3.2 Relative dating
9 min read•august 20, 2024
Relative dating is a crucial technique in paleontology for establishing the chronological order of events and rock layers. It relies on principles like superposition, original horizontality, and to determine the sequence of geological events without assigning specific ages.
Various techniques, including , , and , are used in relative dating. These methods help scientists correlate rock layers across different locations, identify unconformities, and reconstruct Earth's history. However, relative dating has limitations, such as the lack of numerical ages and dependence on preserved features.
Principles of relative dating
- Relative dating is a foundational concept in paleontology that establishes the chronological order of events and rock layers without assigning specific numerical ages
- The principles of relative dating are based on observations of natural processes and the relationships between rock layers, fossils, and geologic features
Law of superposition
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- In an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest layers are at the top
- This principle allows paleontologists to determine the relative ages of rock layers and the fossils they contain
- Exceptions to this law can occur due to tectonic events, such as overthrusting or overturning of
Principle of original horizontality
- are deposited in nearly horizontal positions, conforming to the underlying surface
- If sedimentary layers are found tilted or folded, it indicates that they have been deformed after deposition by tectonic forces
- This principle helps paleontologists recognize deformation events and reconstruct the original depositional environment
Principle of lateral continuity
- Sedimentary layers extend laterally in all directions until they thin out or reach the edge of the depositional basin
- Layers that are continuous over a large area are likely to represent widespread depositional events
- Discontinuities in lateral continuity can be caused by erosion, faulting, or changes in depositional environment
Principle of cross-cutting relationships
- Geologic features that cut across or intrude into rock layers are younger than the layers they intersect
- Examples of cross-cutting features include faults, dikes, and erosional surfaces
- This principle allows paleontologists to establish the relative ages of intrusive igneous rocks, faults, and other features that disrupt the original sequence of rock layers
Principle of inclusions
- Fragments of one rock included within another rock must be older than the host rock
- Inclusions can be used to determine the relative ages of the included fragments and the host rock
- Examples of inclusions are xenoliths (foreign rock fragments) in igneous rocks and clasts (rock fragments) in sedimentary rocks
Principle of faunal succession
- Fossil assemblages in rock layers follow a specific order of appearance and disappearance over time
- Certain fossil species are characteristic of specific geologic time periods and can be used as for correlation
- Changes in fossil assemblages reflect evolutionary changes and environmental conditions at the time of deposition
Relative dating techniques
- Relative dating techniques are methods used to determine the chronological order of events and rock layers without assigning specific numerical ages
- These techniques rely on the principles of relative dating and the physical and chemical properties of rocks and fossils
Stratigraphy
- Stratigraphy is the study of rock layers and their distribution in time and space
- It involves the description, interpretation, and correlation of rock layers based on their lithology, thickness, and depositional environment
- Stratigraphic principles, such as superposition and lateral continuity, are used to establish the relative ages of rock layers
Biostratigraphy
- Biostratigraphy is the study of the distribution of fossils in rock layers to establish relative ages and correlate strata
- It relies on the , which states that fossil assemblages follow a specific order of appearance and disappearance over time
- Index fossils, which are species with short geologic ranges and wide geographic distribution, are particularly useful for biostratigraphic correlation
Lithostratigraphy
- is the study of the lithologic characteristics of rock layers, such as composition, texture, and structure
- It involves the description and classification of rock units based on their lithologic properties
- Lithostratigraphic correlation is based on the similarity of rock units across different locations, assuming that similar rock types were deposited under similar conditions and at approximately the same time
Magnetostratigraphy
- Magnetostratigraphy is the study of the magnetic properties of rock layers to establish relative ages and correlate strata
- It relies on the principle that the Earth's magnetic field has reversed polarity multiple times throughout geologic history, and these reversals are recorded in the magnetic minerals of rocks
- The pattern of magnetic reversals can be used as a timescale for correlation, as it is assumed to be globally synchronous
Chemostratigraphy
- is the study of the chemical composition of rock layers to establish relative ages and correlate strata
