RESULTS OF ONGOING WORK ON EOCENE BASALTIC PHREATOMAGMATIC VENTS IN BIG BEND NATIONAL PARK AND ADJACENT AREAS, TRANS-PECOS IGNEOUS PROVINCE, WEST TEXAS

Recent work has revealed the presence of a series of Eocene phreatomagmatic vent complexes in and near Big Bend Park, which record explosive eruptions driven by interaction between rising alkaline basaltic magma and groundwater-rich sediment during early stages of Eocene basaltic volcanism in this part of the Trans-Pecos province. We have mapped in detail three vents in or near the northern part of the park, a single vent ~300 m across farther south that forms a prominent hill just east of the main park road near Hannold Hill, and a complex dike-sill-vent network to the SW, in the Study Butte area. The main vents have roughly cylindrical shapes and are filled by chaotic masses of tuff-breccia, lapilli-tuff and lapillistone containing variably vesicular, angular to fluidal basaltic pyroclasts intermixed with disaggregated terrigenous mud and sand derived partly from strata making up the vent walls. Similar material also fills fissure vents in the Study Butte area. Irregular basaltic masses and dikes intruded the vent-filling material at the close of eruptions and developed intrusive pillows and peperite during nonexplosive interaction between magma and water-saturated debris.

Vents in different areas cut across mudstone and sandstone of the Upper Cretaceous to Eocene Aguja, Javelina, Black Peaks and Hannold Hill Fms. Disrupted masses of sediment up to 60 m long within the vents were derived from younger Eocene strata, including rhyolitic tuff of the Canoe and Chisos Fms, that are not presently exposed in the areas of the vents. These sediment masses must have slumped downward for as much as 500 m into the vents during vent excavation. These features indicate that the vents fed maar volcanoes, which have now been removed by erosion, and which underwent progressive widening and deepening during subsurface explosive activity. Basaltic intrusions within four of the vents have yielded ⁴⁰Ar/³⁹Ar dates of 47-46 Ma and 43-41 Ma, which overlap respectively with age constraints for the Alamo Creek and Ash Spring Basalts, suggesting that phreatomagmatic processes were important during eruption of these basaltic fields. Monogenetic vents of the type described herein generally occur in clusters, and additional Eocene basaltic phreatomagmatic vent complexes almost certainly are present in other parts of the Big Bend region.