Sedimentary structures - DOC

Document Sample
Sedimentary structures - DOC Powered By Docstoc
					Sedimentary structures
Sedimentary structures are those structures formed during sediment deposition.

Sedimentary structures such as cross bedding, graded bedding and ripple marks are
utilized in stratigraphic studies to indicate original position of strata in geologically
complex terrains and understand the depositional environment of the sediment.

Contents

   1.   Unidirectional Flow structures
   2.   Bedforms vs. Flow
   3.   Ripple Marks
   4.   Biological structures
   5.   Soft Sediment Deformation Structures
   6.   Bedding Plane Structures
   7.   Within Bedding Structures



Unidirectional Flow structures

There are two kinds of flow regimes in single-direction (typically fluvial) flow,
which at varying speeds and velocities produce different structures, called
bedforms. In the lower flow regime, the natural progression is from a flat bed, to
some sediment movement (saltation etc.), to ripples, to slightly larger dunes. Dunes
have a vortex in the lee side of the dune. As the upper flow regime forms, the
dunes become flattened out, and then produce antidunes. At higher still velocity,
the antidunes are flattened and most sedimentation stops, as erosion takes over as
the dominant process.

Bedforms vs. Flow

Typical unidirectional bedforms represent a specific flow velocity, assuming
typical sediments (sands and silts) and water depths, and a chart such as below can
be used for interpreting depositional environments, with increasing water velocity
going down the chart.

Flow                               Preservation
            Bedform                                     Identification Tips
Regime                             Potential
                                                    Flat laminae, almost lack of
           Lower plane bed      High
                                                    current
Lower      Ripple marks         High                Small, cm-scale undulations
                                                    Rare, longer wavelength than
           Sand waves           Medium to low
                                                    ripples
           Dunes/Megaripples Low                    Large, meter-scale ripples


                                                    Flat laminae, +/- aligned
           Upper plane bed      High
                                                    grains (parting lineations)
Upper
                                                    Water in phase with bedform,
           Antidunes            Low
                                                    low angle, subtle laminae
           Pool and chute       Very low            Mostly erosional features



Ripple Marks




Wave ripple or symmetric ripple, from Permian rocks in Nomgon, Mongolia. Note
"decapatation" of ripple crests due to change in current.

Ripple marks usually form in conditions with flowing water, in the lower part of
the Lower Flow Regime. There are two types of ripple marks:
Ripple marks in a siltstone (Carmel Formation, Middle Jurassic, near Gunlock,
Utah).

      Symmetrical ripple marks - Often found on beaches, they are created by a
       two way current, for example the waves on a beach (swash and backwash).
       This creates ripple marks with pointed crests and rounded troughs, which
       aren't inclined more to a certain direction. Three common sedimentary
       structures that are created by these processes are herringbone cross-
       stratification, flaser bedding, and interference ripples.
      Asymmetrical ripple marks - These are created by a one way current, for
       example in a river, or the wind in a desert. This creates ripple marks with
       still pointed crests and rounded troughs, but which are inclined more
       strongly in the direction of the current. For this reason, they can be used as
       palaeocurrent indicators.

Biological structures




Skolithos trace fossil. Scale bar is 10 mm.
A number of biologically-created sedimentary structures exist, called trace fossils.
Examples include burrows and various expressions of bioturbation. Ichnofacies are
groups of trace fossils that together help give information on the depositional
environment. In general, as deeper (into the sediment) burrows become more
common, the shallower the water. As (intricate) surface traces become more
common, the water becomes deeper.

Soft Sediment Deformation Structures




Soft-sediment deformation in etched section of carbonaceous sandstone bed of
Reedsville Formation from along PA Rt. 36 near Loysburg, Bedford Co., PA.

Soft sediment deformation or SSD, is a consequence of the loading of wet
sediment as burial continues after deposition. The heavier sediment "squeezes" the
water out of the underlying sediment due to its own weight. There are three
common variants of SSD:

      load structures or load casts (also a type of sole marking) are blobs that form
       when a denser, wet sediment slumps down on and into a less dense sediment
       below.
      pseudonodules or "ball and pillow" structures, are pinched-off load
       structures; these may also be formed by earthquake energy and referred to as
       seismites.
      flame structures, "fingers" of mud that protrude into overlying sediments.

Bedding Plane Structures
Mudcracks in rock at Roundtop Hill, Maryland.

Bedding Plane Structures are commonly used as paleocurrent indicators. They are
formed when sediment has been deposited and then reworked and reshaped. They
include:

      Sole markings form when an object gouges the surface of a sedimentary
       layer; this groove is later preserved as a cast when filled in by the layer
       above. They include:
           o Flute casts are scours dug into soft, fine sediment which typically get
              filled by an overlying bed. Measuring the long axis of the flute cast
              gives the direction of flow, with the tapered end pointing upcurrent.
              The convexity of the flute cast also points stratigraphically down.
           o Tool marks are a type of sole marking formed by grooves left in a bed
              by objects dragged along by a current. The average direction of these
              can be assumed to be the axis of flow direction.
      Mudcracks form when mud is dewatered, shrinks, and leaves a crack. This
       tells you that the mud was saturated with water and then exposed to air or
       hypersaline water. Mudcracks curl upwards, so they can be used as geopetal
       structures.
      Raindrop impressions form on exposed sediment by raindrop impacts.
      Parting lineations are subtly aligned minerals that form in the lower part of
       the Upper Flow Regime within plane beds.

