CARLTON RHYOLITE ON ZODLETONE MOUNTAIN, SOUTHWEST OKLAHOMA

Zodletone Mountain in the Slick Hills of southwest Oklahoma is the northernmost outcrop of the regionally extensive Carlton Rhyolite, which shows A-type geochemical signatures and was emplaced during Cambrian rifting in the Southern Oklahoma aulacogen. Detailed mapping reveals that the eastern part of Zodletone exposes two tabular lava flows unconformably overlain by the Upper Cambrian Reagan Sandstone. The flows show a consistent vertical structural and textural zonation, which is best developed in the upper flow. This flow is ~190 m thick and has altered, originally glassy flow margins showing relict perlitic texture and local lithophysae. The glassy margins grade into transitional zones displaying flow banding defined by color changes in the groundmass and by concentrations of spherulites. The center of the flow is felsitic rhyolite that coarsens inward and shows rough columnar jointing perpendicular to flow margins. Randomly oriented tridymite needles, now replaced by quartz, are abundant in the groundmass and are inferred to have formed during early, high-T primary devitrification. The contact between this flow and the underlying flow is defined by a laterally discontinuous vitric ash-fall tuff ~30 cm thick. The underlying flow is ~140 m thick and shows extensive flow brecciation in its upper part. Lenses of devitrified rhyolite 10-15 m across within the flow breccia are inferred to represent flow lobes. This flow passes laterally across an obscure contact into undifferentiated rhyolite that forms the entire western part of Zodletone Mountain and consists primarily of a complex assemblage of flow breccia and flow-banded lava. Peperite is locally present and records interaction between lava and underlying wet, unconsolidated tuffaceous sediment during flow emplacement, but lack of consistent vertical zonation prevents mapping of individual flows in this area. The two well defined flows on the eastern part of Zodletone are comparable to flows mapped elsewhere in the Carlton Rhyolite in the Slick Hills. Outcrops of rhyolite in this region appear to represent parts of tabular, laterally extensive lava flows similar to those documented from better exposed, more recent A-type volcanic fields (e.g., Miocene rhyolites of the Snake River Plain).