Formation of scalloped cross-bedding without unsteady flows

Abstract

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 bedforms, but scalloped cross-bedding also originates by a process that does not require fluctuating flow: migration of small bedforms across the lee slopes or along the troughs of larger bedforms. Intersections of the troughs of the two sets of bedforms form topographically low scour pits, and cyclic passage of these scour pits through the outcrop plane--the plane that later becomes an outcrop surface--causes the lower-set boundary to rise and fall. Scalloped cross-bedding formed by fluctuating flow superficially resembles that formed by superimposed or intersecting bedforms, but, as illustrated in three-dimensional computer plots, the two kinds of structures commonly can be distinguished by directional properties of the bedding. Scallops deposited by alongslope-migrating, superimposed bedforms have cross-bed and bounding-surface dip patterns that lack bilateral symmetry and have cross-bed dips that are asymmetrically distributed relative to bounding-surface dips. Scallops with dip patterns that are bilaterally symmetrical and with cross-bed dips that are symmetrically distributed relative to the bounding-surface dips can be produced either by fluctuating flow or by downslope or upslope migration of superimposed bedforms. 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 and directional properties of the bedding suggest that the deposit was produced by a series of small bedforms migrating offshore, down a rip channel that was bounded on one side by a migrating oblique bar.