PALEOENVIRONMENTAL CHANGES DURING THE EOCENE-OLIGOCENE TRANSITION, CHESAPEAKE BAY IMPACT CRATER, WESTERN ATLANTIC

The Chesapeake Impact Basin on the Western Atlantic continental shelf provides an expanded succession of post-impact late Eocene (Chicahominy Fm.) – early Oligocene (Delmarva Beds) strata. We are conducting a high-resolution multi-proxy mineralogical, geochemical, and rock-magnetic study to reconstruct environmental conditions, including paleoclimate, sea-level changes, and tectonic events. Specifically, we investigate, whether the proposed late Eocene cold-warm intervals and the early Oligocene cooling are detectable, and intend to establish a relationship between our proxy data and the sequence stratigraphy provided by other ICDP groups.

Our magnetic susceptibility (MS) measurements have shown low paramagnetic values, though; several very high MS values may indicate the occurrence of inhomogeneous distributed ferromagnetic detritus, specifically for the Delmarva Beds. The non-clay mineralogy of the Chickahominy Fm. and the Delmarva Beds reveals variable amounts of quartz and calcite as major constituents added by feldspars, pyrite, and apatite. The clay fraction yielded a complex mineralogy with variable proportions of smectite, illite, and kaolinite as well as mixed-layers, glauconite and zeolites. The presence of distinct trends in the abundance of mineral phases as well as in the MS suggests a 5-fold subdivision of the Chickahominy Fm. and a two-fold subdivision of the Delmarva beds. These intervals correspond, however, only in part to concomitant changes in the GR log. Fine-fraction bulk rock stable isotope analysis also shows distinct trends: The δ¹⁸O record reveals a positive trend during the Chicahominy Fm. (–0.5 to 0 ‰), culminating during the lowermost Delmarva Beds at +0.5 ‰. The δ¹³C record shows a distinct negative (+0.25 ‰) followed by a positive excursion (+1.5 ‰) in the lower part of the Chicahominy Fm. and a subsequent decrease to more negative values that culminate at the base of the Delmarva beds (–0.5 ‰).