COMPLEX HYPABYSSAL INTRUSIVE NETWORKS ASSOCIATED WITH WIDESPREAD EOCENE BASALTIC PHREATOMAGMATIC VOLCANISM IN THE BIG BEND REGION OF WEST TEXAS

Initial stages in development of the Trans-Pecos igneous province in west Texas included extrusion of Eocene basalt lavas in the area now occupied by Big Bend National Park (BBNP). Our ongoing work has revealed a series of phreatomagmatic vent complexes spatially associated with the basaltic lava fields and present over an area >400 km². Rugged desert outcrops allow 3-D views of the magma plumbing systems to the volcanic vents. We have obtained ⁴⁰Ar/³⁹Ar dates of 47-42 Ma for hypabyssal intrusions associated with five separate vent complexes, which overlap with ages of the Alamo Creek and Ash Spring Basalts, two of the main basaltic units in BBNP.

The vent complexes fed small volcanoes now eroded away. They contain basaltic pyroclastic debris chaotically mixed with disrupted sediment and were generated by explosive interactions between rising magma and groundwater-rich Upper Cretaceous to Eocene fluvial sedimentary strata. Basaltic magma continued to rise in the vents after explosions ceased, forming intricate networks of small intrusive tongues and pillows compositionally similar to basalt pyroclasts in the vents. In the Study Butte area on the west side of BBNP, an array of upward-stepping dikes and sills that fed several central and fissure vents extends laterally for >3 km over a vertical stratigraphic distance of ~500 m. Dikes and sills have complex billowed, locally peperitic margins, recording fluidal interactions with sediment that was still wet and poorly consolidated at the time of intrusion. The dikes have markedly irregular, sinuous trends, and some turn back upon themselves, producing unusual hook-like shapes.

East of Peña Mountain inside BBNP, vent complexes related to the Alamo Creek Basalt were partly obliterated by an extensive intrusive basaltic sheet tens of meters thick that has ⁴⁰Ar/³⁹Ar dates of 34 Ma. The main intrusion connects to a network of smaller sills, tongues, irregular dikes and plug-like bodies that show similar evidence of emplacement into wet sediment, although without explosive behavior. All these examples reveal the geometric complexities that can arise in hypabyssal settings where magma propagates through low-density, mechanically weak sedimentary sequences that may remain unlithified and groundwater-rich for millions of years after deposition.