HIGH-RESOLUTION SEISMIC REFLECTION IMAGES ACROSS THE 1.76-KM-DEEP EYREVILLE COREHOLE WITHIN THE MOAT OF THE CHESAPEAKE BAY IMPACT STRUCTURE

In September 2006, the USGS acquired two perpendicular 1.5-km-long, high-resolution (5-m spacing) seismic reflection and refraction profiles across the ICDP-USGS Eyreville corehole, which was drilled into the moat of the Chesapeake Bay impact structure. The seismic survey was designed to provide a semi 3-D view of the stratigraphy and structures associated with the corehole. Seismic sources consisted of Betsy Seisgun blasts and 1-lb explosions in 0.5- and 3-m deep holes, respectively. The data were recorded on an array of seismographs with 240 channels. Preliminary processing and data analysis show excellent correlation with the core stratigraphy. Most major post-impact sedimentary units identified in the core can be traced hundreds of meters laterally from the corehole with minor disruption, whereas the underlying synimpact lithologic units are highly disrupted. In comparing the reflection images with the core, we find: (1) wedges of relatively continuous reflectors pinch-out in various directions with a mixture of chaotic, discontinuous reflectors (some imbricated) that match a clast-supported lower-resurge section (867- 618 m, principally Lower Cretaceous blocks); (2) the mixture of discontinous, chaotic reflectors and lenses of relatively continuous reflectors correlates with a combination of blocks (50%) and resurge breccias (50%)(618- 527 m); (3) subhorizontal, overlapping-to-shingled reflectors dip and pinch-out in various directions, correlating with several fining upwards packages of the matrix-supported breccia (527- 444 m); (4) continuous subhorizontal reflectors characterize the postimpact sediments up to 90 m depth and correlate with marine deposits; (5) from 90-60 m short to relatively continuous reflectors form a series of prograding structures and correlate with shelly sands; (6) continuous reflectors characterize the top 60 m, with reflectors dipping into a paleochannel that is consistent with the shallow marine and fluvial-paludal deposits.

These images suggest that synimpact lithologic units are highly variable laterally, such that similar units occur at different depths (10's of m) 10's to100's of meters from the corehole. This complex lithologic sequence indicates the 3-D nature of the impact process, whereby materials are deposited from multiple directions.