PHREATOMAGMATIC VOLCANIC PROCESSES RECORDED IN A PALEOCENE TUFF-RING COMPLEX, BIG BEND NATIONAL PARK, TRANS-PECOS IGNEOUS PROVINCE, WEST TEXAS

The Trans-Pecos igneous province (TPIP) spans much of west Texas and is mostly Eocene-Miocene in age. We have recently documented near-vent basaltic pyroclastic deposits > 10 m thick within fluvial strata of the Black Peaks Formation (BPF) containing Paleocene mammal fossils. These deposits crop out in a 1.2 km² area in a remote part of Big Bend National Park within the southeastern portion of the TPIP. The northern margin of the pyroclastic succession is obscured by Eocene felsic intrusions, but elsewhere along the margins of the succession the deposits are overlain and underlain by terrigenous BPF strata and thin to the east. The deposits consist mostly of thinly bedded tuff, lapilli tuff, and lapillistone, with clasts < 1 cm across. Dispersed, poorly vesicular bombs < 1.1 m across with asymmetrical bomb sags also occur. Planar bedding and lamination are most common, but low-angle antidune-type cross-bedding occurs in some areas as well, indicating deposition from both pyroclastic fall and pyroclastic surges. In thin section, lapilli and ash have low vesicularity and fluidal or droplet-like to subangular and angular shapes. They mainly consist of light-brown altered sideromelane glass derived from rapid quenching of magma in contact with water. These characteristics indicate the pyroclasts were formed by explosive phreatomagmatic eruptions. Up to 20 % terrigenous quartz and feldspar silt and sand grains are intermixed with the juvenile pyroclasts and are sharply angular, suggesting that they were shattered during explosive subsurface interactions between uprising magma and groundwater-rich sediments. The pyroclastic deposits typically have gentle dips characteristic of ejecta rims around tuff rings, a common type of phreatomagmatic volcano. Over 20 mostly upright petrified tree trunks have been discovered within the deposits and show no signs of having been burned prior to fossilization, indicating pyroclastic surges were not hot enough to ignite the trees during their passage. Anomalously steep dips (up to 25^o) in the pyroclastic deposits and adjacent parts of the BPF occur locally and likely represent collapse of parts of the succession due to development of one or more vents in weak, unconsolidated strata.