602 SEDIMENTARY PROCESSES/Post-Depositional Sedimentary Structures Rocks: Rudaceous Rocks) Where gravel moves as a traction load, on a stream bed or on a beach, it may become lodged on the bed, particularly where clasts land amongst similar sized clasts If the clasts are flattened, they will tend to be deposited in an imbricated fabric with an upstream dip This is the most stable position beneath the prevailing flow and, when seen in the rock record, ‘imbrication’ (Figure 16) is a valuable palaeocurrent indicator Gravel is also deposited in bars on river beds and these commonly have slipfaces at their downstream ends Cross-bedding results from the avalanching that takes place there Depending on the overall sediment population, sandy and gravel-rich foresets may be intermixed See Also Sedimentary Environments: Depositional Systems and Facies; Lake Processes and Deposits Sedimentary Processes: Erosional Sedimentary Structures; Post-Depositional Sedimentary Structures; Aeolian Processes; Particle-Driven Subaqueous Gravity Processes Sedimentary Rocks: Rudaceous Rocks Trace Fossils Unidirectional Aqueous Flow Further Reading Allen JRL (1982) Sedimentary Structures: Their Character and Physical Basis Developments in Sedimentology 30A & B, pp 593, 663 Amsterdam: Elsevier Allen JRL (1985) Principles of Physical Sedimentology ch London: Allen & Unwin Brookfield ME and Alhbrandt TS (eds.) (1983) Eolian Sedi ments and Processes Developments in Sedimentology 38, p 660 Amsterdam: Elsevier Collinson JD and Thompson DB (1989) Sedimentary Struc tures, 2nd edn., ch London: Chapman & Hall Leeder MR (1982) Sedimentology; Process and Product, ch London: Allen & Unwin Leeder MR (1999) Sedimentology and Sedimentary Basins, ch 12 Oxford: Blackwell Science Mutti E (1992) Turbidite Sandstones Milan: Agip Pettijohn FJ and Potter PE (1964) Atlas and Glossary of Primary Sedimentary Structures Berlin: Springer Pye K and Lancaster N (eds.) (1993) Aeolian Sediments Ancient and Modern Special Publication of the Inter national Association of Sedimentologists 16, p 167 Oxford: Blackwell Scientific Publications Reineck HE and Singh IB (1980) Depositional Sedimentary Environments, 2nd edn Berlin: Springer Selley RC (2000) Applied Sedimentology, 2nd edn., Ch San Diego: Academic press Post-Depositional Sedimentary Structures J Collinson, John Collinson Consulting, Beech, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Primary sedimentary structures and lamination in clastic sediments result from deposition under different hydrodynamic regimes However, there are other secondary structures, which originate during or very soon after deposition as a result of sediment instability, extreme climatic effects, or chemical changes during burial The last group involves the precipitation of mineral cements within the sediment Some of the early physical structures deform primary depositional structures and lamination, whilst others originate entirely as a result of the deformation processes Those associated with cementation often help to preserve original lamination and other delicate features, particularly in fine-grained sediment, that would otherwise have been destroyed by compaction Soft-Sediment Deformation Processes Early formed soft-sediment deformation structures are typically features of sandstones and finer sediment Deformation in gravels and conglomerates is rare, except in glacial and periglacial settings, or where there is abundant muddy matrix The structures have an entirely physical origin and must be distinguished from organic deformation structures, such as burrows and trails of animals and disturbance by plant roots (see Trace Fossils) In addition, tectonic structures are often geometrically identical with syndepositional structures, and the boundaries between widespread tectonic deformation and more local, sediment-induced deformation may be blurred Deformation structures are quite difficult to classify as they result from rather diverse processes However, all attempts at interpreting physical soft-sediment deformation structures must consider two discrete questions First, why did the sediment lose its strength and become susceptible to deformation? Second, what forces operated on the weakened sediment to cause the deformation? In most cases, the sediment loses frictional shear strength and behaves, briefly, as a plastic or a fluid In other cases, a loss of tensile strength can lead to brittle deformation as soft-sediment faults Loss of frictional shear strength may result in total remobilization of the sediment and transformation into a gravitational