Sedimentary Structures Chapter 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.

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 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.

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.

 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.

Flow pattern of sediment movement over migrating ripples or dune

Laboratory flume show the trajectories of sand

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

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

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.