sedstructer introduction to earth science

Physical sedimentary structures  Physical inorganic structures are sedimentary features formed by physical processes without the influence of organism.. Plane bedding The simplest se

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.

bedding (less than 1

cm thick)

alteration of light and dark layers such as glacial varves

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

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

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.

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 stratification from alternating tidal currents.

cross-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.