Physiography of bed-load meandering streams 2: deposition of channel-phase sands and the dispersion of ripple- and dune-generated cross-lamination

Abstract

In mountain meandering streams sand is transported as bed-load mostly in local areas confined to large separation bubbles. Separation of flow takes place on a large scale in bank niches and at the inner banks of meander bends just downstream the axis of bend. In such areas channel sand bars accumulate owing to sediment discharge in the form of dunes, dunes with ripples superimposed, and ripples. The direction of movement of the bedforms is controlled by rather a complex flow system existing in a separation bubble and - in so being - is largely unrelated, to local (paleo) slope. Systematic field measurements of ripple-, dune-, and bar slip-faces as well as bar stoss-sides have demonstrated that: (1) Almost all currents in a separation bubble may ripple sand and the dispersion in dip directions of ripple slip-faces is the greatest with mean dip of 23° and standard deviation of 7.5°; (2) Countercurrents in sand-bottom separation bubbles are too weak to produce typical backflow dunes. Consequently, dune slip-faces dip mostly bankwards or obliquely upchannel at a mean angle of 27° (σ = 7°); (3) Bar slip-faces dip either bankwards or upchannel with mean angle of dip similar to that for dunes; and (4) Bar stoss-sides dip channelwards at a mean angle of 15° (σ = 6°). The measurements have been also analyzed using cumulative contour diagrams and „current roses” in order to visualize the distribution of slip-face dip directions (i.e., azimuths of dip of cross-lamination) in relation to a local downchannel direction. The resultant current roses are strikingly bimodal with the least number of readings in the direction of flow and grand vector means poiniting upchannel (= upslope). It has been found that preservation potential of such channel sand bars is fairly good and they were commonly observed both in the Quaternary and the Lower Carboniferous fluvial deposits of the Central Sudetes. Generally, channel-phase sands are typically associated with bar-platform gravels, i.e., lower portion of a point bar sequence between meander pool and vegetated supraplatform bar. Deposits of similar origin may also occur embedded in fine-grained overbank alluvia 0.