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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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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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Walther’s 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
 Walther’s 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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Walther’s Law: TransgressionRegression
• 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--> 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 Walther’s
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?
• can’t tell from this information!
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•
•
•
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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Relative
Change
Eustatic
Change
Causes of Sea Level
Change
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Sea Level Cycles
• 1st Order Cycles
• 100’s my
• 100’s of meters
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2nd
Order Cycles
• 10’s my
• 100’s of meters
• 3rd Order Cycles
• 1-10 my
• 10’s 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 fossil’s
usefulness? (i.e., What makes a good index
fossil?)
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Illustration of Principle of Faunal
Succession
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.
• 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.uakron.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