Docstoc

Clear_ non-clastic shallow marine environments

Document Sample
Clear_ non-clastic shallow marine environments Powered By Docstoc
					 Deep marine environments
 The specification says that you need
  to be able to:
 describe the characteristic products
  of sedimentation in deep marine
  basin environments: formation of
  turbidites of greywackes and shales,
  and calcareous and siliceous oozes
  from microfossils and explain the
  processes which formed them.
     Sandstones:
     Greywacke
   Greywackes are
    texturally immature
    with > 15% matrix.
   They are also
    compositionally
    immature.
   Greywackes have a
    diverse mineralogy;
    they may contain
    quartz, feldspar, mica,
    clay minerals, mafic
    minerals, rock
    fragments (volcanic,
    metamorphic), with a
    matrix of carbonates,
    clay, pyrite, and
    organics.
Greywacke Formation:
   The poor sorting
    suggests rapidly
    changing energy
    conditions and rapid
    deposition.
   The classic mechanisms
    are turbidity flows.
   These are rapid debris
    flows down the
    continental slope onto
    the abyssal plain.
   As the flow slows down
    sediment is deposited,
    coarse first and then
    fine last.
   This produces graded
    bedding.
                 Shale
   Definition:
   A fine grained
    sedimentary rock that
    is fissile (can be split
    easily).
   A bit like slate but
    much softer.
   The clay particles have
    been aligned by
    compaction.
          Deep Ocean Shales
   These need low energy conditions like
    those found in deep oceans away from
    any currents.
    Calcareous and silicious oozes
    An ooze is a very fine grained
     sediment usually made of
     microfssils.
    These can be:
1.   Calcareous: coccoliths,
     foraminifera.
2.   Silicious: diatoms
              Carbonaceous oozes
Such oozes can form in deep water
  but no deeper than 5 km above
  the carbonate compensation
  depth.
                Chalk:
   This is a very fine grained
    limestone with a micrite texture.
   No grains can be seen and it is
    even difficult with a hand lens.
   They are rich in calcite usually >
    90% and sometimes as high as
    99%.
   As a sediment it was a calcareous
    ooze consisting of the skeletons
    of planktonic organisms:
                         Chalk
   Coccoliths calcareous algae
   Foraminifera micro-organisms
   As they died they settled out
    from the water and as they
    were pelagic (free swimming or
    floating) creatures.
   They settle out in a quiet water
    environment and over time
    large accumulations of these
    skeletons can accumulate.
   It is thought that chalk formed
    in a shelf environment well
    away from land where there
    were no currents to disturb the
    ooze/mud.
                 Silicious Oozes


   Silicious microfossils
    like diatoms sink
    gently to the seabed
    to produce an ooze.
   When solidified these
    form chert.
   These will form
    particularly below the
    carbonate
    compensation depth.

				
DOCUMENT INFO