Sedimentary structures

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sedimentary structure The external shape, the internal structure, or the forms preserved on bedding surfaces, generated in sedimentary rocks by sedimentary processes or contemporaneous biogenic activity. Internal sedimentary structures include: those formed by physical depositional processes (cross-stratification, flat bedding (see PLANE BED), lamination, and heterolithic structures); those due to post-depositional deformation (convolute bedding, slump structures, dish and pillar structures, flame structures, ball and pillow structures, etc.); those caused by organic disturbance (bioturbation, trace fossils); or by post-depositional chemical disturbance (enterolithic structures, collapse and solution structures, concretions, etc.) Structures preserved on the tops of beds include: those formed by depositional processes (ripple marks, primary current lineations); erosional structures (flutes and scour marks, see SCOUR AND LAG); structures caused by the transportation of an object over the bed (tool marks); and other features such as desiccation and syneresis cracks, sand volcanoes, adhesion ripples and warts, rain prints, and biogenic traces and trails. Structures preserved on the bases of beds (sole marks) include load casts, the casts of flutes, trails and tool marks, and the fill of erosional scours. The external form of sedimentary units (sheet-like, channel-fill, reef or mound (see MUD MOUND), lenticular, etc.) is a function of the depositional environment and sometimes of post-depositional compaction.

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cross-stratification A family of primary sedimentary structures formed by the migration of the slip-faces of rippled bedforms or of bars. It is characterized by inclined laminations (foresets) bounded by planar surfaces (planar or tabular cross-stratification), or by scoop-shaped surfaces (trough cross-stratification). The foresets dip at the angle of repose of the sediment on the ripple slip-face and are oriented in the direction of migration of the ripple (see PALAEOCURRENT ANALYSIS). Tabular cross-stratification is produced by the migration of straight-crested, asymmetrical ripples or sand waves. Trough cross-stratification is generated by the migration of linguoid ripples or dunes. The term ‘cross-lamination’ is applied to cross-stratification formed by the migration of ripples; ‘cross-bedding’ is used for cross strata formed by the migration of large-scale forms such as dunes, sand waves, or bars. The term ‘cross set’ is used to define the cross-stratification preserved between any upper and lower bounding surface. Where the original bedform which produced the cross set is preserved and forms the upper bounding surface to the set, the term ‘form set’ is used. A number of cross sets preserved within a single bed are called a ‘coset’.