PALEOGENE BASALTIC PEPERITES DEVELOPED IN CRETACEOUS TO PALEOGENE STRATA IN THE BIG BEND AREA OF WEST TEXAS

Abundant Tertiary basalt dikes and plugs intrude Cretaceous to Paleogene strata in the Big Bend area of West Texas. Detailed studies of the intrusions near the northern boundary of Big Bend Park indicate that the host strata were unconsolidated and rich in pore water at the time of intrusion, forming an array of peperite types where the intruding magma was quenched, disrupted, and commingled with host sediment. Basalt dikes cutting terrestrial strata of the Upper Cretaceous Javelina Formation and the overlying, Paleocene Black Peaks Formation show cuspate, billowy, and pillowed contacts against mudstone. Peperite masses up to 10 m across extend from dike margins and consist of blocky to fluidal basalt clasts showing in situ quench fragmentation and supported by interstitial sediment. In other areas, intrusive pillows with quench-fragmented rims grade into areas of chaotic blocky peperite, inferred to record small, contained steam explosions. Isolated, discordant, plug-like masses of peperite, intrusive pillows, and coherent basalt also cut the Javelina Formation and are not directly associated with basalt dikes. The largest of such masses are up to 300 m across and are inferred to represent the roots of small phreatomagmatic volcanoes. One mass studied in detail contains tabular, steeply dipping masses of sandstone and irregular masses of conglomerate intimately intruded by basalt. Clast compositions indicate that the conglomerate is derived from the Eocene Hannold Hill Formation, which occurs at higher stratigraphic levels. We infer that these strata were derived from parts of the volcano rim that subsided into the volcanic conduit as explosive activity waned. An unusual peperite type formed where basalt penetrated into the conglomerate as globular bodies and thin tendrils extending between cobbles.

A common assumption in the literature is that peperite records penecontemporaneous sedimentation and intrusive activity. Our observations, however, indicate that basalt magma that was intruded after deposition of the Black Peaks and Hannold Hill Formations was able to develop complex peperitic textures against underlying strata as old as the Cretaceous Javelina Formation, parts of which must have remained unlithified for a significant length of time.