NONEXPLOSIVE MAGMA/WET-SEDIMENT INTERACTION AND EXPLOSIVE PHREATOMAGMATISM ASSOCIATED WITH EOCENE BASALTIC INTRUSIONS, BIG BEND AREA, WEST TEXAS

Tertiary basaltic intrusions are abundant in the Tran-Pecos igneous province of West Texas, but little previous attention has been given to interactions between these intrusions and the Cretaceous and Tertiary strata that host them. Here we document an array of features recording both nonexplosive and explosive magma/wet-sediment interaction associated with Eocene emplacement of alkaline basaltic to trachyandesitic magmas into unlithified fluvial strata of the Upper Cretaceous Javelina Formation and the Paleogene Black Peaks and Hannold Hill Formations on the southern flank of the Rosillos Mountains near the northern boundary of Big Bend National Park.

Numerous dikes radiating from the ~40 Ma Christmas Mountains igneous complex to the west show fluidal, billowed, quenched margins against Javelina and Black Peaks mudstones. These billowed margins pass into zones of intrusive pillows associated with peperite, in which hyaloclastite shards derived from pillow margins are intermixed with disrupted sediment. Some partly pillowed dikes show complex forms, with sill-like apophyses that foundered into underlying sediment. These features provide insight into precursory stages of magma/wet-sediment interaction that may lead to explosive phreatomagmatism.

Discordant, roughly cylindrical intrusive masses up to 250 m across of basalt, peperite, and disrupted sediment are interpreted to represent diatreme root zones that fed maar volcanoes. Intensely disrupted zones in the Javelina Formation along diatreme margins record collapse of Cretaceous strata in the vent walls. Masses of Eocene Hannold Hill conglomerate within the diatremes slumped downward from higher levels during vent excavation. Coherent to pillowed basalt intrusions emplaced at the close of explosive activity formed complex zones of peperite within a mixture of sand, mud, and quenched basaltic debris, inferred to represent the phreatomagmatic slurry that filled the vents during explosive volcanism.

This initial work suggests that the Trans-Pecos province may be an important locale for the study of subvolcanic phreatomagmatic processes. Eocene intrusions in the study area underwent complex interactions with wet sediment in strata as old as Maastrichtian, which must have remained unlithified and rich in pore water for ~30 Ma.