THE STRUCTURE OF SEDIMENTARY DEPOSITS OF BRAIDED RIVERS

Abstract

SummaryA description is given of the sedimentary structures and pebble orientations of two braided rivers, the Durance and Ardkhe, whose origin, hydrology, mineralogy, and mechanical composition are already well known.Microtopographic measurements indicate uneven bedding planes. Macrostructures of the deposits show many channel or festoon laminations alternating with bar deposits, showing parallel bedding or oblique, foreset laminations.Oblique laminations (foreset lamination) occur mainly at the downstream ends of the bars along the upstream side of steep concave banks of channels running obliquely to the general valley trend.Pebbles, gravel, and coarse sand dominate over silty deposits which occur as the top layer of the flood plain and occasionally as channel fill. These silty deposits (loams) are very sandy and contain many irregular current ripple marks, distinguishing them from the heavy clay deposits of the flood plain of meandering rivers.Graded bedding is rare. Structures resembling the flute casts and load casts of turbidity currents have been observed but are uncommon. Irregular current ripple marks of horseshoe shape and erosion flow marks parallel to the current direction are common.The average orientation of the long axis of cobbles, pebbles, and coarse sand grains is normal to the current direction. The inclination of flat pebbles is upstream. Points aiding in discrimination between deposits of braided rivers and of turbidity currents are given in Table II.