Abstract

An array of three-dimensional bedforms is present on bedding planes within a 20-35-cm-thick medium-grained sandstone unit in the braided-fluvial South Bar Formation (Pennsylvanian), near Sydney, Nova Scotia. The unit is near the top of a 4.7-m-thick channel sandstone and is attributed to a late stage of fill prior to the abrupt abandonment of the channel. The bedforms comprise radially symmetric mounds and intervening curvilinear hollows, and they contain concordant strata. They closely resemble hummocks and swales but are identified as three-dimensional antidunes based on their fluvial setting and the ubiquitous presence of well-defined current lineation, oriented parallel to unidirectional paleocurrent indicators within the channel fill. The antidunes secreted vertically in paleoflows with mean depth and velocity of about 0.12 m and 1.0 m/s. Antidune bedforms are rare in the stratigraphic record, because such conditions are seldom maintained long enough to accrete a preservable set of strata. Three-dimensional antidunes such as these could easily be mistaken for hummocks and swales and the environment misidentified as storm-influenced if the nature and relationships of current lineation were not observed, or if incomplete exposure did not permit recognition of shallow-water origin.

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