Department of Petroleum Geology & Sedimentology,
Faculty of Earth Sciences,
King Abdulaziz University,
Jeddah, Saudi Arabia
EPS 421
CLASTIC SEDIMENTARY ROCKS
Lecture 10: Classification of sandstones,
Provenance and Tectonic Setting
Prof. Dr. Mahmoud A. M. Aref
Classification of Sandstones
Most sandstone classifications are based on the composition of the rock.
Four Major Components Of Sandstone
Framework
Sand Grains That Are Made Up Crystals Of Two Or More Different
Minerals. Sand (And Silt) Size Detrital Grains
Matrix
Fine-grained (<0.03mm) Material That Are Associated With The
Sand Grains. Silt And Clay Size Detrital Material
Cement
Material Precipitated Post-depositionally, During Burial. Cements
Fill Pores And Replace Framework Grains
Pores
Voids Among The Above Components
EPS 321 Lecture 10
FOUR COMPONENTS OF SANDSTONE
Geologist’s Classification
1.
2.
3.
4.
Framework
Matrix
Cement
Pores
Note different use of “matrix”
by geologists and engineers
Engineering
“matrix”
PORE
FRAMEWORK
CEMENT
(QUARTZ)
MATRIX
FRAMEWORK
(FELDSPAR)
0.25 mm
EPS 321 Lecture 10
CLASSIFICATION OF SANDSTONES
™
Classifications of sandstones are petrographic, based on
microscopic studies and requiring approximate determinations
of the modal composition.
™
Most classifications use a triangular diagram with end
members of quartz (+ chert), feldspar and rock fragments. The
triangle is divided into various fields, and rocks with a modal
analysis falling within a particular field are given a particular
name.
™
In this scheme, sandstones are divided into two major groups
based on texture, that is whether the sandstones are
composed of grains only, the arenites, or contain more than
15% matrix, forming the wackes.
EPS 321 Lecture 10
EPS 321 Lecture 10
EPS 321 Lecture 10
Arenites Group
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Of the arenites, the term quartz arenite is applied
to those with
95% or more quartz grains, a rock-type formerly referred to as quartzite or
orthoquartzite.
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Arkosic arenite refers to an arenite with more than 25% feldspar, which
exceeds the rock fragments content, The arkosic arenites can be divided
into arkoses and lithic arkoses.
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Litharenite is applied where the rock fragments content exceeds 25% and is
greater than feldspar.
„
Specific names which have been applied to litharenite are phyllarenite where
the rock fragments are chiefly of shale or slate, and calclithite where the rock
fragments are of limestone.
„
Two rock-types are transitional with quartz arenite are subarkose and
sublitharenite.
EPS 321 Lecture 10
Wackes Group
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The wackes are the transitional group between arenites and
mudrocks. The most familiar is the greywacke.
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Two types are distinguished: feldspathic and lithic greywacke.
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The term arkosic wacke is used for arkoses with a significant
proportion of matrix.
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Quartz wackes, not a common rock-type, are dominant quartz
plus some matrix.
EPS 321 Lecture 10
Failure in the classification of sandstones
„
This classification is primarily concerned with the mineralogy
of the sediment and presence or absence of a matrix.
„
It is independent of the depositional environment, although
some lithologies are more common in certain environments.
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The nature of the cement in arenites is not taken into account.
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With regard to fine-grained interstitial material (matrix), a
basic feature of the wackes, there is often a problem of origin.
Some is deposited along with the sediment grains, or could
be authigenic, a cement, and some a diagenetic alteration
product of unstable grains. The latter is particularly the case
with greywackes.
EPS 321 Lecture 10
Hybrid sandstones
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In addition to terrigenous clastic sandstones, there are many hybrid
sandstones. These contain a non-clastic component derived from within the
basin of deposition.
The three main types are calcarenaceous, glauconitic and phosphatic
sandstones.
In glauconitic sandstones, the glauconite occurs as sand-sized pellets.
With phosphatic sandstones, the phosphate may be present as a cement,
nodules, coprolites or bone fragments.
Calcarenaceous sandstones contain up to 50% CaCO3, present as
carbonate grains. The latter are chiefly ooids, often with quartz nuclei, and
carbonate skeletal fragments. Calcarenaceous sandstones are developed in
carbonate-producing areas where there is a large influx of terrigenous
clastics. They will often pass laterally into limestones or into purer
sandstones towards the source of the siliciclastic sediment.
