PHYSICAL VOLCANOLOGY OF THE CARLTON RHYOLITE GROUP IN THE WEST TIMBERED HILLS, ARBUCKLE MOUNTAINS, SOUTHERN OKLAHOMA

Here we report initial results of studies in progress on Cambrian rhyolites in the West Timbered Hills in the Arbuckle Mountains, which are exposed for 9 km along strike and represent the only major outcrops of the Carlton Rhyolite Group in the Southern Oklahoma aulacogen that have not yet been mapped in detail. Four flows have so far been recognized in the study area, labeled 1 through 4 from east to west. The main phenocrysts are plagioclase and mafic silicates (probably pyroxene and fayalite) replaced by green clay. Trace-element contents indicate A-type affinities for the rhyolites. Individual flows are > 50 m thick and are distinguished by differences in chemical composition and phenocryst proportions and textures. Complete sections of individual flows are not exposed, but Flows 1, 2 and 4 show vertical columnar jointing and have originally glassy lower flow margins that exhibit perlitic texture, abundant elongate vesicles and flow lamination. Higher in the flows the groundmass develops a mottled appearance and grades up into homogeneous felsite representing the more slowly cooled flow interiors; a zone rich in lithophysae typically occurs at the transition into the felsitic interiors. Flow 3 differs from the other flows in that it exhibits flow banding and flow breccia throughout its exposed thickness, with pervasive spherulitic texture.

Stratigraphic relations between some of the flows are unclear. However, Flow 1 overlies planar-bedded rhyolitic tuffaceous rocks that probably accumulated in a lake, and peperite occurs locally along the contact where the rhyolite quenched against and mixed with wet unlithified sediment. Flow 2 abuts against Flow 1 along a relatively steep contact that is inferred to represent the original lateral boundary between the flows. The western margin of Flow 2 is defined by a paleochannel filled with a massive, polymict debris-flow deposit > 30 m thick containing rhyolite clasts up to 2.6 m long dispersed in a finer grained matrix of rhyolitic and less common basaltic lithic detritus, with abundant interstitial mud and silt. Irregular, meter-scale domains within the deposit show fluidal, peperitic relations between quenched rhyolite debris and mud and silt, suggesting that the debris flow may have been initiated when rhyolite lava poured across unlithified sediment on an unstable slope.