NEW EVIDENCE FOR LATE CRETACEOUS VOLCANIC ACTIVITY IN TRANS-PECOS TEXAS

Strata preserved on the southern edge of the Rosillos Mountain laccolith in the Big Bend area of Texas record the earliest known volcanism in the Trans Pecos region. The strata contain intermixed basaltic pyroclastic debris and terrigenous detritus. Beds of basaltic-scoria lapillistone and bomb sags beneath basalt blocks up to 30 cm across indicate a nearby source of ejecta.

Twenty-six zircon grains separated from a basaltic bomb and analyzed using the Stanford/USGS SHRIMP-RG yielded Cretaceous to Paleoproterozoic U-Pb ages. Analyses for the five youngest grains are tightly clustered, with error ellipses that overlap with concordia ( ± 1 sigma uncertainties). The four most concordant analyses yield a concordia age of 72.6 ± 1.5 Ma (2 sigma errors; MSWD=0.76), which is interpreted as the igneous crystallization age of the rock. The grains that produce this age are euhedral, with generally well-developed pyramidal terminations, and exhibit well-defined, concentric zonation. The older grains are interpreted to be xenocrysts.

Biostratigraphic evidence indicates a Campanian age for the strata, consistent with the U-Pb age. A diverse assemblage of freshwater turtles (including Aspideretes), crocodile teeth, and dinosaur bones have been recovered from beds directly overlying the volcanic deposits. The fauna indicates a Late Cretaceous, most likely Campanian age,based on the abundance and diversity of the turtle fauna in general and the presence of Aspideretes in particular. The strata containing the volcanic deposits and fossils are preserved in a block ?250 m across bound by outward-dipping normal faults. The strata in the hanging walls of the faults belong to the Javelina Formation (middle to late Maastrichtian) or possibly the upper Aguja Formation (late Campanian). Strata in the footwall must be older than those in the hanging wall that are in contact with them across the fault, again consistent with a Campanian age for the basaltic rocks.

The strata in the fault-bound block provide the first evidence for Late Cretaceous volcanism in the Trans-Pecos region. Previously the onset of igneous activity in the area was thought to be no more than 64 Ma. We infer that the near-vent basaltic pyroclastic rocks represent an extension of the Late Cretaceous Balcones igneous province into Trans-Pecos Texas.