Terrigenous clastics cemented by calcite have been referred to as
calcareous sandstones.
EPS 321 Lecture 10
Petrography and origin of principal sandstone types
„
The four common types of sandstone are quartz arenite, arkose,
litharenite and greywacke. They are frequently typical of a particular
depositional environment, but because of the provenance control on
sandstone composition, they are not restricted to that particular
depositional setting.
Four categories of sandstone as seen in thin section under the microscope.
Diameter of field of view is about 4 mm.
EPS 321 Lecture 10
1. QUARTZ ARENITES
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Sandstones with at least 95% quartz grains are the most
compositionally mature of all sandstones. In addition, they frequently
consist of well-rounded and well-sorted grains so that textural
maturity is also very high.
„
Cements are typically quartz overgrowths, but calcite is also
common. Monocrystalline quartz grains dominate. Common heavy
minerals are tourmaline, zircon and ilmenite.
EPS 321 Lecture 10
ARKOSES-1
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Arkoses contain more than 25% feldspar, much quartz and some rock
fragments. Detrital micas are also present and some fine-grained matrix.
„
The feldspar is chiefly potassium feldspar and much of this is microcline.
The feldspar is usually fresh although some may be altered to kaolinite and
sericite.
„
Polycrystalline quartz and quartz/feldspar rock fragments are common.
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Arkoses are typically red or pink, through the feldspar's color, but also
through the presence of finely disseminated hematite, since many arkoses
occur in red-bed sequences.
EPS 321 Lecture 10
ARKOSES-2
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Arkose texture is typically poorly-sorted to well-sorted, with very
angular to sub-rounded grains, the precise texture dependent
on the degree of transportation.
„
Grain-supported arkoses are cemented by calcite or quartz,
while others are cemented by a matrix, often containing much
kaolinite.
EPS 321 Lecture 10
3. LITHARENITES
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Litharenites are characterized by a rock fragment content which is in excess of
feldspar.
„
They range widely in composition, both in terms of mineralogy and chemistry,
depending largely on the types of rock fragment present. These are chiefly
fragments of mudrock and their low grade metamorphic equivalents; other
components are flakes of mica, some feldspar and much quartz.
„
There is little primary matrix, otherwise they are similar to greywackes in
composition and in fact they have been referred to as subgreywackes.
„
Cements are usually either calcite or quartz.
EPS 321 Lecture 10
4. GREYWACKES
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As indicated by the name, greywackes are dark grey or black rocks, often
resembling dolerite
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The characteristic feature of greywackes is the fine-grained matrix, which
consists of an intergrowth of chlorite, sericite and silt-sized grains of quartz
and feldspar.
„
Of the sand fraction, quartz dominates over rock fragments and feldspar. Many
different rock fragments are usually present. Feldspar grains are chiefly sodic
plagioclase and these are usually fresh in appearance.
„
The origin of the matrix has been referred to as the 'greywacke problem'. There
are two possibilities: fine grained sediment deposited along with the sand
fraction (primary origin), and diagenetic alteration of unstable rock fragments
(diagenetic origin).
EPS 321 Lecture 10
Genetic Implications of Sandstone Composition
a)Maturity of a sandstone
i) Textural Maturity
ii) Compositional Maturity
b) Provenance of a sediment
i) Tectonic setting
ii) Climate
EPS 321 Lecture 10
EPS 321 Lecture 10
Provenance of a sediment
Provenance: where something originated.
The Provenance of a sediment is determined by aspects of
composition that reflect the source rock and tectonic and
climatic characteristics of the source area for the sediment.
i) Tectonic setting
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.
EPS 321 Lecture 10
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.
EPS 321 Lecture 10
Provenance & Tectonic setting of Quartz Arenites
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In many cases, quartz arenites are the product of extensive periods of
sediment reworking, so that all grains other than quartz have been broken
down.
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The majority of the quartz grains are second cycle, derived from pre-existing
sediments. Quartz arenites of this type, often deposited on shallow-marine
shelves, with little tectonic movement and the basin subsides very slowly.
EPS 321 Lecture 10
Provenance & Tectonic setting of Arkoses
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Arkoses are derived from granites and gneisses and vary from
in-situ weathering products, to stratified and cross-bedded
arkoses where there has been substantial sediment transport.
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Arkoses are clearly derived from feldspar-rich rocks but apart
from an appropriate provenance geology, climate and source
area relief are also important factors. Under humid conditions,
feldspars weather to clay minerals, so that semi-arid and glacial
climates favor arkose formation. If erosion is very rapid,
however, i.e. the source area has a high relief, then arkoses
can accumulate in spite of an adverse climate. Many arkoses
were deposited in fluviatile environments.
