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System (stratigraphy)
System (stratigraphy)
System (stratigraphy)
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Units in geochronology and stratigraphy[1]
Segments of rock (strata) in chronostratigraphy Time spans in geochronology Notes to
geochronological units
Eonothem Eon 4 total, half a billion years or more
Erathem Era 10 defined, several hundred million years
System Period 22 defined, tens to ~one hundred million years
Series Epoch 38 defined, tens of millions of years
Stage Age 101 defined, millions of years
Chronozone Chron subdivision of an age, not used by the ICS timescale

A system in stratigraphy is a sequence of strata (rock layers) that were laid down together within the same corresponding geological period. The associated period is a chronological time unit, a part of the geological time scale, while the system is a unit of chronostratigraphy. Systems are unrelated to lithostratigraphy, which subdivides rock layers on their lithology. Systems are subdivisions of erathems and are themselves divided into series and stages.

Systems in the geological timescale

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The systems of the Phanerozoic were defined during the 19th century, beginning with the Cretaceous (by Belgian geologist Jean d'Omalius d'Halloy in the Paris Basin) and the Carboniferous (by British geologists William Conybeare and William Phillips in 1822). The Paleozoic and Mesozoic were divided into the currently used systems before the second half of the 19th century, except for a minor revision when the Ordovician system was added in 1879.

The Cenozoic has seen more recent revisions by the International Commission on Stratigraphy. It has been divided into three systems with the Paleogene and Neogene replacing the former Tertiary System though the succeeding Quaternary remains. The one-time system names of Paleocene, Eocene, Oligocene, Miocene and Pliocene are now series within the Paleogene and Neogene.

Another recent development is the official division of the Proterozoic into systems, which was decided in 2004.

References

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Further reading

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System (stratigraphy)

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In stratigraphy, a system is a major chronostratigraphic unit that encompasses all rocks formed worldwide during a specific interval of geologic time, serving as the primary division for correlating strata across the globe and corresponding directly to a period in the geochronologic time scale. Systems form a key part of the hierarchical framework of chronostratigraphy, positioned below the erathem and above the series, within the broader sequence of eonothem > erathem > system > series > stage, which allows for the standardized organization of Earth's rock record based on relative age rather than lithology or fossils. Boundaries between systems are defined as synchronous horizons, typically established through boundary stratotypes—reference sections in continuous depositional sequences—ensuring global applicability and facilitating precise temporal correlation using evidence from biostratigraphy, magnetostratigraphy, or isotopic dating. Named often after geographic localities or key fossil assemblages (e.g., the Devonian System, spanning approximately 419 to 359 million years ago), systems represent significant episodes in Earth history, with time spans generally ranging from 30 to 80 million years in the Phanerozoic Eon, though shorter for units like the Quaternary System (about 2.58 million years). This unit contrasts with lithostratigraphic categories like groups or formations, which emphasize rock composition and physical , by focusing instead on time of deposition to reconstruct paleoenvironments, evolutionary events, and major geological processes. The formal recognition and ratification of systems are overseen by bodies such as the (ICS), which maintains the International Chronostratigraphic Chart as the authoritative global standard, updated periodically to incorporate new geochronologic data.

Definition and Scope

Formal Definition

In chronostratigraphy, a system is defined as a unit of rock strata formed anywhere on Earth during a specific geological period, serving as the fundamental chronostratigraphic division between an erathem and a series (subdivisions of a system). This unit encompasses all rocks, layered or unlayered, that were deposited or formed within the corresponding interval of geologic time, providing a standardized framework for organizing Earth's rock record by age rather than by lithology or other physical properties. Chronostratigraphy, the branch of that deals with the relative time relations and ages of rock bodies, relies on as its core formal units to correlate strata globally based on their time of formation. enable the precise integration of sedimentary layers across continents by employing methods such as index fossils, which are distinctive with limited temporal ranges used for , and radiometric techniques like uranium-lead or potassium-argon dating for absolute age determination. These approaches ensure that boundaries represent synchronous surfaces worldwide, facilitating the reconstruction of and environmental changes. The International Commission on Stratigraphy (ICS), under the International Union of Geological Sciences, has formalized the definition and boundaries of systems since its establishment in 1974, ratifying them through Global Stratotype Sections and Points (GSSPs)—specific, well-preserved stratigraphic sections designated as international reference standards. Each GSSP marks the base of a system with a precisely defined point, often tied to biostratigraphic, chemostratigraphic, or magnetostratigraphic markers, ensuring unambiguous global correlation.

