NONTRONITIC CLAY PSEUDOMORPHS OF CRETACEOUS-PALEOGENE (K-T) BOUNDARY MICROTEKTITES, SHELL CREEK, ALABAMA, USA

摘要

The impactoclastic layer (< 1 m) at the base of the Paleocene Clayton Formation at Shell Creek, Alabama, U.S.A., consists of two parts of approximately equal thickness: a lower poorly sorted and graded layer of coarse, conglomeratic, microtektite pseudomorph-bearing quartzose sand and an upper layer consisting of thinly laminated and hummocky cross-stratified medium to fine quartzose sand.; The lower part of the impactoclastic layer at the base of the Clayton Formation is composed of glauconitic, coarse to fine sand containing a significant component of pseudomorphs of Cretaceous-Paleogene (K-T) boundary microtektites that are composed of nontronitic clay. Previous work has shown the precursor microtektites, which are found in several other places in North America and the Caribbean area, were glassy objects that formed during the Chicxulub impact crater-forming event of approximately 65 million years ago. The Chicxulub impact structure, located on the Yucatan Peninsula of Mexico, is approximately 1200 km south of the Shell Creek locality reported on here.; The Shell Creek microtektite pseudomorphs are mostly in the range of similar to 0.5 to 3 mm, (averaging less than 1 mm) and consist of splash-form shapes, including spheres, prolate and oblate spheroids, dumbbells, and disks. The original glass has been altered to nontronitic clay, which is green-black. Nontronitic clay is indicative of a more mafic melt source than has previously been reported for K T boundary pseudomorphs of microtektites.