Tidally influenced barrier island and estuarine sedimentation in the upper carboniferous of southern West Virginia

Abstract

Upper Carboniferous orthoquartzites exposed in the Guyandotte Valley of southern West Virginia are interpreted as barrier island and backbarrier deposits. They overlie fossiliferous marine limestones and shales, and are overlain in turn by fluvio-deltaic subgraywacke sandstones, siltstones and coals. Only the upper one-third of the 180 m thick orthoquartzitic interval is exposed, but these rocks outcrop over a distance of 16 km, and reveal landward transition of the barrier orthoquartzites into backbarrier and alluvial or delta-plain deposits. Within the exposed orthoquartzitic interval are three major orthoquartzite bodies, separated by subordinate siltstones containing smaller sandstone bodies. The major orthoquartzites are 10–25 m thick and are lenticular in transverse section, with a north-south elongation approximately perpendicular to the palaeoslope. These sandstone units are subdivisible into three facies. Facies 1 is most widespread and comprises mainly upward-thinning sets of cross-stratification, with a silty interval occasionally preserved towards the top. Palaeocurrent indicators reflect increasing dispersal from the base of the sequence upward. These deposits are interpreted as originating within laterally migrating tidal inlets, in which ebb currents were dominant. Facies 2 contains low-angle bedding suggestive of wave swash processes, with patterns of minor sedimentary structures similar to those observed in modern ebb tidal deltas. Facies 3 consists of sandstone with a preponderance of landward-directed cross-stratification, and is attributed to a flood tidal delta, flood-channel and washover berm origin. The orthoquartzites merge eastward and southeastward (landward) into inferred estuarine or lagoonal siltstones, but in places they are in lateral contact with fluvio-deltaic subgraywackes. Thus the barrier sediments are thought to have been derived at least in part by the reworking of abandoned delta lobes. Longshore drift created spit extension towards the south and southwest, thereby enclosing lagoons within which accumulated siltstones and sandstones with brackish or marine fossils and a variety of structures suggestive of tidal processes and local subaerial exposure. Washover sands were deposited to landward of the barrier. Both barrier and backbarrier deposits are incised by small tidal channels. Occurrence of washover features in association with small but well-developed tidal deltas suggests moderate to strong wave activity and a mesotidal (2–4 m) range.