NEWLY DISCOVERED EOCENE BASALTIC PHREATOMAGMATIC VENTS IN THE DOGIE MOUNTAIN AND BLACK PEAKS AREAS, BIG BEND NATIONAL PARK, WEST TEXAS

Over the past several years we have documented a series of Eocene basaltic phreatomagmatic vent conduits near and in Big Bend National Park. The vents have yielded ⁴⁰Ar/³⁹Ar ages of 47–42 Ma, indicating they are coeval with eruption of the widespread Alamo Creek and Ash Spring Basalts. They are present over an area > 400 km², suggesting that explosive interactions between groundwater and basaltic magma in the shallow subsurface played an important role in the evolution of the basaltic fields. Some of the vents that fed small maar volcanoes have subvertical, roughly cylindrical forms, but narrow, elongate, discordant masses of pyroclastic vent fill that fed fissure eruptions also occur.

Here we document two more examples of these vents, one of which is associated with a prominent basaltic dike 7 km long that cuts fluvial strata of the Maastrichtian–Paleocene Black Peaks Formation SE of Dogie Mountain in the western part of the park and has yielded a conventional K-Ar date of 47 ± 2 Ma (Henry et al., 1989). Peperite along the dike margins consists of angular, jigsaw-fit basaltic clasts within a sediment host, recording nonexplosive quench fragmentation of magma against unlithified groundwater-rich strata. Zones ≥ 20 m long and ≥ 8 m across of massive lapilli tuff along the eastern margin contain angular to fluidal, variably vesicular basaltic pyroclasts chaotically mixed with sediment. We interpret this material to represent part of a fissure vent formed during phreatomagmatic explosions prior to intrusion of the main part of the dike, which obliterated most of the original vent fill along the fissure.

The second example occurs ~ 35 km to the east at Black Peaks, which are three small isolated hills within the type area of the Black Peaks Formation. The hills are held up by intrusive basalt that forms pillows and areas of peperite against disrupted sediment and basaltic pyroclastic rock similar to that along the margin of the Dogie Mountain dike. Although the geometry of the Black Peaks vent is unclear, it underwent a similar sequence of events to that seen in all the vent conduits we have so far studied, where initial phreatomagmatic eruptions were followed by continued uprise of basaltic magma at the close of explosive activity. ⁴⁰Ar/³⁹Ar dating is now in progress to better define the age range of the various basaltic vents in the region.