Influence of superimposed bedforms and flow unsteadiness on the formation of cross strata in dunes and unit bars.

摘要

Cross stratification formed by dunes and unit-bars is the most common sedimentary structure in river-channel deposits, and is common in many other depositional environments. The grain-size variation that allows cross strata to be recognized is determined by three main factors: (1) presorted sediment that arrives at the brink point of the bedform, related to superimposed bedforms and longer-term unsteadiness in water flow and sediment transport; (2) sorting due to differential deposition of sediment on the bedform lee slope and associated grain flows, and; (3) movement of sediment on the lee side by water currents in the lee-side flow separation zone. Superimposition of bedforms (e.g., small dunes on large dunes, or dunes on unit bars) is ubiquitous, and the arrival of a superimposed bedform at a host-bedform crest results in a temporary change in flow dynamics, sediment transport, bedform migration, and grain-size sorting of the sediment that is deposited on the lee slope. The arrival of a superimposed bedform with a height exceeding 25% of the host bedform height reduces the host bedform lee slope and forms a low-angle reactivation surface. A superimposed bedform with height smaller than 25% of the height of the host bedform does not reduce the host lee slope and forms angle-of-repose cross strata. The geometry and grain-size sorting of cross strata formed by superimposed bedforms can be used to interpret the geometry and grain-size sorting of the host and superimposed bedforms, hence flow and sediment-transport conditions on the back of the host bedform. Longer-term flow unsteadiness (i.e. floods) also causes changes in the transport rate and grain size of the sediment supplied to the host-bedform lee slope, but is primarily expressed as a change in superimposed bedform type.