Stratigraphic concepts and methods Part 1

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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?

Facies?

Sedimentology?

Geochemistry?

Geophysical properties?

What methods?

Contact

Laminations

Laminae

Clayey laminae

BedLaminations

and laminae

Alternation of

beds of different

lithologies

< 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

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

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

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

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.

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)

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

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

rally C

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

Member

Formation

Super-

Lithostratigraphic units

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

Tuarkyricus

Sarasini

Giraudi

Feraudianus

Sartousiana

Vandenheckii

Caillaudianus

Nicklesi

Angulicostataauct.

Balearis

Ligatus

Nodosoplica-tum p.p. H a3

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).

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

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

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)

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

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

Cretaceous Early

Barremian

Aptian

(C34n)

M"- 1"r

(C34n)

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

Leupoldina cabri (consistent)

Leupoldina cabri (rare)

Globigerinelloides blowi

Leupoldina cabri

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.

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.

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).

Stratigraphic concepts and methods Part 1

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