CHAPTER 6 SEDIMENTS AND SEDIMENTARY ROCKS

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CHAPTER 6 SEDIMENTS AND SEDIMENTARY ROCKS Powered By Docstoc
					GEOLOGY 12                NOTES                                   Name __________________
CHAPTER 6
SEDIMENTS AND SEDIMENTARY ROCKS

Sediments are unconsolidated accumulations of rock and mineral grains and organic
matter that have been transported and deposited by wind, water, or ice. Sedimentary
rocks are formed when sediments become consolidated into a cohesive mass.

Three main sources for the sediments that form sedimentary rocks are:
1.    Clastic – formed from fragments of other rocks such as clay, sand, or gravel.
2.    Chemical – formed from mineral grains that precipitate out of solution by
      evaporation or chemical action.
3.    Organic- formed from remains of plant and animals.


Clastic
In General:

•    Fragments range in size from large to small as: pebbles, gravels, sand, silt, and clay.
•    Fragments are carried by winds, waves, rivers, and glaciers and form in great layers
     of sediment.
•    Ocean, lake, and ground water all contain dissolved minerals; silica (from quartz),
     lime (from calcite), and iron (from iron containing minerals) may form cements.
•    When cement minerals precipitate into pore spaces between sand, gravel, or pebbles,
     they bind the fragments together, turning loose sediments into firm cemented rock.
•    In fine sediments such as clay or silt, sometimes the pressure of the later deposit is
     enough to make the particles stick together, but usually cement is required.
•    Silica and lime give white or grey cement; iron cement is red, brown or rust-colored.
•    Water (or wind) carrying fragments deposit the fragments when the flow slows down.
     The heaviest/largest fragments are deposited first. These are heavy pebbles and
     gravels. Next come the lighter sands and finally the silts and clays.


      Particle Size Range        Sediment              Rock

      over 25.6 cm               boulder        conglomerate (if fragments are rounded)*
      0.2-25.6 cm                gravel         breccia (angular fragments)*
      0.00625- 0.02 cm           sand           sandstone
      0.00039- 0.00625 cm        silt           siltstone**
      < 0.00039 cm               clay           claystone**

      * if particle size is over 0.2 cm, rock is called conglomerate if the fragments are
         rounded, or breccia if the fragments are angular.
      ** both siltstone and claystone are also known as mudstone, commonly called shale if
         the rock shows a tendency to split on parallel planes.
                                               2

Classification/Grouping is done by:
    a) particle size - see chart above.
    b) roundness or angularity of fragments - see breccia and conglomerate
    c) sorting - well-sorted means all grains are about the same size. The spaces
       between the grain particles are usually filled with finer materials called the matrix.

Chemical
•   Precipitates - sediments which have been precipitated from or fallen out of solution.
    (limestone, dolomite)
•   Evaporites - formed when salt water is trapped and evaporated, leaving dissolved
    minerals like halite. Also note: chert (silica), and banded ironstones (deposits due to
    oxidation of iron-bearing groundwaters)

Organic / Biogenic
•   Biological sediments that are formed when an organism precipitates a shell or
    skeleton from the water. When the organism dies, the soft body parts decay, leaving
    a shell or skeleton behind. eg. coral reefs give carbonate rocks, silica skeletons of
    diatoms and radiolarians give cherts.
•   Coal, peat, and lignite result from the burial of plant materials. These rocks are
    distinguished by their texture and percentage carbon content.


From Sediment Into Rock
1. Consolidation is the transformation of loose sediment into a coherent sedimentary
   mass.
2. Diagenesis is the set of processes by which sediment becomes lithified or turned into
   rock. These include:
   • Compaction – squeezes sediments closer together
   • Cementation – glues the sediments together
   • Dolomitization – changing of calcium carbonate into calcium-magnesium
      carbonate


Sedimentary Environments
1. Clastic sediments:
   • rivers, deltas, beaches, tidal flats
2. Chemical sediments:
   • carbonate platforms
   • deep evaporites
   • siliceous bottoms of ocean
3. Other factors that affect the maturity or formation and type of sediments:
   • nature of source rock
   • distance from source
   • effect of climate
   • effect of topography
                                              3

The Facies Concept
Facies - a set of conditions that leads to the formation of a particular type of sediment or
sedimentary rock. (i.e. transgression vs regression p.123-125)


Paleogeographic Reconstruction

Paleogeography is ancient geography and is constructed through the interpretation of
sedimentary rocks.
Paleocurrents are ancient currents that prevailed when the sediments were deposited.
The direction of these currents can be deduced from sedimentary structures.
Paleoclimates can be deduced from sedimentary rock formations and the fossil evidence
found in sedimentary structures.
Shifting shorelines
•   Transgression is when the shoreline moves inland as the ocean level rises and
    advances over what was dry land. The sequence of rocks formed in this “sedimentary
    facies” would be (bottom to top) sandstone, siltstone, claystone and limestone. The
    limestone deposits are created when the ocean moves in and slates, shales and
    mudstone deposits are created when the ocean moves out and the area is covered by
    silts washing into the retreating ocean (Regression). See page 124.


Sedimentary Structures
Stratification (or layering) is the most obvious feature of sedimentary rocks. The layers
(or strata) are visible because of differences in the color or texture of adjacent beds.
Strata thicker than 1 cm are commonly referred to as beds. Thinner layers are called
laminations or laminae . The upper and lower surfaces of these layers are called
bedding planes. Clastic sediments display a wide variety of bed structures that help in
determining such factors as "way-up" and depositional environment.

Bedding - may include
1. Planar bedding has little diagnostic importance
   but animal burrows or bed form irregularities
   might give an indication of way-up and
   environment.


2. Cross-bedding forms beneath ripples and
   dunes. The layering is inclined, dipping
   downward in the downcurrent direction. Hence,
   cross-beds may be used as paleocurrent
   indicators, or indicators of ancient current flow
   directions. Cross-beds
                                             4

   usually curve at the bottom edge. The upper
   edge of individual inclined cross-beds is usually
   at a steep angle. Hence, cross-beds may also
   be used as "up indicators".


3. Ripple-marked bedding is a feature which forms on
   the surface of a bed of sediment. At the time of
   formation, the "surface of a bed" is equivalent to
   the sea floor, or the bottom of a lake or river.
   Ripples which formed in streams or rivers tend to
   be asymmetrical and may be used to determine
   ancient current directions or paleocurrent
   directions. Symmetrical ripples are produced by
   waves or oscillating water.


4. Graded bedding results when a sediment-laden
   current begins to slow down. The grain size within a
   graded bed ranges from coarser at the bottom to
   finer at the top. Hence, graded beds may be used as
   "up indicators".


5. Varves are a special type of lamination which forms
   in glacial lakes. Varves represent deposition over
   one year, and their formation is related to seasonal
   influences. Varves are generally graded, with the
   coarser material at the bottom (silt or sand)
   representing the spring and summer meltwater
   runoff, and the finer material at the top representing
   slow settling of clays and organic matter from
   suspension during the winter months when the lake is covered with ice. Counting of
   varves has been used to measure the ages of some sedimentary deposits.

   Mudcracks are a polygonal pattern of cracks
   produced on the surface of mud as it dries. The mud
   polygons between the cracks may be broken up later
   by water movement, and redeposited in lime muds.


   Fossils (more details later)
      • replacement
      • molds
      • casts
      • trace fossils                                      Several types of burrows.
      • oolites