Classification of Stratigraphic Sequences and their Interrelationships | Geology Optional Notes for UPSC PDF Download

• Principles of Stratigraphic Classification:
  • Stratigraphy involves studying rock successions historically, defining and describing them.
  • A stratum refers to a layer of rock; multiple layers are called strata, each indicating sediment deposition at a specific time.
  • Stratigraphy is crucial for understanding the timing of rock deposition.
• Types of Stratigraphic Classification:
  • Lithostratigraphic Classification: Focuses on rock characteristics like composition, texture, and color to classify strata.
  • Biostratigraphic Classification: Involves studying fossils within strata to determine their age and environmental conditions.
  • Chronostratigraphic Classification: Organizes rocks based on their geological time scale, correlating them with time units like eons, eras, and periods.

Explanation:- Principles of Stratigraphic Classification: - Stratigraphy is the study of rock layers over time to understand their history. - Strata, or layers of rock, signify specific points of sediment deposition, helping determine the sequence of events. - This discipline is essential for interpreting the temporal aspects of rock formations.- Types of Stratigraphic Classification: - Lithostratigraphic Classification: This classification method focuses on the physical properties of rocks, such as their composition, texture, and color. For example, sandstone may be distinguished from limestone based on these characteristics. - Biostratigraphic Classification: By examining fossils found in rock layers, this classification helps determine the age of the rocks and the environmental conditions when they were deposited. For instance, the presence of trilobite fossils can indicate a specific geological period. - Chronostratigraphic Classification: This classification organizes rocks based on their position in the geological time scale. It correlates rock layers with specific time units like eons, eras, and periods. For example, the Cambrian period represents a distinct time interval characterized by specific rock formations.By understanding these principles and types of stratigraphic classification, geologists can interpret the Earth's history and the processes that have shaped it over millions of years. Each classification method provides valuable insights into the composition, age, and temporal context of rock formations, contributing to a comprehensive understanding of geological evolution.

Understanding Stratigraphic Classifications

• Lithostratigraphy

  • Definition: Lithostratigraphy is a classification method based on the physical characteristics of rock units.
  • Importance: It helps in understanding the composition and arrangement of rock layers in a specific area.
  • Example: The identification of limestone layers based on their unique fossil content and mineral composition.

• Chronostratigraphy

  • Definition: Chronostratigraphy involves classifying rocks based on their age and the time of deposition.
  • Significance: It aids in determining the chronological order of rock formations and events in Earth's history.
  • Illustration: Assigning the Jurassic period to a specific sedimentary layer based on its fossil assemblage.

• Code of Stratigraphic Nomenclature

  • Explanation: The code provides guidelines for formal classification in stratigraphy.
  • Requirement: Adhering to the rules outlined in the code is crucial for creating a standardized classification system.
  • Clarification: Failure to comply with the code results in an informal classification lacking scientific rigor.

• Stratotype

  • Definition: A stratotype is a specific section of rock that represents a particular stratigraphic unit.
  • Role: It serves as a reference point for identifying and correlating similar rock sequences globally.
  • Example: Designating a well-exposed cliff in a designated type area as the stratotype for a specific geological formation.

Code of Stratigraphic Nomenclature

• Development of the Code: The Code of Stratigraphic Nomenclature was created by the International Subcommission on Stratigraphic Classification (ISSC) under the International Commission on Stratigraphy (ICS), established by the International Union of Geological Sciences (IUGS). This code has been widely accepted globally, including in countries like India.
• Importance of the Code: The primary purpose of this code is to facilitate a consistent and formal division of rock sequences worldwide.
• Stratotype Definition: A 'stratotype' is considered as a representative example of a specific stratigraphic unit. It is a particular section that exhibits all the defining characteristics of the stratigraphic unit.
• Identification of Stratotypes: It is mandatory to identify stratotypes for all lithostratigraphic units. While it is desirable for chronostratigraphic units, it is not typically utilized for biostratigraphy.

Examples:

• For instance, in a specific geological formation, a stratotype would be a distinct section that showcases all the key features used to define that particular unit.
• In practice, geologists might designate a stratotype within a rock layer that displays unique fossil assemblages and sedimentary characteristics characteristic of that geological time period.

Lithostratigraphy

• Definition: Lithostratigraphy, which combines the words "litho" (rock type) and "stratum graphia" (description of rock bodies), is a branch of stratigraphy focusing on describing, defining, and naming Earth's rocks based on their lithology and stratigraphic relationships.
• Description: The classification in lithostratigraphy primarily hinges on the types of rocks present in a sequence, making it synonymous with rock stratigraphic classification. It involves organizing rock sequences into distinct units based on their lithological properties and how they relate to other rocks.
• Importance: Lithostratigraphic classification is crucial as it provides a more precise understanding of stratigraphy and serves as the foundation for various stratigraphic branches. Additionally, lithostratigraphic units form the basic building blocks of geological mapping.
• Units Division: The sequence is segmented based on lithology, with each rock type or a cluster of rock types forming a unit. These units can then be further subdivided or grouped together.