Scope and Duration

In stratigraphy, a represents a major chronostratigraphic unit encompassing all rocks formed during a specific interval of geologic time, with typical durations for systems ranging from approximately 30 to 80 million years, except for the System (2.58 million years). These boundaries are often demarcated by globally significant events, such as mass or major paleontological turnovers, which provide clear markers for across rock records. For instance, the end-Cretaceous boundary at 66 Ma coincides with the Chicxulub impact and associated , while the Permian-Triassic boundary at 251.9 Ma marks the most severe mass in Earth's history. The spatial scope of a system is inherently global, including all strata deposited worldwide during the corresponding geochronologic period, regardless of local variations in thickness, lithology, or depositional environment. This universality allows systems to integrate diverse rock types—from marine carbonates to terrestrial sediments—while maintaining temporal coherence, though lateral changes can result in unconformities or facies variations that do not alter the unit's overall extent. Systems thus serve as a framework for synthesizing Earth's rock record on a planetary scale, accommodating regional differences without compromising the unit's time-bound integrity. Boundaries of systems are precisely defined using Global Stratotype Sections and Points (GSSPs), which establish the lower limit through a designated reference section and horizon, while the upper boundary is inferred as the GSSP of the succeeding system. This method ensures non-overlapping, abutting intervals in time, with each GSSP selected for its global correlatability based on biostratigraphic, chemostratigraphic, or magnetostratigraphic criteria. The oldest ratified system boundary GSSP is that of the base, located at Fortune Head, Newfoundland, , dated to 538.8 ± 0.6 Ma and marked by the first appearance of the Treptichnus pedum.

Position in Stratigraphy

Chronostratigraphic Hierarchy

In chronostratigraphy, the hierarchy of units organizes stratified rocks based on their age of formation, providing a framework for correlating global rock successions. The primary divisions, from largest to smallest, are , , , series, and . The encompasses all rocks formed since approximately 539 million years ago and is subdivided into three erathems: , , and . represent the next rank below erathems and serve as the fundamental divisions of the , with 12 formal systems recognized in total: , , , , , Permian (Paleozoic); , , (Mesozoic); and , , (Cenozoic). Each system is typically divided into 2 to 6 series, although the exact number varies; for instance, the System includes two series (Lower and Upper), while the System has four. Series are further subdivided into stages, the lowest formally defined chronostratigraphic units, with most series containing 2 to 6 stages, though some, like the , have up to 10. These subdivisions enable precise correlation of rock layers worldwide, with stage boundaries often defined by Global Boundary Stratotype Sections and Points (GSSPs) ratified by the (ICS). Systems themselves span tens to hundreds of millions of years, providing broad temporal brackets for geological events. Chronostratigraphic systems correspond directly to geochronologic periods but differ in focus: chronostratigraphy classifies bodies of rock formed during a specific time interval, whereas measures those time intervals themselves, often in absolute years using . This parallelism ensures that a system's rock record aligns with its corresponding period's temporal span, facilitating integration of relative and numerical dating methods. For example, the System comprises rocks deposited during the Period, approximately 419 to 358 million years ago. Below the stage level, chronozones represent informal, non-hierarchical chronostratigraphic units of unspecified rank, defined by the time span of a particular stratigraphic marker, such as a or event. In modern ICS practice, chronozones are not part of the standard hierarchy and have largely been supplanted by stages for finer , as stages provide globally standardized boundaries suitable for precise .

