THE RELATION BETWEEN THE CHESAPEAKE BAY IMPACT STRUCTURE AND THE STRUCTURE AND STRATIGRAPHY OF CENOZOIC DEPOSITS IN EASTERN VIRGINIA

Surficial and subsurface geologic mapping in eastern Virginia over the last century documented complex structural and stratigraphic relations in Tertiary and Quaternary deposits. Localized reversals of dip direction were recognized on the York-James Peninsula by Harris (1890), Roberts (1932), Mansfield (1943), Johnson (1972 and later), and Ward and Blackwelder (1980). Abrupt lateral variations in lithology and thickness in Eastover and Yorktown strata were observed by Johnson and Coch (1968) and Johnson (1972). In addition, unusual concentric stream and scarp patterns were also noted in this region. These anomalies occur in an arcuate belt extending from Virginia Beach across the York-James Peninsula onto the Middle Neck Peninsula. Darton (1902) and Cederstrom in the 1940’s and 1950's also noted complexities in Cretaceous and Tertiary deposits, as did regional studies by Brown and others (1972), Newell and Rader (1982) and Meng and Harsh (1988). To explain these anomalies, mechanisms such as faulting, folding, and compaction were suggested, but none were satisfactory.

With the discovery of the Chesapeake Bay impact structure (CBIS) by Bruce, Powars, and Poag in the early 1990's, a connection between these anomalies and the impact structure became apparent. The CBIS is a 135-km-wide impact structure that was created about 35 million years ago when a 1.2- to 3.0-km-diameter asteroid or comet plummeted into the continental shelf near the mouth of the present Chesapeake Bay.

These anomalies lie on or near the outer rim of the annular trough of the CBIS. Mega-crossbedded biofragmental sands, which thicken and intertongue with silty fine sands westward, and landward dipping beds, occur in arcuate belts around the outer annular trough and surrounding outer fracture zone. The anomalies are caused by rotation of buried listric fault blocks defined by concentric and radial faults. Aperiodic movement of the blocks and differential compaction of crater fill folded and faulted overlying strata, resulting in stratigraphic and structural anomalies. Recorded earthquakes attest to continued movement of the blocks.