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

Abstract

The Catoctin Formation infills a Neoproterozoic-age rift basin associated with the development of the Laurentian continental margin, one of several similar basins that are distributed along the length of the southern to central Appalachian Basin. The Catoctin Formation consists of a flood basalt sequence with intercalated sedimentary deposits. Two types of architectural elements, channel element (CE) and sheet element (SE), are recognized within these sedimentary deposits. Massive (structureless) sandstones and conglomerates are the dominant facies within the CE and SE and are interpreted to record deposition from hyperconcentrated flows. Lack of vegetative binding of sediment, coupled with a relatively high stream gradient and possibly a high water table enhanced the probability of developing hyperconcentrated flows by the process of bulking (addition of sediment to a flow). Additional facies present are the products of normal stream flow that developed from waning hyperconcentrated flows. Basaltic effusive rate, sediment supply, and extension (subsidence), controlled the generation of CE and SE. All CE reflect deposition in isolated fluvial channels incised into lava flows and are present within the lower member of the Catoctin Formation. The presence of CE most likely indicates a high rate of lava extrusion, low sediment supply, and possible high subsidence rates. The SE, composing the sedimentary member of the Catoctin Formation, reflects a decrease in effusive and subsidence rates and a increase in sediment supply from the hinterland. Overlying the SE, is the last phase of basin infill characterized by basalts with minor interbedded clastic rocks. The final phase of infilling was effected by an increase of effusive and subsidence rates and a potential cutoff of sediment supply. The Catoctin Formation basin displays a more complex infill history than generally associated with the rift deposits of the Laurentian margin.