PROBLEMS SOLVED AND MYSTERIES REMAINING: DINOCYSTS FROM COASTAL PLAIN CORES

Outcrops of marine sediments in the Atlantic Coastal Plain are limited. Fortunately, at USGS, I have had access to some of the best core material in the world. One of the biggest “problems” in Virginia Coastal Plain geology has been solved. The peculiar assemblages of mixed-age fossils, salty ground water, and abrupt thickening/thinning/local absence of subsurface units are the results of the processes that created the largest known impact crater in the United States. The late Eocene Chesapeake Bay impact event also caused a wide variety of abnormal dinocyst preservation. Each new core examined has had new types of strange preservation and/or new combinations of strange preservation types. Some are unique to the crater; others are not.

The dinocysts in the crater deposits also are important in interpreting the events of the first minutes after impact. Where mixed-age dinocysts are found hundreds of meters below any of the levels of original deposition, they must be the result of removal and downward transport. Any simulated model or seismic interpretation that cannot explain the transport must be rejected.

Lessons learned from the dinocysts of the impact crater carry over into other studies. The dinocysts from the material directly above the impact-generated deposits are dominated by relatively small, nondescript Heteraulacacysta specimens. Others have noted that the genus, in high abundance, reflects unusual conditions within restricted marine paleoenvironments. The impact event certainly represented unusual conditions (nutrients, salinity, temperature, competition). In palynological terms, perhaps Heteraulacacysta cysts are analogous to a “fern spike” – the first forms back after an ecological crisis. Heteraulacacysta specimens dominate in the basal Miocene sands of a new core in Maryland as well.

The preservation in impact-related deposits has led to closer examination in other normal and abnormal settings. Curled processes around otherwise undistorted shapes are observed in variable abundance in many samples, if one is looking for them.

A mystery still remaining is that of dinocysts that look like “shrunken heads.” These forms appear as smaller caricatures of their former selves. They are extremely rare in Coastal Plain sediments but not restricted to impact deposits. More observations are sorely needed.