CRETACEOUS-TERTIARY DISTAL IMPACT EJECTA IN ALABAMA -- SHELL CREEK STRATIGRAPHIC SECTION

At the Shell Creek stratigraphic section, Wilcox County, Alabama, the upper surface of the Maastrichian Prairie Bluff Chalk is truncated and an impact-related basal sand couplet (consisting of a microtektite-rich sand and an overlying cross-laminated sand unit) rests directly on top. This basal sand couplet comprises the lower two beds of the Paleocene Clayton Formation in this vicinity. At Shell Creek, a sharp contact can be seen between the grey, bioturbated shelfal chalk and an overlying grey, microtektite-rich, coarse to medium sand layer, ~15 to 25 cm thick. The basal few centimeters of this microtektite-rich layer contains significant amounts of hematite in the matrix, but the upper part is mainly cemented by clay and calcite. This layer does not appear to be graded, but is very densely packed with microtektites, which make up as much as 25% of the grains in some parts of the rock. The microtektites are mostly in the size range of 0.5 to 3 mm, averaging about 1 mm. They have many different shapes, including spheres, "dumbbells," "teardrops," disks, and others. The most common shapes are spheres and "teardrops" (which may be broken "dumbbells"). Most of the microtektites appear to be altered to smectitic clays and many are encased in a shroud of calcite cement. In cross-section, some spherical microtektites display an internal structure than includes intact smaller spherical droplets adhered to the inner walls of the microtektite. The medium to coarse sands comprising the microtektite host bed are quartzitic (no shocked grains have been found thus far) and not remarkable except that they are quite coarse, and thus "out of place" within the interpreted shelfal depositional realm of this material. The microtektite host bed also contains some rip-up clasts of chalk and phosphatic molds of fossils (snails, straight cephalopods, and pelecypods), both in the size range of 0.3-0.6 cm. The overlying layers of tan medium to fine quartzose sand (0.3 to 0.5 m) contain no microtektites, and there is a rather sharp, flat contact between the overlying sand and the grey, microtektite layer. At this contact, clasts and molds are strongly aligned. The main features of these upper sands are thin laminations, which are more nearly horizontal near the base and grade upward into hummocky-type cross-laminations near the top.