INSIGHTS INTO IMPACT PROCESSES AND BASEMENT ROCKS OF THE CHESAPEAKE BAY IMPACT STRUCTURE FROM THE ICDP-USGS EYREVILLE CORES

The ICDP-USGS Eyreville cores from the late Eocene Chesapeake Bay impact structure provide a 1766-m section through the central-crater moat. The lowest unit (215 m thick) consists of basement-derived, multiply deformed, mid-to-upper amphibolite facies mica schists, calcsilicate rock, mylonite, and pegmatitic granite (Hatteras terrane?). These rocks host polymict impact breccia dikes, including rare suevite, but are generally unshocked, suggesting origin of a block by inward collapse from the outer transient-cavity wall (OTCW). A fault zone separates these rocks from overlying polymict impact breccias and associated rocks (PIBAR, 154 m) that contain abundant crystalline and subgreenschist sedimentary clasts that display all stages of shock metamorphism, including melt particles of varied composition. The PIBAR are interpreted as: (1) a lower block-rich, melt-poor section formed by outward ground surge of variably shocked debris followed by inward slumping and mixing with large blocks of unshocked cataclastic gneiss from the OTCW; and (2) an upper section in which suevites record the transition from slumped ground surge to melt-rich deposition from the vapor-rich ejecta plume with turbulent mixing and an upward increase in aerodynamically quenched melt particles. Entrained clast-rich impact melt pods, interpreted as remnants of the transient-cavity melt lining, have chemistry suggesting a 40/60 mix of sedimentary/crystalline components. Suevite deposition ended with the arrival of rock and sediment avalanches represented successively by quartz sand with an amphibolite block and lithic boulders (26 m), a slab of Neoproterozoic and Permian granitic rocks (275 m), and sediment boulders and sand (229 m). The amphibolite block and granitic slab are unshocked, have no impact breccia dikes, and could have originated at or outside the OTCW. Partly fluidized sediment breccias below and above the slab originated from different target sediment layers. In the overlying Exmore interval (423 m), two polymict diamictons are interpreted as ocean-resurge debris flows in which melt particles record continued ejecta-plume collapse, interlayered avalanche deposits indicate temporal overlap, and stratified turbidites and laminated sediments represent the transition to normal shelf sedimentation (top 444 m).