Structural Studies of Sedimentary Deposits

Abstract

The apposition fabric in different sedimentary deposits is discussed from genetic and structural points of view. The preferred orientation of most particle sizes, transported in contact with a frictional substratum, is usually transverse to the direction of transport. Particles immersed in the transporting medium, such as glacier ice, water, gravitating masses etc., tend to align themselves parallel with the direction of movement, owing to the shearing stress of the moving medium. Heavy collisions or obstacles to the particles disrupt their trajectories and final position.

The actual dip of flattened particles in fluvial imbrication beds, calculated graphically and mathematically or measured directly against a standing water level, indicates the direction of transport more adequately than the long-axis orientation of prolate particles. The fabric has a typical monoclinic symmetry, if the bed and flow geometry is not asymmetric. The preferred dip is between 10° and 30° to the flow direction in the observed imbrication pavements of flat pebbles and cobbles. The variations are probably due to particle shape and size and to different hydrodynamic conditions.

In the subaqueous glaciofluvial and wave-built foreset beds investigated, the preferred orientation of pebbles and cobbles generally coincides well with the dip direction of the strata, representing a monoclinic fabric. Deviations from this alignment are thought to result from the influence of a swift backflow up the distal slope of the delta. The inclination of the stones is usually 10–20° less than that of the bed, probably owing to special shearing conditions.

The spread of particle dip is apparently due to the frequency of heavy collisions and contact between the fragments. The necessity of observations under controlled laboratory conditions is stressed upon.