PALEOCENE BASALTIC MAAR/TUFF-RING COMPLEX IN BIG BEND NATIONAL PARK, TRANS-PECOS IGNEOUS PROVINCE, WEST TEXAS

The Trans-Pecos igneous province (TPIP) occupies a large area of western Texas. Almost all the igneous activity is Eocene-Miocene in age, based on available isotopic dating, except for 61-Ma (Paleocene) high-level granitoid rocks in the western part of the TPIP. Recent field work in Big Bend National Park in the south east part of the TPIP has led to the discovery of a basaltic maar/tuff-ring complex interfingering with Paleocene fluvial strata of the Black Peaks Formation. The complex represents the only known Paleocene basaltic proximal volcanic succession in the entire TPIP. Pyroclastic deposits within the complex crop out within a 0.65 km²area. Particles are typically ≤1 cm in size and range from ovoid, poorly vesicular, fluidal lapilli to nonvesicular angular coarse ash and fine lapilli. Tuff, lapilli tuff, and lapillistone are randomly interbedded, implying pulsatory eruptions. Parts of the succession mostly show planar bedding and lamination, but in other areas low-angle antidune-type cross-stratification is common. These features indicate a combination of pyroclastic fall and deposition from base surges. Up to 5% of the pyroclastics consist of bombs as much as 0.5 m across with well-developed bomb sags. Three upright fossilized tree stumps are encased within the pyroclastic deposits and show no evidence of carbonization, implying that base surges in this area were not hot enough to ignite wood. These characteristics indicate explosive phreatomagmatic interactions between basaltic magma and external water. There is no evidence for significant lacustrine deposits within the Black Peaks Formation, suggesting that the eruptions were driven by interactions between basaltic magma and unlithified, groundwater-rich sediments. Discontinuous calcareous mudstones (≤6 cm thick) are inferred to represent deposition in transitory small lakes formed within craters, but more detailed mapping is needed to prove this interpretation. Anomalously steep dip angles (≥ 20^o) in both the pyroclastic deposits and the Black Peaks strata in the immediate area indicate that parts of the succession underwent collapse due to construction of the volcanic complex on an unstable substrate. This model is consistent with local, small-scale, soft-sediment reverse and normal faulting in the pyroclastic succession.