Distinction from Other Units

In , systems are chronostratigraphic units that encompass all rocks formed during a defined interval of geologic time, irrespective of their lithologic composition or geographic distribution. By contrast, lithostratigraphic units—such as formations, members, and groups—are defined and classified based on shared physical properties, including rock type, texture, and stratigraphic relationships, without regard to the age of deposition. This distinction ensures that chronostratigraphic systems provide a temporal framework for correlating rocks globally, while lithostratigraphic units facilitate mapping and description of rock bodies in specific regions. For instance, a lithostratigraphic group may extend across multiple systems if its characteristic remains consistent over time boundaries, potentially encompassing rocks from adjacent geologic periods. Biostratigraphy, another correlative method, relies on the vertical and lateral distribution of s to define units like biozones, which are primarily tools for and environmental reconstruction rather than formal time-rock divisions. Systems incorporate biostratigraphic data, such as biozones, to aid in boundary definition and , but they are not limited to or defined by fossil content alone; biozones function as informal, non-hierarchical aids that may vary in duration and are not always precisely isochronous due to factors like faunal migration or unconformities. Thus, while supports the establishment of system boundaries, it operates independently as a biotic criterion separate from the time-based essence of . Geochronologic units, such as periods, represent abstract intervals of time measured in years, whereas chronostratigraphic systems denote the corresponding bodies of stratified rocks that record those intervals. For example, the comprises the global succession of Jurassic-age rocks, while the Jurassic Period signifies the time span during which they formed, typically calibrated using techniques. Although numerical ages from methods like uranium-lead dating apply to both frameworks, they are formally housed within to quantify the duration of periods, reinforcing the material (rock-based) versus temporal (time-based) distinction. Within the chronostratigraphic hierarchy, systems form a key intermediate level between erathems and series, linking rock sequences to broader history.

Historical Development

Early 19th-Century Establishments

The concept of a stratigraphic system emerged in the early 19th century as geologists sought to classify fossiliferous rock sequences amid intense debates over the ordering of Paleozoic strata. Influential figures such as William Buckland, who mentored early field workers, and Adam Sedgwick, who began mapping northern Welsh rocks in 1831, contributed to this foundational work by emphasizing biostratigraphic correlations. Roderick Impey Murchison joined these efforts in the mid-1820s, focusing on southern Wales and the borderlands, which laid the groundwork for systematic nomenclature. Key systems were established during this period, beginning with the Carboniferous System, named in 1822 by William Conybeare and William Phillips for coal-bearing strata in , marking the first use of "system" for a major chronostratigraphic unit. In the same year, Julien d'Omalius d'Halloy defined the Cretaceous System, or "Terrain Crétacé," based on chalk-dominated sequences in the , derived from extensive mapping across and adjacent regions. The Silurian System followed in 1835, proposed by Murchison for fossil-rich rocks in the Welsh borderlands, named after the ancient tribe and formalized in his 1839 monograph. By 1839, Murchison and Sedgwick established the Devonian System for intermediate strata in Devonshire and , characterized by distinctive shelly fossils and equivalents, resolving uncertainties in transitional rock classifications. By the 1840s, an initial framework coalesced with the recognition of the and eras, proposed by John Phillips in 1840 to encompass the older, primarily marine systems from to Permian and the middle systems including the , respectively; these divisions relied heavily on European type sections for global correlation, particularly from Britain and . A significant refinement occurred in 1879 when Charles Lapworth introduced the System to resolve the long-standing -Silurian boundary dispute between Sedgwick's upper and Murchison's lower Silurian, using biostratigraphy in southern and to delineate a distinct intermediate unit named after the tribe.

20th- and 21st-Century Revisions

In the 20th century, the establishment of the International Commission on Stratigraphy (ICS) in 1974 marked a pivotal advance in global stratigraphic standardization, succeeding earlier bodies like the International Subcommission on Stratigraphic Classification to promote unified principles for chronostratigraphic units worldwide. This framework facilitated the integration of radiometric dating techniques, which gained prominence from the early 1900s onward, allowing for absolute age assignments that refined the correlation of rock layers beyond relative biostratigraphy and magnetostratigraphy. By mid-century, methods such as uranium-lead dating on zircon crystals enabled precise calibration of the geologic time scale, transforming systems from qualitative divisions into a quantitative hierarchy. Revisions to the systems in the early addressed longstanding terminological ambiguities. In 2004, the ICS's Geological Time Scale excluded the Tertiary as a formal period, elevating the and to system status to better reflect evolutionary and climatic transitions, though this sparked debate over hierarchical consistency. The was retained but redefined in 2010, with its base lowered to 2.58 million years ago at the Global Stratotype Section and Point (GSSP) of the Stage in , , ratified by the (IUGS) to encompass the intensified glacial cycles and align with the Pleistocene base. For equivalents, the 2004 ICS chart formalized subdivisions of the Eon into periods such as the (approximately 2.5–2.3 billion years ago), using Global Standard Stratigraphic Ages (GSSAs) based on radiometric dates rather than GSSPs, though these lack the biostratigraphic precision of systems and serve primarily as chronometric markers. By the 2020s, the ICS had ratified over 70 GSSPs for stage boundaries, enhancing global correlations, yet ongoing refinements persist, particularly for the Ediacaran-Cambrian boundary, where ambiguities in chemostratigraphic signals like carbon isotope excursions and the absence of key ichnofossils such as Treptichnus pedum in regions like Morocco's continue to fuel debates on precise positioning.

