bed

depositional surface

graded beds

ichnofacies

ichnofossils

The sedimentary structures found in sedimentary rocks are important keys to the interpretation of their “depositional setting”, and provide critical versus general clues as to the depositional setting of the sedimentary rocks in which they occur. They are features found within the sedimentary section, and/or on, and/or between, bedding plane surfaces subdividing that section. They have characteristic fabrics or features formed at the surface of beds of sedimentary rock (bedforms) that often extend from the surface to within the rock. sedimentary structures are formed in response to the processes that deposited the sediment (primary sedimentary structures) or modified them during or following deposition (secondary sedimentary structures). Primary sedimentary structures provide critical information on the depositional setting of the sediment and may be the result of in situ physical, biogenic, or chemical processes.

Physical processes responsible for bedforms and associated layering and cross-stratification include the wind, water driven currents and waves. These bedforms commonly migrate across the sediment surface forming the cross-stratification common to sandstones. Both current and wave produced bedforms are extremely diverse but are valuable indicators of depositional process, water depth, current velocity, and current. Some develop in quiet water low energy conditions, and others form in moving water or high energy conditions. sedimentary structures associated with physical processes are related to scale and hierarchy of features they occur in, whether in sediments that have been confined (as in a channel) or unconfined settings (as on a shelf), & associated but similar sized structures.

Structures vary from plane bedding, graded beds, imbricate bedding, bedforms and cross-beds generated by unidirectional currents, bedforms and cross-beds generated by multidirectional flow currents, bedforms and cross-beds generated by waves.

The inter-relationships of physical processes active in depositional and/or erosional setting are responsible for the grain size of the sediments. As Hjulstrom & Sundborg have shown a critical current velocity is required to move sediment of a specific grain size for a fixed water depth, while sediment entrainment is also found to be dependent on sediment cohesion and consolidation

Post depositional events include: hydrations/dehydration; compaction and chemical signals. For instance deformational sedimentary structures form when soft, highly water saturated sediment undergoes deformation before lithification. These structures include convolute bedding, flame structures and load casts. In contrast biogenic sedimentary structures are expressed by the modification of depositional fabric by burrowing and/or other organisms at and post deposition by biological activity and include trace fossils, Stromatolites, escape structures and reefs. Finally chemical sedimentary structures reflect the setting in which they form by chemical processes including oxidation-reduction, precipitation, evaporation, etc. Examples include stylolites, concretions, evaporites, physico chemical carbonate grains including pisolites, ooids, and grapestons, etc.

Reference
Allen, J. R. L. 1983, Studies in fluviatile sedimentation: bars, bar complexes and sandstone sheets (low sinuosity braided streams) in the Brownstonews (L. Devonian), Welsh Borders. Sedimentary Geology, 33, 237-293.
Einsele G., Ricken W., and Seilacher A., (editors), 1991, "cycles and events in stratigraphy", Springer-Verlag, Berlin, Heidelberg, New York 1991. 955p.