stratigraphic concepts and methods part 1
TRANSCRIPT
Stratigraphic concepts and methodsPart 1
Stratigraphy
What we learnt so far about rocks and their depositional environments need to be set in the context of a time framework. This is fundamental
in order to use them to understand Earth processes and evolution.
Stratigraphyis the discipline of the Earth Science that help us to define such time framework recorded by the spatial
and temporal evolution of the rocks and their related depositional environments, using different
stratigraphic methods.
What is stratigraphy?
From the Latin ‘stratum’,
bed, and the Ancient
Greek ‘γράφειν’,
‘graphein’, to write
Study the succession of
sedimentary deposits in
order to reconstruct the
Earth’s history in all its
aspects: measurement of
time, evolution of life,
climate and environmentCuvier & Brogniart, 1831
What methods?
STRATIGRAPHY - Study of sedimentary layers (strata),
their lithology, mineralogy, geometry, distribution,
correlation, dating, fossils, chemical composition,
geophysical properties
From C. Dupraz, UConn
What lithology and mineralogy?
What type of limit?
Thickness?
Lateral continuity?
Fossils?
Age?
Facies?
Sedimentology?
Geochemistry?
Geophysical properties?
What methods?
From C. Dupraz, UConn
Contact
Laminations
Laminae
Clayey laminae
BedLaminations
and laminae
Alternation of
beds of different
lithologies
1.5 m
< 10 cm
A question of scale, of lithology…
+ sedimentary structures!Castille Fm., Permian
What methods?
A question of scale, of lithology…
Trailblazers in Stratigraphy
Xenophane (philosopher, ca. 570-475 BC) – Fossils as
evidence for an ancient life. Fossils-bearing rocks must be old
and ‘transported’ from the oceans to the mountains
Herodotus (historian, ca. 484-425 BC) – Recognition of
fossils, identification of the importance of sedimentation and of
sediments superposition in the Nile delta
Aristotle (philosopher, ca. 384-322 BC) – Recognition of
fluvial deposits, discovery of some fossils, interpretation as sea
level changes that lasted for a long period
Strabo (geographer, ca. 64 BC-24 AD) – Recognition of
emersion and inundation phases of the Earth
Trailblazers in Stratigraphy
Leonardo da Vinci (Italy, 1452-1519) – Cycle of sedimentary
rocks with transport by rivers, deposition, lithification and
transfer into mountains. Fossils are witness of ancient life.
Geological times must have been long
Georgius Agricola (Germany, 1494-1555) – « Father of
Geology » with important contributions in mineralogy and
mineral ressources, general geology and paleontology. Two
important books: « De Natura Fossilium » (1546) and « De Re
Metallica » (1556)
James Usher (Scotland, 1581-1665) – The first to
approximate the age of the Earth: follwing the genealogy in the
Bible, it has been created on Sunday October the 23rd, 4004
BC, 9 am
Trailblazers in Stratigraphy
Nicolas Steno / Niels Stenson (Denmark, 1638-1686) –
« Father » of stratigraphy, he defined three fundamental
principles in Geology:
1. Law of superposition: sedimentary layers are younging
upward through a stratigraphic succession
2. Principle of original horizontality: all sedimentary layers
were deposited horizontally
3. Principle of lateral continuity: sedimentary layers are
laterally continuous, any lateral discontinuity must be
explained (basin limit, erosion, earthquake, etc…)
Main contribution: « De solido intra solidum naturaliter contento
dissertationis prodromus », Preliminary discourse to a
dissertation on a solid body naturally contained within a solid
published in 1669
Trailblazers in Stratigraphy
Georges Cuvier (France, 1769-1832) – Pioneer in anatomy
and paleontology, he compared the anatomy of living and fossil
species, showing that the latter were extincted (e.g.,
mammuth). He disagreed with the theory of evolution, instead
he proposes 6 catastrophies as the triggers for extinctions and
subsequent appearance of new species. The last catatrophy
was the Flood
Trailblazers in Stratigraphy
Charles Lyell (England, 1797-1875) – Followed on
Hutton’s theory on uniformitarism, with his formula « The
Present in the key to the Past ». Published « Principles of
Geology » in 1830, a success. Geological processes (uplift,
erosion) are slow and uniform mechanisms, and each
geological period last for a long time, eventually hundreds of
million years.
