Instantaneous sediment deposition rates deduced from climbing-ripple cross-lamination

Abstract

A mathematical analysis is made showing how the geometrical properties of climbing-ripple cross-lamination, a common sedimentary structure in fluviatile and turbidity current deposits, depend on the ripple height and the prevailing rates of sediment deposition and bed load transport. This structure is very commonly found in the field in vertical association with parallel laminated sand-grade deposits. The orientation of the ripple structures and of the parting lineations and[or grain fabric associated with the parallel laminae can be accounted for only if the two kinds of deposit were formed during consecutive stages in the change of a single current. Because the hydraulic conditions attending the transition between current ripples (giving cross-lamination) and a plane bed (giving parallel laminations) are known, the sediment deposition rate at the moment of transition can be calculated, from field measurements of ripple height, angle-of-climb and sediment size. Examples of deposition rates calculated in this way are discussed from the Old Red Sandstone and the Aberystwyth Grits of Wales.