GEOLOGIC COLUMN OF THE ICDP-USGS EYREVILLE-B CORE, CHESAPEAKE BAY IMPACT STRUCTURE: BRECCIAS, BLOCKS, AND CRYSTALLINE ROCKS, 1,095-1,766 M DEPTH

A preliminary analysis of the ICDP-USGS Eyreville-B core below 1,095.74 m indicates the presence of four lithotectonic assemblages. (1) The deepest assemblage 1 (1,766.22-1,560.24 m) consists predominantly of schists that contain muscovite, biotite, graphite, and sillimanite in varied proportions with quartz-feldspar lenses in some layers, and abundant granite pegmatite that locally grades into coarse-grained granite. A zone (1,655.08-1,640.75 m) of variably mylonitic muscovite-rich schist, gneisses, and amphibolite contains disseminated calcite, minor epidosite, and rare tourmalinite. All of assemblage 1 is variably overprinted by cataclastic fabrics. Dikes (≤2.5 m thick) of suevite and breccias not yet studied and classified are most common in the upper 51 m (above 1,611.23 m), and minor breccia veins are found sporadically at deeper levels. (2) Assemblage 2 (1,560.24-1,397.16 m) consists of a basal graphite-rich breccia (1,560.24-1,551.19 m) overlain by a middle section (1,551.19-1,474.05 m) of lithic-clast-rich suevite that contains blocks (up to 20 m thick) of cataclastic, fine-grained, quartz-feldspar gneiss locally grading into cataclasite, and an upper section (1,474.05-1,397.16 m) of melt-rich suevite that locally contains boulders (≤1.5 m) of similar cataclasite. (3) Assemblage 3 (1,397.16-1,371.11 m) is a mixed assemblage of gravelly sand and lithic blocks. Only the basal part of the gravelly sand contains minor altered melt clasts and is a reworked suevite (1,397.16-1,396.44 m). Lithic blocks in assemblage 3 include cataclasite consisting of micaceous quartz-feldspar gneiss fragments (1,396.44-1,393.42 m), suevite (1,393.42-1,393.00 m), and amphibolite (1,389.71-1,376.39 m). (4) Above the gravelly sand and its contained blocks, assemblage 4 (1,371.11-1,095.74 m) is an allochthonous slab of mixed granitic rocks. It includes a basal zone (ca. 7 m thick) of red altered biotite granite, medium-to-coarse-grained biotite granite (mainly below 1,216.53 m), fine-grained biotite granite, and gneissic (layered) biotite granite (mainly above 1,216.53 m). A prominent brittle fracture zone in the gneissic biotite granite is at 1,158.63 to 1,153.05 m. The granitic slab is overlain by gravelly sand derived from the Cretaceous Potomac Formation.