Trailblazers in Stratigraphy
Charles Darwin (England, 1809-1882) – Enuntiated the
theory of evolution, « On the origin of species by means of
natural selection » (1859), « The Descent of Man » (1871)
Trailblazers in Stratigraphy
Betram Boltwood (USA, 1870-1927) – A chemist,
he proposed an age of 2.2 Gy for the Earth, based
on radioactive desintegration
Alfred Wegener (Germany, 1880-1930) – Theory
of the continental drifting based on geological and
paleontological evidences
Trailblazers in Stratigraphy
Arthur Holmes (England, 1890-1965) – In 1913 he
proposed an age of 4 Gy for the Earth, and explained
continental drift by convection inside the Earth
Harry Hess (USA, 1906-1969) – In 1960 he
proposed the seafloor spreading hypothesis,
using Wegener’s theory applied to a solid
basement
Walter Alvarez (USA, 1940-) – In 1980 he
proposed that a meteoric impact at the K-T
boundary may explain the mass extinction
The stratigraphic revolution
All these pioneering studies lead to three primordial
discoveries, essential for our understanding of the Earth
system:
1. The Evolution (Hutton, Lamarck, Darwin, Wallace)
2. Deep Times (Boltwood, Holmes)
3. Continental Drift (Wegener, Hess)
Press & Siever, 1994
Principles (Laws) of Stratigraphy
• Superposition• Original Horizontality• Lateral Continuity• Cross-cutting relationship• Unconformity• Faunal Succession• Walther’s Law
Space- and relative time-relationships between rock strata are defined by some fundamental principles of stratigraphy
Principle of stratigraphic superposition:
“Sedimentary layers are deposited in a time sequence,
with the oldest on the bottom and the youngest on the top”. (N. Steno, 1669)
Youngest Strata
Oldest Strata
Principle of original horizontality:
“Sediments are deposited horizontally or nearly horizontally in layers that lie parallel or nearly parallel to the Earth's surface”. (N. Steno, 1669)
Principle of original lateral continuity:
“Sedimentary rocks are laterally continuous over large areas.
A stratum has the same age along its horizontal extent”.
Similar rocks which are now separated by valleys, faults, etc., can be considered to have been originally continuous.
Stratigraphic geometries of strata
Late
rally C
on
tin
uo
us
Un
its
La
tera
lly
Dis
co
nti
nu
ou
s U
nit
s
Principle of faunal succession:“Fossils show a progressive development through time. The same succession of fossils
can be found at different places and indicate similar ages”.
Principle of cross-cutting relationships:“Any layer that is cut by another geological feature has to be older
than such geological feature”. (J. Hutton, 1795)
Can be applied to igneous intrusions, faults, veins,
erosional surfaces
Included fragments:
The fragments in a clastic rock must be made up of a rock that is
older than the strata in which they are found.
Sometimes conglomerates directly overlie erosional surfaces and are called
basal conglomerates
Principle of Unconformity:“A stratigraphic section can be incomplete due to a gap in the stratigraphic record.
When the stratigraphic continuity is missing we define an unconformity”
Different types of unconformities can be defined. They normally reflect either erosion or non-deposition of sediments.
Angular unconformityRelated to erosion and/or non-sedimentation with deformation of underlying sediments.
Angular Unconformity between Proterozoic and Cambrian rocks. 100s of million years (MY) are missing. G. Canyon
Angular unconformity
DisconformityRelated to erosion and/or non-sedimentation but no deformation of
underlying sediments.
Disconformity
ParaconformityIt is characterized by beds above and below the unconformity contact that are parallel and with
no physical evidences of erosion or non-deposition.
Determined from biostratigraphic evidences such absence of faunal zones or abrupt faunal changes.
NonconformityUnconformities that separate igneous or metamorphic rocks from overlying sedimentary rocks.
What methods?
