Chapter 6 Chapter 7 current textbook Sedimentary Rocks

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• Chapter 6 (Chapter 7 current textbook)
  Sedimentary Rocks
• Sediments Transport Ocean Deposition
• Weathering Soil Review
• Rocks Weathering Erosion Sediments
• The history of Earths Internal Processes is
  recorded in the Igneous and Metamorphic rocks.
• The history of Earths External Processes is
  recorded in the Sedimentary rocks.

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• After rocks are degraded by weathering, the
  weathering products (sediments) are transported
  by river systems and then deposited in ocean
  basins. Longshore currents distribute sediments
  along the coast forming the continental shelves.
  Wave action separates the sediments by grain
  size. As more sediments accumulate, compaction
  cementation sedimentary rock.
• A few sedimentary rocks are preserved from lakes
  and other continental environments, but most were
  deposited on continental shelf environments.

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• Processes that produce sedimentary rocks
• Diagenesis Changes that occur after
  deposition, includes Recrystallization,
  Lithification, Compaction, Cementation.
• Recrystallization Less stable minerals
  change to more stable, e.g., Aragonite becomes
  Calcite. Mild heat and pressure may also cause
  recrystallization of existing minerals grains.
• Lithification sediments become sedimentary
  rock through compaction cementation

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• Compaction Overlying weight causes downward
  pressure, closing pore spaces between particles,
  decrease in volume. Clays may lose 40 of pore
  space with compaction. This parallel packing of
  clay plates decrease porosity permeability
  (the ability to pass liquids).

Clay plates
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• Cementation Ground water dissolved minerals
  percolate through compacting sediments. Minerals
  are deposited along grain margins until grains
  become cemented. Fe oxide, silica, and Calcite
  are most common cements.

Sand grains
Cement
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• Most common cements for sandstones are silica,
  iron oxide, calcite.
• Diagnostics
• Fe oxide reddish brown color,
• Silica - hardest cement, will not fizz
• Calcite - effervesces on exposure to HCl.
• The texture of a sedimentary rock, its grain
  size, sorting, and rounding is a function of its
  origins.

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• Major types of Sedimentary Rocks Clastic and
  Chemical
• Clastic (detrital) sedimentary rocks are composed
  of mineral and rock fragments and clays derived
  from pre-existing rocks. Physical weathering
  yields the fragments, while chemical weathering
  of silicates yields the clays.
• Major types of clastics Shale, Siltstone,
  Sandstone, Conglomerate or Breccia based on
  Particle Size (Table 7.1, pg. 214)

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• Major constituents Clay and Quartz are most
  common.
• Less common Feldspar and Mica, accessory
  minerals.
• Grain size is related to energy conditions during
  transport. Larger size Higher energy.
• Sorting similarities of grain size, related to
  length of transport.
• Roundness degree of angularity of grains,
  relates to type and length of transport.

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• Shale deposited in quiet waters, composed of
  minute particles, indistinguishable w/o High
  Magnification. Clay refers to size (lt1/256 mm)
  and also to a class of minerals. Shale has the
  characteristic of being fissile, i.e.,
  splitting into thin, tabular layers. Claystone
  or Mudstone usually breaks with a concoidinal
  (curving) fracture.
• Shales Clays are the most common
  sediments/sedimentary rocks.

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When clay particles drop out of suspen-sion
in quiet water, initially the grains may be
randomly oriented.
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As compaction increases, the flat clay particles
assume a nearly parallel alignment, producing
fissility.
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• Common quiet environments for clay deposition
  include lakes, river floodplains, lagoons, outer
  continental shelf.
• Shale color - related to depositional
  environment.
• Light colored oxidizing environment, i.e.,
  well-oxygenated with good circulation.
• Dark colored reducing environment, contains
  organics. High organic content may serve as
  source bed for petroleum.
• Red colored likely deposited in a continental
  environment, river delta, tidal flat.

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Shale commonly splits easily and forms talus
slopes. Shales and clays are mined to
produce pottery, brick, tile, ceramics, and
absorbents.
In some cases, shale layers may serve as glide
plains for horizontal thrust faulting in areas of
lateral compression.
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• Siltstones are composed of particles between
  1/256 mm 1/16 mm. Clays feel smooth when
  rubbed between fingers, silts feel slightly
  gritty to the touch. Individual grains are
  difficult to identify.
• Siltstones are deposited in slightly higher
  energy environments than clays, usually in tidal
  flats and continental shelf environments
  (shoreward of clays). Siltstones composed of
  eroded re-deposited ash may also be deposited
  in calderas (remember sample from the Eagle Mts.,
  west Texas).

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• Sands and sandstones are composed of particles
  1/16 mm to 2 mm in diameter, i.e., sand refers
  to size w/o regard to mineralogy.
• The larger size (compared to clay) allows
  observation of individual grain characteristics,
  e.g., sorting and roundness.
• Sorting, the degree of size similarity, is
  related to mode of transport (wind, water, ice),
  energy level, and length of time in transport.

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Beach sands are usually well-sorted because of a
long transport and washing by wave
action. Fluvial/alluvial sands are less
well-sorted, with occasional gravel beds (the
result of occasional floods).
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• Quartz is the dominant mineral in most
  sandstones, these are called quartz sandstones.
  Source area mature, well weathered continent.
  Other varieties of sandstone include
• Arkose sandstone with gt25 feldspar.
  Angular, poorly-sorted grains suggests nearby
  granitic source rocks in a relatively dry
  environment.
• Graywacke a dark sandstone with gt15 silt/clay
  matrix and rock fragments. Very poor sorting and
  angular rock fragments suggests rapid erosion,
  short transport, rapid burial.

