Stratigraphy

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Stratigraphy

• The study of strata (layers) of rocks with an eye
  toward interpreting the geologic history of the
  region
• Closely tied to dating methods
• Uses a variety of methods - fossils, stable
  isotopes, paleomagnetics, sedimentary cycles - to
  correlate and distinguish layers
• Very important for oil exploration and mining

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Basin Analysis

• Use stratigraphic methods to work out sequence
  and timing of deposition of rocks
• usually sedimentary
• Synthesis of data from multiple disciplines
• sedimentology to determine environment of
  deposition
• paleontology to get time
• Used to be main objective
• Petroleum industry

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Correlation Correlation is determining that
rocks are the same formation (may mean rocks are
the same age)
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B
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CHRONOSTRATIGRAPHY Geochronological units
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LITHOFACIES
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Stratigraphic Contacts

• Contacts
• Plane or irregular surfaces between different
  types of rocks
• Separate units
• Conformable
• Unconformable

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Types of Contacts

• Conformable boundaries
• Conformable strata form unbroken depositional
  sequences
• Layers are deposited by uninterrupted
  deposition
• Abrupt or gradational
• Abrupt
• Sudden distinctive changes in lithology
• Often, local change
• Gradational
• Gradual change in depositional conditions with
  time progressive gradual contact
• One lithology grades into another
• e.g., ss becomes finer upsection until it becomes
  a siltstone

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Lithostratigraphic units

• Supergroup
• Group
• Formation a mapable unit with distinctive
  lithic characteristics
• Member
• Bed

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Facies

• aspect or appearance
• Can be genetic (fluvial facies) or descriptive
  (sandstone facies)
• Lithofacies a constant lithological character
  within a formation E.G an evaporite
• Walther Law (1894)- facies that occur in
  conformable vertical succession also occur in
  laterally adjacent environments

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Preservation potential of rocks that are deposited

• Majority of sediments in fossil record
• Marine
• Most sub-aerial environments
• Erosional
• WHY?
• Accommodation space!
• Space available controls accumulation
• no place to put it, then no deposition
• base level
• balance between erosion and deposition

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Walthers Law of Correlation of Facies

• Relationship between vertical and lateral
  variations
• The fact that there is lateral variation in
  facies leads to vertical variation in facies
• Walthers Law of Correlation of Facies
• Lateral variations are expressed in the vertical
  due to the succession of facies

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Walther's Law of Correlation of Facies

• Only those lithofacies which are a product of
  sedimentary environments found adjacent to one
  another in the modern can be occur superimposed
  in continuous, uninterrupted stratigraphic
  succession.

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Walthers Law Transgression-Regression

• Transgression
• Landward movement of shoreline (progessive
  deepening)
• Stand on beach
• Over time, you would be under water as shoreline
  moved landward
• Regression
• Seaward movement of shoreline
• (progessive shallowing)
• Results in lateral and vertical changes

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Transgression

• Geometric relationship of "graded, shore parallel
  facies belts
• Fining Upwards Sequence FUS
• More basin-ward facies overlie more landward
  facies
• Compared to depositional systems models

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Transgression and Regression

• Shallowing upwards, shoreline moves basinward
  through time--gt Regression
• Sea level drop /- uplift /- sediment supply
• Progradation
• excess sediment supply relative to accommodation
  space
• Forced Regression
• Relative sea level drop and formation of erosion
  surfaces Unconformity (surface of subaerial
  exposure)
• Soils kaolinitized, clay-rich layers
• Angular discordance with underlying units
  (disconformity)
• Plant remains, rooted zones
• Non-genetic stratal relationships basinward
  shift in sedimentary facies
• Strata across lithologic boundaries NOT in
  accordance with Walthers law

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Regression

• Geometric relationship of "graded, shore parallel
  facies belts
• Coarsening Upwards Sequence CUS
• More landward facies overlie more basin-ward
  facies
• Compared to depositional systems models

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Transgression - Regression

• What drives transgression/ regression?
• cant tell from this information!
• sea level change has so many components
• relative local
• eustatic global
• sediment supply
• can drive a regression/ transgression
• ONLY KNOW that shoreline has shifted position
• multiple factors responsible for sea- level
  change
• Say sea level rise or fall and you are WRONG!
• Say transgression or regression!

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Causes of Sea Level Change
Relative Change

Eustatic Change
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Sea Level Cycles

• 1st Order Cycles
• 100s my
• 100s of meters
• 2nd Order Cycles
• 10s my
• 100s of meters
• 3rd Order Cycles
• 1-10 my
• 10s of meters

Falling
Rising
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Biostratigraphy
If two rocks contain the same fossils they must
be the same age
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Evolution

• Variations exist within a population
• Result from mutations and other genetic accidents
• Some variations are advantageous but others are
  not
• Some are neutral
• Natural Selection works on these variations
• Characteristics of population shift through time
  evolution

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Bio-Events

• First appearances of new species
• First appearances of new higher taxa
• Extinctions of species
• Mass extinctions of multiple taxa
• Bio-events are unique points in geologic time

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Index Fossils

• Some fossils are more useful than others for
  relative age determinations
• Fossils that are most useful are called INDEX
  FOSSILS
• What factors would maximize a fossils
  usefulness? (i.e., What makes a good index
  fossil?)

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Illustration of Principle of Faunal Succession
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What makes a good index fossil?

• Distinctive appearance/easy to recognize
• Short duration between first appearance and
  extinction (a.k.a. RANGE)
• Widespread geographic distribution (makes
  correlation possible across a wide area/multiple
  continents)

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Characteristics of Index Fossils

• Limited Stratigraphic Range
• Widespread Geographic Distribution
• Commonly Pelagic
• Or tolerant of a wide variety of environments
  (found in many facies)

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•  
• Unconformities
• Unconformities are surfaces in rock that
  represent periods of erosion or non-deposition.
  In other words, time has been left out of the
  physical geologic rock record.
• There are three (3) principal types of
  unconformities
• Angular Unconformity
• Rocks above and below unconformity have
  different orientations. Shows that there was a
  period of deformation, followed by erosion, and
  then renewed deposition. Easiest of the three
  types to recognize because the units are at an
  angle truncated with the units above them.

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•  
• Nonconformity
• Rocks in a horizontal fashion were eroded down
  to igneous bedrock material at which time
  subsequent deposition of sedimentary layers
  commenced. Shows that there was a period of
  deformation, followed by erosion, and then
  renewed deposition. Represents the greatest
  amount of time left out of the geologic rock
  record.
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• Disconformity
• Rocks in a nearly horizontal fashion were eroded
  and an erosional profile remains covered by
  subsequent sedimentary deposition. Shows that
  there was a period of erosion and then renewed
  deposition in nearly horizontal layers. Most
  difficult to recognize because the units are
  nearly horizontal and only a small discontinuous
  layer can be observed (rubble zone or soil
  profile).

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Angular Unconformity
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Disconformity
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Nonconformity
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Unconformity Types Using Grand Canyon as Example
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Stratigraphic Thinking
From D. McConnell, Geologic Time, http//lists.uak
ron.edu/geology/natscigeo/Lectures/time/gtime1.htm
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One possible interpretation...
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Look Bob a shooting star! Lets make a wish.
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Were The Dinosaurs Failures?

• Dinosaurs 150,000,000 years
• Recorded History 5000 years
• For every year of recorded history, the dinosaurs
  had 30,000 years
• For every day of recorded history, the dinosaurs
  had 82 years
• For every minute of recorded history, the
  dinosaurs had three weeks