HANN, Crystal L., and CASTLE, James W.,jcastle@clemson.edu,
 Department of Geological Sciences, Box 340976, Clemson University, Clemson, SC 29634

Detailed sedimentological analysis indicates that various shelf, shoreline, and terrestrial environments are represented by the Dry Branch, Tobacco Road, and Altamaha Formations in the vicinity of the Savannah River Site, South Carolina.  Sedimentological investigation of these formations provides information that is not available paleontologically because of the scarcity of organic remains due to oxidizing conditions in the updip passive-margin depositional setting.  The interpretations of this study are based on core descriptions from 16 wells and examination of nearby surface exposures.
 The Irwinton Sand Member of the Dry Branch Formation, which overlies a sequence-bounding unconformity, is well sorted coarse sand that grades downdip to fine sand.  The depositional environment is interpreted as predominantly shoreface.  Within this member, grain size decreases upward to a laterally extensive clay bed, which represents maximum flooding during transgression.
 The overlying Tobacco Road Formation is moderately to well sorted, very fine to medium sand that initially coarsens upward above transgressive clay.  The upper part of the formation fines upward and contains bi-directional cross-bedding, ripples with clay drapes, interlaminated clay, and burrows, which suggest tidal influence.  The overlying Altamaha Formation is poorly to very poorly sorted argillaceous sand containing quartz granules, pebbles, and mud intraclasts.  This formation becomes finer grained upward above a basal erosional surface and is interpreted as a fluvial-channel deposit.
 The stratigraphic pattern indicates that initial transgression was followed by regressive progradation of the Tobacco Road and Altamaha Formations.  The overall succession thickens downdip in response to increased accommodation space, which along with lateral facies changes helps to explain the stratigraphic architecture of the interval studied, including the Griffins Landing Member of the Dry Branch Formation.  This member, which occurs downdip from the Irwinton Sand Member and directly above the lower sequence boundary, consists of fossiliferous limey clay with only minor sand.  The Griffins Landing Member represents transgressive to highstand shelf deposition, and its occurrence can be explained by the effect of relative sea-level change combined with increased accommodation space related to configuration of the passive-margin shelf.