LITHOSTRATIGRAPHIC FRAMEWORK OF THE CRATER-FILL DEPOSITS: WESTERN ANNULAR TROUGH, CHESAPEAKE BAY IMPACT CRATER

Over the last decade, continuous cores and seismic-reflection surveys have revealed the buried, 90-km-wide, complex, late Eocene Chesapeake Bay impact crater. A 38-km-wide, 1.6-km-deep excavated inner basin is surrounded successively by a crystalline-rock peak ring, a 21- to 31-km-wide, flat-floored annular trough (that contains disrupted sediments, a slumped terrace zone, and a steep gullied escarpment), and a 35-km-wide outer fracture zone. Good correlation exists between the first high-resolution seismic-reflection survey across the structure’s southwestern margin and three recently drilled deep coreholes in the western part of the annular trough (located 23, 20, and 8 km outside the inner basin). The cores confirm the presence of thick (up to 430-m) impact-generated debris beneath 140-350 m of post-impact sediments. The impact-generated debris consists of a variable pile of fluidized, fractured, and catastrophically jumbled sediments characterized by a three-part physical stratigraphy. Unit A, which overlies crystalline basement rocks, consists of up to 200 m of parautochthonous Lower Cretaceous sediments disrupted by relatively short, subvertical to low-angle faults interpreted to represent tensional faulting during late-stage gravitational collapse. Unit A contains fluidized sands in its upper part. Marine seismic data show an outer-rim, slump or fault block up to 2 km long and 1 km thick where unit A apparently becomes much thicker. Middle unit B consists of a clast-supported breccia up to 340 m thick containing disruption zones of strongly deformed clasts (4 mm to 1 m), disaggregated sediments from the Lower Cretaceous section, disaggregated glauconitic Tertiary sediments, and sporadic crystalline-rock clasts separated by zones of moderately deformed Lower Cretaceous megaclasts (1-25 m). Unit B shows extensive fluidization, mixing, and localized scour and fill. Above unit B, the Exmore beds consist of 30 to 92 m of matrix-supported, muddy, quartz-glauconite sand having clasts of pre-impact Tertiary and Cretaceous sediments (granules to meters), brecciated crystalline-rock clasts, rare shocked quartz, and partially melted dinoflagellates. Multiple Exmore depositional sequences appear as shingled reflections on the high-resolution seismic survey.