Evidence for Late Cretaceous Volcanism in Trans-Pecos Texas

摘要

Cretaceous basaltic pyroclastic strata have been discovered in a fault block on the southern edge of the Rosillos Mountains laccolith in the Big Bend area of Trans-Pecos Texas.The sequence comprises base-surge and pyroclastic-fall deposits inferred to have accumulated on the flanks of a small phreatomagmafic volcano.A diverse assemblage of freshwater turtles(including Aspideretes),crocodile teeth,and dinosaur bones have been recovered from the uppermost part of the sequence.The fauna indicate a Late Cretaceous,probably Campanian age.An outward-dipping normal fault bounding the pyroclastic strata on the southeast juxtaposes sediments of the Upper Cretaceous Javelina Formation in the hanging wall with pyroclastic rocks in the footwall,indicating the pyroclastic strata must be Maastrichtian or older.U-Pb SHRIMP-RG analyses of zircons separated from a basaltic block thrown out of the volcano yield an inferred igneous crystallization age of 72.6 ±1.5 Ma,consistent with the biostratigraphic and structural evidence.The pyroclastic strata in the fault 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 older than 64 Ma.We speculate that the basaltic pyroclastic rocks represent an extension of the Upper Cretaceous Balcones magmatic province into Trans-Pecos Texas.