Lithostratigraphy – description and
correlation of lithology
Biostratigraphy – description and use of
fossils to correlate and date successions
Chemostratigraphy – use of geochemical
proxies/parameters to correlate
successions and reconstruct
biogeochemical changes
Magnetostratigraphy – Use of reversals
of magnetic polarity to date and correlate
Cyclostratigraphy – Use of cyclicity to
date and reconstruct paleoclimatic
changes
Sequence stratigraphy – Reconstruction
and interpretation of sequences defined by
discontinuities in terms of eustatic changes
Chronostratigraphy – Establishing time
scales
Absolute vs. Relative Time
Relative dating
Subdivisions of the Earth's Geology in a specific order based upon relative age relationships. Only the sequential order of events can be inferred through relative dating (litho-bio-magneto-chronostratigraphicapproaches)
Stratigraphy
Absolute datingRocks are dated assigning numerical ages (My, Ky, y). These are obtained via radiometric dating performed on appropriate rock types (geochronology).
What methods?
Absolute vs. relative ages
• Absolute: we aim at estimating
an age in years (Ma, Ga…) using
‘geochemical chronometers’
applied to specific minerals (e.g.,
K/Ar method on glauconite, Pb/Pb
on zircon)
• Relative: we aim at defining the
chronology of events recorded by
sedimentary archives as well as
the chronology of strata, based
on criteria characteristics of
geological time periods
Relative Time: litho-bio-chronostratigraphic
correlation
In this case, similar rocks contain similar fossils assemblages, deposited at the same time
Lithostratigraphy
In lithostratigraphyrock units are
considered in terms of the lithological
characteristics of the strata and their
relative stratigraphic positions.
There´s a hieralchicalterminology to define
lithostratigraphic units
Supergroup
Group A
Formation A
Member A
Member B
Member C
Formation B
Member D
Member E
Member F
Formation CMember G
Member H
Group B
Formation D
Member I
Member J
Member K
Member L
Member M
Formation E
Member N
Member O
Member P
Member Q
Formation F
Member R
Member S
Member T
LithostratigraphyThe basic unit of lithostratigraphic division of rocks is the Formation, which is a set of similar beds that can be identified by its lithological
characteristics and by its stratigraphic position.
A Formations must be1. Lithologically homogeneous. 2. Distinct and different from adjacent rock units above and below. 3. Traceable from exposure to exposure, and of sufficient thickness to be mappable.4. Formations must have names. They are usually named by some geographic
locality where they are particularly well exposed (type section/ type locality)Also, to be valid, the name of a formation must be published in the geological literature.
A Bed is defined as lithostratigraphic Unit in case it shows relatively large lateral continuity and particular characteristics, different from the over- and underlying
strata.
The smallest Lithostratigraphic Unit is the Bed:
Supergroup – Two or more associated groups
Group - Two or more associated formations
Formation - Distinct and mappable rock unit
Member - Subdivisions of a formation. (At least two must be present to be accepted)
Bed - Smallest recognized unit. Distinctive portion of a member. (Ex: distinct fossil beds or coal beds)
Lithostratigraphic Units
Bed
Bed
Bed
Member
Member
Formation
Formation
Formation
Formation
Formation
Group
Group
Super-
Group
Lithostratigraphic units
From C. Dupraz, UConn
Lithostratigraphy of the Helvetic Carbonate platform
Föllmi & Gainon, 2008
BiostratigraphyPart of stratigraphy that uses fossils or fossil assemblages preserved in sediments and
sedimentary rocks to establish correlations (temporal and spatial) between strata and define
relative ages.
The concept of biostratigraphy is based on the evidence that the living organisms showed across geological time unique and (almost) irreversible
evolutionary trends and pattern of diversification
Biostratigraphy
Evolution generates genetic similarities and differences between the organisms. Such genetic relationships are at the base of a hierarchical
biological classification which is represented by the so called Linnaean Hierarchy or Linnaean Taxonomy
TaxonomyScience that deals with the study of identifying, grouping, and naming organisms
according to their established natural relationship.
Kingdom: Rhizaria
Phylum: Foraminifera
Class: Textularidia
Order: Textulariida
Family: Orbitolinidae
Genus: Mesorbitolina
Species: parva
Biostratigraphy
Time-space distribution and diversity of fossil assemblages in the rocks can be given by:
-Phyletic transformation or gradualism (ancestral to descendant members of the same phylogenetic lineage)
-At the scale of species, phyletic trasformation is called speciation
-Extinction (pseudo extinction)
Strictly controlled by: -the depositional environment (continental/marine realm; water chemistry, depth,
temperature...facies)
-the mode of life of the organisms (planktonic-nektonic-benthonic)
BiostratigraphyE
arl
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p.p
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Tuarkyricus
Sarasini
Giraudi
Feraudianus
Sartousiana
Vandenheckii
Caillaudianus
Nicklesi
Hugii
Angulicostataauct.
