The Sedimentary Archives

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Chapter 5

• The Sedimentary Archives
• What Sedimentary Rocks Tell Us About The Past

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Chapter 4 Content

• Clues that Sedimentary Rocks provide about
  Earths History.
• Factors that affect the formation of Sedimentary
  Rocks.
• Where Sedimentary Rocks form.
• Small Scale evidence of Earths History.
• Large Scale evidence of Earths History.
• Interpreting Earths History

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• Clues That Sedimentary Rocks Provide About
  Earths History

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Clues that Sedimentary Rocks Provide about
Earths History

• How the rock was formed.
• Composition
• Minerals Present and Absent
• Combinations of Minerals
• Grain Size
• Grain Shape
• Grain Sorting
• Crystal Growth
• Color

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• Factors That Affect the Formation of Sedimentary
  Rocks

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Factors That Affect the Formation of Sedimentary
Rocks

• Tectonic setting.
• Physical, chemical, and biological processes in
  the depositional environment.
• Method of sediment transport.
• Source Rock of the Sedimentary Rock.
• Climate (and its effect on weathering).
• How the rock was lithified or turned into a rock
  (cementation, compaction).
• Time.

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• Tectonic Setting

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Tectonic Setting

• Tectonics The forces controlling deformation or
  structural behavior of a large area of the
  Earth's crust over a long period of time.
• An area may be
• Tectonically stable - like the midwestern US.
• Subsiding (sinking) - like New Orleans or Mexico
  City.
• Rising gently - like New England and parts of
  Canada after glacier retreat.
• Rising actively to produce mountains and plateaus
  - like parts of Oregon in the Cascade Mountains .
  

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Tectonic Setting

• Tectonics influences the grain size and thickness
  of sedimentary deposits.
• Recent uplift of the source area leads to rapid
  erosion of coarse-grained sediment.
• Subsidence in the depositional basin leads to the
  accumulation of great thicknesses of sediment.

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Principle Tectonic Elements of a Continent

• Craton - Stable interior of a continent,
  undisturbed by mountain-building events since the
  Precambrian.
• Shields - Large areas of exposed crystalline
  rocks.
• Platforms - Ancient crystalline rocks are covered
  by flat-lying or gently warped sedimentary rocks.
  
• Orogenic belts - Elongated regions bordering the
  craton which have been deformed by compression
  since the Precambrian Mountain belts.

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• Environments Where Deposition Occurs

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Environments Where Deposition Occurs

• Environment of Deposition All of the physical,
  chemical, biological, and geographic conditions
  under which sediments are deposited.
• Sedimentary rocks may be
• Extrabasinal in origin - Sediments formed from
  the weathering of pre-existing rocks outside the
  basin, and transported to the environment of
  deposition.
• Intrabasinal in origin - Sediments form inside
  the basin includes chemical precipitates, most
  carbonate rocks, and coal.
• By comparing modern sedimentary deposits with
  ancient sedimentary rocks, the depositional
  conditions can be interpreted.

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Environments Where Deposition Occurs

• There are three broad categories of depositional
  environments
• Marine environments (ocean).
• Transitional environments (along contact between
  ocean and land).
• Continental environments (on land).

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Environments Where Deposition Occurs
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Marine Depositional Environments

• Continental shelf - The flooded edge of the
  continent.
• Exposed to waves, tides, and currents.
• Covered by sand, silt, and clay.
• Larger sedimentary grains are deposited closer to
  shore.
• Coral reefs and carbonate sediments in tropical
  areas.
• Continental slope - The steeper slope at edge of
  the continent.
• Deeper water.
• Rapid sediment transport, muddy turbidity
  currents.
• Continental rise - At the base of the continental
  slope.
• Water depths of 1400 to 3200 m.
• Submarine fans form at mouth of submarine
  canyons.
• Turbidity currents deposit thick accumulations of
  sediment
• Abyssal plain - Deep ocean floor.
• Water depths of 3 to 5 km (2 - 3 miles), or more.
  
• Covered by very fine-grained sediment and shells
  of microscopic organisms.

