Sedimentary Rocks

1. Chapter 3: The Classification of Clastic Sedimentary Rocks
A very basic classification of
all sedimentary rocks is
based on the type of material
that is deposited and the
modes of deposition.

2. Grain
Size1
(mm)
Sediment
name
Rock Name Adjectives
> 2 Gravel Rudite Cobble, pebble, well
sorted, etc.
0.0625-2 Sand Arenite Coarse, medium, well
sorted, etc.
< 0.0625 Mud Mudstone
or
Lutite
Silt or clay
1For the purposes of this general classification we will assign the rock or sediment
name shown if more than 50% of the particles are in the range shown. More
detailed classification schemes will limit terms on the basis of different proportions
of sediment within a given grain size.
A simple classification of terrigenous clastic rocks and sediment is based
on the predominant grain size of the material:
Classification based on grain size

3. Classification of Sandstones
Most sandstone classifications are based on the composition of the rock.
Dott’s classificaton scheme is used in most courses at Brock.
It is based on the relative proportions of:
Martrix (fine-grained - <0.03mm - material that is associated with the sand grains).
Quartz
Feldspar
Rock fragments (sand grains that are made up crystals of two or more different minerals).

6. To classify sandstones using Dott’s scheme the first step is to determine
composition of the rock.
Point counting is a method whereby a thin section on a petrographic
microscope is examined by stepping across the thin section at equal
intervals and identifying the material (quartz, feldspars, rock fragments
or matrix) that lies immediately beneath the cross hairs. Counting 250
to 300 grains will accurately yield the proportion of each component.

7. Component Number of Proportion
Grains counted (%)
Quartz 73 26
Feldspar 56 20
Rock fragments 34 12
Matrix 118 42
Total: 281 100
A first order classification is
based on the proportion of
matrix that is present:
% matrix Rock Name
< 15
15 - 75
>75
Arenite
Wacke or
Graywacke
Mudstone
Example Point Count Data:

8. Component Number of Proportion
Grains counted (%)
Quartz 73 26
Feldspar 56 20
Rock fragments 34 12
Matrix 118 42
Total: 281 100
A first order classification is
based on the proportion of
matrix that is present:
% matrix Rock Name
< 15
15 - 75
>75
Arenite
Wacke or
Graywacke
Mudstone
Example Point Count Data:

9. A. Total Rock
Component Proportion
(%)
Quartz 26
Feldspar 20
Rock fragments 12
Matrix 42 a graywacke
Total: 100
Total Q, F, and Rf: 58
A. Quartz, feldspars and rock fragments.
Component Proportion1
(%)
Quartz 45
Feldspar 34
Rock fragments 21
Total: 100
1Calculated as the proportion of each
component in the total rock divided by the
total proportion of quartz, feldspars and
rock fragments (in this case that total is 58).
To classify Arenites and Graywacke’s on the basis of their specific
compositions the data must be “normalized” to 100% quartz, feldspars
and rock fragments.

10. The next step is to plot the
normalized data on a ternary
diagram to determine the specific
field in which the data fall.

11. The next step is to plot the
normalized data on a ternary
diagram to determine the specific
field in which the data fall.

12. If the proportion of matrix is less
than 15% plot the data and use
Dott’s diagram for the
classification of arenites.

13. If the proportion of matrix is less
than 15% plot the data and use
Dott’s diagram for the
classification of arenites.

14. If the proportion of matrix is less
than 15% plot the data and use
Dott’s diagram for the
classification of arenites.

15. If the proportion of matrix is less
than 15% plot the data and use
Dott’s diagram for the
classification of arenites.

16. This classification is based on the major component of most sandstones
and provides a basis for a consistent nomenclature for sandstones.
Specific types of rock fragments may also be important in determining
the history of the sediment.
Fragments of limestone or dolomite are simply classed as “rock
fragments” using Dott’s scheme.
Such grains break down rapidly with transport so that their presence
suggests that the sediment was deposited very close to the area that it
was produced.
The names can be modified to reflect other components of the rock:
e.g., Calcareous quartz arenite: a quartz arenite with a calcite cement.

17. I. Genetic Implications of Sandstone Composition
In addition to providing a basis for sandstone nomenclature, the
composition of a sandstone also indicates something of its history.
a) Maturity of a sandstone
Maturity refers to the cumulative changes that particles go through as it
is produced by weathering and is transported to a final site of
deposition.
Given that the source rocks for many sediments are pre-existing
sedimentary rocks, a very mature sediment may have been through the
rock cycle several times.

18. Clastic sedimentary rocks
can be made up of
“multicycled” particles.
i.e., have passed through the
rock cycle several times.
Each time through the cycle
the sediment becomes more
and more mature.

19. Sediment texture and mineralogical composition all reflect the maturity
of a sediment.
Most changes are related to transport distance, nature of weathering at
the site of sediment formation and number of passes through the rock
cycle.
i) Textural Maturity
Changes in grain size and shape.
Increasing textural maturity
Increased sorting
Increased sphericity
Increased rounding

20. Demir, 2003, Downstream changes in
bed material size and shape
characteristics in a small upland stream
Cwm Treweryn, in South Wales,
Yerbilimleri, v. 28, p. 33-47.
From: Gomez, Rosser, Peacock, Hicks
and Palmer, 2001, Downstream fining i
a rapidly aggrading gravel bed river.
Water Resources Research, v. 37, p.
1813-1823.

