Geochemistry and geophysical framework of the shallow sediments of Assawoman Bay and Isle of Wight Bay in Maryland
1994, Wells, D.V., Conkwright, R.D., and June Park
Open-File Report 15
For the 8th year of the Mineral Management Service's Continental Margins Program, the Maryland Geological Survey conducted a sedimentological and geochemical study of the sediments of Isle of Wight and Assawoman Bays. The objectives of the study were to delineate the shallow stratigraphic sequence of the coastal bays, relating the stratigraphy to late Quaternary sea level fluctuations, and to document the geochemical character of the shallow sediments, providing preliminary base-line data for comparison for future studies.
Thirty-three kilometers of 7 kHz seismic profile surveys and eleven sediment cores were collected. Surficial sediments grab samples were collected at three other stations. The cores were X-rayed, described and sampled at various intervals. A total of 96 sediment samples were analyzed for texture (SAND, SILT, CLAY contents), water content, total nitrogen, carbon, and sulfur concentrations, and six metals: Cr, Cu, Fe, Mn, Ni and Zn.
Seismic records feature several shallow paleo channels defined by a very strong reflector. Depths to the reflector were mapped, allowing the structure of a pretransgression surface beneath the bays to be contoured. The surface reveals a simple paleodrainage system which is traceable to the present tributaries. Maximum depths of the paleochannels are approximately 8 meters below MSL. Thalweg depths, particularly for the St. Martin paleochannel, are much shallower than previously projected based on well log and bridge boring data.
The coastal bay sediments are predominately SILTY. SILT contents averaged 44% for all samples. Averages for SAND and CLAY contents are 3 1% and 25%, respectively. SAND contents generally are higher for those samples collected along the eastern margin of the bays. CLAY becomes an important component in cores collected in the tributaries.
Concentrations for nitrogen, carbon and sulfur for most of the sediments are within ranges expected for marine sediments and are comparable with those found in the Chesapeake Bay and other Atlantic coast estuaries. Nitrogen contents range from 0 to 1.39%, averaging 0.22%; carbon contents range from 0.02 to 30%, averaging 2.8%; andsulfur contents range from 0 to 5.28%, averaging 1.05%. Nitrogen, carbon and sulfur contents are strongly related to the texture of the coastal bay sediments, with higher values associated with finer-grained sediments.
Metal concentrations are within the ranges of other coastal bays not subject to heavy industry. The behavior of the metals were determined by two methods. The first method utilized enrichment factors referenced to average continental crust (Taylor, 1964). Enrichment factor values for Cu, Mn and Ni are less than one for most of the sediments suggesting that the reference material used may not represent the coastal bay sediments. Nevertheiess, enrichment factors indicate that the upper 20 to 30 crn of sediment column are enriched with Cr, Cu, Ni and Zn compared to sediments deeper than 30 cm. The metal concentrations in the deeper sediments are interpreted to represent historical or background levels.
The second technique employed to assess metal concentrations in the sediments correlated metal content with the grain size composition. Sediment deeper than 30 cm were used to obtain the relationship between texture and metal contents to determine background metal concentrations. Background levels were calculated for all samples and compared to measured levels, obtaining variation factors. Variation factors showed the same trends in the behavior of Cu and Zn as did the enrichment factors.