PETROLOGY LAB 4: Sedimentary Rocks � Textures and Structures

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PETROLOGY LAB 4: Sedimentary Rocks � Textures and Structures Powered By Docstoc
					                                                                      Francis, 186-212, 2010


PETROLOGY LAB 5: Sedimentary Textures and Structures

The process of sedimentation begins with erosion to produce sediments in high standing
areas, followed by transportation of the sediments by water, wind, or ice to sites of
deposition and accumulation in low standing areas, and finishes with compaction and
cementation into sedimentary rocks. An examination of textures and structures of
sedimentary rocks enables one to constrain the transport mechanisms and the nature of
the deposition environment of the sediments.


Station 1 - Sedimentary Textures: grain size, shape, and sorting
The most important sediment textures include the size, shape, and degree of sorting of the
clastic grains making up a sediment or sedimentary rock.



Grain Size:
In a broad sense, the grain-size of siliclastic
sediments reflects the hydraulic energy of the
environment: coarser sediments are transported
and deposited by faster flowing currents than
finer sediments. Thus grain-size often reflects
distance from the source where the sediments
originated. For example, pebbles that are found
in conglomerates are often transported over
shorter distances than silt or clay. With sand
and silt-sized sediments you cannot do much
more in a hand-sample than estimate grain-size
and comment on sorting and roundness of
grains. A widely used grain-size scale is that of
Udden-Wentworth. Note that  (phi) refers to
a logarithmic transformation:  = -log2 S,
where S is grain-size in millimeters.




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                                                                        Francis, 186-212, 2010




Sorting and Roundness:
      The degree of sorting of a sandstone reflects the transportation mechanism and the
      nature of the depositional environment, and increases with increasing agitation
      and reworking experienced. For example, aeolian and beach sandstones typically
      are well sorted, contain well rounded grains and no matrix (they are said to be
      ‘texturally mature’), whereas fluviatile sandstones are moderately to poorly sorted
      and may contain angular grains and matrix (they are said to be ‘texturally
      immature’).
      Conglomerates and breccias can be distinguished by the roundness versus
      angularity of the clasts in the rock: if the clasts are rounded the rock is referred to
      as a conglomerate, if they are angular it is a breccia. A conglomerate or breccia
      can be either:
             monomictic - the clasts are all the same type of rock.
             polymictic – there are different types of rock clasts.

          It is also important to note the ‘maximum clast size’, since it is often a
          reflection of the hydraulic energy of the transporting current. In addition you
          should examine the clast-matrix relationships in these samples:

             clast-support fabric is typical of fluvial and beach gravel.
             matrix-support fabric is typical of debris flows and glacial tills.




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Colour:
      Color can give useful information on lithology, depositional environment, and
      diagenesis. Two factors determine the color of many sedimentary rocks: the
      oxidation state of iron and the content of organic matter. Iron exists in two
      oxidation states: ferric (Fe3+) and ferrous (Fe2+). Where ferric iron is present it
      frequently occurs as the mineral hematite and even in concentrations of less than
      1% this imparts a red color to the rock. Where the hydrated forms of ferric oxide
      (goethite and limonite) are present the sediment has a yellow-brown color. The
      formation of hematite requires oxidizing conditions and these are frequently
      present within sediments of semi-arid and continental sub-aerial environments.
      Sandstones and mudrocks of these environments (e.g., desert, lakes and rivers) are
      frequently reddened through hematite pigmentation and such rocks are referred to
      as ‘red beds’. Many red marine sediments are, however, also known. Where
      reducing conditions prevailed within a sediment the iron is present in the ferrous
      state and is contained in clay minerals, imparting a green color to the rock.
      Organic matter within a sediment gives rise to grey colors and with increasing
      organic content the rock becomes black. Such organic-rich sediments generally
      form in anoxic conditions. Thus color may provide you with additional
      information about the depositional environments and should be included in your
      considerations when examining a sedimentary rock.
      For the samples in Station 1, estimate grain-size, sorting and roundness, look at
      the color and think about what conclusions you can draw about depositional
      environment and distance from the source rocks.




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                                                                      Francis, 186-212, 2010


Station 2 - Sedimentary Structures: Bedding


Bedding and lamination:
      The characteristic feature of sedimentary rocks is the presence of stratification or
      bedding, typically produced by changes in the pattern of sedimentation, such as
      changes in sediment composition and/or grain size. Bedding is generally defined
      as layering thicker than 1 cm, whereas finer scale layering is termed lamination.
      Lamination is commonly an internal structure of a bed and arises from changes in
      grain size between laminae, size-grading, or changes in composition between
      laminae. Each laminae may be the result of a single depositional event. Which of
      the specimens from this station show bedding and which lamination? What do
      these features indicate for the depositional environment in which the specimens
      were formed?