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Marine Depositional Environments
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Transitional Depositional Environments

• Transitional environments are those environments
  at or near the transition between the land and
  the sea.
• Deltas
• Fan-shaped accumulations of sediment
• Formed where a river flows into a standing body
  of water, such as a lake or the sea
• Coarser sediment (sand) tends to be deposited
  near the mouth of the river finer sediment is
  carried seaward and deposited in deeper water.
• The delta builds seaward (or progrades) as
  sediment is deposited at the river mouth.

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Transitional Depositional Environments

• Beaches and Barrier Islands
• Shoreline deposits
• Exposed to wave energy
• Dominated by sand
• Marine fauna
• A few km or less in width but may be more than
  100 km long
• Separated from the mainland by a lagoon (or salt
  marsh)
• Commonly associated with tidal flat deposits

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Transitional Depositional Environments

• Lagoons
• Bodies of water on the landward side of barrier
  islands
• Protected from the pounding of the ocean waves by
  barrier islands
• Contain finer sediment than the beaches (usually
  silt and clay)
• Lagoons are also present behind reefs, or in the
  center of atolls.

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Transitional Depositional Environments

• Tidal flats
• Nearly flat, low relief areas that border
  lagoons, shorelines, and estuaries
• Periodically flooded and exposed by tides
  (usually twice each day)
• May be cut by meandering tidal channels
• May be marshy, muddy, sandy or mixed sediment
  types (either terrigenous or carbonate)
• Laminations and ripples are common
• Sediments are intensely burrowed

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Transitional Depositional Environments

• Estuaries
• Mouth of a river drowned by the sea
• Brackish water (mixture of fresh and salt water)
• May trap large volumes of sediment
• Sand, silt, and clay may be deposited depending
  on energy level
• Many estuaries formed due to sea level rise as
  glaciers melted at end of last Ice Age
• Some formed due to tectonic subsidence, allowing
  sea water to migrate upstream

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Continental Environments

• Continental environments are those environments
  which are present on the continents.
• Rivers or fluvial environments
• Alluvial fans
• Lakes (or lacustrine environments)
• Glacial environments
• Eolian environments

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Continental Environments

• Rivers or fluvial environments
• Include river and stream systems
• Channel deposits consist of coarse, rounded
  gravel, and sand.
• Bars are made of sand or gravel.
• Levees are made of fine sand or silt.
• Floodplains are covered by silt and clay.

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Continental Environments

• Alluvial fans
• Fan-shaped deposits formed at the base of
  mountains.
• Most common in arid and semi-arid regions where
  rainfall is infrequent but torrential, and
  erosion is rapid.
• Sediment is typically coarse, poorly- sorted
  gravel and sand.

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Continental Environments

• Lakes (or lacustrine environments)
• May be large or small.
• May be shallow or deep.
• May be filled with terrigenous (eroded
  particles), carbonate, or evaporitic sediments.
• Sediments are typically fine grained but may be
  coarse near the edges.

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Continental Environments

• Glacial environments
• Sediment is eroded, transported, and deposited by
  ice (glaciers).
• Glacial deposits called till contain large
  volumes of unsorted mixtures of boulders, gravel,
  sand, and clay.

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Continental Environments

• Eolian environments
• Wind is the agent of sediment transport and
  deposition.
• Dominated by sand and silt.
• Common in many desert regions.

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Small Scale Evidence of Earths History
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Small Scale Evidence of Earths History

• Color
• Texture
• Grain Size
• Grain Shape
• Grain Sorting
• Grain Arrangement

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Rock Color
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Color of Sedimentary Rocks

• Color of sedimentary rocks provides useful clues
  to the depositional environment.
• Black
• Red
• Green and Gray

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Color of Sedimentary Rocks

• Black and dark gray
• Indicates the presence of organic carbon and/or
  iron.
• Organic carbon in sedimentary requires anoxic
  environmental conditions (lacking free oxygen)
  quiet water marine, deep lakes, and estuaries.
• In these environments, iron combines with sulfur
  to form the mineral pyrite (FeS2), which can also
  contribute to the black color.
• Black, organic-rich sediments may also form in
  environments where the accumulation of organic
  matter exceeds the capacity of the environment to
  oxidize it.

