INSIDE THE CRATER, OUTSIDE THE CRATER: COMPARISON OF TWO CORES FROM THE CHESAPEAKE BAY IMPACT STRUCTURE

In 2002, the U.S. Geological Survey (USGS) drilled a 300-m-deep corehole at Dorothy L. Watkins Elementary School, Newport News, Va., just outside the outer margin of the Chesapeake Bay impact structure’s annular trough. The Watkins section can be compared with that in the USGS-NASA Langley corehole, which is located within the annular trough, about 6.5 km from Watkins School. At the top of the Langley crater section, 173 m of parautochthonous, sediment-clast-supported breccia (rotated, locally fluidized megablocks of Cretaceous Potomac Formation separated by glauconitic matrix zones) are overlain by 32.3 m of allogenic, matrix-supported polymict breccia. In contrast, the section of the Potomac Formation in the Watkins core lacks the deformation features seen at similar depths in the Langley parautochthonous breccia. Instead, the Watkins Potomac section shows abundant primary horizontal laminations and low-angle cross laminations and contains in-place Albian to Cenomanian pollen assemblages. The Watkins core has a 7.3-m-thick section of allogenic polymict breccia that consists of sediment clasts derived from preimpact Cretaceous and Tertiary sections set in a matrix of disaggregated preimpact sediment. Sparse crystalline-rock clasts in the breccia include ejected basement rocks that contain shocked quartz and have cataclastic fabrics (variably porphyritic felsites, fine-grained granitoids) in addition to rounded, undeformed pebbles of quartz, quartzite, and diabase (of detrital origin). Microfossil taphonomy and assemblage compositions suggest that the Watkins polymict breccia is genetically comparable to the upper part of the parautochthonous breccia and to the polymict breccia of the Langley core. The polymict breccias in both cores are interpreted as ocean-resurge deposits. Neither core contains a nearfield ejecta blanket. Surprisingly, the postimpact lower lower Oligocene Delmarva beds and upper middle Miocene Choptank Formation are present in the Watkins core but absent in the Langley core. The Delmarva beds are present in other cores within the Chesapeake Bay impact structure; the Choptank Formation is not. The Choptank may be preserved in a locally down-dropped fault block that provided protection from erosion by subsequent marine transgressions.