NONEXPLOSIVE AND EXPLOSIVE MAGMA/WET-SEDIMENT INTERACTION IN THE SHALLOW SUBSURFACE DURING EOCENE MAGMATISM IN THE TRANS-PECOS IGNEOUS PROVINCE, TEXAS

The earliest Cenozoic magmatism in most of Trans-Pecos Texas is represented by Eocene hypabyssal intrusions and basaltic lavas. Detailed studies of some of these intrusions on the southern flank of the Rosillos Mountains show that they were emplaced into, and interacted with, unlithified wet sediment at shallow levels beneath the surface. Near the northern boundary of Big Bend Park, three intrusive complexes up to 250 m across cut Upper Cretaceous to Paleocene fluvial strata of the Javelina and Black Peaks Formations. The complexes contain coherent to fragmented basalt that intrudes, or is intermixed with, disrupted sandstone and mudstone on a variety of scales. We infer that the complexes were feeder conduits for maar volcanoes produced by explosive interaction between rising basaltic magma and groundwater-rich Javelina and Black Peaks strata. Two of the complexes contain masses of Eocene Hannold Hill conglomerate that slumped downward from higher levels during vent excavation. Groundmass concentrates from basalt in two of the complexes have yielded Ar/Ar dates of 47-46 Ma, indicating they are the same age as the Alamo Creek Basalt and related lavas in nearby areas. These data suggest that explosive phreatomagmatism may have played an important role in development of parts of the Alamo Creek volcanic field.

A series of younger basaltic to trachyandesitic dikes, sills, and irregularly shaped intrusions also occur on the southern flank of the Rosillos Mountains and belong to a dike swarm extending from the Christmas Mountains farther west. Ar/Ar dates on groundmass concentrates from two of these intrusions indicate emplacement at ~42 Ma, consistent with published K-Ar dates for other parts of the dike swarm. The intrusions have fluidal, billowed margins against Javelina and Black Peaks strata, which grade into zones of intrusive pillows associated with pockets of peperite where quenched igneous debris underwent nonexplosive intermixing with water-rich sediment.

The evidence indicates that Late Cretaceous strata remained unlithified and rich in pore water into the Eocene in this part of Trans-Pecos Texas. Subsurface magma/wet-sediment interaction may have been common during Eocene intrusive activity elsewhere in the region, and additional Eocene phreatomagmatic vents probably remain to be discovered.