2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 6
Presentation Time: 1:30 PM-5:30 PM

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


HORTON, J. Wright1, GOHN, Gregory S.1, GIBSON, Roger L.2, REIMOLD, Wolf Uwe3 and EDWARDS, Lucy E.1, (1)U.S. Geological Survey, 926A National Center, Reston, VA 20192, (2)School of Geosciences, University of the Witwatersrand, PO WITS, Johannesburg, 2050, South Africa, (3)Museum of Natural History, Humboldt University, Invalidenstrasse 43, Berlin, D-10115, Germany, whorton@usgs.gov

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.