Within Bedding Structures




Cross-bedding and scour in a fine sandstone (Logan Formation, Mississippian,
Jackson County, Ohio).
These structures are within sedimentary bedding and can help with the
interpretation of depositional environment and paleocurrent directions. They are
formed when the sediment is deposited.

      Cross bedding - This can include ripples and dunes, or any cross
       stratification caused by currents. The "cross" refers to the angle between flat
       bedding and the inclined bedding of the cross bed, typically about 34
       degrees. Paleocurrents are best found from cross beds that have 3D
       architecture exposed so you can measure the axis of the trough of the cross
       bed.
      Hummocky cross-stratification is formed by storm waves, but is not a good
       paleocurrent indicator.
      Imbrication is the stacking of larger clasts in the direction of flow.
      Normal graded bedding occurs when current velocity changes and grains are
       progressively dropped out of the current. The most common place to find
       this is in a turbidite deposit. This can also be inverted, called reversed graded
       bedding, and is common in debris flows.
      Bioturbation - Biological stirring of sediment (i.e. burrowing); typical of
       shallow water, finer-grained sediment.

See also

      Reynolds number
      Froude number

References

Prothero, D. R. and Schwab, F., 1996, Sedimentary Geology, pg. 43-64, ISBN 0-
7167-2726-9



features of sedimentary rocks

       Sedimentary structures are the larger, generally three-dimensional physical
       features of sedimentary rocks; they are best seen in outcrop or in large
       hand specimens rather than through a microscope. Sedimentary structures
       include features like bedding, ripple marks, fossil tracks and trails, and mud
       cracks. They conventionally are subdivided...
Sedimentary structures are features found in the rock that were created at the
time of deposition. Sedimentary structures exclude fossils and diagenetic
changes to the rock. These structures often help evaluate the environment of
deposition.

This web site page contains images of various sedimentary structures and an
explanation of what they are and how they formed.




These two images of Salt Casts are from the Triassic Moenkopi Formation of
northern Arizona. The salt casts represent periods of aridity during the time of the
deposition of this silty sandstone. Drying of the sediments is an indication of very
arid conditions allowing the development of the salt crystals. The salt crystals
were later dissolved away and the voids were then filled with sand to produce the
casts.




RIPPLE MARKS are a very common type of sedimentary structure. This on is
also from the Triassic Moenkopi Formation of northern Arizona. These
symmetrical ripple marks simply indicate that a current was present - in this case a
back and forth current from the top of the picture to the bottom. Symmetrical
ripple marks indicate one of two possible current direction and are associated with
deeper, calmer waters.




Sedimentary structures can be of either physical (e.g. wave action) or biological
(e.g. disruption of sediments by animals) origin.

Sedimentary structures are very important as they provide us with information on
the palaogeography and palaeoclimate of the areas in which they occur. They can
also indicate the direction of palaeocurrents of rivers and seas.

Physical structures

The movement of sand grains in a current creates ripples and dunes on the stream
bed. These are known as bedforms. Ripples are the low narrow ridges that are
separated by wider troughs. Physical structures form on sand dunes, on underwater
sandbars in rivers and streams, and under the waves at beaches. They come in a
wide variety of shapes and sizes which are characteristic of the currents that form
them:

      Asymmetrical ripples: ripples that have a gentle slope upstream and a steep
       slope downstream.
      Cross-bedding: inclined bedding and commonly forms in alluvial
       environments.
      Potholes: rounded depressions caused by swirling currents and eddies.
      Mud cracks: formed by evaporation on mudflats or in shallow lakes.

Biological structures

These are made by living things and include:

      worm burrows (usually in soft-sediment, particularly at high tide levels
       along beaches)
      crab burrows (particularly those of hermit crabs)
      tubeworm colonies
      animal tracks on soft sediment
Sedimentary rocks tell stories about the geologic processes that made them.
These processes are familiar—waves and currents, winds and weather, living
things, landslides and more. All of them leave their signs in sedimentary
structures like these.
Enter Gallery




                                                  Concretions, Nevada
                            Channel Cast,                                  Convolute Lamination,
   Bedding, California        California                                         California




                         Flute Casts, Mongolia
                                                 Foreset Beds, Australia   Gas-Escape Structures,
    Flame Structure,
       California                                                                Colorado




                                                                            Imbrication, Nevada
   Geopetal Structure,                             Hummocky Cross-
       Wisconsin           Graded Bedding,
                                                    Bedding, Utah
                              California
A structure in sedimentary rocks, such as cross-bedding, ripple marks, and sandstone dikes, produced
either contemporaneously with deposition (primary sedimentary structures) or shortly after deposition
(secondary sedimentary structures).

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:49
posted:3/6/2012
language:English
pages:10