Description and interpretation of fluvial deposits: a critical perspective



A critique is given of recent methods proposed for the standardized description, classification and interpretation of fluvial deposits in terms of: (1) hierarchies of strata and their bounding surfaces; (2) lithofacies; (3) lithofacies associations (architectural elements); (4) geometry of sedimentary bodies. Much of the descriptive information can be conveyed using clear, explicit diagrams without recourse to complicated terminology. Any classification that is used should be based on easily measurable parameters which are used to define mutually exclusive classes. Terms used to refer to these classes should be explicit. Little agreement exists on the best terms to describe strata, mainly because existing terminology was not designed to account for all the different superimposed stratal scales. A simple terminology is suggested here whereby the relative scales of hierarchies of strata and cross-strata are specified. Methods of describing hierarchies of strata by numerically ordering stratal bounding surfaces (rather than the strata themselves) are difficult to use in practice. The use of standardized lithofacies codes tends to discourage close observation and recognition of varieties and superpositions of lithofacies, has led to proliferation of acronyms, and to the suggestion that a particular lithofacies has a unique interpretation. In fact, many interpretations associated with these lithofacies codes are misleading or wrong. In a commonly used classification system of lithofacies associations (architectural elements) the different classes are not mutually exclusive, they are referred to using a mixture of descriptive and interpretive terms, and each element is represented graphically by only a single two-dimensional section. Terms used to describe the three-dimensional geometry of a sediment body should only be used to refer to two-dimensional sections if three-dimensional form can be reconstructed unambiguously. Fluvial ‘facies models’ are constructed to relate lithofacies associations and geometry to parameters such as channel and floodplain geometry, modes of channel migration and deposition rates. Most are of limited use or misleading because insufficient three-dimensional information is shown. In addition, they commonly incorrectly associate channel planform (e.g. meandering, braided) with a characteristic lithofacies association and geometry, whereas other factors may play a dominant role.