2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 3
Presentation Time: 8:40 AM

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


POWARS, David S.1, GOHN, Gregory S.1, EDWARDS, Lucy E.1, CATCHINGS, Rufus D.2, BRUCE, T. Scott3, JOHNSON, Gerald H.4 and POAG, C. Wylie5, (1)U.S. Geol Survey, 926A National Center, Reston, VA 20192, (2)US Geol Survey, 345 Middlefield Rd, MS-977, Menlo Park, CA 94025-3591, (3)Virginia Dept. of Environmental Quality, Richmond, VA 23240, (4)Geology Department, College of William and Mary, Williamsburg, VA 23187, (5)US Geol Survey, 384 Woods Hole Rd, Woods Hole, MA 02543-1598, dspowars@usgs.gov

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.