In this article, we will correlate different borehole data on the basis of their lithology. Different types of beds are taken in all boreholes and lithological correlation is shown diagrammatically.
In geology, the term correlation refers to the methods by which the age relationship between various strata of Earth’s crust is established. Such relationships can be established, in general, in one of two ways: by comparing the physical characteristics of strata with each other (physical correlation); and by comparing the type of fossils found in various strata (fossil correlation).
Correlation is an important geological technique because it provides information with regard to changes that have taken place at various times in Earth history. It also provides clues as to the times at which such changes have occurred. One result of correlational studies has been the development of a geologic time scale that separates Earth history into a number of discrete time blocks known as eras, periods, and epochs.
There are few types of correlations.
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand, or core samples with low magnification microscopy, such as color, texture, grain size, or composition. In lithologic correlation, a unit is recognized by its lithology (rock type) or a sequence of lithologies.
Fossil correlation is a principle that geologists use to determine the age of rock. They look at the rock surrounding fossils with unique characteristics, such as a geologically short lifespan and easily identifiable features, and use this information to estimate the age of a rock layer in other areas that contain the same type of fossil or group of fossils.
The principle of fossil correlation states that the strata containing a group of fossils that are all the same age must be of similar age to the fossils. Strata are layers of rock, and every single layer is known as a stratum. The principle works because each species has a finite life span, and these eventually become extinct and after extinction does not reappear.
The geologic study is concerned with establishing geochronological relationships between different areas, based on geologic investigations of many local successions.
The surface represents a break in the geologic record or deposition.
The rock unit(s) immediately above the break is/are much younger than that/those below.
There are four types of unconformities.
Unstratified igneous or metamorphic rock may be uplifted to Earth’s surface by crustal movements. Once the rock is exposed, it erodes. Sediments may then be deposited on the eroded surface. The boundary between the new sedimentary rock and the igneous or metamorphic rock is nonconformity. The boundary represents an unknown period of time during which the older rock was eroded.
2. Angular Unconformity
An angular unconformity form when rock deposited in horizontal layers is folded or tilted and then eroded. When erosion stops, a new horizontal layer is deposited on top of a tilted layer. When the bedding planes of the older rock layers are not parallel to those of the younger rock layers deposited above them, an angular unconformity results.
Sometimes, layers of sediments are uplifted without folding or tilting and are eroded. Eventually, the area subsides and deposition resumes. The layers on either side of the boundary are nearly horizontal. Although the rock layers look as if they were deposited continuously, a large time gap exists where the upper and lower layers meet. This gap is known as a disconformity. Sediments on sediments (same orientation) with obvious erosion.
A para-conformity is a type of unconformity in which strata are parallel; there is little apparent erosion and the unconformity surface resembles a simple bedding plane. It is also called a non-depositional unconformity. Sediments on sediments (same orientation) no obvious erosion surface.
Transgression and Regression–Lithological correlation
A Marine Transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking of the ocean basins filling with water (or decreasing in capacity). Transgressions and regressions may be caused by tectonic events such as orogenies, severe climate change such as ice ages, or isostatic adjustments following removal of ice or sediment load. In either case, seawater rises farther up onto land than it did before.
In this case, we will have deeper sea sediments (shales and limestones) being deposited on top of continentally-derived beach sediments (sand). This forms a sequence (from bottom to top) of sand ► shale ►limestone. A maximum transgression occurs where the finest sediments reach the farthest landward.
Marine Regression is a geological process occurring when areas of submerged seafloor are exposed above sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously exposed land.
During a transgression, the sequence of rocks will show an onlap sequence (the facies will become deeper-water environments as you move up through the sediments).
During a regression, the sequence of rocks will show an off lap sequence (the facies will become shallower environments as you move up through the sediments).
Transgression-regression events allow geologists to see contemporary marine and terrestrial rocks due to intertonguing of facies.
Relative and Absolute dating
Geological specimens that are unearthed need to be assigned an appropriate age. To find their age, two major geological dating methods are used. These are called relative and absolute dating techniques.
Absolute dating, also called numerical dating, arranges the historical remains in order of their ages.
Whereas, relative dating arranges them in the geological order of their formation.
The relative dating techniques are very effective when it comes to radioactive isotope or radiocarbon dating. However, not all fossils or remains contain such elements. Relative techniques are of great help in such types of sediments.
From this experiment, we conclude that some beds are missing in the specific sections of the borehole. It shows that some geological event has occurred in the past which causes removal of these beds.
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