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Color of Sedimentary Rocks

• Red, Brown, Purple, or Orange
• indicates the presence of iron oxides.
• In well-oxygenated continental sedimentary
  environments, the iron in the sediments is
  oxidized to form hematite or ferric iron oxide
  (Fe2O3), which colors the sediment red, brown, or
  purple.
• Typically deposited in continental (or
  transitional) sedimentary environments such as
  flood plains, alluvial fans, and deltas.
• Also formed in marine environments (due to
  oxidation of the iron in the sediment after
  deposition), or to erosion of red sediment from
  the land.

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Color of Sedimentary Rocks

• Green and gray
• also indicates the presence of iron, but in a
  reduced (rather than an oxidized) state.
• Ferrous iron (Fe2) generally occurs in
  oxygen-deficient environments.

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Rock Texture
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Rock Texture

• Texture refers to the size, shape, sorting, and
  arrangement of grains in a sedimentary rock.
• There are three "textural components" to most
  clastic sedimentary rocks
• Clasts - the larger grains in the rock (gravel,
  sand, silt).
• Matrix - the fine-grained material surrounding
  clasts (often clay).
• Cement - the "glue" that holds the rocks
  together.
• Silica, Calcite, Iron oxide, Other Minerals

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Rock Texture

• The texture of a sedimentary rock can provide
  clues to the depositional environment.
• Fine-grained textures typically indicate
  deposition in quiet water.
• In general, it takes higher energy (higher water
  velocity) to transport larger grains.

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Grain Size

• Sedimentary grains are categorized according to
  size using the Wentworth Scale.
• Gravel gt 2 mm
• Sand 1/16 - 2 mm
• Silt 1/256 - 1/16 mm
• Clay lt1/256 mm
• The grain sizes in a sediment or sedimentary rock
  can provide clues to help interpret the
  depositional environment.
• Stronger currents are required to move larger
  particles than to move smaller particles.

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Sorting

• Sorting refers to the distribution of grain sizes
  in a rock.
• If all of the grains are the same size, the rock
  is "well sorted."If there is a mixture of grain
  sizes, the rock is "poorly sorted."
• The range of grain sizes in a sediment or
  sedimentary rock can provide clues to help
  interpret the depositional environment.
• Windblown sediments are better sorted than
  wave-washed sediments.
• Well-sorted sands have higher porosity and
  permeability than poorly-sorted sands (if they
  are not tightly cemented).
• Poor sorting is the result of rapid deposition of
  sediment without sorting by currents.

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Grain Shape

• Grain shape is described in terms of rounding of
  grain edges and sphericity (equal dimensions, or
  how close it is to a sphere).
• Rounding results from abrasion against other
  particles and grain impact during transport.
• Very well-rounded sand grains suggest that a sand
  may have been recycled from older sandstones.
• Shape of clasts is important in naming the
  coarser-grained sedimentary rocks (those with
  gravel-sized clasts).

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Orientation of Grains

• A study of grain orientation or arrangement may
  indicate whether the grains are clustered into
  zones or mixed up.
• This relates to the method of transport and
  deposition of the grains.
• Grain orientation may also be used to interpret
  ancient current or wind directions.
• The long axis of the grain becomes oriented
  parallel to the flow direction.

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Large Scale Evidence of Earths History
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Sedimentary Structures

• Sedimentary structures are larger features which
  form during (or shortly after) deposition of the
  sediment, but before lithification.
• Some sedimentary structures are created by the
  water or wind which moves the sediment. Other
  sedimentary structures form after deposition
  such as footprints, worm trails, or mudcracks.
• Sedimentary structures can provide information
  about the environmental conditions under which
  the sediment was deposited some structures form
  in quiet water under low energy conditions,
  whereas others form in moving water or high
  energy conditions.

