ABSTRACT
The term facies, derived from a Latin (also French) word, implies a facial expression characteristic of a particular condition. In modern sedimentology the term is used as an expression for ‘the total field aspect of the rocks themselves …. The key to the interpretation of facies is to combine observations made on their spatial relations and internal characteristics (lithology and sedimentary structures)…’ (Middleton 1978a). The process of construction of a facies model for an environment involves identification of the diagnostic or common characters of the particular environment. This is done by studying a large number of modern and ancient situations and then formulating general principles about that environment after elimination of the local details. The purpose is to obtain ‘a norm for comparison, a guide for future observation and a predictor for new geological situations’ (Walker 1984a). With this end in view, considerable effort has been made during the last few decades to collect information about the modern environments of sedimentation. The following pages summarise the present-day knowledge of the more important environments—marine, non-marine and mixed. A genetic explanation for an ancient phenomenon should come by way of analogy with these modern environments.
