Geologic Constraints on Modeling of Complex-Crater Collapse: Data from the Chesapeake Bay Impact Structure, Virginia
Abstract
Numerical models of complex impact-crater collapse frequently invoke Bingham fluids as the rheologic model for the behavior of rocks and sediments surrounding the transient crater during gravity-driven collapse. Acoustic fluidization is increasingly relied on by modelers to produce the necessary reduction in overburden pressure at depth that leads to the transient viscous flow of the Bingham fluid. Geologic and geophysical field studies of the buried, 90-km-wide, late Eocene Chesapeake Bay impact structure (CBIS) (Virginia Coastal Plain) provide constraints for model parameters. The target protolith of the CBIS consisted of Proterozoic and Paleozoic crystalline rocks overlain by hundreds of meters of Cretaceous and Tertiary sediments. The USGS-NASA Langley corehole (Hampton, Va.) and an adjacent high-resolution seismic-reflection survey, both located inside and near the outer margin of the CBIS, reveal the vertical variation in impact disruption within the sedimentary section outside the collapsed transient crater. The observed sediment responses to the impact and their vertical distance above crystalline rock are: deepest fluidization of susceptible sands (68 m); deepest downward infiltration of dissociated Tertiary sediments into Cretaceous sediments (183 m); deepest widespread fluidization, widespread downward infiltration of Tertiary sediments into Cretaceous section, and authigenic resedimentation (198 m); deepest occurrence of sparse shocked ejecta (350 m); inferred position of spall ejection (missing Tertiary sediments) (357 m); base of allogenic resurge deposits (357 m); and top of impactites (391 m). Preliminary analysis of additional cores suggests that these horizons become absolutely and stratigraphically deeper toward the crater's center. The general upward increase in disruption of the impact-modified sedimentary section likely reflects the role of overburden pressure in increasing the Bingham yield stress with depth. The seismic survey shows pervasive high- to moderate-angle, small-offset faults in the impact-modified sedimentary section. In the Langley core, faults of this type have slickenside striae that record normal dip-slip movement. We infer that these faults began as tensile fractures produced early in the impact process that served to reduce the strength of rocks and sediments in the vicinity of the impact. There is a predictable lithologic control on the effects of fluidization and fracturing in this heterogeneous sedimentary section. Water-saturated, better sorted sands were most susceptible to fluidization, whereas clay layers were fractured into clasts and megaclasts that retained their internal cohesion.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- Bibcode:
- 2002AGUSM.T21A..07G
- Keywords:
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- 5420 Impact phenomena (includes cratering);
- 9350 North America