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Sedimentary Structures

• Types of Sedimentary Structures
• Stratification
• Graded bedding
• Cross-bedding or cross-stratification
• Ripple marks
• Mud cracks
• Scour marks

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Sedimentary Structures

• Stratification ( layering or bedding) is the
  most obvious feature of sedimentary rocks.
• The layers (or beds or strata) are visible
  because of differences in the color or texture of
  adjacent beds.
• Strata thicker than 1 cm are commonly referred to
  as beds.
• Thinner layers are called laminations or laminae.
  
• The upper and lower surfaces of these layers are
  called bedding planes.

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Sedimentary Structures

• Graded bedding results when a sediment-laden
  current (such as a turbidity current) begins to
  slow down.
• The grain size within a graded bed ranges from
  coarser at the bottom to finer at the top. Hence,
  graded beds may be used as "up indicators."

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Sedimentary Structures

• Cross-bedding or Cross-stratification is an
  arrangement of beds or laminations in which one
  set of layers is inclined relative to the others.
  
• The layering is inclined at an angle to the
  horizontal, dipping downward in the downcurrent
  direction.

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Sedimentary Structures

• Ripple marks are undulations of the sediment
  surface produced as wind or water moves across
  sand.
• Symmetric ripple marks are produced by waves or
  oscillating water.
• Asymmetric ripples form in unidirectional
  currents (such as in streams or rivers).
• Asymmetric ripples have a steep slope on the
  downstream side, and a gentle slope on the
  upstream side.
• Because of this unique geometry, asymmetrical
  ripples in the rock record may be used to
  determine ancient current directions or
  paleocurrent directions.

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Sedimentary Structures

• Mud cracks are a polygonal pattern of cracks
  produced on the surface of mud as it dries.
• The mud polygons between the cracks may be broken
  up later by water movement, and redeposited as
  intraclasts (particularly in lime muds).

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Sedimentary Structures

• Scour marks are depressions or erosional features
  formed as a current flows across a bed of sand.
• Sediment may be deposited over the scoured layer,
  filling the depressions.
• When the overlying sediment becomes consolidated,
  you can see positive-relief casts on the base of
  the overlying bed.
• These casts are termed "sole marks," because they
  appear on the bottom (or sole) of a bed of
  sediment.

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Sedimentary Structures

• Determining "up direction"
• Sedimentary structures can be used to determine
  "up direction."
• Sedimentary structures such as graded beds, cross
  beds, mudcracks, flute marks, symmetrical (but
  not asymmetrical) ripples, burrows, and tracks
  can be used to establish the original orientation
  of the beds.
• Features which can be used to determine "up
  direction" are called geopetal structures.
• Fossils can also be used to establish up
  direction, if they are present in the rock in
  life position.

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Compositional Evidence Provided by Sedimentary
Rocks
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Sands and Sandstones

• Sandstones are classified on the basis of the
  composition of their grains.
• Three components are generally considered
• Quartz grains.
• Feldspar grains.
• Rock fragment grains.
• The particular minerals present provide
  information on the amount of weathering and
  transport experienced by the sand grains.
• Intense weathering and long transport tend to
  destroy the feldspars and ferromagnesian minerals
  because they are less stable, and produce a
  sandstone dominated by quartz. Such sandstones
  are referred to as compositionally mature.
• Sandstones with abundant feldspars, and
  ferromagnesian minerals, on the other hand,
  indicate relatively little weathering and
  transport. These sandstones are compositionally
  immature.

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Sands and Sandstones

• Major types of sandstone described by Levin
• Quartz sandstone (also called quartz arenite) -
  dominated by quartz grains.
• Arkose - contain 25 or more feldspar, with
  quartz.
• Graywacke - contains about 30 dark fine-grained
  matrix (clay, silt, chlorite, micas) along with
  quartz, feldspar, and rock fragments.
• Lithic sandstone (or subgraywacke) - dominated by
  quartz, muscovite, chert, and rock fragments with
  matrix less than 15. Feldspars scarce.