Lithostratigraphic Units

• The fundamental organization of lithostratigraphic units can be understood through a hierarchical structure, as outlined in the table.
• In lithostratigraphic classification, units are arranged in descending order of thickness, with the hierarchy typically consisting of Supergroup, Group, Formation, Member, and/or Beds.
• It is important to note that within this classification system, the primary and mandatory unit is the Formation.

Explanation:

• Supergroup represents the highest level of the lithostratigraphic hierarchy, encompassing vast geological units that share common characteristics.
• Groups are subdivisions of Supergroups and are characterized by specific lithological attributes and depositional environments.
• Formations are fundamental lithostratigraphic units that are distinctive in terms of lithology, sedimentary structures, and fossil content. They form the basic building blocks of the classification system.
• Members and/or Beds are finer subdivisions within Formations, often distinguished by variations in lithology, sedimentary features, or fossil assemblages.

Examples:

• An example of a Supergroup could be the "Columbia River Basalt Group" in the Pacific Northwest, known for its extensive basaltic lava flows.
• A Group within this context might be the "Saddle Mountains Basalt," a specific subdivision of the Columbia River Basalt Group with distinct lithological characteristics.
• For a Formation, we can consider the "Ellensburg Formation" in Washington State, recognized for its sedimentary layers rich in fossilized flora and fauna.
• Within the Ellensburg Formation, a Member could be the "Manastash Ridge Member," identified by its unique sedimentary structures and fossil content.

Formation in Lithostratigraphy

Definition of Formation

• A Formation serves as the fundamental unit in lithostratigraphy for classification purposes.
• It represents a uniform set of layers comprising a specific rock type or a collection of rock types.
• This unit is horizontally consistent and possesses adequate thickness to be depicted on maps.

Characteristics of Formations

• Homogeneity within a Formation can be based on lithological traits, color variations, or distinct weathering features.
• For instance, the Talchir Formation within the Damuda Group of the Gondwana Supergroup exemplifies a Formation.

Classification of Lithostratigraphic Units

• Each lithostratigraphic Formation can be further categorized into Members.
• Members are formal units within lithostratigraphy, ranking below formations.
• Members are distinguished by specific lithologic properties that set them apart from adjacent parts of the formation.
• Distinctive lithology characterizes each member.
• Not all formations need to be divided into members; it depends on the purpose.
• Some formations may have multiple members, while others may not be divided at all.
• Certain parts of a formation can also be designated as members.

Understanding Beds in Lithostratigraphy

• Definition of Bed: A bed is the smallest formal unit of lithostratigraphy. It refers to distinctive layers found in sedimentary sequences that hold stratigraphic significance.
• Characteristics of Beds:
  • Key Beds: These are specific beds within a sequence that serve as important markers for stratigraphic correlations.
  • Marker Beds: Similar to key beds, marker beds are layers that aid in identifying and correlating different rock units.
• Formal Lithostratigraphic Units: Beds such as the Boulder bed of the Talchir Formation are recognized as formal lithostratigraphic units due to their distinctiveness and significance in geological studies.
• Significance of Beds: Beds are crucial for understanding the geological history of an area and are used to establish correlations between different rock formations.

Group in Geology

• A group in geology refers to a collection of two or more formations that share common significant and diagnostic lithological properties.
• An example of a group is the Damuda Group, which is part of the Gondwana Supergroup and comprises four formations: Karharbari, Barakar, Barren measures, and Raniganj.
• The formations within a group are characterized by similarities in their geological composition and can be distinguished from other formations based on these properties.
• Groups help geologists classify and study rock layers based on shared characteristics, aiding in the interpretation of geological processes and history.
• Understanding groups is essential in geological mapping, stratigraphic analysis, and interpreting the environmental conditions that existed during the formation of these rock layers.

Supergroup

• Definition: A Supergroup is a combination of various related groups and formations sharing significant lithological properties.
• Example: The Gondwana Supergroup serves as an illustration of a Supergroup.

