size is largest and bathymetry is deepest toward one side, typically seen in the backscatter maps as the more well defined of the two CSZ edges where that distinction can be made. These observations are consistent with earlier studies suggesting that sorted bedforms are a response to a transverse, alongshore flow. Within the MVCO survey area, the sense of asymmetry changes polarity going from west/shallow water to east/deeper water, suggesting a complex hydrographic regime. Our time series data demonstrate variability in the location of the boundaries between coarse and fine sands, with movements of tens of meters over spans of months, but great stability in the bathymetric features, with little or no migration seen over the same time span and little detectable movement observed for larger features over a span of nearly four decades. Furthermore, the direction of migration of the coarse/fine sand boundaries is often at odds with expectations based on the asymmetries of the sorted bedforms. We speculate that sorted bedform migration may, in the short term, be controlled by small-scale ripple migration forced by wave orbital velocity skewness, and in the long term by alongshore currents. Beneath the sorted bedforms lies a shallow, horizontal seismic reflector, a few tens of centimeters below the seafloor in the shallower waters, and>1 m in deeper water. This reflector is consistently present below the fine sands and is often observed, although less defined, beneath the coarse sands. It is often continuous beneath transitions between fine and coarse sands at the surface. In sediment cores, this reflector appears to correlate to a variable-thickness layer of gravel/very coarse sands that is frequently present beneath both coarse and fine surface sands. This surface also caps a buried fluvial channel system. We interpret this horizon as an erosional lag delineating a transgressive ravinement surface and the contact between poorly sorted glacio-fluvial sediments below and reworked, well- to moderately well-sorted fine and coarse sands above.