NEWLY RECOGNIZED CRETACEOUS AND EOCENE BASALTIC PHREATOMAGMATIC VOLCANISM IN THE BIG BEND AREA, WEST TEXAS

Recent work has documented previously unknown Cretaceous and Eocene episodes of explosive basaltic phreatomagmatism in and near Big Bend National Park. The Cretaceous episode is recorded by proximal base-surge and pyroclastic fall deposits that intertongue with or are overlain by terrigenous Campanian strata of the upper part of the Aguja Formation. These deposits have been found in a fault block on the southern flank of the Rosillos Mountains just outside the park, and on Peña Mountain, 40 km to the SW inside the park. They contain coarse ballistic clasts with impact sags, and are interpreted to have accumulated on the flanks of maar volcanoes. Deposits of at least two coalescing maars and a partly preserved crater margin are present at Peña Mountain. The basaltic tephra is intermixed with large amounts of Aguja sand and mud, indicating that explosive eruptions were initiated when uprising magma encountered groundwater-rich sediments in the shallow subsurface.

The Eocene episode is represented by basaltic diatreme complexes up to 250 m across that occur on the southern flank of the Rosillos Mountains and are associated with a dike swarm extending from the Christmas Mountains igneous center to the west. The diatremes show typical features of root zones to phreatomagmatic vents and are inferred to be feeders of maar volcanoes now eroded away. They contain coherent to peperitic basalt that intrudes intensely disrupted Upper Cretaceous to Eocene fluvial strata, some of which slumped several hundred meters downward into the conduits during vent excavation. The associated dikes have yielded published K-Ar dates of 44-40 Ma; 40Ar/39Ar dating of samples from the diatremes is in progress.

Monogenetic volcanoes of the type described here typically occur in clusters, and additional phreatomagmatic vents of similar ages almost certainly remain to be discovered in the region. The Cretaceous maars appear to represent a westward continuation of the intraplate Balcones igneous province along the buried frontal zone of the Ouachita belt. The Eocene diatremes were emplaced during early stages of Cenozoic Trans-Pecos magmatism. Both episodes record climatic and depositional regimes in which sediments at shallow levels beneath the surface contained sufficient groundwater to drive explosive phreatomagmatism.