Transcript Sedimentary Structures

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
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.