ANATOMY OF AN A-TYPE CAMBRIAN FELSIC VOLCANIC FIELD: CARLTON RHYOLITE IN THE BLUE CREEK CANYON AREA, WICHITA MOUNTAINS, SOUTHERN OKLAHOMA

The Carlton Rhyolite is an extensive A-type felsic extrusive unit occurring at the top of a suite of rift-related Cambrian igneous rocks in the Southern Oklahoma aulacogen. The rhyolite is quite extensive in the subsurface but crops out only in limited areas in the Wichita and Arbuckle Mountains. The Blue Creek Canyon area in the Wichita Mountains contains some of the best exposures of the rhyolite but has received little previous study. A minimum stratigraphic thickness of 800 m of rhyolite is present in the area; the base is not exposed and the top is overlain unconformably by Upper Cambrian strata. Detailed mapping has identified at least five major flow units ranging from 160 to 190 m in thickness. These extend laterally for greater than 3 km and appear to be tabular. Most flow boundaries are locally defined by peperites or bedded tuffaceous deposits 8-30 cm thick. Limited sediment accumulation between flows implies rapid eruption of a series of flow units in a narrow time frame. The peperites formed when rhyolite flowed over wet sediments deposited between eruptions and consist of angular clasts or irregular globs of rhyolite set in a matrix of disrupted tuff. The flows consist dominantly of lithoidal, nonvesicular rhyolite, but finely flow-laminated glass (now devitrified and preferentially altered) with relict perlitic texture forms zones up to 35 m thick along the upper and lower flow margins. A distinct zone up to 5 m thick rich in lithophysae occurs inward from the chilled margins. These regular vertical textural changes indicate that each flow is a single cooling unit. No textural evidence for a pyroclastic origin for any of the flows has yet been found, implying that they are not rheomorphic ignimbrites and instead may represent flood rhyolites produced by effusion of relatively low-viscosity, high-T, A-type magmas.