CRETACEOUS-TERTIARY BOUNDARY STRATIGRAPHY IN BELIZE AND ADJACENT MEXICO

In Belize and adjacent parts of Mexico, Cretaceous-Tertiary boundary stratigraphy consisting of several facies of in-situ and possibly re-worked ejecta from Chicxulub impact crater occur in a series of outcrops ~ 340 to 480 km from the crater. The outcrops comprise potentially mappable stratigraphic units, ranging from a few cm to approximately 15 m in thickness. In the vicinity of the Rio Hondo (i.e., the Belize-Mexican border area), the Albion Formation crops out in road cuts and quarries. The Albion Formation ranges from a few meters to 15 m thick and consists of two main facies, a basal spheroid-bearing clay layer ~ 1 to 2 m thick (i.e., the "sperhoid bed") and an overlying carbonate-rich, coarse impactoclastic breccia layer (i.e., the "diamictite bed"). Both of these units contain altered glass shards, and thus are classified as suevites. The Albion Formation rests upon an eroded Maastrichtian bedrock surface (Barton Creek Formation) and the upper contact of the Albion is either (1) a paleosol overlain by Paleocene carbonates of the El Cayo Group or (2) a subaerial unconformity. About 100 km south of the Rio Hondo, in central Belize, an outcrop of the Albion "spheroid bed" is found resting upon deeply weathered Maastrichtian bedrock, and is overlain by a coarse impactoclastic unit (informally called the Pook's pebble bed). The latter is a conglomerate with sub-rounded pink limestone pebbles and cobbles possessing a peculiar surficial polish, fine striations, penetration features, and pits. These pebbles and cobbles are also found as a minor component within karst-filling diamictites of non-impact origin.