DISTRIBUTION, COMPOSITION, AND ORIGIN OF THE CHICXULUB SPHEROID BED IN SOUTHERN QUINTANA ROO, MEXICO AND NORTHERN BELIZE

The basal Chicxulub ejecta layer found in southern Quintana Roo, Mexico and northern Belize, named the Albion Formation spheroid bed, was deposited on a terminal Cretaceous karst land surface. Outcrops occur along the Belize-Mexico border, ~360 km from the center of Chicxulub, and at one location in the foothills of the Maya Mountains, ~480 km from Chicxulub. The spheroid bed ranges in thickness from 1 to 5 m and contains variable amounts of altered glass (palagonite and smectite), accretionary lapilli, and angular to subrounded carbonate clasts in a fine (1-10 micron) carbonate matrix. The glass comprises 20-30% of the bed and is mostly composed of angular, vesicular shards 0.5-1 cm in diameter with rare glass bombs up to 8 cm in diameter. The accretionary lapilli range in size from <1 cm to 6 cm in diameter and vary in composition. The larger lapilli (>1 cm) typically have cores of single carbonate clasts, agglomerations of clasts, lapilli, or altered glass. The cores are surrounded by a concentric band or bands of small carbonate clasts and crystals. The carbonates in the lapilli include both calcite and dolomite, although the dolomite is probably secondary. Small dolomite lapilli (<1 cm) in some outcrops do not have cores and instead exhibit vermicular textures (under cathodoluminescence) indicative of a precursor crystalline composition with mineral intergrowths. Such intergrowths may derive from the alteration of CaO oxides. Limestone and dolomite clasts comprise about 1-3% of the bed and range is size from <1 cm to 8 cm in diameter. Stratification in the spheroid bed includes tabular beds, low-angle cross-bedding, and massive beds. No graded beds were observed. We interpret the Albion formation spheroid bed as a deposit from the rapidly expanding impact vapor plume, which entrained fine debris and condensed as it flowed out from the crater at high velocity. This turbulent blast of hot gasses laid down a thin layer of lapilli and glass prior to the arrival of the main mass of the ejecta blanket (Albion Formation diamictite bed), which overlies the spheroid bed.