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Sandstone Environmental Interpretation

• Each type of sandstone implies something about
  depositional history and environment
• Quartz sandstone implies a long time of
  formation. Deposition typically in shallow-water
  environments.
• Common sedimentary structures are ripple marks
  and cross-bedding.
• Arkose implies a short time of formation(because
  feldspar typically weathers quickly to clay).
  Also implies rapid erosion, arid climate,
  tectonic activity, steep slopes.
• Commonly deposited in fault troughs or low areas
  along granitic mountains. Often has a pinkish
  color due to oxidized iron, suggesting
  continental deposition.
• Graywacke implies a tectonically active source
  area and rapid erosion. Graded bedding is common.
  
• Associated with volcanic rocks, shales, and
  cherts of deep water origin.
• Lithic sandstone found in deltaic coastal plains,
  and may be deposited in nearshore marine
  environments, swamps, or marshes.
• Associated with coal and micaceous shales.

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Carbonate Rocks and Sediments

• Carbonate rocks consist of limestone and
  dolostone. Limestones are the most abundant
  carbonate rocks.
• Carbonate rocks are chemical or biochemical in
  origin.
• The minerals present in carbonate rocks are
• Limestone
• Calcite
• Aragonite
• Dolostone
• Dolomite

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Carbonate Rocks and Sediments

• Depositional Conditions
• Shallow marine environment, lakes, caves and hot
  springs.
• Direct or indirect result of biologic activity.
• May contain shells or the remains of other marine
  organisms
• May precipitate from seawater as a result of
  biologic activity
• Characteristics of most marine carbonate
  environments
• Warm water
• Shallow water (less than 200 m deep)
• Tropical climate (30 N - 30 S of equator)
• Clear water (low to no terrigenous input)
• Sunlight required for photosynthesis by algae

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Carbonate Rocks and Sediments

• Origin of carbonate sediments
• Much lime mud forms from the disintegration of
  calcareous algae.
• When the calcareous algae die, their skeletons
  break down and disintegrate producing aragonite
  needle muds.
• These lime muds lithify to form fine-grained
  limestone.
• Blue-green algae are involved in the formation of
  oolites (or ooids).
• Oolites form in warm shallow seas with constant
  wave agitation.
• Precipitation of calcium carbonate from seawater
  as a result of biologic activity
• Abrasion of shells
• Coquina,
• Fossiliferous Limestone,
• Accumulation of fecal pellets produced by
  burrowing organisms.

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Carbonate Rocks and Sediments

• Dolomite
• Dolomite is a calcium-magnesium carbonate mineral
  (CaMg(CO3)2) that comprises the sedimentary rock
  dolostone. (Sometimes the rock is also called
  dolomite.)
• Dolomite is interpreted to form when magnesium
  that has been concentrated in sea water replaces
  calcium in calcium carbonate in a previously
  deposited limestone.

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Clays and Shales

• The word Clay" has two definitions.
• It is both a grain size term, and
• a term referring to a layered silicate mineral
  which behaves plastically when wet and hardens
  upon drying or firing.
• Shale is a very fine-grained rock composed of
  clay, mud, and silt.
• Shale is fissile this means that it splits
  readily into thin, flat layers.
• There are quartz shales, feldspathic shales,
  chloritic shales, and micaceous shales, based on
  the composition of the silt-sized grains.
• The environmental interpretations of these shales
  are similar to those of the various types of
  sandstones.

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Clays and Shales

• Claystone is a very fine-grained rock composed of
  tiny (less than 1/256 mm) clay minerals, mica,
  and quartz grains.
• The individual grains are too small to see with
  the naked eye or a hand lens, and the rock feels
  smooth to the touch (not gritty).
• Claystone is not fissile, and breaks irregularly.
  
• Mud is defined as a mixture of silt and clay.
• Rocks with both silt and clay are referred to as
  mudstones or mudshales, depending on whether or
  not they are fissile.

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Clays and Shales

• Deposition of clays
• Because of its fine grain size, clay tends to
  remain suspended in the water column. It will
  settle out of still, quiet water, given enough
  time.
• Clays and shales typically indicate low energy
  environments, sheltered from waves and currents.
  They are commonly found in lacustrine, lagoon,
  and deeper water marine deposits.

