THE CHESAPEAKE BAY IMPACT STRUCTURE: MIOCENE EFFECTS OF THE EOCENE IMPACT

The well-preserved lower and middle Miocene section from the South Dover Bridge core on the Eastern Shore of Maryland contrasts sharply with the lower and middle Miocene section preserved in the “moat” of the Chesapeake Bay impact crater. Located about 125 km from the center of the 85-km-diameter crater, the South Dover Bridge core yielded a complete section of dinocyst zones from DN2a to DN6 and produced an age-depth model with relatively consistent sedimentation rates and little missing time. In contrast, the ICDP-USGS Eyreville A, about 9 km from the center of the crater, yielded an incomplete section of dinocyst zones and a large variation in sedimentation rates. Lower Miocene deposits, throughout the Salisbury embayment, suggest sediment progradation from the north with local thinning or absence. Within the crater, patchy preservation may be enhanced by (1) sediment starvation, (2) a complexly irregular surface due to initial deposition, crater-related faulting and differential compaction, and (3) submarine scour influenced by irregular seafloor shape and bathymetry. Coarse-grained, shelly lower Miocene deposits are preserved on the edge of the crater's outer rim and may represent sand bars in a near shore shoaling environment. Middle Miocene deposits are thicker and more regularly distributed, generally showing a thickening near the axis of the Salisbury Embayment and thinning towards the south, west and north margins of the embayment. Within the crater, the middle Miocene deposits prograde southward across the crater and are abnormally thick, due to (1) increased accommodation space caused by continuing compaction of the heterogeneous crater fill, and perhaps to (2) sudden influx of material due to breaching of a raised rim or to (3) increased sediment load from a newly developing drainage system in the Appalachians or to (4) sediment bypassing of the tectonic low to the north. Calcareous nannofossils are well represented but present in low-diversity assemblages that may indicate an abnormal paleoenvironment.