Transcript Lithostratigraphy

Stratigraphy
Concepts Related to Subdivision of
the Rock Record
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
Modern Stratigraphy
• Still focuses on synthesis
• Has evolved to include many new techniques +
deep marine realm
• many of the advances are driven by need to have
very precise age control
• Can resolve:
» events in some section to the order of < 20 ky
» patterns of events within a basin (regional)
» patterns by correlating between basins (need more
than 1 data point) (global)
Stratigraphy
• Initially used informally
• Now, governed by
– North American Stratigraphic Code
– International Stratrigraphic Code
History of Stratigraphy
• Based on idea that strata (layers of sediment) are
deposited one on top of another
• oldest at bottom and youngest at top, over long period of time
(ky - my) (1000s to 1,000,000s of years)
• Relatively recent concept
History of Stratigraphy
• Greeks and Romans
– Recognized world was ever changing and thought of it
as eternal
• Fall of Rome and Rise of Church
– Developed idea of world based on literal interpretation
of bible
• Attributed variations to Noah’s flood
– e.g., seashells high in Italian Appenines (near Florence)
» Either Noah’s flood brought them or God’s mystery
History of Stratigraphy
• Renaissance and age of Reason
– Began to move away from extreme views and
began to develop scientific principles
– Da Vinci (1452- 1519)
• Suggested rivers had carried the shells down from
the Alps
• Mix of mud and fossils turned to stone, uplifted
wrong, but significant in that it showed beginning of
change in thought
Da Vinci’s Insight
• In 1500 C.E. Leonardo Da Vinci recognized
that fossil shells in the layered rocks
represented ancient marine life
• Observed that many fossil rich layers were
separated by unfossiliferous layers thereby
repudiating the concept of one flood
• Had the idea that seasonal events
responsible
History of Stratigraphy
• Nicolaus Steno
– Niels Stensen (1638-1686)
• famous for Stenos’ laws
– Worked near Tuscany
• First to suggest IN PRINT that rocks enclosing fossils had at
one time been soft
• Sharks’ teeth convinced him
– Noticed living sharks had teeth identical to ‘tonguestones’
– Petrified tongues of dragons and snakes
• Realized shark’s teeth were deposited in mud, which encased
the teeth and then hardened
History of Stratigraphy
• Steno’s Laws
– Original horizontality
– Original continuity
– Superposition
• Provides set of rules for organizing or ordering
strata into relative ages
• Idea caught on but was derailed by the Neptunians
History of Stratigraphy
• Neptunians
• Arduino
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Ordered same Tuscan rocks as Steno into 4 groups
Primitive (Schists, basalts, granite cores of mountains)
Secondary (ls, shale; fossiliferous sedimentary rocks)
Tertiary fossiliferous sedimentary rocks found in hills
• Not universally applicable!
History of Stratigraphy
– Werner
• Rocks form from flood and then precipitated in flood
– (goofy, but not that unreasonable!)
» Primitive
» Transition
» Stratified
» Alluvial
History of Stratigraphy
• Plutonists
– Fought for fiery, volcanic rock
– Hutton
• Father of Geology!
• Uniformitarianism
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No begninning and no end
Saw Earth as an engine
1795- published the Theory of the Earth
“anti- church”; not easy to read
– Lyell
• Took extreme view to defend plutonisms against neptunists
• Actualism (uniform natural laws and processes)
• Gradualism (uniform RATES of change)
History of Stratigraphy
• William Smith
– Engineer; surveyor (late 1700’s)
– Analyzed rocks exposed in mines, canals of England
– Realized rock units had distinct fossils and could use
them to recognize rock units, and named them
• Law (Principle of ) Faunal Succession
– Can divide rock record based on fossils
• Cuvier
– 1769 – 1832
– Also recognized faunal succession but didn’t use it like
Smith for correlation
• (Focused on Paris Basin)
William Smith, Father of
Stratigraphy
• The Map that Changed the World, Simon
Winchester
• 1796 wrote “wonderful order and regularity
with which nature has disposed of these
singular productions [fossils] and assigned
to each its class and peculiar stratum”
• 1815 Publication of the 1st geologic map of
England intended for the development of
canals, quarries and mines as well as natural
resources
William Smith’s "Strata Identified by
Organized Fossils"
• Soon after the first issue of his great geological map of
England in 1815, William Smith published the Strata
Identified by Organized Fossils.
