3.1 Principles of stratigraphy

Stratigraphy is the backbone of paleontology, providing crucial context for fossil discoveries. It uses principles like superposition and cross-cutting to determine relative ages of rock layers. Understanding these concepts helps piece together Earth's history and the evolution of life.

Stratigraphic methods like biostratigraphy, magnetostratigraphy, and chemostratigraphy offer powerful tools for dating and correlating rocks globally. By integrating multiple approaches, scientists can build high-resolution timelines of past environments and biological events, unlocking the secrets of our planet's past.

Principles of stratigraphy

Sedimentary rocks and layers

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• Form through the deposition and lithification of sediments over time
• Sediments can be derived from weathering, erosion, and biological processes
• Layers (strata) represent distinct depositional events or environments (fluvial, lacustrine, marine)
• Studying sedimentary rocks and layers provides insights into past environments, climates, and life

Law of superposition

• In an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest at the top
• Allows for relative dating of strata based on their vertical position
• Forms the basis for establishing the relative age of fossils and geologic events
• Exceptions can occur due to tectonic disturbances (thrust faults) or intrusions (dikes)

Original horizontality

• Sedimentary layers are deposited in nearly horizontal positions
• Any significant deviations from horizontal indicate post-depositional deformation (folding, tilting)
• Helps in reconstructing the original orientation of strata and interpreting tectonic events
• Exceptions include cross-bedding (sand dunes) and depositional slopes (deltas)

Lateral continuity

• Sedimentary layers extend laterally until they thin out or terminate against an obstacle
• Allows for correlation of strata over wide geographic areas
• Changes in thickness or lithology can indicate variations in depositional environments or sediment supply
• Discontinuities may result from erosion (unconformities) or non-deposition (hiatus)

Cross-cutting relationships

• Younger features (faults, intrusions) cut across and displace older strata
• Provides a relative age relationship between the cross-cutting feature and the affected strata
• Helps in establishing the sequence of geologic events and tectonic activity
• Examples include faults offsetting layers and igneous dikes intruding sedimentary rocks

Inclusions and components

• Fragments of older rocks or fossils included within younger sedimentary layers
• Inclusions must be older than the host rock, providing a maximum age constraint
• Detrital components (minerals, rock fragments) can indicate the provenance of sediments
• Reworked fossils may lead to misinterpretation of age if not recognized as inclusions

Faunal succession and index fossils

• Fossil assemblages change over time due to evolution and extinction
• Distinct fossil assemblages characterize specific time intervals and can be used for correlation
• Index fossils are species with short temporal ranges and wide geographic distribution
• Examples of index fossils include ammonites (Mesozoic) and conodonts (Paleozoic)

Lithostratigraphy

Definition and purpose

• The study and classification of sedimentary rock units based on their lithologic characteristics
• Aims to establish a framework for mapping and correlating strata based on observable physical properties
• Lithologic features include composition, texture, sedimentary structures, and thickness
• Provides a basis for interpreting depositional environments and basin evolution

Lithostratigraphic units

• Defined based on distinctive lithologic features and stratigraphic position
• Hierarchical classification includes (in descending order): group, formation, member, and bed
• Each unit is mappable at a particular scale and has a type section or locality
• Lithostratigraphic units are the building blocks for geologic maps and cross-sections

Formation vs member vs bed

• Formation: the fundamental unit of lithostratigraphy, a mappable body of rock with distinct lithologic characteristics
• Member: a subdivision of a formation, distinguished by lithologic features or stratigraphic position
• Bed: the smallest formal lithostratigraphic unit, a single layer with unique lithologic properties
• Formations can be divided into members, and members can be divided into beds

Naming conventions and type sections

• Lithostratigraphic units are named after geographic features, localities, or distinctive lithologies
• Names consist of a geographic term and a lithologic term (Navajo Sandstone, Chattanooga Shale)
• Type sections serve as the standard reference for defining and describing a lithostratigraphic unit
• Type localities are designated where the unit is well-exposed and easily accessible

Biostratigraphy

Definition and purpose

• The study and application of fossil content in sedimentary rocks for correlation and age determination
• Based on the principle of faunal succession and the temporal distribution of fossil species
• Biostratigraphic units (biozones) represent intervals characterized by specific fossil assemblages
• Enables global correlation of strata and refinement of the geologic time scale

Biostratigraphic units and biozones

• Biostratigraphic units are defined by the presence, absence, or abundance of particular fossil taxa
• Biozones are bodies of strata characterized by a specific fossil content or assemblage
• Types of biozones include assemblage, abundance, interval, lineage, and concurrent range zones
• Biozones are named after one or more characteristic fossil taxa (Exus albus Assemblage Zone)

