sedstructer introduction to earth science

Sedimentary Structures Chapter Physical sedimentary structures  Physical (inorganic) structures are sedimentary features formed by physical processes without the influence of organism  Primary sedimentary structures are the most important They are mechanical structures formed during deposition of the sediments Plane bedding  The simplest sedimentary structures is plane bedding They form in practically all sedimentary environments and under a variety of conditions  Three basic mechanism can form plane bedding: sedimentation from suspension, horizontal accretion from a moving bed load, and encroachment into the lee of an obstacle Lamination    Finer scale plane bedding (less than cm thick) It can be form by alteration of light and dark layers such as glacial varves Lamination in mud is usually the result of slow steady deposition  Absence of lamination in mud is due to flocculation (clumping of clays before they settle) or to secondary bioturbation  Laminated sands are the results of rapid deposition, often by a single hydrodynamic event  Lack of lamination may be the result of bioturbation Bedforms generated by unidirectional currents  As soon as flow attains a force sufficient to erode particles from the bed, sediments are transported in a set of structures of the bed called Bedforms  If they are latter buried and preserved, they will form sedimentary structures   Flume studies have shown that their is a predictable sequence of bedforms that depend on velocity, grain size, depth of flow In Sand that is finer than 0.7 mm (coarse or finer) the first feature to form is ripples   Typically their spacing is 10 to 20 cm or less, and their height is less than a few centimeters As flow velocity increase the ripples enlarge until they form sand waves, and finally dunes, which have spacing from 0.5 to 10m or more and heights of tens of cm to a meter or more Flow pattern of sediment movement over migrating ripples or dune Laboratory flume show the trajectories of sand Symetrical ripple marks with A distinctive lenticular x-section Trough cross-strat Develops from migrating Ripples & dunes Tabular cross-strat Is produced by migrating sand waves Bedform generated by multidirectional flow Herringbone crossstratification from alternating tidal currents Interference pattern form In symmetrical ripples from Two coexisting wave sets In a modern tidal flat In tidal regions the most significant features are caused by the mixing of sand- and mud-sized fractions from the asymetrical currents Lenticular bedding occur when sand is trapped in troughs in the mud as sand waves migrate across a muddy substrate If mixing produces minor mud layer in a sandy substrate the pattern is called flaser bedding