EXPLOSIVE BASALTIC PHREATOMAGMATIC ERUPTIONS, DESTRUCTION OF AN ANCIENT FOREST AND SUBSTRATE COLLAPSE DURING MAAR VOLCANISM RECORDED WITHIN PALEOCENE ALLUVIAL PLAIN STRATA OF THE BLACK PEAKS FORMATION, BIG BEND NATIONAL PARK, WEST TEXAS

The only known Paleocene volcano in the Trans-Pecos Igneous Province occurs in the NW part of Big Bend National Park, TX. Our recent work reveals that the volcano consists of proximal ejecta rim deposits that presently extend for 1.4 km², but unknown amounts of additional tephra are covered or have been eroded away. The deposits are intercalated with Paleocene overbank mudstones and channel sandstones of the Black Peaks Formation (BPF) that were deposited in a gently sloping alluvial plain setting. The tephra consists mostly of interbedded tuff, lapilli tuff and lapillistone. Juvenile basaltic lapilli and ash have low vesicularity, range from sharply angular to fluidal in shape, and are intermixed with <30% of terrigenous sand and mud. Pyroclastic fall and base surge deposits are commonly intercalated, with the latter showing antidune-type low-angle cross-stratification and wavy or undulatory bedforms. Small amounts of volcanic bombs (~5%) occur throughout the tephra deposits. Some bombs are cored by sediment or show complex magma-sediment mixing; erupted masses of mud up to 2 m across also occur. Together these features indicate phreatomagmatic eruptions driven by explosive interactions between groundwater-rich sediment and magma in the shallow subsurface. Chaotic bedding attitudes and soft-sediment folding and faulting of the deposits and adjacent BPF strata occur in several areas, and masses of unconsolidated BPF mudstone > 25 m long have collapsed from above into disturbed pyroclastic deposits. We infer that these features indicate the presence of a maar volcano in which the crater cut down into underlying strata, with collapse of the unstable substrate blocking the vent and forcing the eruption to shift laterally. There is no observable pattern in bomb size or distribution, consistent with lateral vent migration or eruptions from several vents. Detailed mapping has identified 36 mostly vertical fossilized tree stumps (<1.5 m tall) encased within both fall and base surge deposits with no signs of ignition prior to burial, indicating cool, wet eruptions in which steam was condensing to liquid water. Juvenile pyroclasts show closely similar chemical compositions indicating that the eruptions occurred within a short time span so that magma within the source chamber had little time to differentiate.