Lithostratigraphic Units Naming

• The naming of lithostratigraphic units is based on specific localities or geographical features where a type section is exposed.
• Example: The Raniganj Formation is named after the town Raniganj in West Bengal, where it is exposed, establishing its type section.
• Identification Requirement: According to the Code of Stratigraphic Nomenclature, it is essential to identify a type section and stratotype for a formation.
• Stratotype Inclusion: A stratotype should encompass the complete range of the formation.
• Naming Convention: The name of the locality or geographical feature associated with a formation is followed by the unit name.
• Example: The Raniganj Formation represents a rock formation with various rock types, where Raniganj in West Bengal serves as the type area.
• Distinct Rock Types: When a formation comprises a unique rock type, the rock name can be used as a suffix instead of "Formation," such as Muth Quartzite and Rohtas Limestone.
• Naming Procedure: The same naming procedure is applied to other lithostratigraphic units like Group and Member.
• Capitalization Rule: The initial letter of all formal stratigraphic units is capitalized.

Biostratigraphy

• Definition: Biostratigraphy, derived from "bio" meaning life and "stratum graphia" meaning description of rock bodies, focuses on the distribution of fossils in a rock sequence and the organization of strata into distinct units based on the fossils present.
• Objective: The systematic subdivision and organization of a rock sequence into named units are based on the fossils contained within the rocks.
• Basic Principle: The fundamental principle of biostratigraphy asserts that evolutionary changes in organisms are irreversible. Fossils found in a specific time interval are unique to that time and cannot be duplicated at any other time.
• Biostratigraphic Zones: Strata are divided into different biostratigraphic zones or biozones based on the presence of characteristic fossils. Biozones are the primary units of biostratigraphic classification.

Biostratigraphic Units or Biozones

• Characteristics: Biozones constitute the primary units of biostratigraphic classification.

Biozone Classification

• Range Zone: This type of biozone is determined by the complete range of a fossil form, spanning from its initial appearance to its final disappearance. Index fossils with short time ranges and broad geographical distribution are crucial for establishing range zones. There are two subtypes: partial range zone and concurrent-range zone.
• Partial Range Zone: It is established within the stratigraphic range of a taxon, with part of its range above another taxon's range and part below another taxon's range.
• Concurrent-Range Zone: This zone covers a rock sequence that corresponds to the overlapping stratigraphic range of two or more specified fossil taxa.
• Interval Zone: Represents the rock sequence between two specific biological events, such as extinction or origination events.
• Assemblage Zone: Characterized by the presence of multiple fossil forms (usually three or more) that must all be present in order to identify the zone. Unlike range zones, assemblage zones are recognized based on the collective association of fossil forms.
• Abundance Zone: Refers to a stratum containing a high population of one or more fossil species, indicating the peak population of those species during that period. These zones are primarily used for local classification, especially in industries like oil exploration for categorizing stratigraphic sections in a specific area.

Biostratigraphic Classification in Fossiliferous Sequences

• The term "Barren Zone" refers to sections within a fossiliferous sequence that lack any fossils.
• In biostratigraphic classification, stratotypes are generally absent, but reference localities can be designated.
• Three main types of zones are utilized in biostratigraphic classification:
  • Range Zone: Named after the generic or trivial name of the index species. For instance, the Ophiceras Zone.
  • Abundance Zone: Named after the peak abundance species, like the Spiriferella raja Zone.
  • Assemblage Zone: Named after significant forms within the assemblage, such as the TonkinellaBailliella Zone.
• Zones may be subdivided further into subzones or even Zonules, which are the smallest biostratigraphic sub-divisions.

Chronostratigraphy

• Lithostratigraphy and Biostratigraphy

  You may have learned that lithostratigraphy focuses on the composition of rock layers (lithology), while biostratigraphy revolves around the study of fossils found in these layers.

• Definition of Chronostratigraphy

  Chronostratigraphy, derived from the words 'chrono' meaning time and 'stratum graphia' meaning the description of rock bodies, is a branch of stratigraphy concerned with determining the relative time relationships and ages of rock formations.

• Abstract Nature of Chronostratigraphy

  Unlike lithostratigraphy and biostratigraphy, chronostratigraphy is more abstract in nature as it deals with interpreting relative time frames primarily through biostratigraphy.

• Determination of Time

  Time in chronostratigraphy is not directly observable within rocks; instead, it is inferred from the fossil content identified through biostratigraphic analyses.

• Purpose of Chronostratigraphy

  The primary objective of chronostratigraphy is to categorize rock sequences globally into chronostratigraphic units, enabling the correlation of local, regional, and global geological events to a unified standard geological scale.

• Focus on Age and Time Relations

  Chronostratigraphy is principally concerned with determining the age of rock sequences and establishing their temporal relationships with each other.

• Chronostratigraphic Classification

  Chronostratigraphic classification involves organizing rocks into units based on their age or time of formation, facilitating a systematic understanding of geological time scales.

