CRETACEOUS-PALEOGENE BOUNDARY STRATIGRAPHY OF BELIZE

The Cretaceous-Paleogene (K-Pg) boundary unit in Belize is a stratigraphic unit, ranging in thickness from about 15 m to less than 1 m, which can be mapped at the surface and traced in well logs in the subsurface. Thickness generally decreases away from the source crater, but there are anomalously thick zones in local fault-bounded grabens. At Albion Island in northern Belize, this informal stratigraphic unit has been named the Albion formation, but elsewhere there is no assigned name. The K-Pg boundary unit commonly lies atop the post-Barton Creek unconformity, and in places fills low areas on that karstic surface. The typical lithic sequence within the K-Pg boundary unit is: a spherule-bearing fine clastic bed; boulder-bearing breccia beds; and a re-worked, pebbly ejecta unit. The basal fine clastic unit lies on eroded limestones of the Barton Creek at Albion Island, but lies atop a paleosol upon Barton Creek at Armenia. Fine clastic matrix of this bed consists of clay and clay-sized carbonate. Spherules range in size from a few mm to a few cm and typically have a lithic fragment at the nucleus and finely layered carbonate surrounding the nucleus. The boulder-bearing breccia beds are impactoclastic deposits that contain a mixture of angular limestone blocks (including boulders up to 15 m) and finely pulverized carbonate matrix containing varying amounts of green glass shards (mainly altered to clay). The boulder-bearing breccia beds do not persist across all of northern Belize, and are limited to areas within a few 10s of km of the Mexican border. The re-worked pebbly ejecta unit consists of matrix and grain-supported conglomeratic beds of rounded and in some places polished (pink to red) limestone pebbles and some grey to tan chert pebbles ranging in size from a few cm to a few dm. The striated polish on the surfaces of these limestone pebbles appears to have been formed by the abrasive action of numerous fine particles. Some limestone pebbles exhibit apparent impact craters and pits. The chert is not so well polished, and commonly exhibits a chalcedonic rim. The re-worked pebbly ejecta is interpreted to have fallen over wide areas in northern Belize and to have been subsequently washed into grabens or other low-lying areas. The upper surface of the K-Pg boundary unit in Belize is a disconformity in most places. Overlying stratigraphic units in northern Belize include the Paleogene Red Bank formation (clays), El Cayo group (micritic limestones), or other Cenozoic formations.