X-RAY MINERALOGY OF THE EXMORE-CHICKAHOMINY BOUNDARY IN THE CHESAPEAKE BAY IMPACT STRUCTURE - EYREVILLE CORE

XRD techniques were used to determine the whole sample and clay fraction mineralogy of 21 samples from depths of 440 to 454m in the Eyreville A core. Thirteen samples were obtained from the upper 10m of the Exmore Breccia; including eight from a silty-shaley, upward-fining sequence just below the boundary, two from a glauconitic sand just below them, and three from a diamicton at the base of the sampled interval. Eight samples were obtained from the lower 4m of the Chickahominy Fm. Several Chickahominy and Exmore samples were provided by Wylie Poag. Analyses were performed on random powders and oriented aggregates of the >2μm, the <2μm, and the <0.5μm size fractions.

Quartz is the dominant mineral in the five samples from the lower section of the Exmore. The two glauconitic sand samples contain heulandite and mica. A fully expandable smectite is the major clay mineral. Illite (glauconite) and a 7A-phase with a very broad peak are minor components. The presence of a 4.45A peak on the 300C-heated analysis suggests that the 7A peak may be due to vermiculitic clay. Feldspars and calcite were absent. The eight samples from the upward-fining sequence are essentially identical. Quartz and heulandite are the major minerals. Ordered microcline, ordered albite, and mica (2M1 muscovite) were also detected. The clay mineral assemblage is the same as in the lower part of the interval. The Chickahominy assemblages are distinguished by the appearance of calcite in the very first sample and an upward increase in its relative abundance. Heulandite and mica exhibit a contrasting decrease in abundance.

Calcite in the coarse and fine fractions of the Chickahominy is a proxy for the return of abundant life to the impact site. Its absence (or low abundance) in the uppermost Exmore deposits is due to unfavorable conditions promoting dissolution or limited productivity. Heulandite, smectite and the 7A-phase are the primary products of glass reactivity in the upper Exmore sediments. Glasses in this interval may have been more crystallized prior to deposition, thus accounting for the presence of feldspars. Decreasing quantities of heulandite in the Chickahominy suggest a diminishing supply of reworked glass as the area recovered from the impact.