Sediment Transport and Deposition Under Upper Flow Regime Plane Bed Conditions

Abstract

<p>Upper plane bed is a stable bedform which develops on mobile sand beds and results in the formation of horizontal laminae. An investigation of some characteristics of upper plane bed and its deposits is described in the thesis.</p> <p>"Heavy mineral shadows" are described for the first time and defined as: irrregular patches of heavy minerals, on bedding plane exposures of sandstones (or sands) deposited on an upper plane bed, which are characterized by the presence of an abrupt upstream boundary, with adjacent lighter minerals, and a gradual transition into the lighter minerals in the downstream direction. The directional significance of shadows is shown with examples from an ancient sandstone and from a flume. Flume experiments also showed that shadows wash out with increasing flow strength, suggesting their possible use in paleohydraulic interpretations.</p> <p>Horizontal laminae formed under upper plane bed conditions are precisely described by a detailed examination of textural variations through laminated sands and sandstones as seen in thin section. The data show two types of structure: (1) "textural laminae", including both fining-upward (F.U.) and coarsening-upward (C.U.) laminae less than 1 mm thick, occuring in random sequence and extending generally less than 40 mm in the direction normal to the depositing flow, and (2) laterally extensive "heavy mineral sheets", consisting of layers (often only a few grain diameters thick) in which opaque heavy minerals are abundant in or making up coarsening-upward laminae. The characteristics of textural laminae are proposed to result from their formation by bursts (forming F.U. laminae) and sweeps (forming C.U. laminae) in the flow near the bed. Heavy minerals in sheets, however, are thought to move in patches (shadows?) on the bed largely in response to large eddies in the outer flow. Local erosion of the patches by sweeps incorporates the heavy minerals into C.U. laminae while the patches are resistant to the effects of bursts, so that heavy minerals are absent in F.U. laminae.</p>