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Classifying Rock Layers
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Rock Facies
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Rock Facies

• Rock Facies a distinctive rock unit that forms
  under certain conditions of sedimentation
• It is used to define a particular process or
  environment.
• Examples
• Sand and Silt facies of a beach environment,
• Shale facies in deeper, quieter water,
• Carbonate facies, far from shore in warm shallow
  seas.
• Coal facies found in a swamp area on a delta,
• When a depositional environment grades laterally
  into other environments it is referred to as a
  facies change.
• The different fossil assemblages in a uniform
  rock unit may be referred to as biofacies.

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Facies Changes Indicate Changes in Sea Level

• Transgression.
• A sea level rise is called a transgression.
• A sea level rise will produce a vertical sequence
  of facies representing progressively deeper water
  environments. As a result, a transgressive
  sequence will have finer-grained facies overlying
  coarser-grained facies. This is sometimes
  referred to as an onlap sequence.
• Regression.
• A sea level drop is called a regression.
• A regression will produce a sequence of facies
  representing progressively shallower water
  environments (shallowing-upward sequence). As a
  result, a regressive sequence will have
  coarser-grained facies overlying finer-grained
  facies (coarsening-upward). This is sometimes
  called an offlap sequence.

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Walther's Law (or Principle)

• Walther's Law (or Principle)
• Sedimentary environments that started out
  side-by-side will end up overlapping one another
  over time due to sea level change (transgressions
  and regressions).
• The result is a vertical sequence of beds. The
  vertical sequence of facies mirrors the original
  lateral distribution of sedimentary environments.
  

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Facies Changes Indicate Changes in Sea Level

• Fluctuations in sea level are caused by
• Plate Tectonic Changes
• Glaciation

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Correlating Rock Units
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Correlation

• The branch of geology that deals with the
  correlation of rock units from one area to
  another is known as stratigraphy.
• Three main types of correlation
• Lithostratigraphic correlation - Matching up rock
  units on the basis of their lithology
  (composition, texture, color, etc.) and
  stratigraphic position.
• Biostratigraphic correlation - Matching up rock
  units on the basis of the fossils they contain.
• Chronostratigraphic correlation - Matching up
  rock units on the basis of age equivalence, as
  determined by radioactive dating methods or
  fossils.
• Geologists can trace beds from one exposure to
  another. This is called lithostratigraphic
  correlation.

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Unconformities
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Unconformities

• Indicates that something has changed
• Three types
• Disconformity
• Nonconformity
• Angular Unconformity

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Unconformities

• Types of unconformities
• Angular unconformities - An erosional surface
  which truncates folded or dipping (tilted)
  strata.
• Nonconformities - An erosional surface which
  truncates igneous or metamorphic rocks.
• Disconformities - An irregular erosional surface
  which truncates flat-lying sedimentary rocks.
• A fourth type of unconformity is the
  paraconformity, separating two parallel units of
  sedimentary rock.
• There is no obvious evidence of erosion.
• A paraconformity is virtually indistinguishable
  from a sharp conformable contact. The fossils
  show that there is a considerable time gap
  represented by a paraconformity.

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Depicting the Past
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Depicting the Past

• Methods
• Geologic Columns Columnar sections show the
  vertical succession of rock units at a given
  location.
• They are used in correlation and construction of
  cross-sections.
• Stratigraphic cross-sections tie together several
  geologic columns from different locations.
• The purpose is to show how rock units change in
  thickness, lithology, and fossil content across a
  given area.
• Structural cross-sections show the timing of
  tilting, folding, and faulting of rock units.
• Geologic Maps show the distribution of various
  layers and types of rocks in an area.
• Paleogeographic Maps are interpretive maps which
  depict the geography of an area at some time in
  the past.
• Isopach Maps show the thickness of formations or
  other units in an area.
• Lithofacies Maps show the distribution of
  lithofacies that existed at a given time over an
  area, or show the percentage of some lithologic
  component (such as clay), or show the ratio of
  one rock type to another within the unit.

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THE END