- It involves the analysis of stable isotopes, trace elements, and other geochemical markers in rocks and fossils
- Changes in the chemical composition of rocks can reflect changes in the depositional environment, climate, or ocean chemistry, which can be used for correlation
Sequence stratigraphy
- is the study of genetically related sedimentary packages (sequences) bounded by unconformities or their correlative conformities
- It involves the analysis of depositional systems and their response to changes in sea level, sediment supply, and tectonic subsidence
- Sequence stratigraphic correlation is based on the identification of key surfaces (sequence boundaries, transgressive surfaces, and maximum flooding surfaces) and the stacking patterns of sedimentary packages
Unconformities in relative dating
- Unconformities are gaps in the geologic record that represent periods of non-deposition or erosion
- They are important in relative dating because they represent missing time and can be used to subdivide the geologic record into distinct units
Types of unconformities
- There are four main types of unconformities: , , , and
- Each type of represents a different relationship between the rock layers above and below the unconformity surface
Angular unconformities
- Angular unconformities occur when tilted or folded strata are overlain by younger, horizontally deposited layers
- The angular relationship between the two sets of strata indicates that the older layers were deformed and eroded before the deposition of the younger layers
- Angular unconformities represent significant gaps in time and often indicate tectonic events, such as mountain building or basin
Disconformities
- Disconformities are unconformities that separate parallel or nearly parallel strata
- They represent a period of non-deposition or erosion, but without significant tilting or folding of the underlying strata
- Disconformities can be recognized by the presence of an erosional surface, a change in lithology, or a gap in the fossil record
Nonconformities
- Nonconformities occur when sedimentary rocks are deposited directly on top of igneous or metamorphic rocks
- They represent a significant gap in time, as the igneous or metamorphic rocks must have been uplifted, exposed at the surface, and eroded before the deposition of the sedimentary rocks
- Nonconformities are important in understanding the tectonic history of an area and the timing of major geologic events
Paraconformities
- Paraconformities are unconformities that are difficult to recognize because the strata above and below the unconformity surface are parallel and have similar lithologies
- They represent a period of non-deposition or very slow deposition, without significant erosion
- Paraconformities can be identified by subtle changes in lithology, gaps in the fossil record, or the presence of hardgrounds or other diagenetic features
Unconformities and missing time
- Unconformities represent missing time in the geologic record, which can range from a few thousand years to hundreds of millions of years
- The amount of missing time represented by an unconformity depends on the duration of non-deposition or erosion and the rates of sediment accumulation before and after the unconformity
- Unconformities can complicate the interpretation of the geologic record and the reconstruction of past environments and ecosystems
Correlation in relative dating
- Correlation is the process of establishing the equivalence of rock units or events across different locations
- It is essential for creating a regional or global framework for relative dating and understanding the spatial and temporal relationships between geologic units
Lithostratigraphic correlation
- Lithostratigraphic correlation is based on the similarity of rock units in terms of their lithologic characteristics, such as composition, texture, and structure
- It assumes that similar rock types were deposited under similar conditions and at approximately the same time
- Lithostratigraphic correlation can be challenging when rock units change laterally in character or when similar lithologies are repeated in the stratigraphic sequence
Biostratigraphic correlation
- Biostratigraphic correlation is based on the presence of index fossils or distinctive fossil assemblages in rock units
- It relies on the principle of , which states that fossil species appear, evolve, and go extinct in a specific order over time
- Biostratigraphic correlation is particularly useful for correlating marine sedimentary rocks, as many marine organisms have wide geographic ranges and rapid evolutionary rates
Magnetostratigraphic correlation
- Magnetostratigraphic correlation is based on the pattern of magnetic reversals recorded in the magnetic minerals of rock units
- It assumes that magnetic reversals are globally synchronous and can serve as a timescale for correlation
- Magnetostratigraphic correlation is useful for correlating both marine and terrestrial sedimentary rocks, as well as volcanic rocks that contain magnetic minerals
Chemostratigraphic correlation
- Chemostratigraphic correlation is based on the similarity of geochemical signatures in rock units, such as stable isotope ratios, trace element concentrations, and organic geochemical markers
- It assumes that changes in the chemical composition of rocks reflect changes in the depositional environment, climate, or ocean chemistry that are broadly synchronous across a region
