THE UPPER MISSISSISSIPPIAN (CHESTERIAN) SEDIMENT SOURCE CONUNDRUM SOUTHERN OZARKS, NORTHERN ARKANSAS

The Upper Mississippian Series in the southern Ozarks, northern Arkansas comprises the Moorefield, Batesville-Hindsville, Fayetteville-Wedington, and Pitkin-Imo Formations (ascending order). They represent a third-order transgressive-regressive cycle bounded by type 1 unconformities. The source and the depositional environment for each are problematic and unanswered at best. The Batesville and the Imo (Upper Mississippian) thin gradually westward from their type areas, indicating an easterly source. The Batesville Sandstone becomes calcareous and the Imo shaly as the sandstones progressively thin westward. In contrast, the Wyman and Wedington sandstones thin eastward and become calcareous and shaly respectively, suggesting a westerly source. The Batesville and the Wyman sandstones have historically been interpreted as deltaic deposits. However, the absence of coals, channel sandstones, paleosols, land-plant fossils, and the presence of marine faunas suggest otherwise. In contrast, the Wedington does exhibit some of those characters, and is regarded as a constructional delta. The source of the Moorefield and Fayetteville shales, the equivalent of the Stanley Shale in the Ouachitas, is equally problematic. The Moorefield Shale represents a lowstand wedge, and the Fayetteville, records transgression, maximum flooding, highstand and initial regression for the Chesterian cycle. However, the general thinning of both units toward the west and north suggests a southeasterly source, perhaps Gondwana, which is closing on Laurasia in the Upper Chesterian. Sandstone source is much more difficult to place. Lower Mississippian platform carbonates covered most of the North American Midcontinent, and did not allow recycling of earlier first or second cycle terrigenous clastics from the craton.