LATE CRETACEOUS PHREATOMAGMATIC VOLCANISM AT PENA MOUNTAIN, BIG BEND NATIONAL PARK, TEXAS

Upper Cretaceous phreatomagmatic deposits have recently been reported near the northern boundary of Big Bend National Park. Here we describe more extensive basaltic phreatomagmatic deposits ~70 km to the south, exposed on Pena Mountain in the SW part of the park. The deposits represent the remains of a composite near-vent complex ~500 m across preserved within the Upper Shale Member of the Aguja Formation, constraining the volcanism to have occurred in the Campanian. Pyroclastic layers in the complex define at least three distinct packages each up to 50 m thick that represent individual eruptive episodes and are separated by sharp truncation surfaces. Bedding within each package shows centripetal dips, but marked changes in dip occur across truncation surfaces. Lacustrine limestone and mudstone occur in places above truncation surfaces and record periods of quiescence. Each package consists of numerous beds ~30 cm thick on average that are composed of tuff, lapilli tuff, and grain-supported lapillistone and were deposited by air-fall and base-surge processes.

Juvenile basaltic ash and lapilli in the deposits are intermixed with up to 30% accidental, sand-sized quartz, feldspar, and lithic grains. Some lapilli have globular shapes and were ejected as fluid droplets of magma. Angular quench-fragmented shards also occur. Basaltic pyroclasts are poorly to moderately vesicular and contain abundant sideromelane/palagonite altered to smectite. Quartz xenocrysts are common and record fine-scale magma-sediment interaction before eruption. Armored lapilli and numerous bomb and block sags indicate that the eruption columns contained water and that the beds were wet and cohesive during and after deposition. All these features point to generation of the tephra from subsurface phreatomagmatic explosions driven by uprise of basalt magma into wet, unlithified sediments. Downward quarrying by the explosions and collapse of unstable vent walls may have forced the vent location to change with time, causing slumping and generation of the truncation surfaces within the sequence. The Pena Mountain deposits significantly increase the known extent of Cretaceous volcanism in the Big Bend area and are inferred to be part of the intraplate Balcones Igneous Province present farther east in Texas.