IMPACT-TO-POSTIMPACT PALEOENVIRONMENTAL TRANSITION IN THE CENTRAL BASIN OF THE CHESAPEAKE BAY IMPACT CRATER

Sixty-five samples record the paleoenvironmental transition from synimpact chaos to full postimpact recovery in the central basin of the Chesapeake Bay impact crater (Eyreville, Virginia, core hole). The transition begins at 444.32 m core depth with 6 cm of structureless, micaceous, pyritic, sandy, silty clay, which contains scattered, white, cm-scale siltstone clasts, and exhibits rare cross laminae. Microfossils (all reworked older specimens) include rare ostracodes and mollusks, and 21 benthic foraminiferal species. This initial unit represents a rapidly deposited (geologically instantaneous) turbidite. Above the initial turbidite is 27 cm of very thin, parallel-to-subparallel, lenticular, convolute, indistinct and regular silt/clay laminae. Abundant thin, platy pyrite crusts, pyrite burrow casts, and pyrite aggregates are present, along with a few siltstone laminae, mica, and rare lignite. No indigenous microfossils are present. Reworked older specimens include a few radiolarians, diatoms, and 8 benthic foraminiferal species. This unit represents a tranquil seafloor environment hostile to marine organisms (dead zone), which lasted ~4 k.y. The next transition deposit above the dead zone is the basal 1 m of the Chickahominy Formation, a dark, dense, richly fossiliferous, silty clay, with generally indistinct lamination and scattered pyrite (crusts, burrow casts, nodules). The abrupt appearance of a rich, indigenous, marine fossil assemblage signals the return of habitable marine conditions to the seafloor. Fossils include bivalves, gastropods, echinoid spines and plates, radiolarians, ostracodes, planktonic foraminifera, and 40 species of calcareous and agglutinated benthic foraminifera. The distinctive agglutinated assemblage indicates, however, that abnormal paleoenvironments still persisted (perhaps a result of elevated bottom-water temperature and salinity), and lasted ~30 k.y. Following disappearance of the agglutinated assemblage, the calcareous benthic foraminiferal assemblage expanded through a 50-m-thick section, and reached a peak abundance of 76 species at 393.13 m drill depth, 51.38 m above the base of the transition interval. This peak abundance represents the termination of impact-altered paleoenvironmental effects at this site, ~900 k.y. after the impact.