Introduction
The branch of geology concerned with the description, correlation, and interpretation of stratified sediments and stratified rocks on and in the Earth. The greatest part of the uppermost zone of the earth’s bedrock is sedimentary rock, stratigraphy is an important branch of Earth science.
Of the rocks exposed on earth’s surface almost 70% consist of sedimentary rocks.
Stratigraphy
(Strata= a set of sedimentary beds, graphy= description) reveals various details of the history of the earth during the different periods of geological past, from the beginning till the present. Through stratigraphy, we can know the past details of climate, geography, glaciations, orogeny, eperogeny, evolution, and migration of plants and animals.
Thus, as this branch of geology reveals the history of our planet, it is called as “Historical Geology”
Correlation:
Matching up exposures of rock that are the same age but are in different places is called correlation. By correlating rocks worldwide, geologists developed the geologic time scale and obtained a fuller perspective on Earth’s history.
Correlation methods may involve the use of:
fossils (*biostratigraphy) Rock units (*lithostratigraphy): physical properties of strata and their organization into units based on lithologic character.
geologic time units or intervals (*chronostratigraphy): Deals with the ages of strata and their time relations. Modern Offshoots of Stratigraphy
- Sequence stratigraphy.
- seismic stratigraphy,
- Magnetostratigraphy
- chemostratigraphy
Note: No single locale exhibits the entire sequence, but correlation reveals a more complete picture of the sedimentary rock record.
Index fossils are particularly useful in correlation because they are widespread and associated with a relatively narrow period. Index fossils represent widespread and abundant species but have a short life span.
Facies: Sum total of features that reflect the specific environmental conditions under which a given rock was formed or deposited. Or bodies of sediment with distinctive characteristics are called facies. The features may be lithologic, sedimentological, or faunal. In a sedimentary facies, *mineral composition *sedimentary structure, and bedding characteristics are all diagnostic of a specific rock or lithofacies.
Principles of Stratigraphy
Uniformitarianism(J. Hutton):
The principle of uniformitarianism states that physical processes we observe operating today also operated in the past, at roughly comparable rates. Study of the present is the key to the study of the past.
Law of Superposition:
younger rocks overlie on older rocks. Steno proposes it
Law of Original horizontality:
strata were close to being horizontal when they were originally deposited because sediments accumulate on surfaces of low relief (such as floodplains or the sea floor) in a gravitational field. If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified. With this principle in mind, geologists conclude that examples of folds and tilted beds represent the consequences of deformation after deposition. (fig d)
Law of original lateral continuity:
strata were originally laterally extensive relative to their thickness when they were deposited. (Fig e)
Principle of Crosscutting Relationship
It states that if one geologic feature cuts across another, the feature that has been cut is older. For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the Dike. If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers. But if a layer of sediment buries a fault, the sediment must be younger than the fault.
Principles of Inclusions
The principle of inclusions states that a rock containing an inclusion (fragment of another rock) must be younger than the inclusion. For example, a conglomerate containing pebbles of basalt is younger than the basalt, and a sill containing fragments of sandstone must be younger than the sandstone.
Principle of Faunal Succession
In the early 1800s, William Smith in England and Georges Cuvier in France used the successions of faunas in stratified rocks for mapping, This led to the development of the principle called the law of faunal succession: distinct faunas succeed one another regularly in the rocks.
Principles of Baked Contacts
The principle of baked contacts states that an igneous intrusion “bakes” (metamorphoses) surrounding rocks, so the rock that has been baked must be older than the intrusion.
Conformability
Conformable strata are characterized by unbroken depositional assemblages, generally deposited in parallel order, in which layers are formed one above the other by more or less uninterrupted deposition. The surface that separates conformable strata is conformity, that is, a surface that separates younger strata from older rocks but along which there is no physical evidence of non-deposition. A conformable contact indicates that no significant break or hiatus in deposition has occurred. A hiatus is a break or interruption in the continuity of the geologic record. It represents periods of geologic time (short or long) for which there are no sediments or strata.
Contacts between conformable strata: Contacts between conformable strata may be either abrupt or gradational.
Abrupt contacts :
Abrupt contacts directly separate beds of distinctly different lithology.
minor depositional breaks, involving only short hiatuses in sedimentation with little or no erosion caused by floods, storms, etc before deposition is resumed, are called diastems. Post-depositional chemical alteration of beds, producing changes in color owing to oxidation or reduction of iron-bearing minerals, changes in grain size owing to recrystallization or dolomitization, changes in resistance to weathering owing to cementation by silica or carbonate minerals
Gradational contacts:
Conformable contacts are said to be gradational if the change from one lithology to another is less marked than abrupt contacts, reflecting gradual changes in depositional conditions with time. Gradational contacts may be of either the progressive gradual type or the intercalated type Progressive gradual contacts occur where one lithology grades into another by progressive, more or less uniform changes in grain size, mineral composition, or other physical characteristics.
Intercalated contacts are gradational contacts that occur because of an increasing number of thin inter-beds of another lithology that appear upward in the section.
Principle of Unconformities
When we observe layers of rock that have been deposited essentially without interruption, we call them conformable.
Throughout Earth’s history, the deposition of sediment has been interrupted over and over again. All such breaks in the rock record are termed unconformities. An unconformity represents a long period during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed.
Unconformities are important features because they represent significant geologic events in Earth’s history. There are four basic types of unconformities, and their recognition helps geologists identify what intervals of time are not represented by strata and thus are missing from the geologic record.
Walther Law of Facies
The principle that facies that occur in conformable vertical successions of strata also occur in laterally adjacent environments The various deposits of the same facies-area and similarly the sum of the rocks of different facies-areas are formed beside each other in space, though in a cross-section we see them lying on top of each other.
Sedimentary rock types record the environment of their deposition. Depositional environments can shift laterally as conditions change. (Regression and Transgression)
