Meteoritical Micropaleontology: An Example From the Late Eocene of Virginia

C. Wylie Poag

Micropaleontology, as applied to the study of meteorite impacts, has come into sharp focus recently, highlighted by the K-T impact controversy. The principal applications of microfossils to the K-T problem have been toward testing the mass extinction hypothesis and interpreting paleoenvironmental changes across the boundary. Micropaleontology played no role, however, in identifying the K-T impact crater. In contrast, initial identification of the late Eocene Chesapeake Bay impact crater arose directly (though inadvertently) from micropaleontological studies of core samples. The USGS and the Virginia State Water Control Board, in 1986-89, discovered a unique subsurface breccia layer (Exmore breccia) at four sites around the lower bay. Microfossils (foraminifera, bolboformi s, calcareous nannofossils, dinoflagellates, and pollen) showed that individual clasts in the breccia came from eight different coastal plain formations ranging in age from Albian to late Eocene. A single sample of the breccia matrix could contain mixtures of microfossils from all these formations. Youngest microfossils in the breccia indicate that the unit accumulated in Biochronozone P15 (planktonic foraminifera) and NP19-20 (nannofossils), correlative with an impact ejecta layer (containing shocked quartz, tektites, coesite) cored on the New Jersey continental slope. Graphic correlation between the Virginia and New Jersey sections indicates that normal marine sedimentation resumed simultaneously in the late Eocene at each location. Subsequent geophysical and petrographic evidence has onfirmed what the microfossils originally indicated -- that the Exmore breccia and its source crater are geological consequences of a hypervelocity bolide impact.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California