CHESAPEAKE BAY IMPACT: INTERNAL STRUCTURE AS REVEALED BY THE GRAVITY FIELD

The Chesapeake Bay Impact structure is an ~85 km crater buried beneath the Atlantic coastal plain of Virginia. As it is buried, its overall dimensions and structure must be investigated using geophysical techniques. Gravity data, reported here, were collected on the Delmarva Peninsula and mainland Virginia and in the bay. The western coast of the Peninsula cuts diagonally across the structure resulting in irregular data distribution; the greatest number of stations are in the northeast quadrant. The impact anomaly is characterized by a central (~7 km diameter) gravity high surrounded by an annular (35 km diameter) gravity low that correlate with the central uplift and interior basin. The central high rises 8 mGal above the surrounding annular low; and the annular low is ~10 mGal below the surrounding block-faulted margin. The presence of significant density anomalies in the crystalline basement makes it difficult to separate the crater's anomaly from regional anomalies. The block-faulted margin does not appear to exhibit a gravity anomaly; consistent with blocks of sediment rotated along listric faults that sole into the sediment-basement contact, such a geometry produces little or no density contrast. Density data derived from core and sample analysis of the ICDP Eyreville core indicate the Exmore breccia and impact suevite have densities of ~ 2.2 - 2.3 g cm^-3; crystalline basement ranges from 2.6 - 2.8 g cm^-3. The density contrast between the breccia and suevitic material and the surrounding basement is at least 0.25 g cm^-3 and could be as high as 0.5. Thus, the central uplift gravity high and the surrounding low are the result of the large density contrast between the basement and fill. Several small positive anomalies are occur in the interior basin; due to irregularities of the floor or large crystalline blocks, similar to the 250 m block encountered in the Eyreville drill hole.