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• Conglomerate consists of rounded pebbles
  (river rock), suggesting a long transport time,
  deposited in a high energy environment (large
  river, mountain stream, accumulation of gravel
  along a rapidly eroding seashore.
• Breccia consists of angular pebbles,
  suggesting a nearby source and rapid erosion
  deposition.
• Most conglomerates breccias are deposited in
  continental settings.

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• Chemical Sedimentary Rocks derived from the
  precipitation of dissolved minerals in water.
• Inorganic minerals precipitate because of
  evaporation and/or chemical activity. Examples
  Evaporites (salt, gypsum), chert, travertine or
  cave onyx (stalactites, etc.), oolitic limestones
• Organic minerals precipitate because of organic
  activity. Examples Limestone, Dolostone,
  Diatomites, bedded cherts. Coal is included in
  this category.

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• Evaporites minerals that precipitate from
  super-saturated (mineralized) water in enclosed
  basins.
• Salt NaCl
• Gypsum CaSO4

Inflow of water
Evaporation from coastal basin
No external drainage, only evaporation
Playa lake basins be-tween mountain ranges,
especially in Basin and Range Province.
Shallow sea inflow during high tide, storms
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• Coal composed of compressed, altered, but not
  decayed plant remains.
• Deposited in a reducing (oxygen-poor)
  environment, often in coastal delta settings.
  Sulfurous, acidic setting is inhospitable to most
  bacteria. Sulfur is usually deposited as pyrite
  or marcasite. This oxygen-poor environment may
  also be conducive to deposition of some uranium
  compounds.

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• Progression of organic maturation (see p. 226)
• Peat poor energy source, better for mulch
• Lignite use only when necessary
• Bituminous most common
• Anthracite best, but least common
• Because anthracite has undergone some
  metamorphism, the rocks containing the layer may
  be deformed along with surrounding rocks, more
  difficult to mine.

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• Limestone primarily formed by the organic
  activity of certain algae and other organisms
  that remove calcite (CaCO3) from water for their
  skeletons or exo-skeletons.
• Limestones are deposited under the following
  conditions Warm, clean, shallow water, within
  the photic zone. There are a few freshwater
  limestones.
• Usual sites of deposition are Outer margins of
  continental shelves Shallow carbonate platforms.
  At the edge of continental shelf or carbonate
  platform, biologically built structures
  (bioherms, reefs), composed of skeletal debris
  and living corals, may be present.

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• Modern reefs are dominated by Scleractinid
  Corals. Other organisms, e.g., Rugose and
  Tabulate Corals, archaeocyathids, bryozoans,
  calcareous algae, tubular molluscs (rudistids),
  have been responsible for past reefs. Reefs
  attract other living organisms, yielding greater
  species diversity (biodiversity).
• A good example of an ancient reef is the
  Permian-aged Capitan Reef, Guadalupe Mts., Texas
  New Mexico (west of Carlsbad Caverns).

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This reef was built by algae, sponges, and
bryozoa. Skeletons help trap sediments, aid in
build-up.
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Back reef Reef
Fore reef
Basin facies
http//www.nps.gov/gumo/gumo/geology.html
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The Permian Reef encircled the Delaware Basin
(left), a portion of the shallow sea covering
parts of western N.A. duringcont.
the Permian Period. The reef facies in the
Guadalupe Mts. are exposed because of faulting.
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Coquina texture limestone with sand matrix,
probably deposited along a shoreline.
Dolostone composed of the mineral dolomite,
originally deposited as a lime-stone, groundwater
conditions cause the partial removal of Ca and
its replacement by Mg to form CaMg(CO3)2
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• Chert microcrystalline silica (chert, flint),
  precipitated from seawater (minor) or
  biochemically by siliceous, microscopic diatoms
  (below) and radiolaria.
• Volcanic ash may also serve as a source of silica
  (for chert).
• Sometimes silica will replace the calcite in
  limestone (including the fossils).

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Environments of Deposition geographic
setting for sediment accumulation. Facies (pg.
234) characteristics relate to Depositional
Environment
Decreasing Energy
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Other examples of sedimentary environments are
shown on Fig. 7.19, pp. 230 - 231. Box 7.2
article discussion of recovery of continental
shelf cores for paleoclimatic study. The
thickness of layers, the nature of the sediments,
the organics (including fossils and
microfossils), and isotopic study of certain
minerals proxy data. The sedimentary
characteristics fossils offer evidence of
environmental conditions.
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Examples of Sedimentary Environments
Red continental environments
Blue transitional environments
Black marine environments
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• Continental environments Eolian (windblown),
  fluvial, alluvial, alluvial fans, playa lakes,
  paludal (cave), glacial settings, fresh water
  lakes, early rift deposits.
• Marine environments continental shelf,
  continental slope and rise, abyssal plain, deep
  sea trench (not often preserved).
• Transitional environments beaches, tidal flats,
  deltas, lagoons.

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Sedimentary structures Internal and External.
External those structures seen on the
surface. Mudcracks Ripple marks
See also Figures 7.25 and 7.26 pg. 237 In
sedimentary rocks, raindrop impressions, salt
casts, and trace fossils (footprints, feeding
trails, etc.) can be preserved on bedding plane.
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Internal sedimentary structures those that
are seen in cross-section, i.e., from the side.
Examples are cross-bedding (below) and Figure
7.22, the horizontal layering w/in the Grand
Canyon (Chapt. 1 and Fig. 7.3, pg. 216), and
graded bedding.
The cross-beds at left were deposited in sand
dune deposits. Fossils in sedimentary rocks
evidence of past life.
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Major types of fossils Body fossils remnants
of organisms preserved, e.g., shells, bones,
etc.. Trace fossils evidence of life activities
preserved, e.g., footprints, burrows, fecal
pellets.