MAPPING HEAVY MINERAL SANDS IN CHESAPEAKE BAY WITH SHIPBOARD MAGNETIC FIELD DATA

We use shipboard magnetic field data to map shallow concentrations of metal-rich sands in Chesapeake Bay. Black sands are frequently observed along the shore of the Bay, as well as within sand layers of sediment cores that represent sediments deposited during lower global sea level ~15-10 Ka. Samples of black sands exhibit significant magnetic properties and are denser than their Si- or Ca-rich counterparts, with susceptibilities ranging between 1-2 x 10^-3 SI. Natural remanent magnetization measurements of samples can be as high as 0.7-0.8 A/m, but any degree of magnetic grain alignment in the natural setting is undetermined. Shipboard magnetic field measurements collected near the Choptank River and Parker’s Creek inflow areas exhibit short-wavelength (< 50 m) anomalies ranging from 2 to 10 nT. These anomalies are of spatial scales too fine to represent features of 500-m-deep crystalline basement rock, and must instead indicate properties of the overlying sediments. The majority of anomalies do not persist from track to track (separation 450 m), but are clustered in subsections of the survey area. Using spectral methods for characterization, we find that the short-wavelength anomalies are clustered in areas where sands dominate the seabed surface (as determined by previous grab sample analyses), especially near fluvial inputs of both the Choptank River and Parker’s Creek, and along topographic slopes. These distributions suggest that dense, metal-rich sediments are concentrated in sandy areas through resuspension and deposition in higher-energy environments associated with wave action and/or fluvial inflow. Magnetic field models suggest that the concentrations may be one to several meters thick, depending on the degree to which magnetic grains are able to align and contribute remanent magnetization.