- Chemostratigraphic correlation is particularly useful for correlating marine sedimentary rocks, as the chemistry of the oceans is influenced by global processes
Sequence stratigraphic correlation
- Sequence stratigraphic correlation is based on the identification of key surfaces (sequence boundaries, transgressive surfaces, and maximum flooding surfaces) and the stacking patterns of sedimentary packages
- It assumes that changes in sea level, sediment supply, and tectonic subsidence produce a predictable pattern of sedimentary packages that can be correlated across a basin or region
- Sequence stratigraphic correlation is useful for understanding the depositional history of a basin and the response of sedimentary systems to external forcing factors
Regional vs global correlation
- Regional correlation involves establishing the equivalence of rock units or events within a limited geographic area, such as a sedimentary basin or a mountain range
- Global correlation involves establishing the equivalence of rock units or events across different continents and ocean basins
- Global correlation is more challenging than regional correlation, as it requires the integration of multiple dating and correlation techniques and the consideration of global-scale processes, such as eustatic sea-level changes and magnetic reversals
Limitations of relative dating
- Relative dating is a powerful tool for establishing the chronological order of events and rock layers, but it has several limitations that can affect the accuracy and precision of the results
Lack of numerical ages
- Relative dating does not provide specific numerical ages for rock units or events
- It only establishes the order of events and the relative time differences between them
- To assign numerical ages to rock units, absolute dating methods, such as radiometric dating, are needed
Dependence on preserved features
- Relative dating relies on the preservation of physical and chemical features in rocks, such as fossils, sedimentary structures, and geochemical signatures
- If these features are not preserved or are altered by diagenesis or metamorphism, the relative dating of the rocks may be difficult or impossible
- The absence of diagnostic features can lead to gaps or uncertainties in the relative age sequence
Influence of erosion and deformation
- Erosion can remove parts of the stratigraphic record, creating gaps in the relative age sequence and making correlation more challenging
- Deformation, such as folding and faulting, can disrupt the original stratigraphic relationships and make it difficult to apply the principles of relative dating
- In areas with complex deformation histories, the relative ages of rock units may be ambiguous or impossible to determine without additional information
Challenges in correlating across regions
- Correlation of rock units across different regions can be challenging due to lateral changes in lithology, fossil content, and depositional environment
- The same rock unit may have different characteristics in different locations, making lithostratigraphic correlation difficult
- Fossil assemblages may vary across regions due to differences in paleoenvironments or biogeographic barriers, complicating biostratigraphic correlation
Relative dating vs absolute dating
- Relative dating and absolute dating are complementary approaches to determining the age of rocks and events
- Relative dating establishes the order of events and the relative time differences between them, while absolute dating assigns specific numerical ages to rocks and events
- Absolute dating methods, such as radiometric dating, can provide a more precise and accurate chronology than relative dating alone
- However, absolute dating methods have their own limitations, such as the need for suitable materials (e.g., igneous rocks with radioactive isotopes) and the assumption of closed-system behavior
Key Terms to Review (27)
Angular Unconformities: Angular unconformities are geological features that represent a significant gap in the geologic record, occurring when horizontally deposited sedimentary rocks are tilted or folded and then eroded, followed by the deposition of younger, flat-lying sedimentary layers on top. This unique structure provides vital clues about the earth's history, revealing periods of tectonic activity and erosion before new sedimentation processes resumed. Understanding angular unconformities is essential for interpreting relative dating and reconstructing the geological timeline.
Biostratigraphy: Biostratigraphy is a branch of stratigraphy that uses the distribution of fossil organisms to date and correlate rock layers. This method is crucial in understanding the history of life on Earth, helping scientists identify and classify different geological time units based on the fossils found within them.
Charles Lyell: Charles Lyell was a prominent 19th-century geologist known for his work in establishing the principles of uniformitarianism, which suggests that the Earth’s features have been shaped by continuous and uniform processes over long periods of time. His ideas revolutionized geology and laid the groundwork for modern stratigraphy, relative dating methods, geological time units, and paleogeography, connecting geological processes to the Earth's historical narrative.