Assemblage vs abundance vs interval zones

• Assemblage zone: an interval characterized by a unique fossil assemblage or association
• Abundance zone: an interval defined by the quantitative abundance of one or more fossil taxa
• Interval zone: the strata between two specified biostratigraphic horizons (first or last occurrences)
• Lineage zone: an interval representing the temporal range of an evolutionary lineage
• Concurrent range zone: the interval of overlap between the ranges of two specified taxa

Correlation using biostratigraphy

• Fossil assemblages are used to correlate strata across different regions or basins
• Correlation is based on the presence of identical or closely related fossil taxa
• Biostratigraphic events (first or last occurrences) serve as tie points for correlation
• Limitations include facies dependence, diachronous ranges, and preservational biases

Chronostratigraphy

Definition and purpose

• The study and classification of sedimentary strata based on time
• Aims to establish a global standard for the subdivision and correlation of geologic time
• Chronostratigraphic units represent intervals of geologic time and their corresponding strata
• Forms the basis for the geologic time scale and facilitates global correlation

Chronostratigraphic units and boundaries

• Hierarchical classification includes (in descending order): eonothem, erathem, system, series, and stage
• Each unit represents a specific interval of geologic time and is defined by a lower and upper boundary
• Boundaries are defined by specific stratigraphic markers (biostratigraphic, magnetostratigraphic, or chemostratigraphic)
• The base of each unit serves as the reference point for global correlation

Stage vs series vs system

• Stage: the basic unit of chronostratigraphy, representing a relatively short interval of geologic time
• Series: a subdivision of a system, comprising several stages and representing a longer time interval
• System: a major subdivision of the Phanerozoic Eon, representing a significant period of geologic time
• Stages are grouped into series, and series are grouped into systems (Cambrian System, Furongian Series, Paibian Stage)

Global boundary stratotype sections and points (GSSPs)

• GSSPs are specific locations where the lower boundary of a chronostratigraphic unit is formally defined
• The boundary is marked by a physical reference point (golden spike) within a stratigraphic section
• GSSPs are selected based on stratigraphic completeness, accessibility, and the presence of reliable markers
• Examples include the Meishan section (Permian-Triassic boundary) and the El Kef section (Cretaceous-Paleogene boundary)

Magnetostratigraphy

Principles of paleomagnetism

• Earth's magnetic field has periodically reversed polarity throughout geologic time
• Magnetic minerals in sedimentary rocks record the direction and polarity of the Earth's magnetic field at the time of deposition
• The study of the magnetic properties of rocks and their changes over time is called paleomagnetism
• Paleomagnetism provides a means for dating and correlating sedimentary sequences

Magnetic reversals and polarity zones

• Magnetic reversals are recorded as changes in the polarity of magnetic minerals in sedimentary rocks
• Normal polarity (magnetic north pole near geographic north pole) and reversed polarity (magnetic north pole near geographic south pole) intervals alternate
• Polarity zones are stratigraphic intervals characterized by a specific magnetic polarity pattern
• Polarity zones are named after prominent geomagnetic reversals (Matuyama Reversed Chron, Brunhes Normal Chron)

Correlation using magnetostratigraphy

• Magnetic polarity patterns can be used to correlate sedimentary sequences across different regions or basins
• Correlation is based on the identification of identical polarity patterns and the matching of reversal boundaries
• The global geomagnetic polarity timescale (GPTS) serves as a reference for magnetostratigraphic correlation
• Limitations include incomplete or discontinuous records, post-depositional remagnetization, and low magnetic mineral content

Chemostratigraphy

Principles of isotope stratigraphy

• Variations in the isotopic composition of sedimentary rocks reflect changes in the global carbon, oxygen, and strontium cycles
• Isotopic ratios (e.g., δ13C, δ18O, 87Sr/86Sr) are measured in specific mineral phases (carbonates, phosphates) or organic matter
• Isotopic excursions or trends can be used as stratigraphic markers for correlation and age determination
• Isotope stratigraphy provides insights into paleoenvironmental conditions and global geochemical events

Carbon vs oxygen vs strontium isotopes

• Carbon isotopes (δ13C) reflect changes in the global carbon cycle, influenced by organic carbon burial and volcanic CO2 input
• Oxygen isotopes (δ18O) record changes in global temperature and ice volume, as well as local evaporation and precipitation
• Strontium isotopes (87Sr/86Sr) reflect the balance between continental weathering and hydrothermal input, and are used for dating and correlation
• Each isotope system has its own characteristic temporal trends and excursions

