Paper No. 9
Presentation Time: 10:35 AM
Fine-Scale Variations in Magnetic Field Measurements Over Chesapeake Bay Delineate Zones of High Magnetic Susceptibility Sediments
Shipborne magnetic field measurements collected over Chesapeake Bay near the Choptank River and Parker's Creek inflow areas reveal short-wavelength (50 m or less) fluctuations of 2-5 nT over portions of the survey area. These fluctuations, collected from altitudes of ~5-18 m above the seabed, are of spatial scales too fine to represent features of ~500 m-deep crystalline basement rock, and must instead indicate magnetic properties of the overlying Cenozoic and Cretaceous sediments. The individual fluctuations do not generally persist from track to track, but maintain a systematic geographic distribution over the survey area. Using spectral methods for characterization, we find that the short-wavelength magnetic anomaly amplitudes are greatest within the shallow Choptank River inflow area (less than 10 m water depth), but are absent over a paleochannel carved during the last glacial maximum (LGM) with a present-day seafloor depth of over 20 m. Additional significant fluctuations are observed within 1 km of the western shore of the Bay. Piston cores were collected from the region but not from areas exhibiting the short-wavelength magnetic field fluctuations. These cores recovered sediments from subbottom depths of up to 24 m and document a post-LGM history that includes fluvial sediments with high magnetic susceptibility deposited during lower global sea level ~15-10 Ka. We attribute the short-wavelength magnetic field anomalies to the presence of high magnetic susceptibility sediments containing Fe and Ti oxide-rich minerals. These were probably reworked through wave-action from surrounding near-shore fluvial deposits and coastal exposures of Miocene sediments. Reworked sediments are limited to seafloor shallower than modern wave base and thus exclude the ~20-m deep Holocene paleochannel, and are preferentially found near the Bay shoreline. We use magnetic field modeling approaches constrained by piston core and CHIRP subbottom profile data to estimate the extent and constrain the depositional history of these sediments.