Clastic Sedimentary Rocks

1
Clastic Sedimentary Rocks

• The specification states that you need to be able
  to
• Identify, describe and explain the origin of
  clastic sedimentary rocks using observation of
  grain size, grain shape, degree of sorting,
  colour, mineral composition and texture.
• Breccia
• Conglomerate
• Sandstones
• Orthoquartzite
• Arkose
• Greywacke
• Micaceous sandstones
• Desert sandstones
• Mudstone
• Clay
• Shale

2
Clastic Sedimentary Rocks

• How might you define a clastic sedimentary rock?
• Made of clasts.
• What is a clast?
• A piece of a pre-existing rock.

3
GRAIN SIZE

• Strictly the length of its intermediate axis
  determines the size of a sedimentary grain.
• With large grains, such as pebbles, this can be
  easily measured.
• However, for finer grained sediments, such as
  sands, it would be a very difficult process.
• An easier method is by visual comparison with
  sand of known grain sizes.
• The grain sizes can be printed on a "grain size
  comparator.

4
GRAIN SIZE

• Sediment grain size is described according to the
  "Wentworth Scale" .
• Coarse gt2 mm Rudaceous
• Medium 1/16 - 2 mm Arenaceous
• Fine lt 1/16 mm Argillaceous

5
GRAIN SHAPE

• The term used to describe the overall shape of a
  grain is sphericity (how like a sphere it is).
• Another, and separate, aspect of grain shape is
  the degree of roundness.
• This relates to the extent to which originally
  angular edges and corners have been rounded off.

6
GRAIN SHAPE

• The range of terms used to describe the rounding
  of grains is
• very angular
• Angular
• subangular
• subrounded
• rounded
• well rounded

7
SORTING

• Very often, sediments contain grain sizes
  belonging to several different classes on the
  Wentworth Scale.
• Sorting is an expression of the overall grain
  size of sediment.
• If most of the grains in sediment are of a
  similar size the sediment is well sorted.

8
SORTING

• If the range of grain sizes is great then the
  sediment is poorly sorted.
• Poorly sorted sediments were usually deposited
  rapidly whilst well sorted have taken time to be
  deposited.

9
Maturity in sedimentary rocks

• This can be divided into
• Textural Maturity
• Compositional Maturity
• If a rock is texturally mature it will be well
  sorted with well rounded clasts.
• What does this suggest about its formation?
• Texturally immature will be poorly sorted with
  angular clasts.
• If a rock is compositionally mature it will only
  contain quartz.
• Why will it only contain quartz?
• Compositionally immature will have a wide range
  of clast compositions.

10
Breccia

• Coarse grained gt 2mm.
• Very poorly sorted.
• Angular.
• Therefore texturally?
• Immature.
• Also tends to have various clast compositions and
  so is compositionally?
• Immature.
• Poor sorting suggests what?
• Angularity of clasts suggests what?
• Suggest environments of deposition.

11
Conglomerate

• Coarse grained gt 2mm.
• Very poorly sorted.
• Rounded clasts.
• Therefore texturally?
• Immature.
• Also tends to have various clast compositions and
  so is compositionally?
• Immature.
• Poor sorting suggests what?
• Rounded clasts suggests what?
• Clast size suggests what?
• Suggest environments of deposition.

12
Sandstones

• These are medium grained arenaceous rocks (1/16th
  2 mm).
• There are various kinds that you need to know
• Orthoquartzite
• Arkose
• Greywacke
• Micaceous sandstones
• Desert sandstones

13
Sandstones Orthoquartzite

• These are just very pure sandstones just made of
  quartz.
• They are therefore compositionally?
• Mature.
• How could they form?
• Lots of transport
• Desert conditions.
• Parent rocks were quartz rich.

14
Sandstones Arkose

• Arkoses contain gt 25 feldspar (mainly
  potash-feldspar).
• They are commonly reddish and cross-bedded.
• They have variable textural maturity.
• They commonly are derived from granite or gneiss.
• Semi-arid environments favour feldspar
  preservation (low chemical weathering), so they
  are commonly formed in warm and cool deserts.

15
Sandstones Greywacke

• Greywackes are texturally immature with gt 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.

16
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.

17
Sandstones Micaceous Sandstone

• These are sandstones that contain micas.
• Why are they unlikely to form in desert
  conditions?
• Because the micas will be blown (winnowed) out of
  the system.
• Therefore they form in water formed deposits
  especially rivers.
• They are usually close to the original rock that
  provided the mica.
• Why?
• Because mica usually breaks down rapidly so must
  be close.
• The mica often allows the rock to split (friable).

18
Dune Sandstones

• As you know desert sands are transported by wind
  (can be called aeolian deposits).
• They typically form sand dunes.
• The windward slope is lower angled lt25 and the
  dip slope is lt 40.

19
Sand Dunes

• Sand is driven up the windward side and then
  falls down the dip slope.
• This moves sand from the windward side and makes
  the dune migrate.
• Former positions of the dip slope are marked by
  cross bedding.
• This is one of the typical features of desert
  sandstone deposits.

20
Cross bedding

• The top of the cross bed is cut across by the bed
  above.
• The base of the cross- bed curves into the bed
  below.
• This allows you to work out way up.

21
Desert Sandstones

• As well as having cross bedding desert sandstones
  have the following features linked to their
  transport by wind.
• Well sorted
• Well rounded
• Frosted
• Millet seed texture
• They will also be red/brown/orange in colour.

22
Sandstones Desert Sandstones

• Therefore desert sandstones are
• Texturally?
• Mature.
• And compositionally?
• Mature.

23
Mudstone

• The constituents of these rocks are varied
• "Rock Flour" comes from glacial erosion or
  abrasion by running water or wind.
• New "Clay Minerals" can be produced by chemical
  weathering of feldspars and ferromagnesian
  minerals.
• Such fine grained sediments settle out only in
  low energy environments which exist in
• currentless lagoons
• Lakes
• estuaries
• or in the oceans beneath the wave base.
• As the original mud deposit has water squeezed
  out it changes to a sticky compact clay and then
  to a hard mudstone or laminated shale.

24
Clay

• By definition
• A smooth, earthy sediment or soft rock composed
  chiefly of clay sized particles and a significant
  content of Clay Minerals.
• A good example is the London Clay.

25
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.