21. The name of a sandstone tells you something of its maturity.
E.g., a Quartz arenite has less than 15% matrix and is better sorted than
a Quartz graywacke.
The quartz arenite is more mature (greater transport distance and/or
more times through the rock cycle) than the Quartz graywacke.
ii) Compositional Maturity
Compositional maturity is reflected by the relative proportion of
physically soft or chemically unstable grains.
The fewer the soft or unstable grains, the more mature the sediment.

22. What is the relative stability of minerals?
Bowen’s Reaction series shows the sequence in which minerals
crystallize from a cooling magma.

23. Mineral stability can also be shown using Bowen’s Reaction series:
The earliest minerals to crystallize are the least stable.
Quartz is the most stable of the common mineral; it resists chemcial
weathering and is the most common mineral in most sedimentary rocks.
Potassium feldspar is
also common but
Muscovite is relatively
soft and breaks down
during transport.
The stability of rock
fragments varies with
their mineralogy.

24. The most “mature” sediment would be made up of 100% quartz
grains.
With increased transport and number of times through the rock cycle
the less stable minerals are lost.
The “average” igneous and metamorphic rocks contain 60% feldspars.
The “average” sandstone contains 12% feldspars.
This reflects the fact that many sandstones are made up of particles
that have been through several passes of the rock cycle.

25. b) Provenance of a sediment
The Provenance of a sediment is inferred from aspects of composition
that reflect the source rock and tectonic and climatic characteristics of
the source area for the sediment.
Provenance: where something originated.
The source rock of a sediment and the tectonic setting are closely linked:
the tectonic setting determines the relative abundance of different types
of rock that is available for weathering and the production of clastic
sediment.
e.g., An arkosic sandstone (rich in feldspars) would have a source area
that is rich in granites.
A mountain chain adjacent to a convergent margin (e.g., modern Andes)?
An exposed craton (e.g., the Canadian Shield)?
i) Tectonic setting

26. e.g., a sandstone with abundant volcanic and low grade metamorphic
rock fragments.
Island arc setting.
Quartz arenite: sedimentary source rocks; uplifted sediments in an
orogenic belt.
Not foolproof! These are two very different tectonic settings.
ii) Climate
Climate exerts a strong control on the type of weathering that takes place
in the source area of a sediment; this, in turn, influences composition.
Cold, arid climate: predominantly physical weathering, producing
abundant detrital grains (unaltered mineral grains and rock fragments).
Sandstones produced in such settings will be relatively immature,
depending on the source rocks.

27. Warm, humid climate: chemical weathering predominates.
Unstable minerals removed from the sediment that is produced by
weathering.
Will produce a more mature sediment than a cold climate.
Plot of the feldspar content
in sands in eastern and
southern North America.

28. Overall, there is a reduction in the proportion of feldspar in sands
towards the south.
Several factors at work:
Source rocks: in the north are more granitic source rocks whereas in
the south the major source rocks are Paleozoic sedimentary rocks.

29. Climate: colder in the north so that physical weathering is important,
producing immature sediment.
Warmer in the south so that chemical weathering produces a more
mature sediment.
Many sediments were produced during glaciation which only breaks
down source rocks by physical processes.

30. Transport distance: the south has many rivers that have transported
sediment over long distances, increasing the maturity of the sands (e.g.,
Colorado River, Rio Grande, Mississippi River).

31. II. Genetic Classification of sedmentary rocks
Classification on the basis of how the rocks were deposited.
Commonly independent of composition, grain size, etc.
a) Tillite
A rock that is made up of lithified till that was deposited from glacial
ice.
Normally very poorly sorted (mud to gravel-size particles) and the
gravel is angular.

32. b) Turbidites
Rocks made up of sediment
that was deposited from a
turbidity current.
http://cima.uprm.edu/~morelock/8_image/7turb.jpg
Turbidity currents are subaqueous flows of water and sediment that
flow down slope under the influence of gravity.

33. http://cima.uprm.edu/~morelock/8_image/7turb.jpg
Turbidites are characterized
by a particular association
of sedimentary structures.
They may include sediment
ranging from silt to gravel
in size and have a wide
variety of compositions.
Note that this classification
is independent of
depositional environment:
turbidites may be deposited
in marine or non-marine
settings (e.g., lakes).

34. c) Storm Beds (Tempestites)
The lithified deposits of
storms influencing a shallow
marine environment.
Independent of grain size or
lithology.

35. III. Which classification should you use?
This depends on the purpose of the study that you are participating in.
Genetic classification of sedimentary rocks requires a knowledge of the
depositonal setting and cannot normally be made on the basis of hand
specimens alone.
Most studies aimed at determining ancient depositional environments can
classify sandstones on the basis of grain size only.
Studies that aim to reconstruct ancient tectonic settings require a detailed
analysis of the composition of the sandstones.
Some studies require compositional classification in order to understand
the mechanical properties of the sandstone (e.g., if the study aims to
determining excavation costs).

