ABOVE THE AVALANCHES, BELOW THE WAVES: THE ICDP-USGS EYREVILLE CORES AND ADJACENT SEISMIC REFLECTION PROFILE WITHIN THE MOAT OF THE CHESAPEAKE BAY IMPACT STRUCTURE

Polymict-breccias of resurge origin blanket the Chesapeake Bay impact structure and consist of unstratified, muddy, glauconitic-quartz sand (matrix) and varying amounts and combinations of mixed-age fossils and clasts, including crystalline ejecta clasts and melt(?). A 13 m-thick section of this kind of breccia down to 866.7 m in the Eyreville A cores is taken to be the base of the resurge, in its broad sense. Comparisons of recovered section in Eyreville A and B, correlated with an adjacent low-resolution seismic reflection profile, show both the scale and the chaotic nature of the deposition that immediately followed the impact event.

The interval from 866.7-618.2 m is dominated by Lower Cretaceous clasts (1-30 m intersected diameter) of sand and oxidized clay-silts. The resurge matrix is absent for 142 m above the basal 13 m and is sporadically present as layers and/or injections in the overlying 106.5 m. Many of the clasts display relict bedding that is highly contorted (again at various scales); clast margins are highly deformed; and within-clast shearing and brecciation are evident. Some preimpact stratigraphy appears to be preserved. This interval is interpreted as a chaotic combination of ocean-resurge reworking and Lower Cretaceous transported blocks.

The interval from 618.2-443.9 m is dominated by unstratified muddy glauconitic-quartz sand with abundant sediment and ejected crystalline clasts. The lower 102 m is about a 50/50 mixture of matrix and clasts, has ~1/3 oxidized clay-silt clasts and common crystalline clasts. The upper 72 m is dominantly matrix with small Cretaceous sediment clast (most <1m), with locally abundant crystalline clasts and rare Tertiary-sediment clasts. This section has several fining-upwards packages with the upper one grading to stratified sand and laminated clay-slit. Although Tertiary clasts are rare, Tertiary microfossils are abundant in the matrix. This section represents thoroughly mixed ocean-resurge deposits possibly with multiple wave oscillations.

The seismic profile correlates well with these stratigraphic units and show reflectors that dip SE toward the central uplift within the slump blocks (1095.7-866.7) and reflectors for the upper 421 m of impactites that dip NE away from the central uplift, substantiating a complex, multi-directional resurge scenario.