Marginal flat deposit

A series of prominent east-west trending oyster beds are found within the marginal flat deposits of the Tappan Zee. These oyster beds are presumably associated with the oyster population once abundant along the river from Sandy Hook to Croton Point and harvested by Native American populations in prehistoric times. Oyster beds are evident in the side-scan sonar imagery as a series of linear, moderate backscatter regions that extend to the channel banks. In the sub-bottom data the beds correspond with strong, often irregular reflections that, within the main parts of the beds, do not permit penetration of sound energy. Oyster shells found within sediment cores and visible in sediment profile image (SPI) photography collected by NOAA confirm our interpretation of oyster beds from the geophysical data. The oyster beds are also evident in the bathymetric data as local regions elevated ~50 cm above surrounding terrain. The oyster beds are now exposed at the river bed along both the eastern and western marginal flats and older oyster beds exist as distinct deposits buried by up to 10 m of sediment. Portions of each exposed oyster bed extend to the channel bank where a steep scarp is developed. Erosional remnants of the beds are found beyond the eastern edge of two of the centrally located beds, embedded within the west channel bank deposits. These remnants form prominent irregular mounds that are partially buried by sediments. A narrow sliver of oyster bed is found along the western edge of the river near Hook Mountain. This bed is bounded by a steep, linear scarp with up to 5 m of relief. Away from the oyster beds, the marginal flat deposits are low backscatter sediments with little internal stratification evident in the sub-bottom data. Sediment cores indicate that these sediments are higher in density than cores recovered from the channel axis and channel banks implying that the marginal sediments are somewhat older than the channel sediments.

Radiocarbon age dating of oyster shells from the marginal flats reveals that oysters at the river bed are ~1,000 years old (Carbotte et al., 2001). Oysters up to 5,500 years old were recovered from near the base of a 10 m core collected offshore at Nyack. Although our data set is limited, it appears that oysters thrived within this region during two distinct time periods from ~5,500 yrs b.p. to 4,500 yrs b.p. and during the more recent time period of2,500 to ~ 1,000 yrs b.p. Small live oysters have been recovered during our sampling program.

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