Current Framework

Phanerozoic Systems

The Eon, spanning from approximately 538.8 Ma to the present, encompasses all rocks containing abundant, complex fossils and is divided into 12 formal systems within three eras: , , and . These systems represent major intervals of Earth history marked by significant evolutionary, climatic, and tectonic developments, with boundaries defined by Global Stratotype Sections and Points (GSSPs) ratified by the International Commission on Stratigraphy (as of the International Chronostratigraphic Chart v2024/12). The total duration covers about 539 million years, during which life diversified dramatically from early to modern ecosystems.

Paleozoic Era Systems

The Paleozoic Era (538.8–251.9 Ma) features the initial proliferation of multicellular life and ends with the largest mass extinction in history.
SystemAge Range (Ma)Key Defining Features
538.8 ± 0.6 – 485.4 ± 1.9Marked by the , a rapid diversification of marine life including trilobites, early arthropods, and the first complex ecosystems; associated with continental rifting and shallow seas.
485.4 ± 1.9 – 443.1 ± 0.9Diversification of , emergence of the first corals, bryozoans, and primitive vertebrates; culminates in the linked to global glaciation and sea-level changes.
443.1 ± 0.9 – 419.62 ± 1.36Recovery from extinction with colonization of land by vascular plants and arthropods, alongside the of jawed fishes; characterized by stable, warm climates and reef-building organisms.
419.62 ± 1.36 – 358.86 ± 0.19Known as the Age of Fishes with diversification of bony fishes and early tetrapods; spread of early forests, tectonic collisions forming Euramerica, and multiple extinction events.
358.86 ± 0.19 – 298.9 ± 0.15Dominated by lush coal-forming swamps and forests supporting giant insects and early reptiles; extensive glaciation and the assembly of the Pangea.
Permian298.9 ± 0.15 – 251.902 ± 0.024Rise of synapsids and early therapsids as dominant vertebrates; formation of Pangea leads to arid interiors, ending in the Permian-Triassic mass extinction that eliminated over 90% of marine species.

Mesozoic Era Systems

The Mesozoic Era (251.9–66.0 Ma), often called , saw the dominance of and the breakup of Pangea amid greenhouse climates.
SystemAge Range (Ma)Key Defining Features
251.902 ± 0.024 – 201.4 ± 0.2Recovery from Permian extinction with early , pterosaurs, and mammals; arid conditions, volcanic activity from , and initial rifting of Pangea.
201.4 ± 0.2 – 145.0 ± 0.8Peak of dinosaur diversity, emergence of birds from theropod ancestors, and first angiosperm precursors; warm, humid climates with widespread shallow seas and ongoing continental fragmentation.
145.0 ± 0.8 – 66.0Flourishing of flowering plants, diverse marine reptiles, and large herbivorous ; ends with the Cretaceous-Paleogene triggered by impact and Deccan .

Cenozoic Era Systems

The Era (66.0 Ma–present) is defined by mammalian radiation, cooling climates, and the rise of modern biota, including humans.
SystemAge Range (Ma)Key Defining Features
66.0 – 23.04Rapid diversification of mammals and birds post-extinction, evolution of early and whales; transition from to icehouse world with Antarctic glaciation.
23.04 – 2.58Expansion of grasslands, evolution of great apes and early hominins, uplift of mountain ranges like the ; onset of glaciation cycles.
2.58 – presentCharacterized by repeated glacial-interglacial cycles, and migration, megafaunal extinctions; divided into the Pleistocene (2.58–0.0117 Ma) and (0.0117 Ma–present) series, with ongoing discussions.

