VARIATION OF IMPACT RESPONSE WITH DEPTH AND LITHOLOGY, OUTER ANNULAR TROUGH OF THE CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA COASTAL PLAIN

Geologic and geophysical studies of the Chesapeake Bay impact structure during the past three years focused on the structure's annular trough and outer margin. Three new coreholes and a high-resolution seismic-reflection survey augment earlier work on this buried, ca. 90-km-wide, late Eocene, "wet-target" impact structure. Collectively, this work provides an enhanced understanding of the impact response in unexcavated materials located outside the transient crater but within the limits of major gravity-driven collapse. The impact target consisted of three layers that decrease upward in density: crystalline basement, water-saturated Cretaceous and lower Tertiary sediments (variable thicknesses of hundreds of meters), and a ca. 300-m-deep seawater column.

No shocked minerals or other unequivocal structural or thermal impact features were seen in cores of the crystalline basement. In the heterogeneous sedimentary section, parauthochthonous Lower Cretaceous fluvial sediments are nearly pristine near the basement contact. However, water-saturated sand beds were increasingly fluidized, and clay beds were increasingly fractured into internally cohesive clasts and megaclasts, with decreasing depth. Upper Cretaceous and lower Tertiary marine sediments were thoroughly dissociated at the scale of individual sedimentary particles and mixed into the upper part of the underlying Lower Cretaceous section. Beds of the marine sediments are never seen in normal stratigraphic position within parauthochthonous sections of the annular trough. Shocked quartz grains and coherent clasts of the marine beds are absent in the mixed interval, and unequivocal crystalline ejecta are very sparse in this interval. Resedimented backsurge deposits (Exmore diamicton) at the top of the syn-impact section contain shocked crystalline ejecta, shocked quartz grains, and coherent Cretaceous and Tertiary sediment clasts. These components of the backsurge deposits suggest scouring of the nearfield ejecta and underlying sediments outside the structural outer margin and the sediments exposed at the margin. The general upward increase in impact disruption within the outer annular trough (prior to the backsurge) likely reflects, in part, the role of overburden pressure in increasing yield stress with depth.