Age of Chesapeake Bay Impact Crater

Abstract

The biochronological age of the Chesapeake Bay impact crater has been determined by documenting the stratigraphic ranges of three groups of microfossils (planktonic foraminifera, bolboformids, and calcareous nannofossils), sampled in two different geographic areas. We compared these ranges to the global geochronostratigraphic framework constructed by Berggren et al. (1995; Fig. 8.1). Sample area one is in and near the crater itself, where Poag and Aubry (1995), Poag and Commeau (1995), and Poag (1997a) first established the detailed planktonic biostratigraphic framework of the Exmore breccia and Chickahominy Formation [Fig. 8.2; see also Poag and Norris (in press) for correlative planktonic foraminiferal biostratigraphy of the Chickahominy Formation in the NASA Langley corehole]. The second sample area for micropaleontological analysis is the New Jersey Continental Slope, where several different investigators have documented microfossil assemblages from impact ejecta cored at DSDP (Deep Sea Drilling Project) Site 612 and ODP (Ocean Drilling Program) Sites 903 and 904 (Fig. 8.3; see Poag and Aubry 1995, for the most recent synthesis and references). The youngest diagnostic foraminifera and nannofossils within the Exmore breccia represent an ∼0.8 myr interval (35.2–36.0 Ma) in which the upper part of planktonic foraminiferal Biochron P1 5 overlaps the lower part of calcareous nannofossil Biochron NP19–20 Fig. 8.1. This same biochronologic overlap interval is present in the Exmore breccia in all seven coreholes drilled inside and outside the crater, and extends into the lower third of the overlying Chickahominy Formation. The remaining upper two-thirds of the Chickahominy Formation incorporates biochrons P16-P18 (forams) and NP 19-20 and NP21 (nannofossils), which represent the final ∼1.5 myr of the late Eocene.