DINOCYSTS OF THE CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA, U.S.A

The late Eocene Chesapeake Bay impact structure is entirely buried beneath southeastern Virginia, where an asteroid or meteor slammed into wet, unconsolidated sediments on the continental shelf. Upon impact, fossil dinoflagellate cysts were heated, shocked, and redistributed from these unconsolidated sediments. A study of these fossils reveals a surprising amount of information about the crater's history.

From a biostratigraphic standpoint, the mixed-age assemblages of fossil dinocysts in impact deposits are striking. Fossils are present from ages that were previously unknown in the subsurface of the Virginia Coastal Plain. These fossils must have been transported from various stratigraphic levels from the east. Fragmented, but recognizable Eocene fossils are found in localized occurrences of glauconitic sand more than 600 m below the surface. These occurrences confirm injection to great depths. Lateral and vertical variations in the particular combinations of fossil ages reflect differences in contributions from individual target components. Lateral and vertical variations in dinocyst preservational states attest to differences in temperature, pressure, and transport of the target material.

In the broader stratigraphic picture, the absence of stratigraphic units is as informative as their presence. Previously existing units that are represented only as parts of mixed-age slurries indicate that tens of meters of regional erosion occurred at the crater margin, virtually instantaneously.

The basic biostratigraphy of the dinocysts in the sediments overlying the impact deposits reveals two episodes of rapid deposition in the 35 million years since the impact. An isolated pocket of upper middle Miocene sediments, most likely the result of selective preservation in a down-dropped block, confirms that faults are present and that fault motion continued long after the impact.

Some of the preservation modes of dinocysts in the impact deposits are unique. Originally hollow features on the cysts are swollen up like balloons. Dinocysts and other organic debris are welded together to form a “dino-ignimbrite.” These welded clumps are only locally present and may be useful in distinguishing airfall from waterborne components of impact deposits.