Clastic facies models, a personal perspective

Abstract

Facies models evolved from classifications that were mainly descriptive, based on observable, measureable features such as the composition and texture of sedimentary rocks. As our understanding of sedimentary processes expanded, genetic facies models were developed based on the inferred process of formation. Since individual facies cannot be interpreted in isolation, they must be studied with reference to their neighbours, emphasizing the association of facies and sequences, in particular those that coarsen and fine upward. Environmental facies models are based on the interaction of studies on modern environments and ancient rock facies. Earlier facies models tended to invoke intrinsic, autocyclic controls. The advent of sequence stratigraphy led to greater emphasis on the surfaces that separate sequences and to external allocyclic controls. These were, initially, sea-level changes; later, changes in climate, tectonic movements and sediment supply were invoked. Over time, simple, all embracing models have given way to increasingly complex ones as our knowledge of the variability of nature has increased. Complex though these models are, they are only simplifications of reality. In nature there are no models and the majority of past environments differed in some respect from any modern environment. Each environment and rock sequence is unique.