THE GAITHER MOUNTAIN SANDSTONE: EARLY PENNSYLVANIAN EXOTIC COMPOSITION IN A CRATONIC SETTING, ARKOMA SHELF, NORTHWEST ARKANSAS

The Gaither Mountain Member, Bloyd Formation, Morrowan Series (Early Pennsylvanian) in northwest Arkansas is a fluvial sandstone deposited by south and southwest flowing braided stream systems. Granules and occasional pebbles of vein quartz are abundant along foreset laminae and as lag conglomerate at the base of individual channels. Sandstone intervals range from 3 to 35 meters thick with an outcrop breadth perpendicular to depositional strike of approximately 70 kilometers. Westward the Gaither Mountain grades into coastal plain and swamp deposits of the Woolsey Member (Bloyd Formation) and southward into marine deposits of the incipient Arkoma basin in south-central Arkansas. The member is stratigraphically positioned within a largely cratonic marine succession of Mississippian and Early and Middle Pennsylvanian strata. Sandstone composition of the Gaither Member contrasts dramatically with the composition of older and younger sandstone units in the Mississippian and Pennsylvanian section of northwest Arkansas. In the medium to fine sand size range, polycrystalline quartz and metamorphic rock fragments of phyllite and schist form 20 to 25 percent of the framework grains in basal samples. Plagioclase, microcline and orthoclase comprise from 1 to 3 percent. Mississippian and older Pennsylvanian sandstone units are composed dominantly of monocrystalline quartz with occasional grains of chert and other sedimentary rock fragments. Sandstone units in the lower part of the Atoka Formation are also composed of quartzarenite dominated by monocrystalline quartz. Middle Atoka sandstone units, approximately 200 meters above the base of the formation, record the first appearance of metamorphic rock fragments above the base of the Gaither Mountain. The member may represent the initial incursion of far-traveled sediment derived from orogenic uplift in the northern Appalachians into the northern Arkansas cratonic setting.