Antidunes on Modern and Ancient Washover Fans

Abstract

ABSTRACT Antidune deposits on a washover fan on Seabrook Island, South Carolina display a variety of bedding types. The predominant stratification form comprises nested, very thin lenses which are internally cross-laminated and which become thinner and more closely spaced downflow. In addition, cross laminae within the lenses tend toward backset forms in the bermward portions of the flow, and become more foreset-oriented in a downfan direction. These characteristics, together with the antidune field's relationship to other bed features on the fan, suggest that antidune formation is related to the occurrence of an undular hydraulic jump just downfan from the berm crest. Ancient deposits interpreted as washover sequences, including Ordovician and Pleistocene sandstones in South Africa and a Carboniferous sandstone in Kentucky, display sedimentary structures similar to those observed in the Seabrook antidune field. Because washed-out antidunes and their associated cross laminae are very subtle features, antidunes are possibly responsible for many of the gentle irregularities in otherwise flat beds and are therefore possibly far more common in the geologic record than previously recognized. Their sedimentary structures are distinct from degraded forms of ripple cross-lamination, and in many cases are identical to features usually described in the literature as low-angle truncation surfaces.