36. Conglomerate A rudite composed
predominanty of
rounded clasts.
Rounded clasts may indicate considerable
distance of transport from source. The
significance will vary with the lithology of the
clast (i.e., limestone clasts will become round a
short distance from their source whereas
quartzite will require much greater transport).
Rudites are classified on the basis of particle shape, packing and
composition.
Classification of Rudites
http://www.geographyinaction.co.uk/Assets/Photo_albums/Seven/pages/Conglomerate_jpg.htm

37. Generally indicates that the clasts have not
traveled far from their source or were
transported by a non-fluid medium (e.g.,
gravity or glacial ice).
A rudite composed
predominantly of angular
clasts.
Breccia
http://homepage.smc.edu/robinson_richard/rocktest/igneous_web/pages/breccia.html

38. Commonly refers to sediment deposited from
glaciers or sediment gravity flows, particularly
debris flows.
A rudite composed of
poorly sorted, mud to
gravel-size sediment,
commonly with angular
clasts.
Diamictite
http://www-eps.harvard.edu/people/faculty/hoffman/Snowball-fig11.jpg

39. Note: in the following the rock names are given for rudites consisting of rounded clasts
(conglomerates) but the term conglomerate may be replaced with the term "breccia" if the clasts
comprising the rock are angular.
Orthoconglomerate
(clast-supported
conglomerate)
Clast-supported framework is typical
of gravels deposited from water flows
in which gravel-size sediment
predominates. Open framework
suggests an efficient sorting
mechanism that caused selective
removal of finer grained sediment.
Closed framework suggests that the
transporting agent was less able to
selectively remove the finer fractions
or was varying in competence,
depositing the framework-filling
sediment well after the gravel-size
sediment had been deposited.
A conglomerate in which all clasts
are in contact with other clasts
(i.e., the clasts support each
other). Such conglomerates may
have no matrix between clasts
(open framework) or spaces
between clasts may be filled by a
matrix of finer sediment (closed
framework).

40. http://seis.natsci.csulb.edu/rbehl/cong.htm
Orthoconglomerate with open
framework.

41. Paraconglomerate
(matrix-supported
conglomerate)
Typical of the deposits of debris flows or
water flows in which gravel size clasts were
not abundant in comparison to the finer grain
sizes.
A conglomerate in
which most clasts
are not in contact;
i.e., the matrix
supports the clasts.
http://www.science.uwaterloo.ca/course_notes/earth/earth390/6.GIF

43. Polymictic
conglomerate
Conglomerates that include clasts from a
wide-variety of source rocks, possibly derived
over a wide geographical area or a smaller but
geologically complex area.
Oligomictic
conglomerate
Suggests that the source area was nearby or
source rock extended over wide geographic
area.
A conglomerate in which
clasts include several
different rock types.
A conglomerate in which
the clasts are made up of
only one rock type.
http://graduate.eas.ualberta.ca/rhartlaub/Rae/QPL.JPG

44. Intraformational
conglomerate
http://www.yuprocks.com/ilist/ic1.html
A conglomerate in which
clasts are derived locally
from within the
depositional basin (e.g.,
clasts composed of local
muds torn up by currents;
such clasts are commonly
termed "rip-up clasts" or
"mud clasts").
Deposition in an environment where muds
accumulated. Muds were in very close
proximity to the site of deposition as the clasts
would not withstand considerable transport.

45. Extraformational
Conglomerate
Clasts derived from a distant
source.
A conglomerate in which clasts
are exotic (i.e., derived from
outside the depositional basin).
Clasts are normally very well
rounded and well sorted.

46. Classification of Lutites
For our purposes, familiarity with terminology will suffice:
Shale: The general term applied to this class of rocks (> 50% of particles are
finer than 0.0625 mm).
Lutite: A synonym for "shale".
Mud: All sediment finer than 0.0625 mm. More specifically used for
sediment in which 33-65% of particles are within the clay size range
(<0.0039 mm).
Silt: A sediment in which >68% of particles fall within the silt size range
(0.0625 - 0.0039 mm).
Clay: All sediment finer than 0.0039 mm.

47. Fissility: Refers to the tendency of lutite to break evenly along parting planes.
The greater the fissility the finer the rock splits; such a rock is said to
be "fissile".

48. Siltstone: A rock composed largely of silt size particles (68-100% silt-size)
Mudstone: A bocky shale, i.e., has only poor fissility and does not split finely.
Argillaceous
sediment: A sediment containing largely clay-size particles (i.e., >50%).
Argillite: A dense, compact rock (poor fissility) composed of mud-size
sediment (low grade metamorphic rock, cleavage not developed).
Psammite: Normally a fine-grained sandstone but sometimes applied to rocks of
predominantly silt-size sediment.

49. Lutite terms based on
proportion of clay, degree of
induration and thickness of
stratification.

50. Terminology related to stratification and fissility (parting).