Correlation using chemostratigraphy

• Isotopic excursions or trends can be used to correlate sedimentary sequences across different regions or basins
• Correlation is based on the identification of similar isotopic patterns and the matching of key chemostratigraphic events
• Examples include the Paleocene-Eocene Thermal Maximum (PETM) carbon isotope excursion and the Cenomanian-Turonian Oceanic Anoxic Event (OAE2)
• Limitations include diagenetic alteration, local environmental influences, and the need for independent age control

Sequence stratigraphy

Depositional sequences and systems tracts

• Depositional sequences are stratigraphic units bounded by unconformities or their correlative conformities
• Sequences are composed of systems tracts, which are genetically related strata deposited during specific stages of relative sea-level change
• The main systems tracts are the lowstand (LST), transgressive (TST), highstand (HST), and falling stage (FSST) systems tracts
• Each systems tract is characterized by distinct depositional environments, facies associations, and stacking patterns

Eustatic sea-level changes

• Eustatic sea-level changes are global variations in sea level, controlled by changes in ocean basin volume or water volume
• Eustatic changes can be caused by tectonic processes (seafloor spreading rates, continental collision) or climatic factors (ice volume, thermal expansion)
• Eustatic sea-level curves are constructed based on the analysis of global stratigraphic data and serve as a reference for sequence stratigraphic interpretation
• Examples of eustatic events include the Messinian salinity crisis and the Eocene-Oligocene sea-level fall

Transgressive vs regressive sequences

• Transgressive sequences are deposited during periods of relative sea-level rise, characterized by landward shift of facies belts and retrogradational stacking patterns
• Regressive sequences are deposited during periods of relative sea-level fall or stillstand, characterized by seaward shift of facies belts and progradational stacking patterns
• The interplay between accommodation space (created by subsidence and eustatic changes) and sediment supply determines the character of sequences
• Transgressive surfaces (TS) and maximum flooding surfaces (MFS) are key stratigraphic markers within sequences

Sequence stratigraphic correlation

• Sequence stratigraphy provides a framework for correlating sedimentary successions across different basins and tectonic settings
• Correlation is based on the identification of key surfaces (sequence boundaries, transgressive surfaces, maximum flooding surfaces) and systems tracts
• Sequence stratigraphic correlation helps in understanding basin evolution, sediment dispersal patterns, and reservoir distribution
• Limitations include the complexity of local tectonic and sedimentary controls, and the need for biostratigraphic or other independent age constraints

Integrated stratigraphy

Combining stratigraphic methods

• Integrated stratigraphy involves the combination of multiple stratigraphic techniques to achieve high-resolution correlation and age control
• Stratigraphic methods include biostratigraphy, magnetostratigraphy, chemostratigraphy, and sequence stratigraphy
• Each method provides unique insights into the age, depositional environment, and global context of sedimentary successions
• The integration of stratigraphic data allows for the construction of robust and reliable stratigraphic frameworks

High-resolution stratigraphy

• High-resolution stratigraphy aims to achieve the highest possible temporal resolution in stratigraphic analysis
• Involves the application of multiple stratigraphic techniques at fine scales (centimeter to meter)
• Enables the identification of short-term events, orbital cycles, and rapid environmental changes
• Examples include the study of Milankovitch cycles in sedimentary records and the analysis of abrupt climate events

Solving complex stratigraphic problems

• Integrated stratigraphy is particularly useful in solving complex stratigraphic problems, such as condensed sections, unconformities, and diachronous facies
• The combination of different stratigraphic methods allows for the resolution of conflicting or ambiguous data
• Integrated stratigraphy helps in understanding the interplay between local and global controls on sedimentation
• Case studies demonstrate the power of integrated stratigraphy in unraveling the geologic history of complex regions (e.g., the Western Interior Basin of North America)

Applications in paleontology and geology

• Integrated stratigraphy provides a robust framework for paleontological studies, enabling precise dating and correlation of fossil occurrences
• Helps in understanding patterns of evolution, extinction, and migration in the fossil record
• Integrated stratigraphy is crucial for the study of global events, such as mass extinctions, oceanic anoxic events, and rapid climate changes
• In the field of geology, integrated stratigraphy is applied in basin analysis, resource exploration, and geohazard assessment
• Stratigraphic data are used to reconstruct paleogeography, sediment routing systems, and the tectonic evolution of sedimentary basins