The location, composition, and origin of oyster bars in mesohaline Chesapeake Bay

Abstract

Sub-bottom profiling sonar surveys in the mesohaline Chesapeake Bay located pre-Holocene, hard terrace structures surrounded by or buried by softer sediments. The descending scarps of these hard terraces frequently correspond to the boundaries of charted oyster bars. The terraces are composed of a base of coarse Tertiary or Pleistocene sediments and may be covered by a layer of oyster shell, shell grit or mud. The presence of soft, methane gas-charged sediments adjacent to descending terrace scarps indicates that terrace/oyster bar configuration is due to the segmentation of the larger regional terrace structure by paleochannel drainage. These sediment filled channels, relicts of the last glacial period, extend into the estuary far beyond the current terrestrial shores and demarcate the oyster bar boundaries. Continual filling of the paleochannels segregating oyster bars, both by fluvial transport as well as in situ production of sediment, has lead to a decrease in relief in the terrace scarp profile. At some locations sediments are burying or have buried oyster bar terraces. Although widespread reduction of oyster bar relief in the Chesapeake Bay has been attributed to harvest activity, statistical analysis of bathymetry does not clearly support this premise. Our findings suggest that oyster habitat restoration should take place only on the clearly identified, hard terraces away from encroaching bottom sediments.