Chapter 5: Sandstone and Conglomerates

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Chapter 5Sandstone and Conglomerates
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Terrigenous

• From the Earth
• Terrigenous clastic sedimentary rocks are
  composed of clasts of pre-existing rocks and
  minerals.
• Also referred as
• detrital (detached from pre-existing rocks by
  erosion or weathering), or
• Epiclastic (derived from the surface).

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Silisiclastic

• Because most terrigenous clastics are especially
  rich in quartz and other silicates minerals, the
  term silisiclastic is also used.
• Clast formed by physical weathering are eroded
  and transported by mass wasting, wind, water, and
  ice and are deposited as discrete, unconsolidated
  fragments that are eventually lithified.

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• Terrigeneous sediments and sedimentary rocks are
  defined on the basis of clast diameter.
• Three distinct groups are recognized
• Conglomerates and breccias
• Sandstones
• mudrocks

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Conglomerates and Breccias

• Conglomerates is lithified gravel made up of
  rounded to subrounded clasts whose diameters
  exceed 2 mm.
• They are also called roundstone or puddingstone.
• Breccias is lithified rubble made up of angular
  clasts coarser than 2 mm.
• They are also called sharpstone.

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Conglomerates and Breccias

• The roundness (angularity) of the grains is
  measured using standard grain silhouettes.
• Very coarse clastic rocks are collectively
  referred to as rudites or rudaceous sedimentary
  rocks.

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Composition

• Most clasts on conglomerates and breccias are
  fragments of rocks and minerals produced by the
  disintegration of bedrock.
• These occur both as coarser-grained fragments and
  finer-grained matrix filling the space between
  fragments.
• Clasts are typically glued together by a small
  amount of siliceous, calcareous, or ferruginous
  cement.

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Conglomerates and Breccias

• Three principal categories of clasts are
  distinguished
• Mineral fragments that occur as major components,
• Mineral fragments that occur as accessory
  constituents, and
• Fragments of rock.

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Mineral fragments occurring as major constituents
(5 or more)

• Clasts of a single mineral such as quartz or
  feldspar tend to be less abundant in
  conglomerates and breccias than in sandstone
  because few igneous, metamorphic, or sedimentary
  rocks have original grains coarse enough to
  disintegrate into pebbles and coarser detritus.
• Source rocks with mineral grain diameters coarser
  than 8 mm (fine pebble) include quartz veins,
  pegmatites, deep-seated plutons, high-grade
  metamorphic rocks, breccias and conglomerates.

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• Quartz is the most abundant major mineral in
  conglomerates and breccias.
• It is harder than other rock-forming mineral
• Has no cleavage
• Practically insoluble
• Large clasts of K-feldspar, plagioclase feldspar,
  and mica can also be abundant but seldom last as
  long as quartz because they corrode,
  disaggregate, and abrade with transport.
• The sand matrix is similar in composition to
  sandstone interbedded with the conglomerate or
  breccia.

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Mineral class occurring as accessory constituents
(less than 5)

• Other fragments occur as accessory, their
  presence is incidental.
• They occur as accessory minerals either because
  their original abundance in source rocks is low
  or because they are easily destroyed by
  weathering.
• Micas and heavy minerals such as olivine,
  pyroxene, amphibole, zircon, magnetite and
  hematite.

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Rock fragments

• Rock fragments are typically the most abundant
  component in very coarse-grained terrigenous
  rocks and are invariably the most interesting.
• Careful analysis of their composition provides us
  with direct information on provenance.

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Texture

• Conglomerate and breccia textures are studied at
  the outcrop using methods of quantitative grain
  size analysis that differ from those used for
  sandstone.
• Grain diameters of particles coarser than sand
  (gt2mm).
• The interstitial space between framework grains
  can be empty filled with finer-grained detrital
  matrix or occupied by cement, fluid (oil, water)
  or natural gas.

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Two distinctive varieties of conglomerates (and
breccias) are defined on the basis of texture

• Orthoconglomerates (literally, true
  conglomerates) consist mainly of gravel-sized
  framework grains. Matrix (sand or finer) is less
  than 15. So, grain-supported framework.
• Paraconglomerate have a matrix of sand and finer
  clasts. Matrix is at least 15 most have 50
  matrix and are actually sandstone and mudrocks in
  which pebbles, cobbles and boulders are scattered.

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Extraformational or intraformational

• They are separated by comparing the composition
  of framework and matrix grains.
• Intraformational- conglomerates and breccias have
  an interior (intrabasinal) source that is they
  are eroded from the same sedimentary rock unit
  they are a part, rather than being derived from
  rocks located outside the depositional basin.
  Consequently intraformational conglomerates and
  breccias have framework grains identical in
  composition to those in the matrix.

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Extraformational or intraformational

• Extraformational- conglomerates and breccias are
  derived from sources areas outside the
  depositional basin. Detritus weathered from
  external sources is carried away and deposited
  elsewhere. As a result framework clasts differ
  markedly in composition from matrix.Framework
  matrix is exotic that is, not derived by the
  erosion and redeposition of matrix material.

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Orthoconglomerate and Paraconglomerates

• (Orthobreccias and Parabreccias)
• They are separated by examining the proportion of
  matrix. Orthoconglomerate are matrix poor,
  Paraconglomerates are matrix rich.

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Orthoconglomerates

• They are matrix-poor (80 or more framework
  grains) and have an intact, stable,
  grain-supported fabric. They are transported and
  deposited on a grain-by grain basis by fluids,
  specifically water or air.
• Oligomict or petromict-cngl are further divided
  into these on the basis of framework grain
  composition.

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Oligomict or petromict

• In oligomict (orthoquartzose) conglomerates (or
  breccias), more than 90 of the framework clasts
  consist of fragments of only a few varieties of
  resistant rocks and minerals as metaquartzite,
  vein quartz, and cherts.
• In petromict (polymict) clasts of many different
  composition of metastable and unstable rocks are
  abundant for example, basalt, slate, and
  limestone.

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• A more precise classification can be given by
  specifying predominant clast size and lithology
• quartz pebble oiligomictic orthoconglomerate
• slate coble petromictic parabreccia

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• Oligomict orthoconglomerates imply wholesale
  decomposition and disintegration of immense
  volumes of rocks, reflecting climate and
  topography that promote chemical decomposition
  and physical disintegration of all but the most
  resistant components.
• Typically stream channels deposits and bars
  deposits, or near shore marine settings.

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• Petromic are much more abundant than oligomict
  orthoconglomerates and are mainly alluvium eroded
  from high-relief areas.

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Paraconglomerates
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Paraconglomerates

• Paraconglomerates and parabreccias are further
  divided on the basis of their inferred origin as
  well as the size and internal organization of
  their matrix.
• Is the matrix sand or mud?
• Is the matrix internally laminated or chaotic?
• Is the framework imbricated, sorted, and
  vertically graded?
• Is the deposit sheetlike or lenticular?
• With what other types of sediment is the deposit
  associated?

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Paraconglomerates

• Paraconglomerates containing a matrix of
  delicately laminated mudrocks in which coarser
  framework grains float are called laminated
  pebbly (or cobbly, or bouldery) mudrock.
• Dropstone- ice rafting

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Paraconglomerates

• Paraconglomerates in which the matric is
  disorganized and non-laminated are either tillite
  (only if glacial origin can be inferred) or
  tilloid (deposited by mass movement).

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