Unit-5: Principle of Stratigraphy
Introduction to Stratigraphy
Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
Definition and Scope
Stratigraphy can be defined as the science of rock strata. It deals with the origin, composition, chronologic succession, and correlation of stratified rocks.
Stratigraphy is the description of all rock bodies forming the Earth's crust and their organization into distinctive, useful, mappable units on the basis of inherent properties or attributes to establish their distribution and relationship in space and time.
Scope of Stratigraphy
- Determining the chronological order of geological events.
- Understanding the Earth's history and paleogeography.
- Locating natural resources such as petroleum, coal, and groundwater.
- Facilitating basin analysis and tectonic history interpretation.
Code of Stratigraphic Nomenclature
The code of stratigraphic nomenclature provides a standardized set of rules and guidelines for naming and defining stratigraphic units. Different units are established based on distinct characteristics of the rocks.
Rock Unit or Lithographic Unit
Lithostratigraphic units are defined exclusively on the basis of lithologic (rock) properties, such as mineral composition, grain size, texture, and color, independent of time.
- Formation: The fundamental unit of lithostratigraphic classification. A formation is a body of rock identifiable by lithic characteristics and stratigraphic position, and must be mappable at the surface or subsurface.
- Member: A named lithostratigraphic unit of subordinate rank comprising a specific part of a formation.
- Group: Two or more associated formations with significant common lithic characteristics.
- Bed: The smallest lithostratigraphic unit, representing a distinct layer within a bed or formation.
Time Unit or Chronological Unit
Chronologic units are the intervals of geological time used to measure the duration of Earth's history. They are abstract time divisions and are not bounded by physical rock bodies, though they correspond to chronostratigraphic units.
- Eon (e.g., Phanerozoic)
- Era (e.g., Mesozoic)
- Period (e.g., Cretaceous)
- Epoch (e.g., Miocene)
- Age (e.g., Maastrichtian)
Time Stratigraphic or Chronostratigraphic Unit
Chronostratigraphic units are bodies of rocks formed during a specific interval of geological time. They serve as the physical reference standards for corresponding time units.
- System (corresponds to Period)
- Series (corresponds to Epoch)
- Stage (corresponds to Age)
- Zone (chronostratigraphic)
Biostratigraphic Unit
Biostratigraphic units are bodies of rock defined and characterized by their fossil content. They are independent of lithostratigraphy and chronostratigraphy.
- Biozone: The fundamental unit of biostratigraphy. It is defined by the occurrence of specific fossils or fossil assemblages.
Comparison of Stratigraphic Units
| Unit Type | Defining Basis | Primary Example |
|---|---|---|
| Lithostratigraphic Unit | Lithology (rock type) | Formation |
| Chronostratigraphic Unit | Time of formation (rock body) | System |
| Time Unit | Geological time duration | Period |
| Biostratigraphic Unit | Fossil content | Biozone |
Methods of Correlation
Correlation is the procedure of establishing equivalence in age and/or lithology between separate stratigraphic successions in different geographic areas.
Key Methods of Correlation
- Lithologic Correlation: Matching rock units based on physical properties, lithology, and sequential position.
- Biostratigraphic Correlation: Matching rock strata using fossil content and index fossils that evolved rapidly and had a wide geographic distribution.
- Chronostratigraphic Correlation: Establishing time equivalence between rock units regardless of their lithology, often utilizing marker beds, radiometric dating, and paleomagnetism.
- Electrofies/Well Log Correlation: Utilizing geophysical borehole data to correlate subsurface strata.
Geological Time Scale
The geological time scale is a system of chronological dating that classifies geological strata in time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history.
Structure of the Geological Time Scale
- Eons: The largest divisions of geological time (e.g., Archean, Proterozoic, Phanerozoic).
- Eras: Subdivisions of eons (e.g., Paleozoic, Mesozoic, Cenozoic within the Phanerozoic Eon).
- Periods: Subdivisions of eras (e.g., Cambrian, Ordovician, Silurian).
- Epochs: Subdivisions of periods (e.g., Paleocene, Eocene, Oligocene).
Exam-Oriented Notes
- Common Mistake: Confusing time units with chronostratigraphic units. Remember, a Period is a unit of time, while a System is the rock body formed during that period.
- Key Definition: Formations are the fundamental units of lithostratigraphy and must be mappable.
- Important Observation: Biostratigraphic units are time-transgressive in many cases because the migration of species takes time, meaning identical fossils can appear in younger rocks in different locations.