SANDSTONES

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MAYA BRADFORD
SANDSTONES
Consist mainly of silicate grains ranging in size from 1/16 to 2 mm, plus cement and matrix grains
smaller than 0.03 mm.
MINERALOGY AND CHEMICAL COMPOSITION (OF FRAMEWORK GRAINS, CEMENTS, and
MATRIX MINERALS):
Only a few principal kinds of minerals make up the bulk of all sandstones. Framework grains can
be individual mineral grains or clasts of parent rock that have not yet broken down to individual
mineral grains. Binding together those framework grains are cements, which are made up of
either silicate minerals (such as quartz) or nonsilicate minerals (such as calcite and dolomite).
Matrix minerals fill in the interstitial spaces and are largely made up of clay minerals.
Mineralogy and Chemical Composition of Framework Grains
- Quartz
- 50-60% of the framework grains of average sandstones
- Superior hardness + chemical stability
- Derived from plutonic rock, particularly felsic plutonic rocks such as granites,
metamorphic rocks, and older sandstones
- Feldspars
- 10-20% of the framework grains of average sandstones
- K(potassium)-feldspar v. plagioclase feldspar
- Softer and chemically less stable than quartz
- K(potassium)-feldspars are generally considered to be more abundant overall in
sedimentary rocks than plagioclase feldspars; however, plagioclase is more abundant in
sandstones derived from volcanic rock
- Accessory minerals
- <1-2% of the framework grains of average sandstones
- Micas (muscovite and biotite): <0.5% of the framework grains of average sandstones,
muscovite is more stable and more abundant in sandstones than biotite, micas are
derived particularly from metamorphic parents and plutonic igneous parents
- Heavy minerals: minerals that have a specific gravity greater than 2.9, include stable (ex.
zircon, which can survive multiple recycling episodes and are commonly rounded,
indicating the last source was sedimentary) and unstable minerals (ex. magnetite,
pyroxenes, and amphiboles, are less likely to survive recycling, are commonly first-cycle
sediments that reflect the composition of proximate source rocks). Heavy minerals are
useful indicators of sediment source rocks because different types of source rocks yield
different suites of heavy minerals
- Rock fragments/ clasts
- 15-20%* of the framework grains of average sandstones
*(the rock fragment content of sandstones is highly variable and ranges from 0 to >90%)
- Pieces of the parent rock that have not yet broken down into individual mineral grains
- Most common rock fragments in sandstones are clasts of volcanic rocks, volcanic glass,
and fine-grained metamorphic rocks such as slate, phyllite, schist, and quartzite
- Chert grains are rock fragments
Mineralogy and Chemical Composition of Cements
- All cements are secondary minerals that form in sandstones after deposition and during burial
(see: diagenesis)
- Silicate cements (i.e., quartz)
- Quartz is the most common silicate mineral that acts as a cement. In most sandstones,
the quartz cement is chemically attached to the crystal lattice of existing quartz grains,
forming rims of cement called overgrowths, which look like lines of impurities or bubbles
marking the surface of an original grain.
MAYA BRADFORD
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In some sandstones, the quartz cement forms a mosaic of very tiny quartz crystals that fill
the interstitial spaces among framework silicate grains and is called microcrystalline
quartz.
- Opal can occur as a cement in sandstones, particularly in volcanic sandstones
Nonsilicate cements (i.e., calcite, dolomite)
- Carbonate minerals are the most abundant nonsilicate mineral cements in sandstones
- Calcite is the most common carbonate cement and is precipitated in the pore spaces
among framework grains, forming a mosaic of smaller crystals which adhere to the larger
framework grains and bind them together
- Dolomite and siderite (iron carbonate) are less common
Hematite and limonite, feldspars, anhydrite, gypsum, barite, clay minerals, and zeolite (volcanic)
minerals can also make up a sandstone’s cement
Minerology and Chemical Composition of Matrix Minerals
- Matrix minerals are grains smaller than about 0.03 mm and fill interstitial spaces in sandstones.
Usually, matrix grains are largely made up of clay minerals but may include micas, quartz, and
feldspars
- Clay minerals form principally as secondary minerals during subaerial weathering and hydrolysis,
but may also form via subaqueous weathering or during burial diagenesis
CLASSIFICATION (mostly) ON THE BASIS OF MINERAL COMPOSITION
Descriptive classification of sandstones is based fundamentally on framework mineralogy.
Relative abundance of matrix also influences some classifications.
