Principles and Concepts of Stratigraphy in Geology
**Stratigraphy**
Stratigraphy is the study of layered (stratified) rocks, primarily sedimentary rocks, focusing on their composition, origin, age relationships, and geographic extent. However, it also applies to certain igneous rocks, such as layered lava flows or volcanic ash beds, which follow the principles of stratigraphy. Additionally, some metamorphic rocks exhibit stratification and can be studied using stratigraphic principles.
Principle of Biological Succession: Different kinds of plants and animals succeed one another in time because life has evolved continuously; therefore, only rocks formed during the same age can contain similar assemblages of fossils. Since these fossil assemblages are unique for particular periods of the past, they can be used to:
- Correlate rocks from around the world, and to
- Order rock layers into a sequence of relative age (i.e., older …. newer).
Law of Faunal Succession – Fossil groups were succeeded by other fossil groups through time. This allowed geologists to develop a fossil stratigraphy and provided a means to correlate rocks throughout the world.
The concept of Golden Spikes (Global Stratotype Section and Point, or GSSP) in stratigraphy defines the boundaries of geological time periods and epochs by marking specific physical points in rock strata. These points, often chosen in fossil-rich, continuous sedimentary sequences, serve as the defining moments for the beginning and end of time units, such as periods or epochs, effectively linking geochronology (time) and chronostratigraphy (rock layers). Golden Spikes are typically located at fossil-rich strata, where specific fossils, like graptolites in the case of the Devonian, distinguish the base of the period. This physical marker allows global correlation of strata from similar age beds elsewhere using techniques like biostratigraphy, lithostratigraphy, and magnetostratigraphy. Gaps in the sedimentary record at the GSSP site are avoided to ensure continuous representation of the time interval.
Intrusive Contact: This occurs when molten rock (magma) forces its way into pre-existing rock layers. It can create features like sills (which are parallel to the bedding) and dikes (which cut across the bedding at high angles).
Fault Contact: A fault contact happens when rocks on either side of a fracture or fault plane have been displaced due to tectonic forces. This results in rocks being offset or shifted, either vertically or horizontally.
Depositional Contact:
- Vertical Contact: A boundary where rock layers are stacked vertically, often reflecting changes in deposition over time.
- Conformable Contact: A boundary where layers of rock are deposited without interruption, showing continuous deposition with no significant time gap or erosion between them.
- Lateral Contact: A boundary where different rock types meet side by side, typically due to changes in the environment of deposition over a horizontal distance.
Unconformity: An unconformity is a contact where there is a gap in the geological record due to either erosion or non-deposition. It represents a period of time during which no sediments were deposited, or existing sediments were eroded, and new layers were deposited on top. There are several types of unconformities (e.g., angular unconformity, disconformity, nonconformity), each describing different relationships between the layers.
Conformity in geology refers to the condition where rock layers are deposited in a continuous and uninterrupted sequence, with no significant gaps, erosion, or folding between them. In a conformable sequence, the layers are essentially “in order,” showing a consistent, smooth progression of deposition over time without any major breaks in the record.
Hiatus: A gap or break in the geological record, typically caused by erosion or non-deposition, where no sediment was deposited for a period of time.
Diastem: A very short, minor hiatus or brief period of non-deposition, usually marked by a thin layer of sediment with no significant erosion, indicating a short interruption in deposition.
Disconformity: A gap in the geological record between two parallel layers of sedimentary rock. The layers above and below are parallel, but there has been erosion or non-deposition between them.
Angular Unconformity: This occurs when tilted or folded older layers are overlain by horizontal younger layers. The contact between the two sets of layers is at an angle.
Nonconformity: This occurs when sedimentary rock layers are deposited on top of much older, eroded, and often metamorphic or igneous rocks, creating a sharp boundary between the two.
Paraconformity is an unconformity where there is a gap in the geological record, but the layers appear continuous and parallel, with no obvious erosion or tilting. It represents a period of non-deposition, but the boundary looks like a normal contact.
A marginal marine depositional environment is a coastal area where land meets the sea, such as beaches, estuaries, and tidal flats. It is influenced by both marine and land processes, with varying conditions like salinity and water depth, and often contains sediments like sand, mud, and fossils from both land and marine sources. Examples include beaches, deltas, and tidal flats.
Walther’s Law of Facies states that the vertical sequence of rock layers reflects the lateral shifts in depositional environments. In other words, the types of rocks found next to each other horizontally will appear in vertical succession over time, as environmental conditions change. Facies refers to a body of rock with distinctive characteristics, such as composition, texture, and fossil content, that reflect the conditions under which it was deposited. It represents a specific environment of deposition, like a beach, river, or deep ocean.
Different types of facies transitions:
- Gradational by continuous change
- Gradational by interbedding
- Abrupt by change in deposition
- Abrupt by erosional break
Walther’s Law: In a sequence of strata without unconformities, facies that lie one atop another were deposited side by side in the depositional environment. Vertical facies sequences tend to reflect horizontal facies arrays.
Sea-level changes at any point can take place by:
- Vertical tectonic movements (uplift or subsidence) of the lithosphere
- Compaction of the sedimentary column below the given locality
- Worldwide changes in sea level caused by changes in the volume of water in the oceans or by rise or fall of the sea floor.
Eustasy– a change in the elevation in sea level worldwide relative to some stationary datum at depth. Relative to sea bottom depth in a given locality. Eustatic sea-level changes occur due to factors like glacier volume (glacio-eustatic changes), mid-ocean ridge activity, and seafloor depth changes. Glacier-related changes happen over centuries to millions of years, while changes due to ridge activity and seafloor depth can span millions to hundreds of millions of years, influenced by plate tectonics and orbital cycles.
When relative sea level fluctuates in cycles, the time in the cycle when sea level is highest is called the highstand, and the time in the cycle when sea level is lowest is called the lowstand.
Transgression is the landward migration of the shoreline (due to either crustal subsidence or sea-level rise). Regression is its opposite. Progradation is the building or shifting of depositional units seaward. Retrogradation is its opposite. Aggradation is the building up of depositional units vertically upwards, neither progradational nor retrogradational.
Accommodation space is used for the space made available for sediment deposition below sea level. Accommodation is developed by crustal subsidence and/or sea-level rise.
Primary data sources for stratigraphy:
- Outcrops (consolidated vs. unconsolidated sediments)
- Cores (hand-operated vs. power-driven)
- Geophysical data (e.g., wireline logs, seismic, ground-penetrating radar)