Balearis
Ligatus
Sayni
Nodosoplica-tum p.p. H a3
H a4
H a5
H a6
H a7
Ba1
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Ba4
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Bd1
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Praedictyorbitolina /
D ictorbitolina
lineage
Valserina /
Eygalierina
lineage
M ontseciella /
Rectodictyoconus
lineage
Eopalorbitolina /
Palorbitolina
lineage
H ighstand Systems Track
Transgressive Systems Track
Lowstand Systems Track
Sequences after
Clavel et al ., 1995
Schroeder et al., 2002
Benthic foraminifera: a rapid evolution of forms in the Barremian - Aptian
Biostratigraphy
Fossil organisms ideal (Index Fossil or Index Species) for biostratigraphy must exhibit:
• A rapid evolution
• Abundant forms
• limited temporal distribution.
• wide geographic distribution (as wider as possible).
• An apparition and disappearance synchronous in all basins
• Well defined FO and LO
• A morphology that is not linked to the environment
• high potential of preservation of the hard parts.
• easy to identify on field, thin section, SEM.
The presence of different fossil assemblages within different rock strata allowed to define temporal (and spatial) subdivisions of rocks in Biostratigraphic Units.
The fundamental unit of biostratigraphy is the Biozone.
Biozone features:-Can be defined on the base of shallow-water or deep-water assemblages.
-Fossil content different from the under- and overlying strata.
-Is usually defined by First- and Last Occurrence (FO and LO) of specific index taxa.
-Lateral and vertical (thickness) variations, ranging from local to nearly global distribution and from few meters to thousand of meters.
-It can be coincident with lithostratigraphic units and chronostratigraphic units.
-The name is usually given by the commonest taxa defining the biozone itself.
Macro and micro fossils used in biostratigraphy
Ammonites
The large size and free-swimming cephalopods. Excellent group for deep-water biostratigraphic purposes. Are widespread in many fully marine environments. Ammonites are the main zone fossils in Mesozoic rocks. They became extinct at the end ofthe Cretaceous.
TrilobitesPalaeozoic arthropods widely used as main zone fossils in the Cambrian rocks and locally in the Ordovician and Devonian rocks. They became extinct at the PTB (Permo-Trias boundary).
Macro and micro fossils used in biostratigraphy
Planktonic foraminiferaVery good zone fossils in Mesozoic and Cenozoic rocks.They are abundant and widespread in marine strataAppear to have evolved rapidly.
Benthic foraminiferaOften used for shallow-water biozonations (together with calcareous algae). Their quality as biostratigraphic indexes is strictly dependent on the palaeoenvironment. They span from the Carboniferous to the recent.
Calcareous Nannofossils
Macro and micro fossils used in biostratigraphy
They are found in marine sediments deposited on the shelf or any depths above the CCD below which they are not normally preserved. They are used biostratigraphically in Mesozoic and Cenozoic rocks (Jurassic-recent).
Dinoflagellatesmarine to fresh water planktonic organismsfound from the Triassic through to the present.Zonation based on dinoflagellates is locally veryimportant, especially in non-calcareous strata of Mesozoic and Cenozoic ages.
Palynomorphs
Macro and micro fossils used in biostratigraphy
Airborne particles such as pollen, spores and some seeds may be widely dispersed and the occurrence of these aeolian palynomorphs within marine strata allows for correlation between marine and continental successions. They can be used as zone fossils, but they rarely provide such a high resolution as marine fossils.
Mammal and reptile teethRarely used to define continental biozonations together with palynomorphs.
Range of the major macrofossil groups used for biostratigraphic zonations
Biostratigraphy – Biozones
Interval zone (partial range)
Concurrent range zone
Range zone
Assemblage zone B
Assemblage zone A
Multi-taxon concurrent
range zone
Oppel zone (defined by FAD or LAD of one
taxon, but characterized by additional taxa)
TIM
E
Example of shallow-water biostratigraphy: from distribution to biozonation
Chronostratigraphy and geochronology
Branch of stratigraphy that studies "the relative time relations and ages of rock bodies". Thus, chronostratigraphy deals with rock bodies, in opposition
to geochronology which deals with the dating and subdivision of geologic time.
