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Sedimentary structures group presentation


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Sedimentary structures group presentation

  2. 2. Brief content  Introduction  Body  Conclusion and Economic Importance  reference
  3. 3. S E D I M E N T A R Y S T R U C T U R E S C L A S S I F I E D
  4. 4. 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.
  5. 5. 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. Bed forms 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.
  6. 6. Lamination  Finer scale plane bedding (less than 1 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.
  7. 7.  Studies have shown that there 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.  In deeper currents, greater flow velocity is required to produce the large bedforms.  With increasing flow velocity, dunes are destroyed and the turbulent flow which was out of phase turns into sheetlike flow in phase with the bedform. It forms plane beds.  At higher velocities plane beds are replaced by antidunes of up to 5m spacing. Low dip angles of 10 degrees or less, eventually chutes and pool.
  8. 8. Trough cross-strat. Develops from migrating Ripples & dunes Tabular cross-strat. Is produced by migrating sand waves Symetrical ripple marks with A distinctive lenticular x- section Terminology for the shape of the crests of ripples and dunes formed by unidirectional currents. Ripple types and variation with flow regime
  9. 9. Interference pattern form In symmetrical ripples from Two coexisting wave sets In a modern tidal flat. Herringbone cross- stratification from alternating tidal currents. Bedform generated by multidirectional flow
  10. 10. 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. Lenticular bedding Flaser bedding
  11. 11. Small-scale erosional features on a bed surface are referred to as sole marks.They are preserved in the rock record when another layer of sediment is deposited on top leaving the feature on the bedding plane. Sole marks may be divided into those that form as a result of turbulence in the water causing erosion (scour marks) and impressions formed by objects carried in the water flow (tool marks).They may be found in a very wide range of depositional environments, but are particularly common in successions of turbidites where the sole mark is preserved as a cast at the base of the overlying turbidite.
  12. 12. Graded Bed
  13. 13. Bio-genic Sedimentary Structures
  14. 14.  Bio-genic Sedimentary Structures Bio-genic structures result from bioturbation, the post-depositional disturbance of sediments by living organisms.This can occur by the organisms moving across the surface of sediment or burrowing into the first few centimeters. It is usually contemporaneous with deposition. The organisms that cause these alterations in the sediments have a dual affect.They physically and chemically alter the original deposit, and they give some useful information about the bottom conditions at the time of deposition. Depending on the location, these animals can affect up to 90% of the sediment surface. As a result, this biological action can completely destroy primary laminations and the original orientation of sediment particles. There are usually significant changes in the formations porosity and permeability.This can help determine the origin and magnitude of any abnormal pore pressures. It can also enhance the tendency for slumping and sliding on steep continental margins, as the sediment shear strength is decreased.
  15. 15.  Conclusion and Economic Importance: Having gone through the study of sedimentary structures, it is necessary to note that they can be used to determine depositional processes, being that depositional processes occurs in several environments, and few structures are immediately diagnostic of a specific environment. Sedimentary structures can also indicate the direction of paleocurrent flow, paleoslope, paleogeography and sand-body trend. They record the processes that occur during deposition and so  help in interpreting sedimentary and post-depositioal processes  Help in defining geological history and surface processes . Cross-beds within ancient rocks helps to tell the direction a stream flowed millions of years ago or the direction of prevailing wind in the past.
  16. 16.  Reference Baker hughes INTEQ, Petroleum geology. Principles of Sedimentation and Stratigraphy. Sedimentary structures, Wikipedia.
  17. 17. THE END