IMPACT BRECCIAS OF THE CENTRAL UPLIFT, CHESAPEAKE BAY IMPACT STRUCTURE: INITIAL RESULTS OF A TEST HOLE AT CAPE CHARLES, VIRGINIA

In 2004, at Cape Charles, Va., the USGS drilled an 823 m test hole into the east flank of the central uplift within the central crater of the late Eocene Chesapeake Bay impact structure. Cores were recovered from depths of 823-744 m and 433-427 m, and cuttings were collected from the uncored intervals. Postimpact sediments (355 m) overlie impactites (300 m of allogenic sediment-clast breccia, above 168 m of polymict crystalline-clast suevite and monomict brecciated gneiss). The sediment-clast breccia is typical of the Exmore beds, interpreted as a seawater-resurge deposit; lithification of clays with depth suggests hydrothermal alteration. The suevite (interpreted as fallback) has clasts of target rocks (mostly metamorphic, fewer igneous and sedimentary) and sparse clasts of impact melt. Aphanitic to partly glassy melt clasts have flow lamination and a few calcite-filled amygdules; the melt clasts are mostly <1 cm (rarely 5 cm) thick. Some melt clasts and matrix were flattened together while plastic, implying hot compaction. Blocks of gneiss >10 m thick were mixed into the fallback suevite, perhaps during central-uplift rise and collapse. Shocked quartz in the suevite is common; cataclastic fabrics are characteristic of the clasts but uncommon in the moderately cohesive matrix. Cuttings of laminated aphanitic rock suggest injection of impact melt at the top of the suevite. Brecciated gneiss has shocked quartz and cataclastic fabrics; extensional fractures and faults are consistent with central-uplift rise and collapse. The gneiss also contains mafic dikes, pseudotachylite(?) veins, and probable shatter cones. The brecciated gneiss and suevite contain similar minerals: albite, quartz, and chlorite; lesser amounts of calcite; minor opaque minerals and muscovite-illite; and traces of epidote, apatite, and zircon. Chlorite (clinochlore) is disseminated and also pseudomorphs euhedral amphibole or biotite. The fallback suevite and brecciated gneiss were albitized and chloritized, indicating lower greenschist-facies, postimpact hydrothermal conditions. Extensional fractures and veins contain abundant calcite, chlorite, minor secondary quartz, and traces of pyrite. Calcite is mostly inclusion-rich, twinned and strained, but is partly recrystallized and overgrown by clear sparry calcite.