Formation of Nearshore-Marine Scalloped Cross-Bedding

David M. Rubin, H. Edward Clifton

Scalloped cross-bedding (compound cross-bedding with internal bounding surfaces that cyclically scoop into the previously deposited foresets and into the sediment below the set) is a common and distinctive structure in eolian, fluvial, tidal, and nearshore-marine sands. Scalloped cross-bedding in shallow marine deposits previously has been interpreted to be produced by cyclic flows, such as neap-spring tidal flows, which are known to cause cyclic fluctuations in the depth of scour in the troughs of migrating bed forms. However, scalloped cross-bedding also originates by a process that does not require fluctuating flow: migration of small bed forms across the lee slopes or along the troughs of larger bed forms. Intersections of the troughs of the two sets of bed forms form topographically low scour pits, and cyclic passage of these scour pits through the plane that later becomes an outcrop can cause the lower set boundary to appear to rise and fall with distance downcurrent. Scalloped cross-bedding formed by fluctuating flow superficially resembles that formed by superimposed intersecting bed forms. But, as illustrated in three-dimensional computer plots, the two kinds of structures commonly can be distinguished by details of the bedding.

An example of nearshore-marine scalloped cross-bedding of Pleistocene age was examined in detail in a coastal terrace of Monterey Bay, California. The three-dimensional structure of the bedding indicates that the deposit was produced by a series of small gravel bed forms migrating offshore, along the base of the lee slope of a larger sandy bed form that was migrating dominantly alongshore. This assemblage of bed forms is dynamically and geographically compatible with the interpretation that deposition occurred within a rip channel bordered on one flank by a migrating oblique bar.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.