Abstract
During a biostratigraphic investigation of a Cretaceous-Tertiary boundary section at Shell Creek, Alabama dark green spherules with distinctive morphologies were serendipitously found in the lower Danian basal Clayton sand. With the exception of in the upper 30 cm, they were found throughout the sand, upwards from an erosional contact separating the sand from the underlying upper Maastrichtian Prairie Bluff Chalk. Additional studies revealed that the spherules exhibit the morphologies of splash-form tektites and that their original composition has been altered to a smectite external layer with a calcite core. The presence of spherules with similar morphologies and size distribution in K-T boundary sections at Beloc (Haiti) and in Arroyo El Mimbral (northeastern Mexico) leads to the deduction that the Shell Creek spherules represent altered impact ejecta. Impact glasses, Ni spinels and shocked mineral grains were searched for extensively, but intense alteration and dilution by detrital quartz sand have made it unlikely that such supporting evidence of an impact origin will be found at this site. Nonetheless, the large sizes of the microtektite pseudomorphs, which are well over 1 mm in diameter, and abundance (the total fluence is about 2 g/cm 2), makes this location an important corroboration of a proximal site for the K-T impact.
Published Version
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