Characterization of fluvial deposits interbedded with flood basalts, Neoproterozoic Catoctin Formation, Central Appalachians, USA

The Catoctin Formation infills a Neoproterozoic-age rift basin associated w ith the development of the Laurentian continental margin, one of several si milar basins that are distributed along the length of the southern to centr al Appalachian Basin. The Catoctin Formation consists of a flood basalt seq uence with intercalated sedimentary deposits. Two types of architectural el ements, channel element (CE) and sheet element (SE), are recognized within these sedimentary deposits. Massive (structureless) sandstones and conglomerates are the dominant facie s within the CE and SE and are interpreted to record deposition from hyperc oncentrated flows. Lack of Vegetative binding of sediment, coupled with a r elatively high stream gradient and possibly a high water table enhanced the probability of developing hyperconcentrated flows by the process of bulkin g (addition of sediment to a flow). Additional facies present are the produ cts of normal stream flow that developed from waning hyperconcentrated flow s. Basaltic effusive rate, sediment supply, and extension (subsidence), contro lled the generation of CE and SE. All CE reflect deposition in isolated flu vial channels incised into lava flows and are present within the lower memb er of the Catoctin Formation. The presence of CE most likely indicates a hi gh rate of lava extrusion, low sediment supply, and possible high subsidenc e rates. The SE, composing the sedimentary member of the Catoctin Formation , reflects a decrease in effusive and subsidence rates and a increase in se diment supply from the hinterland. Overlying the SE, is the last phase of b asin infill characterized by basalts with minor interbedded elastic rocks. The final phase of infilling was effected by an increase of effusive and su bsidence rates and a potential cutoff of sediment supply. The Catoctin Form ation basin displays a more complex infill history than generally associate d with the rift deposits of the Laurentian margin. (C) 1999 Elsevier Scienc e B.V. All rights reserved.