LITHOSTRATIGRAPHIC SECTION OF THE ICDP-USGS EYREVILLE COREHOLES, CHESAPEAKE BAY IMPACT STRUCTURE: POSTIMPACT SEDIMENTS 444-0 M DEPTH

A 443.9-m-thick, virtually undisturbed section of postimpact deposits in the Chesapeake Bay impact structure was recovered in the Eyreville A and C cores, Northampton Co., Virginia, within the “moat” of the structure's central crater. Recovered sediments are fine-grained marine siliciclastics, except for paralic Pleistocene sand, clay, and gravel. The lowest postimpact unit is the upper Eocene (and ?lower Oligocene) Chickahominy Formation (443.9-350.1 m). At 93.8 m, this is the maximum thickness yet recovered for these initial deposits that represent the return to “normal marine” sedimentation. Intepretation of the somewhat coarser-grained Oligocene is complicated. Elsewhere in the subsurface of the Virginia Coastal Plain, three Oligocene units have been recognized: in ascending order, the informal Delmarva beds, the informal Drummonds Corner beds, and the Old Church Formation. In Eyreville A, two or more thin Oligocene units are present (350.1-344.4 m); eight burrowed surfaces of varying significance, all within these 5.7 m, reflect a complex scenario involving variations inaccommodation space, tectonics, sedimentation rates, and erosion that is different from nearby cores both inside and outside the crater. Above the Oligocene is a more typical Coastal Plain succession. The Calvert Formation (344.4-224.3 m) includes a thin lower Miocene part overlain by a much thicker middle Miocene part. The middle Miocene Choptank Formation (224.3-206.0 m), rarely reported in the Virginia Coastal Plain, is a thin unit that is conspicuously sandier than the overlying and underlying silts and clays. The thick upper Miocene St. Marys and Eastover Formations (206.0-61.4 m) are difficult to separate lithologically and biostratigraphically, presumably because they represent a more complete succession than in the type localities. Correlation with the nearby Kiptopeke core suggests that two Pliocene units are present: Yorktown (61.4-29.0 m) and Chowan River Formations (29.0-17.4 m). Sediments at the top of the section fill a late Pleistocene channel and are assigned to the Occohannock Member of the Nassawadox Formation (17.4-0.6 m).