ACCRETIONARY SPHERULES AT THE KT BOUNDARY, VILLAGE OF ARMENIA, BELIZE

The KT boundary stratigraphy exposed on the Hummingbird Highway at the village of Armenia, Belize, includes the upper beds of the Maastrichtian Barton Creek dolomite, which is overlain by a lateritic clay bed (paleosol) containing dolomite boulders. This is in turn overlain by an accretionary spherule-bearing clay layer (~ 2 m), which is the focus of this paper. Above the spherule-bearing layer lies a limestone conglomerate bed. Accretionary spherules at Armenia range in size from less than one millimeter to 1.5 cm. A typical spherule consists of a core (usually a carbonate grain) that is surrounded by concentric layers of clayey calcium carbonate. The surface (or “shell”) of each concentric layer is harder than the lower part of the layer. Concentric layers range in thickness from < 0.001 mm to > 1 mm and are structureless to vaguely laminated. It is likely that the carbonate in each concentric layer was deposited as microcrystalline lime, but has now converted to low magnesium calcite. Aggradational recystallization among calcite crystals is evident in thin section, and this is interpreted to have accompanied the lime to calcite conversion. The spherules at Armenia are interpreted as Chicxulub vapor plume deposits, based upon their stratigraphic position and internal physical structure, which strongly resembles a typical accretionary lapillus. Unlike previously reported accretionary grains from a 1-m thick clay layer at Albion Island in northern Belize, the Armenia spherules are almost all nearly perfectly spherical and nearly all of them display distinctive internal layering.