G342 Sedimentation and Stratigraphy
Lecture 6: Sandstones and Conglomerates
17 February 2005
Assoc. Prof. A. Jay Kaufman
Sandstones and Conglomerates
Because of their formation requirements (coarse detritus, strong current transport) and
their physical properties (high porosity and permeability), the majority of sedimentary research
has focused on sandstones and conglomerates. This is even though they represent, at most, 25%
of the stratigraphic record. Below are sets of criteria and classifications that are handy in
describing these lithologies
Conglomerate and Breccia
Lithified gravel and rubble are called conglomerate and breccia, respectively.
Conglomerates are very special in the sedimentary record for two reasons. First, they are always
associated with high energy systems. Second, they bring large chunks of material right to your
feet. As such, they are very helpful in reconstructing provenance of the hinterland, and are often
linked to nearby tectonic activity.
Orthoconglomerates consist primarily of framework grains and <15% matrix.
Paraconglomerates have a matrix of sand and finer clasts and are matrix-supported. The
systematic orientation of clasts in a conglomerate is termed imbrication. In conglomerate
classification, there main concern is composition, but there are also textural issues around grainsize distribution and roughness of grains.
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Diamictite is another term for a paraconglomerate, and is often used to denote glacial
rocks. Clasts that appear to pierce finely laminated sediments were likely transported by
sediment-laden icebergs and are called dropstones.
In addition to a glacial origin for conglomerate and breccia, these coarse-grained deposits
can also result from subaerial and subaqueous debris flows, subaqueous grain flows, turbidity
currents, and bolide impacts.
Sandstone
Sandstone is the indurated equivalent of unconsolidated sand containing clasts between 2
and 1/16 mm. Sandstones hold over ½ of the worlds oil reserves
There are MANY different classification schemes for sandstones, but the two defining
parameters are the percentage of matrix and the composition of sand framework grains.
Name
Φ
Quartz arenite
Arkose
Grain-size
(mm)
2 – 0.064
2 – 0.064
-1 to 4
-1 to 4
Lithic arenite
2 – 0.064
-1 to 4
Graywacke
2 – 0.064*
-1 to 4*
>2 mm
>2 mm
>2 mm
>2 mm*
< -1
< -1
< -1
< -1*
Conglomerates
Breccias
Orthoconglomerate
Paraconglomerate
Composition
>95% quartz
Surprisingly common
>25% feldspar Often
associated
w/
recycling of continental
cratons
>25% lithic Often
associated
w/
fragments
recycling of arcs and
mountain belts
15-75% mud
Often
associated
w/
recycling of arcs and deep
basins
varied
Round clasts
varied
Angular clasts†
varied
Framework supported
varied
Matrix supported
* actual mean and modal grain size varies with mud content
† there are many different origins to breccias – be careful in interpretation.
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Other
Quartz arenite
Quartz arenites are typically deposited in stable cratonic environments such as eolian,
beach and shelf. They are generally well lithified and well cemented, and texturally mature to
supermature. These sediments are most likely the product of multiple recycling of quartz grains
(polycyclic). If they are first generation it suggests an intense weathering environment.
Feldspathic arenite
A medium to coarse grained lithic arenite with >25% feldspar is known as an arkose.
These sandtonse form from the rapid disentigration of granite to form grus, which is rapidly
deposited, most often in alluvial fan deposits. They originate either 1) in cold or very arid
environments where chemical processes are inhibited, or 2) in warmer, humid environments
where marked relief or local uplift allows rapid erosion of feldspar before they can be
decomposed. A good nearby example is the Old Red Sandstone along barge canal at Great Falls.
Greywacke
Arenites are differentiated from wackes by the percent of matrix grains. Wackes contain
>15% and arenites less. The proportion of rock fragments to feldspar in wackes is believe to be a
reliable index of sandstone provenance, one supracrustal (i.e. volcanics and fine-grained
sedimentary rock) and the other subcrustal (i.e. granite and gneiss)
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Textural maturity of siliciclastic rocks
There are four stages of textural maturity based on 1) the proportion of clay clasts, 2) the
sorting of the sand grains, and 3) the roundness of the sand grains. Sandstone is texturally
immature if there is >5% clay clasts, regardless of sorting or rounding. These also tend to be rich
in feldspar, and are common in alluvial fans, turbidites, and overbank deposits. Submature
sandstones have <5% clay but are otherwise similar to immature sandstones. Mature and
supermature sandstones also have <5% clay, but the sand framework is well rounded and sorted.
These are common in fluvial channels, beaches, and aeolian dunes.
Grain textures are the chief factors that control the porosity and permeability of
sediments in siliciclastic settings. These include 1) grain size distribution (mean, median, and
sorting), 2) shape (sphericity), 3) packing, 4) composition, and 5) cementation. Porosity is the %
of void space,
P = (bulk volume – grain volume)/bulk volume
Porosity is NOT constrained by geometry. The distribution of pores in a volume is
independent of porosity. In general, better sorting results in better porosity, and finer grain sizes
show higher porosity.
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Permeability is the “flowability” of fluids within a volume of rock and is expressed in
Darcy units (cm/s). Permeability is STRONGLY constrained by geometry. The governing
parameter is pore-throat diameter, or the size of the minimum space between grains. As such,
permeability is affected both by sorting and mean grain size. A heterogeneous fabric diminishes
permeability by increasing the tortuosity of the pore system. Permeability is not flow rate, which
varies as a function of viscosity, temperature, and pressure.
Packing is the mutual spatial relationships between grains. This includes packing
proximity, or how many grains another grain touches, and packing density, or the cumulative
number of grains intercepted by a random line through the volume. Rocks with identical grainsize distributions can have different porosity and permeability as a function of packing.
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