• It was intended as a kind of geological users manual with
illustrations to identify fossils.
• But Smith’s work went beyond the mere illustration of fossils.
Smith had deciphered the hieroglyphics of nature-the
distinctive inscriptions borne by the different strata. With the
Strata Identified . . . and its colored plates in hand, anyone
would be able to compare the plates with fossils collected in
the field and immediately identify the strata from which they
came.
• The strata once identified, their place in the orderly succession
of the strata-which lay above and which lay below, as Smith
had determined it - was then known. All this, Smith wrote,
"without the necessity of deep reading, or the previous
acquirement of difficult arts."
Georges Cuvier and Alexandre
Brongniart- Paris Basin
• Development of geologic map of Paris Basin
• The strata of the Paris Basin were close to
horizontal.
• As of 1811, Cuvier and Brongniart employed
fossils but only in the few instances where more
obvious evidences of sequence were absent.
• The title of their work was Géographie
Minéralogique by which they meant the
distribution of what Werner had called the
"external" characteristics of the mineral and fossil
contents, shapes, colors, and textures of the strata
within the Paris basin.
• Today call this lithology. They determined the
order of the strata from their superposition, their
lithology and by tracing them across the basin
• Cuvier firmly established the fact of the extinction
of past lifeforms
History of Stratigraphy
• Time Scale
– A way of formally organizing rocks and time
• Artificial division of time
• Evolved as workers began to apply scientific principles to the
study of rocks
• Smith, Cuvier, others
– Named rocks
– Rock names became basis for time scale
GEOLOGIC TIME SCALE
Subdivision of the Rock Record
• Lithostratigraphy –
– Study of the physical relationship among rock
units;
• No time connotation other than superposition
• Physical properties and stratigraphic position
relative to other lithostratigraphic units
• Chronostratigraphy –
• Integrated approach to establishing the time
relationships among geologic units
Chronostratigraphy:
Integrated Approach to Establishing the Time
Relationships Among Geologic Units
• Biostratigraphy
– Study of the fossil record with emphasis on faunal succession to
establish relative time relationships
– The correlation web
• Magnetostratigraphy
– Study of the magnetic properties of rock units for the purpose of
correlation using magnetic polarity reversals
• Allostratigraphy
– Study of rock units defined by unconformities and other features
generated by base level change
• Geochronology
– Various techniques, especially isotope geochemistry, to establish
the absolute age of rock units
Stratigraphic Correlation
Types of Stratigraphy
• Each has separate role
– Ultimate goal is integration and better
interpretation (more refined)
• Additional divisions of stratigraphy
– Stratigraphic Code
– Divided into sub-disciplines based on physical
limits or geologic age
Lithostratigraphy
• Definition
– Study of the sequence and interpretation of
layered rock sequences, based on PHYSICAL
CHARACTERISTICS (lithology) of the rocks
• Essentially, study of TIME
• Allows us to ORDER events!
Lithostratigraphy
• 2 uses of term lithology
– Study and description of physical
characteristics of rocks
• Especially hand specimen and outcrop
– e.g., investigate the lithology
– Term that refers to these physical characteristics
– e.g., lithology of a stratigraphy unit- such as sandstone
Lithostratigraphy
• Divide rocks on basis of lithology
– Stratotype
• Designated type unit of readily accessible rocks in natural
outcrops, mines or bore holes.
– Type section
• Formal designation
– Lithosome
• Masses of rock with uniform character
– Dissolve away all other rock Sloss (1963)
• Informal designation
Lithostratigraphy
• Some generalizations about lithostratigraphic units
– Sedimentary, extrusive igneous, meta-sedimentary,
meta-volcanic
– Generally conforms to law of superposition
– Commonly stratified (layered) and tabular in form
– Identification is based on observable rock
characteristics
– Boundaries can be either sharp and clear, or gradational
Uses of Lithostratigraphy
• What lithostratigraphy can do for you
– Place unit in geologic framework
– Establish stratigraphic relationship with units
above and below
– Aid in correlation between other
LITHOLOGIC units
• What it CAN’T do for you
– Denote time
Units of Lithostratigraphy
• Lithostratigraphic classification and nomenclature
– System for standardization
• Fundamental unit
– Formation
• Lithologically distinctive stratigraphic unit that is large enough
in scale to be mappable at the surface or traceable in the
subsurface.
