FIELD RELATIONS OF THE CARLTON RHYOLITE IN THE FORT SILL AREA, SOUTHWESTERN OKLAHOMA

The Cambrian Carlton Rhyolite Group (CRG) and related Mt. Scott Granite were emplaced during rifting within the Southern Oklahoma aulacogen. Detailed mapping of the CRG on and adjacent to Fort Sill Military Reservation has revealed the presence of at least two separate lava flows. Remnants of the lower flow, the Davidson metarhyolite, can be traced along strike for 3.7 km but are truncated by discordant intrusive contacts against younger granite. The unit is aphyric, unlike other parts of the CRG, and shows pervasive flow lamination, with complex, meter-scale flow folds and localized pockets of flow breccia. The groundmass has recrystallized to a granoblastic intergrowth of quartz and feldspar due to contact metamorphism during intrusion of the granite. Metamorphic veinlets of magnetite and quartz are common. To the north, this unit is in fault contact with granite directly below Mt. Scott, as indicated by linear zones of cataclastically sheared and brecciated metarhyolite juxtaposed against granite with similar textures. To the south, a discontinuous unit of rhyolitic sandstone and recrystallized tuff up to several meters thick separates the aphyric metarhyolite from an overlying, southward-dipping rhyolite lava rich in quartz and alkali feldspar phenocrysts. Locally, granite has intruded along this contact, resulting in complex interfingering relations between chilled granite and rhyolite. The upper lava, termed the Fort Sill rhyolite, can be traced along strike for 18.5 km and is > 190 m thick. It typically has a felsitic groundmass that coarsens due to metamorphic recrystallization near contacts with the granite. Pervasive, discontinuous flow banding is present near the base of the unit and in thick zones near the top, where it is deformed by steeply plunging, open flow folds with wavelengths up to 3 m. The center of the unit is massive, with poorly developed, subvertical hexagonal columns best exposed at Medicine Bluffs. Near the northern boundary of Fort Sill, an E-W-trending brittle fault zone offsets part of this unit. Further mapping and geochemical and modal analyses are underway in order to determine if all of the outcrops of phenocryst-rich rhyolite on Fort Sill belong to a single, laterally extensive lava flow comparable to those documented from other anorogenic felsic volcanic provinces.