234 Volcanology (Posters)

VOLCANOLOGY, GEOCHRONOLOGY, AND STRATIGRAPHIC FRAMEWORK OF UPPER CRETACEOUS BASALTIC MAAR DEPOSITS IN THE BIG BEND AREA OF WEST TEXAS

Paper number: 234-1
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

It is generally assumed that volcanism began in Trans-Pecos Texas in the Tertiary, when the Cordilleran arc swept eastward into the region. Our recent work has documented near-vent basaltic pyroclastic sequences at two localities in the Upper Cretaceous Aguja Formation in the Big Bend area of West Texas. On the Pitcock Ranch adjoining Big Bend National Park, a sequence >15 m thick of air-fall and base-surge deposits within the upper Aguja Formation is inferred to have accumulated on the flanks of a maar volcano. At Peña Mountain, 40 km to the southwest within the park, similar deposits form a more complex succession ~70 m thick. Steeply inward dipping pyroclastic and nonvolcanic Aguja strata showing intense soft-sediment deformation define the margin of a maar crater. The vent is filled with less disturbed tephra deposits, which are unconformably overlain by a second pyroclastic sequence that was erupted from a nearby vent and passes laterally into typical floodplain deposits of the upper Aguja Formation.

Pyroclastic deposits at both localities contain poorly to moderately vesicular, angular to fluidal olivine- and plagioclase-phyric basaltic ash and lapilli that record diverse styles of magma-water interaction and are intermixed with up to 70% sand- and mud-sized Aguja terrigenous debris. Basaltic cauliflower bombs and juvenile lithic blocks, as well as clasts of Aguja sediment up to 1 m across, are associated with abundant impact sags. Characteristics of the deposits indicate derivation from subsurface phreatomagmatic explosions driven by interaction of uprising basaltic magma with unconsolidated, water-rich Aguja strata.

SHRIMP-RG U-Pb isotopic analyses of zircon grains from coarse basaltic pyroclasts in the maar deposits reveal the presence of abundant xenocrysts as old as ~1.7 Ga. The youngest concordant analyses yield weighted mean ages of 72.6 ± 1.3 and 76.6 ± 1.3 Ma for the Pitcock and Peña samples, respectively, consistent with a Late Campanian age for the upper Aguja Formation based on biostratigraphic constraints. The volcanism may be related to the Balcones igneous province farther east. Further work is needed to determine if basaltic volcanic deposits occur at similar stratigraphic levels elsewhere in the Trans-Pecos region.