The fundamental chronostratigraphic unit is the Stage. Stages are then grouped into a hierarchical structure of higher units, or may be subdivided into substages. Each
chronostratigraphic unit has a corresponding geochronologic unit, that can be seen as the time interval during which that chronostratigraphic unit formed.
A Chronostratigraphic unitincludes all rocks deposited
during a given interval of geologic time.
When the radiometric dating techniques became
available , the ages of boundaries between chronostratigraphic
units could be estimated!
The Geological Time ScaleFrom the beginning of the 20th century, a chronostratigraphic scale was established and the
geologic time was subdivided into chronostratigraphic units of unknown duration (mainly based on relative stratigraphic relationships).
The relative chronostratigraphic scale that integrates absolute radiometric ages is the
Geologic Time Scale
Ma
Standard Chronostratigraphy
Period Epoch Age/Stage
Geomagnetic
Polarity
Marine Macrofossils
(Mesozoic-Paleozoic)Tethyan Ammonoids
Planktonic Foraminifers
Sub-Tropical Zone Foram Zone Marker Nanno Zone Marker
Calcareous Nannofossils
120,5
121
121,5
122
122,5
123
123,5
124
124,5
125
125,5
126
126,5
127
127,5
128
128,5
129
129,5
Cretaceous Early
Barremian
Aptian
M3
M1
M0r
(C34n)
M"- 1"r
(C34n)
M-S
eq
ue
nce
Cre
tace
ou
s N
orm
al S
up
er-
Ch
ron
("C
reta
ce
ou
s Q
uie
t Z
on
e")
Nicklesia nicklesi
Nicklesia pulchella
Kotetishvilia compressissima
Moutoniceras moutonianum
Ancyloceras vandenheckii
Gerhardtia sartousi
Imerites giraudi
Deshayesites oglanlensis
Deshayesites forbesi
Deshayesites deshayesi
Dufrenoyia furcata
Epicheloniceras martinoides
Hedbergella similis
Globigerinelloides blowi
Leupoldina cabri
L. cabri (consistent)
Globigerinelloides ferreolensis
Globigerinelloides algerianus
Globigerinelloides algerianus
Leupoldina cabri (consistent)
Leupoldina cabri (rare)
Globigerinelloides blowi
Leupoldina cabri
CC6
CC7
Eprolithus floralis
Rhagodiscus gallagheri
Hayesites irregularis
Flabellites oblongus (consistent)
Micrantholithus hoschulzii
Conusphaera rothii
Stratigraphic Correlation
•Lithostratigraphic correlation: -physical lateral matching between the same Formations (or other lithostratigraphic units).
Assessment of the equivalence of rock bodies at different locations. Two kinds of correlation can be defined:
• Chronostratigraphic correlation:-matching up rocks of the same relative age, usually through the use of fossils (biostratigraphic correlation). Can be apply on a larger scale (compared to the lithostratigraphic correlation alone)
The lithostratigraphic correlation alone is often not able to give us a time framework because the
boundaries of the lithostratigraphic units are diachronous!!!
That means: no relationship between lithostratigraphy (lithology) and
chronostratigraphy (time).
Example of chronostratigraphic correlation
Ethyopia Italy
Walther’s LawVertical conformable facies changes reflect lateral
adjacent facies changes
Different sedimentary environments and their related facies do not remain fixed through time but move spatially mainly following sea level changes. The results is a specific pattern of vertical
facies variation given by the lateral shift of the related depositional environments.
Marine Transgression
A marine transgression occurs when sea level is rising.
The result is a landward shift of facies zones and a
retrogradation of the coastline.The stratigraphic section will
show facies progressively more distal upward.
t1
t2
t3
Marine Regression
A marine regression occurs when sea level is falling.
The result is a seaward shift of facies zones and a progradation
of the coastline.The stratigraphic section will
show facies progressively more proximal upward.
t1
t2
t3
limestone
shales
sandstones
Transgression
Defining litho- and chronostratigraphic surfaces
Relationships between the boundaries of lithostratigraphic units (defined by lithological characteristics resulting from the depositional environment) and time-lines in a succession of strata formed during
gradual sea-level rise (transgression).