Sedimentary Sedimentary StructuresStructures

Chapter 4Chapter 4

Physical sedimentary Physical sedimentary structuresstructures

Physical (inorganic) structures are Physical (inorganic) structures are sedimentary features formed by physical sedimentary features formed by physical processes without the influence of processes without the influence of organism.organism.

Primary sedimentary structures are the Primary sedimentary structures are the most important. They are mechanical most important. They are mechanical structures formed during deposition of the structures formed during deposition of the sediments.sediments.

Plane beddingPlane bedding The simplest sedimentary structures is The simplest sedimentary structures is

plane bedding. They form in practically all plane bedding. They form in practically all sedimentary environments and under a sedimentary environments and under a variety of conditions.variety of conditions.

Three basic mechanism can form plane Three basic mechanism can form plane bedding: sedimentation from suspension, bedding: sedimentation from suspension, horizontal accretion from a moving bed horizontal accretion from a moving bed load, and encroachment into the lee of an load, and encroachment into the lee of an obstacle.obstacle.

LaminationLamination Finer scale plane Finer scale plane

bedding (less than 1 bedding (less than 1 cm thick).cm thick).

It can be form by It can be form by alteration of light and alteration of light and dark layers such as dark layers such as glacial varves.glacial varves.

Lamination in mud is Lamination in mud is usually the result of usually the result of slow steady slow steady deposition.deposition.

Absence of lamination in mud is due to Absence of lamination in mud is due to flocculation (clumping of clays before they flocculation (clumping of clays before they settle) or to secondary bioturbation.settle) or to secondary bioturbation.

Laminated sands are the results of rapid Laminated sands are the results of rapid deposition, often by a single hydrodynamic deposition, often by a single hydrodynamic event.event.

Lack of lamination may be the result of Lack of lamination may be the result of bioturbation.bioturbation.

Bedforms generated by Bedforms generated by unidirectional currentsunidirectional currents

As soon as flow attains a force sufficient to As soon as flow attains a force sufficient to erode particles from the bed, sediments erode particles from the bed, sediments are transported in a set of structures of the are transported in a set of structures of the bed called bed called Bedforms.Bedforms.

If they are latter buried and preserved, If they are latter buried and preserved, they will form sedimentary structures.they will form sedimentary structures.

Flume studies have Flume studies have shown that their is a shown that their is a predictable predictable sequence of sequence of bedforms that bedforms that depend on velocity, depend on velocity, grain size, depth of grain size, depth of flow.flow.

In Sand that is finer In Sand that is finer than 0.7 mm than 0.7 mm (coarse or finer) the (coarse or finer) the first feature to form first feature to form is ripples.is ripples.

Typically their Typically their spacing is 10 to 20 spacing is 10 to 20 cm or less, and their cm or less, and their height is less than a height is less than a few centimeters.few centimeters.

As flow velocity As flow velocity increase the ripples increase the ripples enlarge until they enlarge until they form sand waves, form sand waves, and finally dunes, and finally dunes, which have spacing which have spacing from 0.5 to 10m or from 0.5 to 10m or more and heights of more and heights of tens of cm to a meter tens of cm to a meter or more.or more.

In deeper currents, In deeper currents, greater flow velocity is greater flow velocity is required to produce required to produce the large bedforms.the large bedforms.

With increasing flow With increasing flow velocity, dunes are velocity, dunes are destroyed and the destroyed and the turbulent flow which turbulent flow which was out of phase was out of phase turns into sheetlike turns into sheetlike flow in phase with the flow in phase with the bedform. It forms bedform. It forms plane beds.plane beds.

At higher velocities At higher velocities plane beds are plane beds are replaced by replaced by antidunes of up to antidunes of up to 5m spacing. Low 5m spacing. Low dip angles of 10 dip angles of 10 degrees or less, degrees or less, eventually chutes eventually chutes and pool.and pool.

Flow pattern of sediment movement over migrating ripples or dune

Laboratory flume show the Laboratory flume show the trajectories of sandtrajectories of sand

Trough cross-strat.Develops from migratingRipples & dunes

Tabular cross-strat.Is produced by migratingsand waves

Symetrical ripple marks withA distinctive lenticular x-section

Interference pattern formIn symmetrical ripples fromTwo coexisting wave setsIn 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.