SOURCE AND DISPERSAL PATTERNS OF AN EARLY PENNSYLVANIAN SANDSTONE UNIT. ARKOMA SHELF, NORTHWEST ARKANSAS

The middle Bloyd sandstone is a fluvial unit within the Morrowan (Pennsylvanian) succession of northwest Arkansas. It occupies a stratigraphic position above the Brentwood Limestone and below the marine Dye Shale Member of the Bloyd Formation. The unit ranges to 40 meters in thickness and crops out along a belt that extends for 128 kilometers from east to west. The orientation of the outcrop belt approximates depositional strike.

The middle Bloyd sandstone displays a strong contrast in composition with sandstone units in adjacent early Pennsylvanian sandstones. The unit is dominantly composed of polycrystalline quartz with abundant metamorphic rock fragments of phyllite and schist. Granule to pebble sized grains of vein quartz are also abundant. Older sandstone units in the Morrowan section are composed of texturally mature quartzarenite but lack metamorphic rock fragments. Stratigraphically higher sandstone units of the lower Atoka Formation also are composed of quartzarenite, however, no metamorphic rock fragments appear in the section below the middle Atoka interval, approximately 150 meters above the top of the Bloyd sandstone. Paleocurrent data derived from tabular and trough cross strata indicates a transport direction towards the southwest. To understand sediment dispersal patterns and to reconstruct the paleogeography during the early Pennsylvanian in the Midcontinent, a series of outcrop samples of the middle Bloyd sandstone were collected from outcrops in northwest Arkansas. LA-ICP-MS U-Pb detrital zircon geochronology in addition to a detailed sandstone petrographic study are utilized to document and correlate sediment provenance. Preliminary results suggest that the middle Bloyd sandstone is from an extra cratonic source to the north and northeast with a limited local source contribution. The sandstone was emplaced by a south-southwest flowing braided stream system on a near-strand coastal plain.