Chemostratigraphy: Chemostratigraphy is a branch of stratigraphy that uses the chemical composition of rock layers to correlate and date them, providing insights into the geological history of an area. This method involves analyzing the abundance of certain chemical elements or isotopes within sedimentary rocks, allowing for the identification of distinct geochemical signatures that can be used to establish relative ages and correlations between different strata. By connecting these chemical variations to changes in environmental conditions, it plays a critical role in understanding Earth's history, especially during significant geological periods.
Chronostratigraphy: Chronostratigraphy is the branch of stratigraphy that focuses on the age relationships and time intervals of rock strata. It helps establish a geological timeline by integrating relative and absolute dating techniques, allowing scientists to understand the history of Earth's layers and the events that shaped them over time.
Cross-cutting relationships: Cross-cutting relationships are a geological principle that states if a geological feature cuts through another, the feature that has been cut is older. This concept is crucial for understanding the chronological order of geological events and helps in reconstructing the geological history of an area. The principle assists in visualizing how different rock layers and structures interact over time, forming a foundation for analyzing stratigraphy and establishing relative dates.
Disconformities: Disconformities are a type of sedimentary rock formation that indicates a gap in the geological record, where layers of sediment are missing due to erosion or non-deposition. They represent a break in time between two layers of sedimentary rock that are parallel to one another, often making it challenging to understand the complete geological history of an area. Recognizing disconformities is crucial for interpreting relative ages and understanding the processes that shape Earth’s history.
Faunal Succession: Faunal succession is the principle that fossil assemblages succeed one another in a recognizable order, allowing geologists and paleontologists to correlate the ages of rock layers. This concept helps scientists understand the relative ages of different strata and the evolution of life through time, linking it closely to the principles of stratigraphy, relative dating, and biostratigraphy, while also shedding light on specific geological periods and environmental indicators.
Formation: A formation is a distinct layer of sedimentary rock that has a recognizable and consistent set of characteristics, often used to identify and correlate rock layers across different geographic areas. Formations are essential in understanding the history of the Earth's crust, as they provide insights into the processes of deposition, erosion, and the environments in which sediments were deposited, contributing to broader geological studies.
Index fossils: Index fossils are remains of organisms that lived during a specific geological time period and are used to identify and date the layers of rock in which they are found. They are crucial for understanding the age of rock strata and can provide insights into the environmental conditions at the time of their existence, linking various geological layers and aiding in relative dating.
Law of Superposition: The law of superposition is a fundamental principle in geology stating that in any undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the younger layers are on top. This principle helps establish the relative ages of rock layers and their contained fossils, playing a crucial role in understanding Earth's geological history and the timeline of life.
Lithostratigraphy: Lithostratigraphy is the branch of stratigraphy that deals with the description and classification of rock layers based on their lithology, or physical and chemical characteristics. This approach is essential for understanding the spatial and temporal distribution of sedimentary rocks, which helps establish correlations between rock units and informs both geological history and paleoenvironmental reconstructions.
Magnetostratigraphy: Magnetostratigraphy is a geochronological dating method that uses the magnetic properties of rock layers to correlate and date sedimentary sequences. It relies on the Earth's magnetic field's historical changes, which are recorded in rocks and sediments, providing a timeline for relative dating and helping to establish the chronology of geological events.
Nonconformities: Nonconformities are geological features that represent a significant gap in the geologic record, occurring when younger sedimentary rocks are deposited on top of older igneous or metamorphic rocks that have been eroded. This relationship indicates that there was a period of uplift and erosion before the deposition of the younger layers, providing important insights into the history of the Earth's crust and its tectonic activities.
Paraconformities: Paraconformities are a type of unconformity in geology where there is a gap in the geological record, indicating a period of non-deposition or erosion, yet without any visible change in sedimentary rock layers. They can complicate relative dating since they may represent significant time periods that lack distinctive features, making it hard to identify when certain geological events occurred. Recognizing paraconformities is essential for understanding the chronology of sedimentary layers and interpreting Earth's history.