• May be just 1 lithosome or multiple
• Subdivision of formation
Lithostratigraphy
• Other formal lithostratigraphic units
– Smaller than a formation
• Member
– Subdivision of a formation
• Bed
– Smallest formal lithostratigraphic unit
– Collections of formations
• Group
– 2 or more adjacent formations that have some unifying
lithological and/ or genetic features
• Supergroup
– Groups of groups
» e.g., Newark Supergroup
Stratigraphic Contacts
• Contacts
– Plane or irregular surfaces between different
types of rocks
– Separate units
• Conformable
• Unconformable
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
Types of Contacts
– Intercalated contact
• Increasing number of interbeds of another lithology
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Laterally adjacent contacts
Pinch-outs
Inter-tonguing
Progressive lateral gradation
Geometry of Facies/Lithostratigraphic
Relationships
Unconformable
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Break in deposition
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Strata do not succeed 1 another as a continuous whole
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Unconformity:
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Surface of erosion or non- deposition separating younger strata
from older rocks
Represents a significant hiatus
Diastem
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Separated by an unconformity
Lack of continuity in deposition
Minor interruption in depositional conditions
Interpretation
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Period of non-deposition, weathering or erosion
Such processes may occur sub-aerially (exposed to air) or subaqueously (under water)
Unconformities
• Types of unconformities
– Angular unconformity
– Nonconformity
– Disconformity
• Erosional surface between // units
– Paraconformity (obscure)• // units with no obvious erosional surface b/n the units
• use obscure traits to identify it
Unconformity
Following Stratigraphic Code
Unconformities
• Hiatus
– Total interval of geologic time represented by
missing strata
• At a specific position along a stratigraphic sequence
• Don’t confuse syndepositional variations
with post-depositional alteration of beds
– Groundwater changes color (Fe) or resistance
(cementation)
Vertical/lateral variations in rock
• Recognition of lateral variation
– Because rocks are not constant through space, you get a
variation in lithology within one unit.
• Initially people didn’t recognize this-
– Thought Noah’s flood left layer cake layers of strata
that extended to ends of Earth without change in
lithology or thickness
• Began to trace beds
– walked them
– looked at how the rocks between the boundaries changed
– realized fallacy of idea
– We retain layer cake to describe stacking pattern; not
infinite nature
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
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.”
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
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
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
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
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!
Relative
Change
Eustatic
Change
Causes of Sea Level
Change
Sea Level Cycles
• 1st Order Cycles
– 100’s my
– 100’s of meters
• 2nd Order Cycles
– 10’s my
– 100’s of meters
• 3rd Order Cycles
– 1-10 my
– 10’s of meters
Falling
Rising
Preservation potential of rocks
that are deposited
• Majority of sediments in fossil record
– marine
• Most sub-aereal environments
– erosional
• WHY?
– Accommodation space!
• Space available controls accumulation
– no place to put it, then no deposition
– base level
» balance between erosion and deposition
• Base level of erosion
– level below which erosion cannot occur ( = sea level)
• not strictly true!
– you do get erosion below sl.
» storms = erosion on shelf
» downcurtting = canyons on slopes
» deep sea currents = erosion of sea floor
• base level of aggradation
– level above which sediments cannot accumulate
permanently
• most marine sediments dominate record
• rise in sea level tends to result in increased preservation of seds
– high rate of erosion explains why there are “more gaps
than record” in geol. record.
Correlation
• Lithostratigraphic Correlation
– Match
• match lithologies
– Correlate
• link units of similar lithology and stratigraphic position
• PROCEDURE
• identify environments of deposition
– (use simple depositional model)
• draw lines of correlation between adjacent facies
• make interpretation- trans/ regression/ no change
Actualism and "Genetic Stratigraphy"
• Recognition of Uniformitarianism
– the relationship between modern processes of
sedimentation and the rock record
Actualism and "Genetic Stratigraphy"
• Sediments with distinct
lithologic aspect are deposited in
only limited areas in a given
time period.
• This results from the limited
lateral extent of contiguous
depositional environments.
• Distinct depositional
environments migrate in space,
through time so that lithofacies
units are inherently diachronous
(variable in age throughout)
Diachronous Stratigraphic Units
Diachronous Stratigraphic Units
Basement