EVIDENCE FOR A LATE EOCENE ATLANTIC IMPACT PRIOR TO THE CHESAPEAKE BAY EVENT

An unusual 0.5 to 2 meter-thick diamictite occurs near the bottom of the upper Eocene succession beneath the Upper Coastal Plain of Georgia and South Carolina. The diamictite represents at least a portion of the Albion Member of the Clinchfield Sand Formation, as defined by Huddleston and Hetrick (1979). Sandy et al. (1966) described the unit as an altered volcanic deposit, based on its remarkable texture in hand sample and thin section derived from abundant white kaolinite and black smectite intraclasts. Despite that appearance and that Allen et al. (2004; 2009) showed that it is part of a fluvial floodplain sequence, the diamictite primarily is composed of siliceous marine sponge spicules (Carver, 1972; 1980) set in a gray kaolin matrix. Carver (1972) notes that the sediment reflects a “…significant change in source, perhaps the introduction of volcanic ash…” The close correlation to strata containing ejecta from the Chesapeake Bay impact (CBI) led Harris et al. (2004) to suggest it might be related to that catastrophe. However, subsequent investigation (Harris, 2009) demonstrated that the diamictite occurs several meters below the CBI horizon. If associated with an impact, the event occurred 0.3 to 0.5 Myr earlier.

Although the diamictite contains goethite spherules similar to altered microkrystites from upper Eocene and K-Pg layers; and the chemistry and petrography of the smectite intraclasts are consistent with altered mafic glass, direct evidence of an impact origin has been lacking. We examined the matrix using a Hitachi SU-3500 VP-SEM at FSC. To avoid contamination, samples were uncoated, unpolished, and surfaces exposed immediately prior to placement in the chamber. Compositions were analyzed using an EDAX Element EDS detector.

We find that a variety of tiny (<10 μm) exotic grains are ubiquitous in the matrix, including Ni-rich chrome spinel, pentlandite, native nickel (~2% Fe), and Ni-rich brass. We interpret these phases as indicative of the high temperature processes during the entry and collision of a large bolide.

We propose that the diamictite represents a separate late Eocene marine impact (~36.0 Ma) in the Atlantic Ocean that caused a wave raking the continental shelf and resulting in the inland deposition of a large volume of sponge spicules, fine-grained silica, and glass mixed with meteoric debris.