Horizontal Laminae Formed under Upper Flow Regime Plane Bed Conditions

Horizontal laminae have been examined from the fluvial (?) Whirlpool Sandstone (Silurian), modern intertidal sand bars, and a flume. Textural variations which define laminae were determined by averaging all grain long axes lengths in contiguous rectangular areas, 2 mm wide by 0.2 mm high, up through thin sections cut normal to bedding. "Textural laminae" were identified as fining-upward (FU) and coarsening-upward (CU) from trends seen in plots of average grain long axes versus height above an arbitrary datum. FU and CU laminae occur in random sequence and have an average thickness of less than 1 mm, with CU laminae being slightly thinner than FU laminae. Heavy mineral "sheets," which appeared as continuous layers in outcrop and in trenches, consistently occurred in CU laminae. Lateral correlation diagrams constructed across thin sections showed that some textural laminae were continuous for the 15 mm over which measurements were made while others pinched out laterally across the measured distance. Heavy mineral sheets were continuous across thin sections, maintaining approximately horizontal positions with respect to associated textural laminae. The probable mechanism for the deposition of FU and CU laminae is the burst\ sweep process: (1) FU laminae form by fallout of progressively finer sand which had been temporarily carried upward by a burst, and (2) CU laminae form because of sorting by dispersive pressure in a layer of high grain concentration close to the bed. This layer is produced by locally high shear stress developed beneath a sweep of high speed fluid approaching the bed: deposition of a CU lamina takes place from the layer as the sweep dissipates. Heavy mineral grains in sheets represent relatively coarse lags left behind by migrating patches of heavy minerals that moved in response to high average shear stress associated with large eddies in the flow. The burst/sweep cycle also operates above heavy mineral patches, but the heavy minerals are resistant to uplift by bursts so FU laminae are not generally formed in lag regions. Heavies, and some of the lights underlying the lag, may however be temporarily entrained by sweeps in lag regions, so they are found predominantly in CU laminae.