Graded Bedding:
      Grading is a gradual change in grain-size upwards through a bed and typically
      develops in response to change in flow velocity during a sedimentation event.
      Beds with coarser grain-size at the bottom and fining towards the top are termed
      normally graded, while beds that show coarser grain-size at the top and finer
      grain-size at the bottom are termed reverse graded. Normal graded bedding is by
      far the most common, and can be an excellent tops indicator. Reverse graded
      bedding is relatively rare, and tends to occur in poorly sorted sediments that have
      been deposited rapidly from sediment-charged flows. Examine the specimens at
      this station from the point of view of whether they exhibit normal grading or
      inverse grading? How did the flow conditions change during sedimentation?




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                                                                      Francis, 186-212, 2010


Station 3 – Sedimentary Structures: Bedforms, and Cross Stratification:

Cross stratification due to current ripples and dunes
The nature of the bottom surface in terms of the sedimentary structures or bedforms, like
ripples and dunes, is dependent on the current flow conditions. Bedforms are
characterized by their wavelength and/or height as ripples (=0.05-0.2 m), dunes (=0.5-
10 m), and sand waves (=5-100 m). In aqueous flows, for a given grain size and water
depth ripples from at the lowest current velocity, followed by dunes, plane beds, and
antidunes with increasing current velocity.
Ripples and dunes are asymmetrical bedforms, which gradually move downstream as
sediment is transported through erosion of the upstream-facing side and deposition over
the bedform crest in foreset beds on the downstream-facing side. They are common in
rivers, tidal flats, delta channels, on shallow-marine shelves, and on the deep-sea floor.
The foreset beds of ripples and dunes are commonly preserved as cross stratification.
These range from planar cross strata formed through the migration of straight crested
ripples to trough cross strata formed through the migration of lunate or sinuous ripples.
In planar cross bedding the foresets (sloping beds) dip at angles up to 30 or more, and
may have an angular basal contact with the horizontal. Trough cross bedding consists of
scoop-shaped beds with tangential bases and dips of 20-30. The asymmetry of trough
cross-bedding, and their cross cutting relationships, give excellent top and current
direction indicators.

The scale of the cross stratification reflects the
bed form that they preserve:
          cross-bedding typically represents the
           preserved foreset beds of sand waves
           and dunes.
          cross lamination typically represents
           the preserved foreset beds of ripples

          Climbing Ripples: Under conditions
           of rapid deposition, successive
           ripples may climb onto the backs of
           previous ripples with little or no
           erosion, producing a ‘false’ cross
           bedding.

Can you identify upstream-facing side and
downstream-facing side in the specimens at this
station? What can you say about the wavelength
of the ripples? Can you determine the flow
direction?




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                                                                        Francis, 186-212, 2010


Station 4 – Sedimentary Structures: Sole Markings and other structures:


Sole Markings:
       The bottoms of sandy beds often have markings that are indicative of current flow
       and/or differential compaction:

       Grove casts           Elogated ridges that represent filled depressions in
                             the underlying bed. Commonly elongated parallel to the
                             current direction, although it is typically not possible to tell
                             the upstream ends from the downstream ends.
       Flute clasts:         Bulbous ridges that are steeper in the upstream
                             direction and flare downstream. They represent filled scour
                             depressions in the underlying bed formed by eddies in the
                             flow that deposited the overlying sands.
       Bounce, Prod          Casts of small gouge marks in the underlying bed
       and skip marks:       produced by the collision of fragments being carried
                             by the flow that deposited the overlying bed. They
                             commonly give the opposite sense of flute casts, that is
                             their steep sides are downstream and their shallow sides up
                             stream.




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                                                      Francis, 186-212, 2010




Load casts:   Ball, pillow, and bulge shaped sand structures that
              have sunk down into an underlying clay bed because of
              liquifaction and de-watering of the clays following the
              loading of the overlying sand bed. Similar to flutes, but
              more irregular, and they do not have any current direction
              implications. Not strictly a cast because they do not fill a
              pre-existing hole.




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De-watering Structures:   Sandy sediments that are deposited very rapidly commonly
                          have a significant proportion of trapped water that is
                                   expelled as the sediment undergoes compaction.
                                   This leads to the formation of dish and pillar
                                   structures. Dish structures are defined by dark
                                   coloured clay-rich laminations that appear saucer
                                   or dish shaped. They are associated with vertical
                                   pillar structures that are also defined by darker
                                   clay rich material.




Flame Structures:         Sandy beds deposited by strong currents over clay-
                          rich beds commonly develop flames of the clay-rich
                          material that have been dragged into the overlying sand.
                          The asymmetry of flame structures can be used to
                          determine the paleo-current direction.




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                                                                      Francis, 186-212, 2010


Fossils, rain spots, mud cracks, and other peculiar features:

      The samples at this station have interesting textures and structures that are easily
      recognized:
         Rain spots are small depressions with rims, formed through the impact of rain
          on the soft exposed surface of fine-grained sediments.
         Mud cracks are polygonal patterns on the bedding surface that formed
          through shrinkage and cracking of the bed or lamina.




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