Sedimentary layers are laid down by deposition of sediment associated with weathering processes, decaying organic matters (biogenic) or through chemical precipitation. These layers are distinguishable as having many fossils and are important for the study of biostratigraphy. Igneous layers are either plutonic or volcanic in character depending upon the cooling rate of the rock. These layers are generally devoid of fossils and represent intrusions and volcanic activity that occurred over the geologic history of the area.
There are a number of principles that are used to explain the appearance of stratum. When an igneous rock cuts across a formation of sedimentary rock, then we can say that the igneous intrusion is younger than the sedimentary rock. The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed stratum is younger than the one beneath and older than the one above it. The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds.
A lithostratigraphic unit conforms to the law of superposition, which state that in any succession of strata, not disturbed or overturned since deposition, younger rocks lies above older rocks. The law of horizontal continuity states that a set of bed extends and can be traceable over a large area.
Lithostratigraphic units are recognized and defined on the basis of observable rock characteristics. The descriptions of strata based on physical appearance define facies. Lithostratigraphic units are only defined by lithic characteristics, and not by age.
Stratotype : A designated type of unit consisting of accessible rocks that contain clear-cut characteristics that are characteristic for a particular lithostratigraphic unit.
Lithosome: Masses of rock of essentially uniform character and having interchanging relationships with adjacent masses of different lithology. E.g.: Shale lithosome, limestone lithosome…
The fundamental Lithostratigraphic unit is the Formation. The formation is a lithologically distinctive stratigraphic unit that is large enough to be mappable and traceable.
Hierarchy of terms: Supergroup – Group – Formation – Member - Bed/Bed sets
Two types of contact: Conformable and Unconformable.
Conformable: Unbroken deposition, no break or hiatus (break or interruption in the continuity of the geological record). The surface strata resulting is called a conformity.
Two types of contact between conformable strata: Abrupt contacts (Directly separate beds of distinctly different lithology, minor depositional break, called Diastems) Gradational contact (Gradual change in deposition, mixing zone).
Unconformable: Period of erosion/non-deposition. The surface stratum resulting is called an unconformity.
Four types of unconformity
Angular unconformity Younger sediment lies upon an eroded surface of tilted or folded older rocks. The older rock dips at a different angle than the younger.
Disconformity The contact between younger and older beds is marked by visible, irregular erosional surfaces. Paleosol might develop right above the disconformity surface because of the non-deposition setting.
Paraconformity The bedding plans below and above the unconformity are parallel. A time gap is present but there is no erosion, just a non-deposition period.
Nonconformity Relatively young sediments are deposited right above older Igneous or metamorphic rocks.
To correlate lithostratigraphic units, geologists define facies, and look for key beds or key sequences that can be used as a datum.