486 SEDIMENTARY ENVIRONMENTS/Depositional Systems and Facies the physical properties of the sediments Grain size is a common starting point for subdivision Not only dominant grain size but also secondary features, such as sorting, might be considered Any differences in composition would also be used for primary subdivision A subdivision based on grain size may be refined by considering sedimentary structures (e.g styles of lamination), so that facies such as ‘finegrained, ripple cross-laminated sandstone’ or ‘horizontally laminated siltstone’ might result Facies defined in this way are called lithofacies, and they can commonly be interpreted in terms of depositional processes by applying knowledge of the relationships between hydrodynamics, particle size, and sedimentary structures However, many depositional processes occur in a range of sedimentary settings, and, therefore, few lithofacies are fully diagnostic of depositional environment Other features, formed soon after deposition, permit more refined facies schemes Bioturbation by animals or plant roots can provide clues about the environment, and highly distinctive facies such as coal may also allow a specific environment of deposition to be deduced Where burrowing is important, the assemblage of burrow types and the intensity of burrowing may necessitate schemes of ichnofacies Early-formed concretions and textures in palaeosols can indicate the drainage conditions of emergent surfaces soon after deposition and can lead to pedofacies If sediments are fossiliferous, the types and diversities of the fossils may lead to biofacies, and consequent facies analysis may incorporate palaeoecological considerations Analysis of very fine plant debris or spores may lead to palynofacies For carbonate sediments, the nature of the constituent grains is important in establishing the depositional setting, and their characterization through petrographical description may lead to microfacies In complex sequences, combinations of these different approaches may be appropriate Where two contrasting lithologies are interbedded, it may be appropriate to recognize event deposition (Figure 1) In many successions of interbedded muds and sands, sand beds commonly have sharp bases and may show internal grading and vertically changing styles of lamination They were clearly deposited by sudden high-energy events in otherwise low-energy settings Establishing the nature of the events may depend on interpreting quite subtle features of both the sand beds and the finer interbeds Where the muddy component lacks evidence of higher energy and the sands show only current-generated structures and lamination, the events are likely to be turbidity currents in a deep-water setting If the finer sediments are strongly bioturbated and show signs of Figure Interbedded sandstones and mudstones showing the distinction between the quiet background sedimentation of muds and the high energy events that delivered the sands In this case, the high energy events were turbidity currents in a deep water marine setting Paleocene, Zumaia, Spain fluctuation energy and the sand beds show evidence of wave action, perhaps reworking their tops, then storm events on a shelf may be the appropriate inference If the finer sediments show evidence of emergence, with soil textures, and the sands are current dominated, then river floods (crevasse events) on a floodplain may be the appropriate interpretation In all these cases, it would be valid to regard the finer and coarser components as separate facies, but it may prove practical to use composite facies based on the thicknesses and proportions of the coarse and fine components This approach is particularly applicable to turbidite successions Facies Analysis Most facies defined according to the criteria outlined above may be interpreted in terms of processes of deposition, but these are seldom unique to a