ERUPTIVE STYLES IN EOCENE BASALTIC PHREATOMAGMATIC FISSURE VENTS, BIG BEND NATIONAL PARK, WEST TEXAS

Our ongoing work indicates that Eocene basaltic phreatomagmatic volcanism was widespread in parts of the Trans-Pecos igneous province in and near Big Bend National Park. In parts of this area, explosive subsurface interactions between basaltic magma and unlithified, groundwater-rich Upper Cretaceous to Eocene fluvial sediments formed roughly cylindrical diatremes that fed small maar volcanoes now eroded away. Near Study Butte in the western part of the park, several such diatremes are linked to a well-exposed network of basaltic dikes and sills that has yielded ⁴⁰Ar/³⁹Ar ages of 43 Ma and extends for at least 3 km laterally over a vertical stratigraphic distance of ~500 m. Some of the dikes were injected along linear zones filled with pyroclastic material that cut across the host strata discordantly and that we interpret as the subsurface parts of phreatomagmatic fissure vents.

The largest fissure vent found so far, which is representative of the vent network as a whole, is 25 m across and extends ≥ 340 m laterally before running under cover. Juvenile basaltic pyroclasts within the vent include ellipsoidal, ribbon, and pancake bombs and pieces of spatter as well as lapilli and ash having fluidal to angular shapes and showing a wide range in vesicularity. In parts of the fissure, basaltic pyroclasts are thoroughly mixed with abundant (~25%) clumps and tendrils of disaggregated sediment, recording fine-scale explosive magma-sediment interaction. Some lapilli are coated with layers of sediment-rich ash-sized material. In other parts of the fissure, bombs are dispersed within a basaltic lapillistone matrix that contains ≤ 2 % sediment, indicating that Hawaiian or Strombolian-style eruptive activity primarily driven by release of magmatic volatiles played a more important role in these parts of the fissure. This example shows that eruptions from basaltic phreatomagmatic vents can be markedly heterogeneous over short distances, due to variations in magma flux or groundwater availability. Coherent basalt geochemically identical to basalt bombs within the vent forms plug-like masses as well as interconnected, fluidal tongues and sinuous tubes intruding the vent fill, recording continued uprise of magma into unconsolidated tephra within the fissure after explosive activity had ceased.