GRAVITY AND MAGNETIC MAPS OF THE CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA

Geophysical surveys are important tools used in the investigations of the buried Chesapeake Bay impact structure. Of these surveys, only the gravity map gives a synoptic view of the central crater, outlined by a 10-12 mgal elliptical low and a local central high of 4 mgal. This compilation combines recent and pre-existing gravity surveys with considerable detail on land. The gravity low (~25-30 km in diameter) coincides with the central crater, as delineated by the absence of a continuous basement reflector on seismic surveys. The mass deficiency indicated by the low probably results from the infilling of porous and impermeable sediments, impact breccias, as well as brecciated rocks to considerable depth. The central high near Cape Charles, Virginia, reflects a physical central uplift (peak). Lithic breccia and suevite or crystalline-megablocks were encountered in an 823 m test hole (STP-2) into the central uplift, at a level 441 m higher than the probable equivalent unit in the 1,766 m ICDP-USGS corehole, 8 km to the NE. Density of blocks of gneissic granite and schist in the 73 m of core at the base of the STP-2 hole is probably higher than the density of sediment-clast breccia at the equivalent level in the ICDP hole, thus providing the source of the central anomaly. Lowest gravity makes a donut-shaped “moat” around the central high. Ground magnetic measurements on land and in the Chesapeake Bay together with a 1972 aeromagnetic survey, unlike the gravity, do not show a sharply defined anomaly associated with the central crater. Rather, a broad low area is punctuated by a series of small (~1 km diameter) magnetic highs (~100 nT amplitude) that fall within the low-gravity moat area. A central high-low magnetic anomaly pair roughly coincides with the gravity high of the central uplift. Modeling of this anomaly indicates a strong remanent magnetization in a direction that differs from the estimated earth's magnetic field at the time of the 35 Ma impact that is still unexplained. Rocks with highest magnetic susceptibility in the STP-2 drill-hole are a 2-ft-thick black melt inclusion (k= ~10x10-3 SI) in suevite, and parts of several gneissic granite blocks (k= ~5x10-3). If present in large enough volumes, disks of the melt fraction of suevite could be the source of the small highs and with gneiss, part of the source of the central anomaly.