EPS 321 Lecture 10
Geologic environment in which an arkose would
be deposited.
EPS 321 Lecture 10
Provenance & Tectonic setting of Litharenites
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Litharenites account for some 20 to 25% of all sandstones.
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Their immature composition implies high rates of sediment
production followed by short transport distances. Many
fluviatile and deltaic sandstones are litharenites.
EPS 321 Lecture 10
Provenance & Tectonic setting of Greywackes
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Many greywackes were deposited by turbidity currents in basins of various
types, often off continental margins and in association with volcanics.
Frequently greywackes are transported by masses of water highly charged
with suspended sediment. Because of the suspended matter, the mass is
denser than surrounding water and moves along the sloping sea floor or down
submarine canyons as a turbidity current.
Greywacke sediment characteristically accumulates in deep-sea fans at the
base of the continental slope.
EPS 321 Lecture 10
Provenance & Tectonic setting of Sandstones
Sedimentary Rock Source area
lithology
Paleoclimate
Quartz ss, well
sorted, well
rounded
Granite
Arkose, poorly
sorted, poorly
rounded
Granite
Quartz ss,
angular, muddy,
poorly sorted
Granite
Arkose, well
sorted, well
rounded
Granite
Litharenite,
poorly sorted, no
mud, angular
Basalt,
Gabbro,
Gneiss,
other mafic
rock
EPS 321 Lecture 10
Tectonic
Activity
Energy Levels
Time
Provenance & Tectonic setting of Sandstones
Sedimentary Rock Source area
lithology
Paleoclimate
Quartz ss, well
sorted, well
rounded
Granite
Humid
Arkose, poorly
sorted, poorly
rounded
Quartz ss, angular,
muddy, poorly
sorted
Arkose, well sorted,
well rounded
Granite
Arid
Litharenite, poorly
sorted, no mud,
angular
Basalt,
Gabbro,
Gneiss,…
Granite
Humid
Granite
Arid
Arid or
Temperate
EPS 321 Lecture 10
Tectonic
Activity
Energy Levels
Time
Provenance & Tectonic setting of Sandstones
Sedimentary Rock
Source area
lithology
Quartz ss, well
sorted, well
rounded
Granite
Arkose, poorly
sorted, poorly
rounded
Quartz ss,
angular, muddy,
poorly sorted
Arkose, well
sorted, well
rounded
Litharenite, poorly
sorted, no mud,
angular
Granite
Granite
Paleoclimate
Tectonic
Activity
Humid
Passive
Arid
Active
Humid
Passive
Arid
Active
Granite
Basalt,
Gneiss,
other mafic
rock
Arid or
Temperate
EPS 321 Lecture 10
Active
Energy Levels
Time
Provenance & Tectonic setting of Sandstones
Sedimentary Rock
Source area
lithology
Quartz ss, well
sorted, well
rounded
Granite
Arkose, poorly
sorted, poorly
rounded
Quartz ss,
angular, muddy,
poorly sorted
Arkose, well
sorted, well
rounded
Litharenite, poorly
sorted, no mud,
angular
Granite
Granite
Paleoclimate
Tectonic
Activity
Energy Levels
Humid
Passive
High and
consistent
Arid
Active
Inconsistent
Humid
Passive
Inconsistent
Arid
Active
High and
consistent
Active
Inconsistent
Granite
Basalt,
Gneiss,
other mafic
rock
Arid or
Temperate
EPS 321 Lecture 10
Time
Provenance & Tectonic setting of Sandstones
Sedimentary Rock
Source area
lithology
Tectonic
Activity
Paleoclimate
Energy Levels
Time
Quartz ss, well
Granite
sorted, well rounded
Humid
Humid
Passive
High and
consistent
Long
Arkose, poorly
sorted, poorly
rounded
Quartz ss, angular,
muddy, poorly
sorted
Arkose, well sorted,
well rounded
Granite
Arid
Arid
Active
Inconsistent
Short
Granite
Humid
Humid
Passive
Inconsistent
Short
Granite
Arid
Arid
Active
High and
consistent
Long
Litharenite, poorly
sorted, no mud,
angular
Arid or
Basalt,
Arid or
Temperate
Gneiss, other Temperate
mafic rock
Active
Inconsistent
Short
EPS 321 Lecture 10