CAMBRIAN VOLCANIC STRATIGRAPHY AND RHYOLITE PEPERITE in THE BALLY MOUNTAIN AREA, SW OKLAHOMA

Southern Oklahoma contains an extensive series of rift-related Cambrian A-type rhyolites that can be traced for ~40,000 km² in the subsurface. Outcrops in the Wichita and Arbuckle Mountains provide insight into the nature of this largely buried volcanic field. The thickest stratigraphic sequence of rhyolites (~2.1 km) occurs in the Bally Mountain area in the Wichitas. Sixteen separate rhyolite flows have been mapped in this area. The largest flow (the big orange flow or BOF) is ~400 m thick but can only be traced ~3 km before going under cover or being truncated by faulting. A possible feeder dike for this flow is poorly exposed a short distance beneath it. Individual flows typically show a characteristic vertical zonation, with upper and lower originally glassy margins that exhibit well-developed flow banding and in some cases pass inward into lithophysal zones. Flow interiors consist of massive, holocrystalline rhyolite containing tridymite needles (now inverted to quartz) that increase in size inward, recording uniform cooling after emplacement.

Intervals of rhyolitic tuff, mudstone, volcaniclastic sandstone, and debris-flow deposits occur between some of the flows and are typically 1-5 m thick. The thickest such interval (~70 m) occurs beneath the BOF and consists of sediment gravity-flow deposits and tuffaceous beds deposited in a lake. Extensive peperite is present where the base of the BOF underwent quench fragmentation and intermixing with wet, unconsolidated lacustrine sediment. The peperite consists of fluidal to blocky rhyolite fragments separated by disrupted sediment. Rhyolite clasts show progressive stages of fine-scale fragmentation, contributing numerous small, angular shards into the adjacent sediment host. One or more tongues ≥ 190 m long from the base of the BOF penetrated into the lacustrine sequence and underwent complete disruption to form zones of peperite ≥ 20 m thick. Less extensive peperite is developed at the bases of four other rhyolite flows in the Bally Mountain area. We infer that ongoing subsidence during Cambrian rifting created environments favorable for peperite formation, as rhyolite lava poured across unconsolidated, water-rich sediments that had accumulated during pauses in eruptive activity within the rift.