EOCENE(?) BASALTIC DIKE-SILL NETWORK AND PHREATOMAGMATIC VENTS EMPLACED INTO UNLITHIFIED CRETACEOUS TO PALEOCENE STRATA, STUDY BUTTE AREA, BIG BEND NATIONAL PARK, WEST TEXAS

An array of basaltic dikes and sills intrudes Upper Cretaceous to Paleocene mudstone and sandstone of the Aguja, Javelina and Black Peaks Formations over an area of ~ 1.5 km² on the west side of Big Bend National Park near Study Butte, Trans-Pecos igneous province. Field relations indicate that the host sediments were unlithified and rich in pore water during basalt intrusion. Sills up to tens of meters thick show complex, intertonguing contacts with adjacent strata. Dikes are 0.2–20 m wide and have irregular trends and sinuous, branching shapes, consistent with emplacement into weak sediment. Chilled margins on dikes and sills have billowed surfaces, recording surface instabilities between magma and sediment; cm- to m-scale pockets of peperite formed where basalt underwent quench fragmentation and intermixing with mud and sand.

Two vertical pipe-like masses measuring 8 m and ~ 100 m across cut Aguja strata in the same area. The pipes are filled with massive tuff-breccia and lapilli-tuff containing variably vesicular, fluidal to angular basaltic pyroclasts (including pieces of spatter) set within a sediment-rich matrix and intermixed with clasts of partially disaggregated mudstone, sandstone and felsic vitric tuff up to 2.5 m long. We interpret these pipe-like masses to represent vents for small maar volcanoes generated from subsurface phreatomagmatic explosions driven by violent interaction between rising basaltic magma and groundwater-rich sediment. Felsic tuff clasts within the vents are identical to air-fall tuff deposits in the Eocene Chisos Formation exposed in nearby areas, and must have slumped downward into the vents from higher levels. Massive lapilli-tuff similar to that within the pipes occurs in zones up to several meters wide along the margins of some dikes in the area. This lapilli-tuff is inferred to record phreatomagmatic eruptions along fissures, prior to dike emplacement at the close of explosive activity. The Study Butte vents likely formed during the same regional basaltic phreatomagmatic episode as vents for maar volcanoes recently documented ~ 35 km to the NE in the Big Bend region. The latter vents have Ar/Ar dates of 47–46 Ma (Befus et al., 2009, Jour. Volcanol. Geotherm. Res.), suggesting a link with the widespread Alamo Creek Basalt, which was extruded in the same time frame.