GEOLOGICAL COLUMN OF THE ICDP-USGS EYREVILLE-A AND -B CORES, CHESAPEAKE BAY IMPACT STRUCTURE: SEDIMENT-CLAST BRECCIAS AND SEDIMENT-BLOCKS 1095.7-443.9 M

In 2005 a 1.76-km-deep corehole was drilled into the Chesapeake Bay impact structure. The upper 652.8 m of impactites consists mostly of unconsolidated sediment-clast breccias derived from the sedimentary target layer, which consisted of thick Lower and basal Upper Cretaceous nonmarine deposits and thin Upper Cretaceous and lower Tertiary marine deposits. The section can be subdivided (in ascending order) into 229 m of clast-supported blocks, 248.5 m of clast-supported breccias and blocks, and 174.3 m of matrix-supported breccias.

The lower section (1095.7-866.7m) contains only Lower Cretaceous sedimentary boulders and blocks (up to 25 m) small-clast zones (up to 10 m thick), and massive sand zones (disaggregated sand clasts), all dominated by feldspathic quartz sands with minor clay-silt. Clasts have relict bedding, dip angles range from subhorizontal to high angle to overturned. Clast boundaries are sharp, jigsaw, diffuse, injected or mixed. Overall clast size increases upwards and high-angle bedding occurs within 1035-1004 m, 928-912 m, and 882-877 m. Thin diluted glauconite sand injections reach down to 928 m.

The middle interval (866.7-618.2 m) is also dominated by Lower Cretaceous sedimentary blocks (up to 38 m). This section contains many of the same features as the lower section. Differences includes abundant (~1/3 of section) highly fractured oxidized clay-silt clasts (paleosols), layers and injections of muddy glauconite-quartz sand with mixed age fossils, and very irregular clast boundaries, including locally disaggregated clasts dispersed into the adjacent matrix. Deformation within clasts includes locally contorted and folded relict bedding, disaggregated clay silt clasts, and pebbles floating in sand.

The upper section (618.2-443.9 m) is dominated by muddy glauconite quartz sand matrix that contains scattered crystalline basement clasts and a variety of soft sediment deformation features. The lower 102 m includes over 50% oxidized clay-silt clasts (up to 12 m) and abundant crystalline clasts (up to 8.5 m). The upper 72 m is dominated by small (<1 m) Lower Cretaceous (up to 2.5 m) and locally abundant crystalline basement clasts and rare lower Tertiary clasts. The section includes several apparent fining upward packages; the uppermost grades to a stratified sand capped by laminated clay-silt.