INSIGHTS INTO MAGMA PLUMBING SYSTEMS FOR PHREATOMAGMATIC VOLCANOES: INTRUSIVE EOCENE BASALTIC FEEDER COMPLEX AND ASSOCIATED EXPLOSIVE VENTS EAST OF PEñA MOUNTAIN, BIG BEND NATIONAL PARK, WEST TEXAS

Eocene basaltic volcanism in the Big Bend region of the Trans-Pecos igneous province included widespread explosive phreatomagmatic interactions between rising basaltic magma and groundwater-rich Cretaceous to Eocene fluvial sediments. Open folding of the succession and Basin-and-Range normal faulting have exposed networks of feeder intrusions which supplied some of the phreatomagmatic volcanoes. Here we describe results of our work in progress on one example of these magma plumbing systems, which illustrates the complexity of the feeder networks that may develop in settings where magma encounters poorly lithified sedimentary sequences at shallow levels beneath the surface.

East of Peña Mountain in the SW part of Big Bend National Park, an interconnected network of hypabyssal basaltic intrusions penetrates gently dipping terrigenous and tuffaceous sediments of the Paleocene to Eocene Black Peaks and Chisos Formations. A complex array of dikes, sills, tongues and irregular, plug-like bodies of basalt occurs over an area at least 1350 m long by 300 m wide. The intrusions show fluidal, billowed, chilled margins against the host sediment, and sinuous, highly irregular, bifurcating dikes extend laterally into the sedimentary strata from larger tongues or sills. The intrusive basalt surrounds three separate zones up to 60 m long and several meters wide that have steep margins and are filled with a chaotic mixture of disaggregated sediment and poorly vesicular to scoriaceous basaltic lapilli, blocks and bombs. The characteristics of the pyroclastic material indicate that these zones are remnants of explosive phreatomagmatic vents. Meter-scale elongate tongues or tubes of the intrusive basalt penetrate parts of the vent-fill material.

We infer that Eocene phreatomagmatic volcanoes in the area east of Peña Mountain were fed by an upward-propagating network of relatively small, subvolcanic intrusions. After explosive activity had ceased (probably when the available groundwater was used up), basalt magma continued to be supplied to the growing intrusive network, obscuring much of the earlier feeder system and the associated phreatomagmatic volcanic vents. It is possible that the larger intrusions fed effusive basaltic eruptions, although we have not yet found field evidence for this.