Precambrian Equivalents

The , encompassing the interval from approximately 4,600 to 538.8 million years ago, lacks formal chronostratigraphic systems as established by the (ICS) primarily because of the scarcity and long-ranging nature of fossils, which preclude the precise biostratigraphic correlations essential for defining such units. Instead, Precambrian subdivisions rely on geochronometric boundaries defined by through Global Standard Stratigraphic Ages (GSSAs), reflecting the dominance of absolute age determinations over methods used in the . In place of formal systems, analogous stratigraphic frameworks emphasize lithostratigraphy, with supergroups serving as major divisions that group associated formations and groups based on shared lithologic characteristics in the and eons. Informal chronostratigraphic period names, such as the (2,500–2,300 Ma), were proposed around 2004 as part of efforts to standardize nomenclature but remain unratified by the ICS as official units. The broader structure includes the informal eon (prior to 4,000 Ma), the eon (4,000–2,500 Ma) divided into four eras— (4,000–3,600 Ma), (3,600–3,200 Ma), (3,200–2,800 Ma), and (2,800–2,500 Ma)—and the eon (2,500–538.8 Ma) subdivided into three eras: (2,500–1,600 Ma), (1,600–1,000 Ma), and (1,000–538.8 Ma), each further partitioned into informal periods like , , , , , , , , and . The Period/System (635–538.8 Ma), the sole formal chronostratigraphic unit in the , was ratified by the ICS and the in 2004, with its base defined by a Global Stratotype Section and Point (GSSP) to facilitate correlation with the overlying and highlight the emergence of complex multicellular . This unit bridges the informal framework to the more refined systems.

References

  1. https://nacsn.americangeosciences.org/code/
  2. https://www.idigbio.org/wiki/images/7/7f/255-271_Murphy_.pdf
  3. https://stratigraphy.org/ICSchart/ChronostratChart2024-12.pdf
  4. https://stratigraphy.org/guide/defs
  5. https://timescalefoundation.org/strat_guide/chro.html
  6. https://stratigraphy.org/chart
  7. https://stratigraphy.org/guide/chron
  8. https://pubs.geoscienceworld.org/gsa/books/book/817/chapter/3913849/Chronostratigraphic-Units
  9. https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2018.00191/full
  10. https://stratigraphy.org/gssps/
  11. https://cambrian.stratigraphy.org/gssps
  12. https://stratigraphy.org/chart/
  13. https://www.geosociety.org/gsatoday/archive/23/3/article/i1052-5173-23-3-4.htm
  14. https://stratigraphy.org/guide/litho
  15. https://stratigraphy.org/guide/bio
  16. https://pubs.usgs.gov/fs/2007/3015/fs2007-3015.pdf
  17. https://earthwise.bgs.ac.uk/index.php/1855_The_Murchison_succession_-_Geological_Survey_of_Great_Britain_%28by_E.B._Bailey%29
  18. https://www.nps.gov/articles/000/mississippian-period.htm
  19. https://www.lyellcollection.org/doi/10.1144/SP544-2023-60
  20. https://zenodo.org/record/1772599/files/article.pdf
  21. https://www.lyellcollection.org/doi/10.1144/SP532-2022-285
  22. https://geology.fandom.com/wiki/International_Commission_on_Stratigraphy
  23. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1502-3885.1972.tb00001.x
  24. https://physicstoday.aip.org/news/the-forgotten-history-of-radiometric-dating
  25. https://pubs.usgs.gov/of/1986/0110/report.pdf
  26. https://doi.org/10.1016/j.earscirev.2009.06.006
  27. https://stratigraphy.org/gssps/files/quaternary-pleistocene.pdf
  28. https://timescalefoundation.org/ICS_chart_archive/Div_GeolTime2004.pdf
  29. https://doi.org/10.1016/j.earscirev.2024.105010
  30. https://timescalefoundation.org/strat_guide/litho.html
  31. https://stratigraphy.org/gssps/files/ediacaran.pdf
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