Chronostratigraphic Units Overview

Definition of Chronostratigraphic Units

• A chronostratigraphic unit refers to a collection of rocks formed during a specific geological time interval, encompassing all rocks created within that timeframe.
• These units are crucial in chronostratigraphy and are identified based on biochrons, which represent the time span of a particular biozone.

Hierarchy of Chronostratigraphic Units

• The lowest unit in chronostratigraphy is the stage, identified on the basis of a biochron.
• Ascending from stages are series, systems, erathems, and eonothems, all determined by their fossil content and relative nature.
• The order of superposition helps establish the sequence of sediments and characteristic fossils in different rock layers, indicating their relative ages.

Geochronology and Geochronologic Units

• Geochronology, a branch of geology, focuses on dating rock formations and geological events using radioactive dating methods to determine their ages in years.
• Geochronologic units are abstract subdivisions of geologic time, defined after chronostratigraphic units.

Hierarchy of Geochronologic Units

• Geochronologic units are established in a hierarchical order, including smaller units within larger ones.
• These units provide a more precise understanding of the timing of geological events and rock formations.

Chronostratigraphic and Geochronologic Classifications

• The primary goal of chronostratigraphic and geochronologic classifications is to systematically organize Earth's rock strata into named units that correspond to specific intervals of geologic time.
• These classifications do not primarily rely on assigning absolute ages in years through radiometric methods; instead, they focus on establishing a structured framework for rock layers.
• By creating named chronostratigraphic units and geochronologic units, these classifications enable the correlation of time across different rock strata and assist in determining the ages of geological formations.

Significance of the Classification

• The arrangement of chronostratigraphic and geochronologic units serves as a foundational reference system for recording and interpreting events in Earth's geological history.
• Each chronostratigraphic unit corresponds to a specific geochronologic unit, facilitating the correlation between rock layers and geologic time frames.

Example Illustration

• For instance, the identification of a particular chronostratigraphic unit, such as the "Jurassic Period," corresponds to a defined geochronologic unit with a specific age range, aiding in dating rock formations.
• Through this systematic organization, geologists can establish a timeline of Earth's history based on the layers of rock and the events they represent.

Chronostratigraphic Units Overview

• Stage

  • A stage is the fundamental unit in chronostratigraphy, representing all rocks formed during a specific age globally.
  • It serves as a subdivision of a series, with its geochronologic equivalent being an age.
  • Example: The Jurassic Stage encompasses rocks formed during the Jurassic Age.

• Series

  • A series is a chronostratigraphic unit above a stage and below a system, with an epoch as its geochronologic equivalent.
  • Example: The Paleogene Series includes rocks formed during the Paleogene Epoch.

• System

  • System is a major chronostratigraphic unit above a series and below an erathem.
  • Its geochronologic equivalent is a period.
  • Example: The Triassic System comprises rocks deposited in the Triassic Period.

• Erathem

  • Erathem consists of a group of systems and is exemplified by periods in geological time scales.
  • Its geochronologic equivalent is an era.
  • Example: The Mesozoic Erathem includes the Jurassic, Triassic, and Cretaceous Systems, corresponding to the Mesozoic Era.

• Eonothem

  • Eonothem is a higher-ranked unit than an erathem and comprises a group of erathems.
  • Its geochronologic equivalent is an eon.
  • Example: The Phanerozoic Eonothem encompasses the Archean, Proterozoic, and Phanerozoic eons.

Magnetostratigraphy

• Magnetostratigraphic classification organizes rock bodies into units based on their magnetic characteristics.
• A magnetostratigraphic unit, known as a magnetozone, groups rocks with similar magnetic properties together, distinguishing them from neighboring rock bodies.
• Magnetostratigraphic polarity classification categorizes rock units based on changes in their polarity, reflecting magnetization linked to Earth's magnetic field reversals.
• Remnant magnetic polarity direction in a stratigraphic sequence divides the sequence into units with the same magnetic polarity, identifying magnetostratigraphic polarity units as distinct from adjacent rocks.

Transitional Strata

• Definition of Transitional Strata:

  Transitional strata refer to the layer between magnetozones, characterized by either magnetostratigraphic polarity-reversal horizons or polarity-transition zones.

• Magnetostratigraphic Polarity-Reversal Horizons:
  • Magnetostratigraphic polarity-reversal horizons are thin layers of strata deposited during relatively short polarity transition durations. They exhibit a distinct difference in magnetic polarity compared to the strata above and below them.
• Polarity-Transition Zones:
  • Polarity-transition zones occur when a polarity change happens through a thicker layer of strata, approximately one meter in thickness. This indicates a longer magnetic transition duration.
• Geochronology and Absolute Age:
  • Age in absolute years, determined through methods like radioactive dating or magnetostratigraphy, is superimposed on time units. This variation does not alter the scale since it is not solely based on absolute years.