ABSTRACT

Since the fining-upward cycle model for meandering-river deposits was published in 1963, about a dozen facies models have been defined for rivers of varying sediment grain size, channel multiplicity and sinuosity. However, this suite of models is inadequate for the interpretation of many fluvial deposits because it does not incorporate the effects of a variety of outside-the-basin (allocyclic) sedimentary controls. The most important of these controls is tectonics, because differential movement between basin and source area, and changes in the rate of movement, can cause major changes in fluvial sedimentary environments. These changes give rise to fining- or coarsening-upward cyclic sequences which are superimposed on the cycles generated by within-basin (autocylic) mechanisms.

Variations in climate can cause variations in sediment grain size and in depositional style as a result of alterations in weathering modes and discharge characteristics. These latter variations have been studied in hot arid, warm temperate and fluvioglacial regimes, but are poorly known in other climatic zones. As a result, many of our preconceptions about fluvial geomorphology and sedimentation may be inappropriate for other climatic conditions such as hot, humid environments.

Channel morphology varies between four basic styles: braided, meandering, anastomosed and straight, but the sedimentological characteristics of intermediate morphological types have yet to be quantified. Many pre-Carboniferous rivers appear to have lacked clearly defined channels with cutbanks. The probable cause was low bank cohesion caused by an absence of land vegetation, a condition that cannot be modelled accurately from observations of modern rivers. The absence of vegetation would have favoured a flash-flood depositional process, which tends to generate sheet-like deposits rather than lenticular, channel-bound units.

Sedimentological studies in modern rivers are beset by the problem of preservability, in that a river's long-term aggradation behaviour is commonly unclear. The problem is compounded by the fact that many studies have been carried out on rivers that are actively degrading.

Cyclic sequences in fluvial sediments can be produced by a variety of mechanisms, and the interpretation of a given deposit solely in terms of a published facies model may be misleading. For most fluvial sequences it is necessary to adopt a multivariate "basin analysis" approach.