FIRST OBSERVATIONS ON EXMORE BRECCIA FROM THE ICDP-USGS EYREVILLE CORE, CHESAPEAKE BAY IMPACT STRUCTURE

The Eyreville A core intersected between ~444 and 940 m depth a sedimentary clast breccia (Exmore Breccia) involving a distinct diamicton and extensive clay and sand beds. The EB (sampled here between 445 and 528 m = main diamicton sequence) consists of a fine-grained, phyllosilicate (much sericite) rich groundmass with a sand-size dominated clast content (with variable amounts of larger clasts up to several cm in size). Main clast types include fossil carbonate, glaukonite, sediment (mudstone, siltstone, minor chert), and mineral/lithic clasts from granitic and mica-schist precursors of the crystalline basement. Shocked quartz (and even more rarely, feldspar) occurs as a minute fraction throughout the sample suite; in previous work on other drill cores, shocked material also constituted only a minor component of EB. Mostly altered melt particles are ubiquitous but rare, but between 457.7 and 467.3 m abundant (10-60 vol%) and then also carry more than normal shocked clasts (quartz + some feldspar with PDF, rare diaplectic quartz glass). Major element analysis of 57 Exmore Breccia samples allows a fourfold division of this breccia sequence: (1) above ~450.7 m, a zone with ,fine- to medium-grained groundmass and rather rare glaukonite, of high SiO2 (75-85 wt%) and complementary low Al2O3 and Fe2O3 contents; (2) to 467 m, a breccia of variable, but sometimes very low (62 wt%) SiO2 content, slightly finer-grained than (1) and with significantly more glaukonite. This section involves the melt particle-rich unit. (3) The major part of the sequence to a depth of about 518 m is fine-grained and of comparatively lower glaukonite content than (2). Individual bands carry grit derived from granitoids, pelitic rocks, and fossil carbonate. Zone (3) has relatively high but rather constant SiO2 (74-80 wt%). (4) From ca. 518-528 m, several samples show relatively reduced SiO2 content (74-66 wt%). Distinct variation of granitoid : mica schist clast ratios is noted throughout the EB. Continuing work attempts to quantify the nature and abundance of variably shocked materials and sediment as well as impact melt.