The Cretaceous-Paleogene ("KT") Boundary In Belize and Alabama - David T. King, Jr., Department of Geology and Geography, Auburn University, Alabama: Belize - At Albion Island in northern Belize, Cretaceous-Paleogene (KT) boundary deposits, also known as the Albion formation, rest upon karsted and fractured Maastrichtian dolostones. These deposits consist of a basal impactoclastic clay layer (~ 1 to 2-m thick) and an upper carbonate-rich, coarse impactoclastic breccia layer (up to 15-m thick). The focus of this paper is the upper layer, the Albion impactoclastic breccia. The Albion impactoclastic breccia shows several important sedimentary structures, including development of discrete sedimentation units (2 to 7-m thick), which are strata that have been enhanced by horizontal shearing, and other sedimentary structures such as normal and reverse size grading, clast imbrication, flow lamination, and isolated and linked aggregates of clasts (i.e., clast clustering).
Most carbonate clasts within the coarse impactoclastic unit show a broad range of angularities and shapes, with the most common being subangular and compact-bladed to compact-elongated, respectively. Surface texture analysis of carbonate clasts shows several types of surface markings, which display a gross sequential order (i.e., facets, polish, striations, cryptographic markings, bruises and pits, and chips). In-situ, apparent-diameter measurements of the carbonate clasts, which ranged in size from 10 to 300 mm (or -3.3 to -8.2 Ø);, resulted in cumulative grain-size (Ø) frequency curves with similar shapes through the interval –3.3 Ø and –6.25 Ø (i.e., 10 to 76 mm). Matrix, the total area comprised of less-than-10 mm (< -3.3 Ø) particles, ranged from approximately 71 to 82 percent. Modified moment measures of these curves show these breccias are “extremely poorly sorted.” The matrix content increase upward through the entire coarse impactoclastic layer, but is slightly lower near its top.
The Albion impactoclastic breccia has sedimentary structures and sedimentologic characteristics suggesting its mode of emplacement during the impact aftermath was similar to that of a very large volcanic debris avalanche. Sedimentation units show evidence of early turbulent flow and a more conspicuous later stage of laminar flow with shearing accompanying emplacement of most breccia sedimentation units. Clasts within these debris flows are not locally derived for the most part. We speculate that each sedimentation unit at Albion may represent a separate emplacement event during the process of ejecta curtain collapse, perhaps owing to variations in atmospheric interaction with the debris.
Alabama - At Shell Creek stratigraphic section, Wilcox County, Alabama, a < 1 m-thick, Cretaceous-Paleogene boundary sand body crops out over an area of ~ 200 m2. This sand body consists of (1) a basal impact spherule-bearing, coarse to medium sand and (2) an overlying fine sand with hummocky-type cross-lamination. This K-T boundary sand body probably represents post-impact, shelf sedimentation events involving (1) gravity-driven resedimentation of reworked impact spherule-bearing sands and (2) energetic wave reworking of the impart spherule-bearing, gravity-driven deposits or other subsequently deposited sands. Most impact spherules from Shell Creek are spherically shaped grains (~ 1 mm in diameter) that are now hollow, or were hollow prior to secondary calcite filling. Most impact spherules from Shell Creek consist of an outer shell, which is composed of smectitic clays, and an inner region of open space or sparry calcite. Most of these impact spherules still retain features like vesicles that attest to their former molten condition. This stratigraphic section is remarkable in that it represents the most easterly U.S. Gulf Coastal Plain occurrence of abundant impact spherules in a Cretaceous-Tertiary (K-T) boundary sand body.
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