Principle of Cross-Cutting Relationships: The principle of cross-cutting relationships states that geological features, such as faults or igneous intrusions, that cut through other rocks must be younger than the rocks they disrupt. This principle is vital for understanding the relative ages of rock layers and is a key concept in relative dating, allowing geologists to piece together the sequence of geological events over time.
Principle of faunal succession: The principle of faunal succession states that sedimentary rock layers contain fossilized organisms that succeed one another in a specific, recognizable order. This principle is essential for understanding relative dating, as it allows geologists and paleontologists to identify and correlate rock layers based on the fossils they contain, establishing a timeline of Earth's history.
Principle of Inclusions: The principle of inclusions states that any rock fragment that is included within another rock must be older than the rock in which it is contained. This principle is crucial for understanding the relative ages of rocks and their formations, providing insight into the geological history of an area. By analyzing these inclusions, geologists can reconstruct sequences of geological events and establish a chronological framework for sedimentary layers and other rock types.
Principle of lateral continuity: The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; they are deposited continuously over large areas unless interrupted by erosion or other geological events. This concept is crucial for understanding how sedimentary rock layers correlate across different geographic locations, allowing geologists to reconstruct past environments and geological history.
Principle of Original Horizontality: The principle of original horizontality states that layers of sedimentary rock are originally deposited in horizontal or nearly horizontal layers due to the influence of gravity. This concept is crucial for understanding sedimentary processes and serves as a foundation for relative dating, helping geologists determine the sequence of geological events and the relative ages of rock formations.
Sedimentary Layers: Sedimentary layers are distinct strata of sediment that accumulate over time, often in bodies of water, and can reveal information about the Earth's geological history. These layers form as sediments settle and compact, resulting in a record of environmental conditions and biological activity at the time of deposition. They play a crucial role in understanding relative dating, as the position of these layers can indicate the relative ages of the fossils and rocks within them.
Sequence Stratigraphy: Sequence stratigraphy is a branch of geology that focuses on the study of sedimentary strata, particularly their arrangement and the processes that control their deposition over time. This approach uses relative changes in sea level and sediment supply to interpret the sedimentary record, allowing geologists to understand the timing of geological events and the environmental conditions during deposition. It is essential for reconstructing past environments and correlating stratigraphic units across different locations.
Strata: Strata refers to the distinct layers of sedimentary rock or soil that accumulate over time, each representing a specific period of geological history. These layers can reveal important information about the Earth's past environments, climate changes, and biological evolution. Understanding strata is crucial for interpreting relative dating and the chronological order of events in Earth's history.
Stratigraphy: Stratigraphy is the branch of geology that studies rock layers (strata) and layering (stratification), primarily to understand the temporal and spatial relationships of geological formations. This field provides crucial insights into the Earth's history, including fossil records, which aid in understanding the processes of fossilization and preservation, and how these layers relate to different geological time units and significant periods in Earth's history.
Trace fossils: Trace fossils are geological records of biological activity that provide evidence of the behavior, movement, and activities of organisms rather than their physical remains. They include footprints, burrows, feces, and feeding marks, showcasing how ancient life interacted with its environment. Understanding trace fossils is essential for reconstructing past ecosystems and connecting various aspects of fossilization, preservation, distortion, dating, and evolutionary biology.
Unconformity: Unconformity refers to a surface of contact between two groups of unconformable strata, representing a gap in the geological record where either erosion or non-deposition has occurred. This geological feature is crucial in understanding the relative ages of rock layers and the history of Earth’s geological processes, as it indicates periods where sediment was not laid down or existing layers were eroded away.
William Smith: William Smith was an English geologist and civil engineer, often called the 'Father of English Geology' for his pioneering work in stratigraphy and the development of the principle of faunal succession. His groundbreaking geological maps laid the foundation for relative dating and biostratigraphy, significantly advancing the understanding of Earth's history and the distribution of fossils throughout different geological periods, including the Silurian.