High-resolution geophysics over Chesapeake Bay: impact crater structures and "magnetized" paleochannels?

High resolution surveys including gravity, magnetics, single-beam bathymetry, chirp and sidescan sonar were conducted over two sections of Chesapeake Bay during May-June 2002. We surveyed ~70 km2 in Maryland waters near the Choptank River and Parker's Creek outflows, where the sub-bottom is dominated glacial-age channels and tributaries of the Susquehanna River. When longer-wavelength trends are removed, the magnetic field appears to show lineations of up to 12 nT corresponding to portions of paleochannels formed by the Susquehanna River during the Wisconsinian glaciation. A possible source of these anomalies is the deposition of higher-magnetization sands by the Susquehanna during the sea-level low associated with the last ice age, where the denser and coarser materials were deposited in channels due to the higher-energy environment there. Igneous and metamorphic rocks outcropping in the Susquehanna watershed are a likely source of such materials. This is consistent with previous density and susceptibility measurements of USGS cores, although the magnetic field anomalies are a bit greater than expected. Bouguer anomalies roughly hint lows of ~1 mgal over the Susquehanna channel, suggesting infill by unconsolidated muds, but the anomalies are close to resolution limits of the instrument. Further south, near the mouth of the Bay, the sub-bottom is dominated by the ~35 m.y.o. Chesapeake impact crater. The crater has been previously described by USGS workers as a complex peak-ring crater with outer escarpments forming a rim of diameter of ~90 km. The diameter of the peak-ring is ~35-40 km, with regional gravity data showing a low near its center. Our data refine the structure of this broad low, showing a step-like drop of ~12 mgal along the peak ring. This drop may be created by a 0.5-1 km-deep density contrast of 300-600 kg/m3 between impact breccia and crystalline basement. Towards the center of the ring, a slight increase of ~2 mgal is suggested, perhaps caused by a central peak. A slight ~2 mgal "rim high" is observed along parts of the peak ring, and is attributed to uplift along the edge of this basin. Magnetic anomalies show sharp variations of up to 100 nT. Although they may be due to basement features, the proximity of the largest gradients to the peak ring suggests a relationship between the two. Such anomalies may have been crated during melt of the impacted crystalline rock, or perhaps alteration associated with later hydrothermal circulation through the brecciated rock.