THE CHESAPEAKE BAY CRATER EJECTA BLANKET

The discovery a little more than two decades ago of an 85-km-diameter impact crater below the lower Chesapeake Bay has gone unnoticed by the general public, largely because virtually all published literature on the subject has described all but a few tiny impact-related areas as “buried.” However, this notion has been challenged by the present author in his 2012 publication (copies available at this poster station). The evidence presented there shows that the so-called “upland deposits” that blanket ~5,000 km² of the U.S. Middle-Atlantic Coastal Plain (M-ACP) display morphologic, lithologic, and stratigraphic features consistent with their being ejecta from the 35.4-Ma Chesapeake Bay Impact Structure (CBIS) and absolutely inconsistent with the prevailing belief that they are of fluvial origin. The author’s 2012 paper includes an idealized calculation of the CBIS ejecta-blanket profile minutes after the impact, following well-established rules for impact-generated craters (cf. H J Melosh. Impact Cratering a Geological Process, Oxford University Press, 1989). This profile has been shown to match the volume of the upland deposits in an arc ≥115 km westward and northwestward of the crater center. However, closer to the crater center much of this “postdicted" ejecta blanket has been removed by erosion. Nevertheless, inside the ~115 km radial limit the ~10-20-m-deep, fossil-free(!) Bacons Castle and the closer-in Shirley Formations have been identified by the present author as curtain and excavation-phase ejecta respectively. The neritic-fossil-bearing Calvert Formation external to the crater is deduced to be of Eocene age, preserved by the armoring effects of the overlying upland deposits. The lithofacies of the in-crater Calvert Formation can only have resulted from inward mass wasting of the postdicted ejecta blanket, vestiges of which (i.e., the Bacons Castle and Shirley Formations) still overlap the crater rim and sag into its interior, consistent with this expectation. Because there appear to be a total of ~10,000 km² CBIS ejecta lying on the present-day surface, future research should center on the stratigraphic, lithologic, and petrologic properties of these ejecta, "in plain sight".