AN UNUSUAL BASALTIC PHREATOMAGMATIC FISSURE VENT IN BIG BEND NATIONAL PARK, WEST TEXAS

Recent work in and near Big Bend National Park, West Texas has documented a series of Eocene phreatomagmatic basaltic volcanic vents that are mostly expressed as roughly cylindrical diatremes representing the feeder conduits to maar volcanoes now eroded away. However, some of these conduits are unusual in that they form basaltic fissure vents, which are underrepresented in the published literature on phreatomagmatism. Here we focus on a newly discovered, well exposed phreatomagmatic basaltic fissure vent that trends north-northwest in the western part of the park between Maverick Junction and Tule Mountain and is likely a continuation of a network of maar conduits, fissure vents and associated feeder intrusions in the Study Butte area to the north. The fissure vent is ~25 m wide and >340 m long but runs beneath cover at both ends. Various types of pyroclasts occur within the fissure, including ovoid, ribbon, cored, and pancake bombs, spatter, scoria, and finer basaltic lapilli mixed with variable amounts of sediment. Evidence for explosive interactions between fluid magma and groundwater-rich sediment includes the presence of angular, glassy pyroclasts, armored lapilli coated by wet ash during the eruption, and differences in pyroclast vesicularity from nearly nonvesicular to scoriacious. Two distinct pyroclastic facies occur in the fissure. Facies A contains abundant disaggregated mud and sand between the pyroclasts and grades northward into Facies B, which contains only minor amounts of sediment and has a basaltic lapillistone matrix. The difference between the facies is significant for it indicates that phreatomagmatic eruptions can be very heterogeneous over a short distance within a single fissure. The transition from Facies A to Facies B is obscured by a large coherent basaltic intrusion in the fissure measuring ~60 m long by 25 m wide. Other smaller intrusions in the fissure, from 15 cm up to 4 m across, occur as tongues and sinuous tubes that have fluidal, billowed margins, suggesting that the magma intruded into wet, unconsolidated vent fill in the fissure shortly after explosive activity ceased. A similar sequence of events is observed in other Eocene phreatomagmatic vents in the region. ⁴⁰Ar/³⁹Ar dating and geochemical studies of bombs and coherent basaltic intrusions within the fissure are now in progress.