Stratigraphy of the Cretaceous-Tertiary and Paleocene-Eocene Transition Rocks of Big Bend National Park, Texas

The marine to terrestrial transition in the Big Bend area falls within the Late Cretaceous Aguja Formation, and, in light of new biostratigraphic data resulting from screening for small vertebrates and magneto-stratigraphic data, the Cretaceous-Tertiary boundary falls within the Javelina Formation, which includes the first red banding produced by oxidation of overbank fluvial mudstones. No record of a catastrophic event is apparent in the Javelina Formation. The Javelina, Black Peaks, and Hannold Hill Formations and the Big Yellow Sandstone Member of the Canoe Formation record increasing uplift in the region, culminating in uplift and volcanism in the Chisos Mountains, the source for upper Canoe Formation sediments. The sequence of changes produced by this trend and by unroofing in source highlands to the west is sufficiently gradual that the Javelina through Black Peaks units are not lithostratigraphically distinct at the formation level and therefore are reduced to member status, and placed, along with the Big Yellow Sandstone Member, within the redefined Tornillo Formation. The Aguja Formation and the Toraillo Formation are united in the Chilicotal Group (new), which spans the deposits from the first significant influxes of terrestrial sediments, formed as the Cretaceous sea retreated, up to the beginning of local volcanism in the Chisos. The volcanic strata of the upper Canoe Formation are reassigned to the Chisos Formation.