SILICICLASTIC SEDIMENTARY ROCKS

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SILICICLASTICSEDIMENTARY ROCKS

• Prepared by Dr. F. Clark
• Department of Earth and Atmospheric Sciences,
  University of Alberta
• August 06

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INTRODUCTION - TEXTURES

• Just as with igneous rocks, the textures of
  siliciclastic sedimentary rocks are involved in
  their classification. As a first pass, the rock
  name depends on the grain size, but other aspects
  of texture, namely shape and arrangement, are
  factors in further refinement of the name. In
  gross terms, three grain sizes, namely 2 mm, 1/16
  (0.0625) mm, and 1/256 (0.0039) mm, divide
  grains, and thus siliciclastic sedimentary rocks,
  into four size classes. Those four size classes
  correspond to conglomerate, sandstone, siltstone,
  and claystone, in order from coarsest to finest
  grain sizes.

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Conglo-merate.A significant proportion of grains
(some sources suggest over 30) is larger than 2
mm in diameter.

• Loose sediment with this grain size
  characteristic is referred to as gravel. In the
  sample above, the larger gravel-sized grains
  yellow arrows constitute the framework, whereas
  the smaller, sand- and silt-sized grains
  constitute the matrix blue arrows.

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Conglo-merate?Perhaps no more than 15 of the
grains exceed the 2 mm lower size limit most of
the grains are sand-sized.

• This sample shows that the distinction between
  the size classes can be somewhat arbitrary. A
  subtle change in the current which produced the
  lamination green arrow in these finer gravels,
  and from which this sediment was deposited, could
  have resulted in all grains being sand purple
  arrows or gravel yellow arrows.

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Conglomerate vs. Breccia

• In more traditional usage, the term conglomerate
  applies to those rocks with rounded clasts
  (left), whereas those rocks with more angular
  grains (right) are referred to as breccia. The
  angularity of the grains on the right specimen is
  not pronounced, so the term breccia might not be
  appropriate in this case.

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Conglomerate vs. Diamictite

• In more current usage, the term conglomerate
  applies to those rocks that are grain-supported,
  such as on the left framework grains are in
  contact light blue arrows. It is said to have
  an intact framework. On the right, the rock is
  matrix-supported dark blue arrows and the
  grains are not touching. This is called a
  diamictite, and is said to have a dispersed
  framework.

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Diamictite.The high matrix content blue arrows
is best seen where framework grains have been
plucked yellow stars.

• The large amount of matrix is sufficient to form
  durable external molds of missing framework
  grains. Such high matrix content is commonly
  associated with glacial activity, which does not
  selectively remove the finer matrix grains, or
  flood episodes.

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Sand-stone.The highest proportion of grains lie
in the range between 2 mm and 1/16 mm. These
rocks are also called arenites.

• The yellow arrows point to individual grains
  which show up slightly darker than their
  neighbouring grains. Virtually all the grains are
  of the stable silicate mineral quartz, and so
  this is a quartz sandstone or quartz arenite.

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More Quartz Sandstones

• The sample on the left shows lamination, parallel
  to the green arrow, reflecting subtle changes in
  colour due to trace amounts of stain in the
  cementing material that holds the grains
  together. Lighter coloured layers, lacking the
  stain, are highlighted by light blue arrows.
  Yellow arrows point to individual grains. The
  right sample shows the uniform light appearance
  of many quartz arenites.

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Lithic Arenite.Lithic (from the Greek lithos,
meaning stone) sandstone or arenite is
characterized by abundant rock fragments.

• Because of the very high mechanical and chemical
  stability of quartz, it will also usually be
  abundant in lithic sandstone. As a result, lithic
  arenites characteristically have a
  salt-and-pepper look to them. This example has
  traces of woody plant fossil matter brown
  arrows.

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Lithic Arenites/Sandstones

• These examples are from the Belly River Formation
  of Cretaceous age (on the order of 80 million
  years old), from the Foreland Basin of Western
  Canada. In these classic salt-and-pepper lithic
  arenites, the pepper is sand-sized grains of
  chert. Although chemically the same as quartz,
  chert is classified as a lithic grain or rock
  fragment by most sedimentologists, rather than as
  a variation of quartz.

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Siltstone.Silt-sized grains are by definition
between 1/16 and 1/256 mm.

• This sample from the Spray River Group of Western
  Canada is quarried near Canmore as Rundle Rock,
  and is used as a facing stone in construction,
  especially common on upscale homes. Clearly,
  individual grains are barely discernible without
  magnification.

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Lamination and Bedding in Siliciclastics

• These two views of the Spray River siltstone
  illustrate lamination parallel to green arrows,
  which is basically a synonym for layering. This
  characteristic of many sedimentary rocks is
  produced by discontinuities (e.g. grain size,
  grain type, colour) in sedimentation. Discreet
  units of sediment are bounded by bedding planes
  blue arrows the layers are called beds if they
  exceed 1 cm in thickness.

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Siltstone.At these fine grain sizes, individual
grains can barely be detected even with a hand
lens, and only if they are coarse silt.

• The next grain size working down from silt is
  clay, less than 1/256 mm. It is not generally
  practical, even with significant magnification,
  to distinguish between fine silt- and clay-sized
  grains. This practical limitation gives rise to
  the two siliciclastic rock types that follow.

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Mudstone.This term embraces rocks with grain
sizes less than 1/16 mm, and what is called
blocky fracture, without distinguishing silt vs.
clay.

• The sample above is relatively thick, bounded
  above and below by bedding planes blue arrows,
  and has broken along irregular failure surfaces
  unrelated to bedding purple arrows into the
  three pieces, producing relatively thick,
  irregular chunks of rock.

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Mudstone.At gt 1 cm, the mudstone slab
constitutes a bed, whose upper and lower bounding
surfaces blue arrows are bedding planes.

• The tendency of mudstones is to break along
  fracture surfaces purple arrow unrelated to
  both bedding blue arrows and lamination green
  arrow. Our understanding is that mudstones do
  this because flat or platy grains are not aligned
  parallel to each other and the lamination.

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Shale.The term shale is applied to those rocks,
with grains less than 1/16 mm, that are fissile,
or split into thin sheets, without regard to silt
vs. clay.

• Again, we may not be able to distinguish
  siltstones from claystones proper, so we classify
  the rock according to a gross textural
  characteristic, namely how it breaks or splits.
  Our understanding is that fissile rocks owe their
  character to parallel alignment of platy grains.

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Fissility Expanded

• In these two views of a shale, we see bedding
  planes blue arrows being exploited as planes of
  weakness yellow arrows that make this rock
  fissile. It must be pointed out that the parallel
  alignment of mineral grains that produces these
  planes of weakness occurs at the time of
  deposition, unlike the parallel alignment that
  produces slaty cleavage in certain similar
  metamorphic rocks, in response to stress.

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Fissility Not Always Planar

• These views of a shale illustrate that the
  lamination of a shale, the bedding planes of that
  shale blue arrows, and the resulting fissility
  purple arrows are not necessarily planar. The
  sea or lake bottom is often characterized by an
  irregular surface that is referred to as a
  bedform (ripples and dunes are examples). Bedform
  development is controlled by the interplay of
  sediment and waves or currents.

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Colour as Environmental Indicator

• The different colours of these shale samples tell
  us something about the conditions at their
  environment of deposition. The black colour of
  the left specimen is due to preserved organic
  matter in an anoxic or anaerobic environment,
  whereas the red sample on the right reflects
  oxidizing conditions that have turned the iron
  content red.