WIDESPREAD EOCENE BASALTIC PHREATOMAGMATIC VOLCANISM IN THE BIG BEND REGION, TRANS-PECOS IGNEOUS PROVINCE, WEST TEXAS

Cenozoic igneous rocks of the Trans-Pecos province (TPP) in west Texas were emplaced from 61-18 Ma in arc- and rift-related tectonic regimes. The first main magmatic phase in the area of Big Bend National Park, which occupies a large region in the SE part of the province, involved extrusion of widespread Eocene (47-41 Ma) lava fields forming the Alamo Creek and Ash Spring Basalts. Evidence for pyroclastic volcanism during this extrusive activity has previously been lacking. Our ongoing work has revealed a series of pyroclast-filled diatremes and fissure-like bodies that have ⁴⁰Ar/³⁹Ar dates overlapping with those for the basalt lavas and represent feeders to monogenetic volcanoes originally present at higher levels. Explosive behavior resulted from violent subsurface phreatomagmatic interactions between rising basaltic magma and unlithified, Campanian to Eocene fluvial strata rich in groundwater.

Diatremes have roughly cylindrical forms < 300 m across and contain juvenile basaltic blocks and bombs set within a matrix of terrigenous sand and mud intermixed with originally glassy basaltic ash and lapilli. Also present are masses of disrupted sediment and felsic tuff up to 60 m long derived from strata as much as 500 m higher in the succession than the present exposure level, indicating that the diatremes fed maar volcanoes at the surface; material subsided to deeper levels in the diatremes during ongoing subsurface explosive activity. In places the diatremes connect with upward-stepping dikes and sills exposed in cross-section, and some of the dikes intrude linear bodies of pyroclastic material that record explosive eruptions from fissures. Zones of phreatomagmatic tephra mixed with sediment in these fissure feeders locally pass into zones of Strombolian bombs in a vesicular lapillistone matrix, indicating heterogeneous eruption styles over short distances, presumably controlled by variations in magma flux and other factors. The diatremes and fissure-vent feeders have so far been traced over an area of ~ 400 km^2­, showing that phreatomagmatic processes played an important role in initial stages of volcanism in this part of the TPP. The region provides useful insights into hypabyssal basaltic plumbing systems and explosive subsurface processes associated with phreatomagmatic eruptions at higher levels.