OCCURRENCE OF SHOCK-METAMORPHOSED QUARTZ AND FELDSPAR GRAINS IN THE EXMORE DIAMICTON AND SEDIMENT MEGABLOCKS, ICDP-USGS EYREVILLE CORES, CHESAPEAKE BAY IMPACT CRATER

Sixty samples from a depth of approximately 450–800 m in the ICDP-USGS Eyreville cores, central Chesapeake Bay impact crater, including the interval of the allogenic Exmore diamicton and underlying Cretaceous sediment megablocks, were examined petrographically to identify the occurrence and abundance of shock-metamorphosed mineral grains. Preliminary observation of quartz and feldspar in several thousand grains and lithic clasts from the Exmore diamicton and Cretaceous sediment megablock intervals has confirmed data from previous cores in the central crater and annular trough indicating that shock-metamorphosed grains are very rare (i.e., shocked to unshocked grain ratio of ?1:1000) and diluted but widespread throughout the interval. The observed shock-metamorphic effects in the Eyreville samples include planar fractures, multiple sets of decorated planar deformation features (PDFs), and patchy, mosaic extinction patterns. In the Exmore diamicton, from a depth of about 450–620 m, shocked quartz and feldspar rarely occur as angular single-mineral grains and as components within crystalline-rock lithic clasts, which were probably derived from the crystalline basement of the target. Shocked grains within sediment clasts, and individual shocked grains derived from a sedimentary source as evidenced by grain-margin rounding, are also present but very rare. In the Cretaceous sediment block interval from a depth of about 620–800 m, shocked minerals are significantly rarer than in the overlying Exmore diamicton and consist of rounded, single-mineral grains, which were recovered only from a diamicton intercalation within the lower part of the sediment block succession. Initial studies of lithic clasts from this interval did not reveal any that contain shocked quartz. The type and relatively greater abundance of shocked grains in the Exmore diamicton compared to those in the Cretaceous sediment-block interval, coupled with observed lithologic and textural differences, support previous interpretations that these two intervals evidence distinct depositional episodes and processes during emplacement of the Chesapeake crater impactite sequence.