PSEUDOMORPHS OF IMPACT SPHERULES FROM A CRETACEOUS-TERTIARY BOUNDARY SECTION AT SHELL CREEK, ALABAMA

During a biostratigraphic investigation of a Cretaceous-Tertiary bound ary section at Shell Creek, Alabama dark green spherules with distinct ive morphologies were serendipitously found in the lower Danian basal Clayton sand. With the exception of in the upper 30 cm, they were foun d throughout the sand, upwards from an erosional contact separating th e sand from the underlying upper Maastrichtian Prairie Bluff Chalk. Ad ditional studies revealed that the spherules exhibit the morphologies of splash-form tektites and that their original composition has been a ltered to a smectite external layer with a calcite core. The presence of spherules with similar morphologies and size distribution in K-T bo undary sections at Beloc (Haiti) and in Arroyo El Mimbral (northeaster n Mexico) leads to the deduction that the Shell Creek spherules repres ent altered impact ejecta. Impact glasses, Ni spinels and shocked mine ral grains were searched for extensively, but intense alteration and d ilution by detrital quartz sand have made it unlikely that such suppor ting evidence of an impact origin will be found at this site. Nonethel ess, the large sizes of the microtektite pseudomorphs, which are well over 1 mm in diameter, and abundance (the total fluence is about 2 g/c m2), makes this location an important corroboration of a proximal site for the K-T impact.