At the end of the day, it is very difficult to name one way to suitably classify all types of
sandstones — classifications that are all inclusive tend to be too complicated and unwieldy for
general use, and classifications that are oversimplified do not convey enough information
Methods for Classifying Sandstones
- Step one: Is it a sandstone? Textural nomenclature to name a rock of mixed sediments:
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The first way to classify sandstones is to use your grain scale card to see what are the
sizes of the particles making up your rock and then use triangular texture diagrams to see
if the ratio of sediment sizes in your rock make it a sandstone, or another rock like a
mudstone, shale, conglomerate, etc…
Step two: Mineralogical classification of sandstones:
MAYA BRADFORD
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Most sandstone classification schemes involve a QFR or QFL plot, which is a triangular
diagram on which quartz (Q), feldspars (F), and rock fragments (R or L) are plotted at the
ends of the triangle. The book recommends the Gilbert model:
Sandstones with <5% matrix are classified as quartz arenites, feldspathic arenites, or
lithic arenites based on the relative abundance of QFL components
- Arkose is an informal term for any feldspathic arenite that has an abundance of
feldspars >25%
Sandstones with 5% matrix are classified as quartz wackes, feldspathic wackes, or lithic
wackes
- Graywacke is another informal term for any matrix-rich sandstones that have
undergone deep burial, have a chloritic matrix, are dark gray to dark green, and
are very hard and dense (controversial)
Another step: Classification of sandstone maturity:
- Compositional maturity: refers to the relative abundance of stable and unstable
framework grains; a sandstone composed mainly of quartz is considered compositionally
mature while a sandstone composed mainly of unstable minerals (i.e., feldspars) or
unstable rock fragments is compositionally immature
- Textural maturity: determined by the relative abundance of matrix and the degree of
rounding and sorting of framework grains
Major Classes of Sandstones
MAYA BRADFORD
QUARTZ
ARENITES
(~15% of all
sandstones)
FELDSPATHIC
ARENITES
(~15-20% of all
sandstones)
LITHIC
ARENITES*
(~50% of all
sandstones)
COMPOSITION
COLOR
MATURITY
ASSOCIATED
SEDIMENTARY
STRUCTURES
ORIGIN AND
DEPOSITIONAL
ENVIRONMENT
EXAMPLE
>90% siliceous
grains that may
include quartz,
chert, and
quartzose rock
fragments
commonly
white or
light gray
but may be
stained
red, pink,
yellow, or
brown by
iron oxides
most are
texturally
mature to
supermature.
wackes
uncommon
cross-bedding and
ripples may be
abundant, fossils
are rarely abundant
Jurassic Navajo
Sandstone,
Colorado;
<90% quartz, more
feldspar than
unstable rock
fragments, and
minor amounts of
other minerals such
as micas and heavy
minerals
some are
pink or red
because of
k-feldspars
and iron
oxides;
others are
light gray
to white
commonly
texturally
immature or
submature
not characterized by
any specific kinds of
structures
could be first-cycle deposits
derived from primary
crystalline or metamorphic
rocks, with vigorous unstable
chemical-eliminating
weathering conditions. more
likely the product of multiple
recycling of quartz grains
from sedimentary source
rocks. typically deposited in
stable cratonic environments
such as eolian, beach, and
shelf environments
although some feldspars may
survive recycling from a
sedimentary source, it’s
unlikely that sedimentary
source rocks can furnish
enough feldspar to produce a
feldspathic arenite or arkose.
mostly derived from granitictype primary crystalline rocks
(i.e., coarse granite or other
rocks abundant in k-feldspar).
feldspathic arenites that are
abundant in plagioclase
feldspar are derived from
igneous rocks (i.e., quartz
diorites) or from volcanic
rocks. deposited in very cold
or very arid climates (where
chemical weathering
processes are inhibited and
large quantities of feldspar
remain), or in warmer, more
humid environments where
marked relief of local uplifts
allows rapid erosion of
feldspars before they can be
decomposed
>90% quartzose
grains and contain
more unstable rock
fragments than
feldspars
all kinds of
gray
texturally
immature to
submature
all kinds of
structures
originate under conditions
favoring the production and
deposition of large volumes
of relatively unstable
materials. probably derived
from rugged, high-relief
source areas. may be
deposited in nonmarine
settings nearby alluvial fans
or other fluvial environments,
in marine foreland basins
adjacent to fold-thrust belts,
or in deltaic or shallow shelf
environments
Pennsylvanian
Pottsville
Formation,
Appalachians
Cretaceous
Dakota
Sandstone,
Colorado
Carboniferous
Old Red
Sandstone,
Scotland
*Volcaniclastic sandstones are a special kind of lithic arenite composed primarily of volcanic detritus and are made
up largely of pyroclastic materials. They are characterized by the presence of euhedral feldspars, pumice fragments,
glass shards, and volcanic rock fragments, and generally have a very low quartz content
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