Upper Devonian Turbidites of Central and Southern Appalachian Basin--A Prodeltaic Clastic Ramp?: ABSTRACT

Abstract

The Brallier Formation (Upper Devonian) of the central Valley and Ridge province is a thick (600 to 900 m) regressive sequence of distal to proximal turbidites composed of interbedded siltstones, olive-gray mudstones and shales, and organic-rich black shales. This sequence is transitional westward to the thinner, distal, Devonian black shale facies. Regional and vertical patterns in sedimentologic features differ from those of most models for turbidite sedimentation. The uniformity of turbidite bed thickness, implying a triggering mechanism of uniform intensity, and the absence of slump structures in the proximal facies suggest that turbidity currents were initiated by means other than localized mass movement. Storm surges or high river discharges are more likely mechanisms. The Brallier depositional sequence differs significantly from existing submarine-canyon-fan models in End_Page 488------------------------------ that it lacks large-scale radial dispersal patterns as well as canyon and channeled inner-fan facies. Rather than radial progradation, characteristic of a large, stable submarine fan, uniform progradation from multiple point sources built a clastic ramp composed of a coalescing series of small, short-lived submarine fans. This uniform progradation is indicated by the disposition of lithofacies and consistent westerly paleocurrent trends. We envision a prodeltaic setting for the formation of this clastic ramp. In spite of a paucity of modern analogs for such a depositional system, the Brallier Formation and other ancient examples attest to the significance of turbidite sedimentation in deltaic settings. End_of_Article - Last_Page 489------------