SEDIMENTARY ENVIRONMENTS/Depositional Systems and Facies 485 SEDIMENTARY ENVIRONMENTS Contents Depositional Systems and Facies Alluvial Fans, Alluvial Sediments and Settings Anoxic Environments Carbonate Shorelines and Shelves Contourites Deltas Deserts Lake Processes and Deposits Reefs (‘Build-Ups’) Shoreline and Shoreface Deposits Storms and Storm Deposits Depositional Systems and Facies J Collinson, John Collinson Consulting, Beech, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction The concept of facies as applied to sediments is a means of classifying and grouping deposits in such a way that objective differences, usually with genetic significance, are highlighted In present-day settings, the relationships between processes and environments are self-evident, but in sequences of ancient sediment various analytical procedures are needed to reconstruct environments ‘Facies analysis’ is such a suite of techniques and is based on an assemblage of facies and the relationships of the facies to one another The results of facies analysis may be expressed as facies models, which summarize and, to some extent, idealize the relationships and allow the facies of a stratigraphical interval to be predicted in areas where it cannot be directly observed Facies analysis also provides a major input to ‘sequence stratigraphy’ (see Sequence Stratigraphy), whereby facies successions are placed within a stratigraphical framework defined by key stratal surfaces that are related to changes in relative sea-level or base-level Facies The concept of facies dates back to the early nineteenth century, when A Gressly introduced the term to summarize those descriptive features of a rock that distinguish it from other rocks Historically, the concept has been applied at a wide range of scales and, therefore, with varying significance At the largest scale, ‘facies’ has been applied to major stratigraphical units in particular tectonic contexts ‘Flysch’ and ‘molasse’ are examples of this and are used particularly in the description of the Tertiary fold belts of Europe At a large scale, ‘facies’ is also used to describe regional differences of depositional environment within a particular stratigraphical interval For example, in Britain, one might contrast the ‘basinal facies’ of the Silurian of Central Wales with the coeval ‘shelf facies’ of the Welsh Borders These usages are generally accepted and understood but, in order to arrive at an interpretation of the depositional environment, it is necessary to go through a process of facies analysis This involves establishing an appropriate facies scheme for the succession being studied and then analysing the spatial relationships of those facies Facies schemes are established using a variety of physical, chemical, and biological criteria, and there is no such thing as a universally applicable facies system Different sedimentary successions demand different schemes because of their contrasting character or because the intended end products of facies analysis may differ A simple facies scheme may suffice for a rather basic interpretation, whilst a more elaborate scheme may be appropriate for a more refined interpretation The facies scheme therefore depends on the scale and detail of the study In successions of dominantly clastic sediments, a facies scheme would commonly be based largely on