PETROGRAPHY AND CHEMISTRY OF SUEVITIES IN THE IMPACTITE SEQUENCE, EYREVILLE B CORE, CHESAPEAKE BAY IMPACT STRUCTURE

Seventy samples from the impactite (suevite) sequence between 1393 and 1538 m in the Eyreville B core through the Late Eocene Chesapeake Bay impact structure were analysed using petrographic and geochemical methods. Qualitative and quantitative data were collected for (1) melt fragments, (2) clasts, (3) breccia matrix, and (4) alteration phases, and shock effects were investigated in clasts. The melt fragments vary with depth in colour, size and shape. Melt fragments in the upper suevite section range in size from 0.5 mm to 3 cm, and are more sub-angular and devitrified compared to those of the lower section (< 0.7 mm in size and increasingly more rounded with depth). Electron microprobe analyses indicate melt compositions of quartz, quartz with varying amounts of feldspar (alkali feldspar and plagioclase), and of mica minerals (mainly biotite). Volume percentage of melt decreases in the lowermost parts. Relative abundance of melt particles involves larger, more closely packed fragments in the upper section and smaller, disseminated ones in the lower part. The lithic clast proportion reflects igneous, sedimentary and metamorphic precursors, with granite, arkose and mica-schist being most important. Size and colour of clasts are variable with no distinct pattern with depth, and shapes are generally rounded to sub-angular. Initial observations do not suggest a regular change of relative proportions of the three lithic clast types with depth. Occurrence of irregularly and planar fractured quartz, PDFs (planar deformation features), and alteration effects vary within the sequence; again, no regular trend has been noted so far. Initial observations regarding clast:matrix ratios show an increase of matrix downwards in the succession. The matrix composition is essentially granitic, and both microscopic melt and clastic phases constitute the groundmass.