A sedimentary layer is more recent than the one it covers

It mean  that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

2. The Principle of Continuity

The principle of continuity, which is the basis for geological correlation, is defined as follows :

A sedimentary layer limited by a base and top and defined by a given facies has the same age  throughout.

In other words, when a layer was laid down the same sedimentary processes occurred at the same time at numerous points in the sedimentary environment. In Practice, it is impossible to follow a layer or series of layers continuously except in places that lack vegetation. The principle of continuity must not be applied unless a facies has been completely defined and its paleontological characteristics are unambiguous. It will be shown later that it is always preferable to define the palaeological identity before asserting the continuity of layer or series of layers.

In the stratigraphy you can find term of

- Stratigraphic classification. The systematic organization of the Earth’s rock bodies, as they are found in their original relationships, into units based on any of the properties or attributes that may be useful in stratigraphic work.

- Stratigraphic unit. A body of rock established as a distinct entity in the classification of the Earth’s rocks, based on any of the properties or attributes or combinations thereof that rocks possess. Stratigraphic units based on one property will not necessarily coincide with those based on another.

- Stratigraphic terminology. The total of unit-terms used in stratigraphic classification.It may be either formal or informal.

- Stratigraphic nomenclature. The system of proper names given to specific stratigraphic units.

- Zone.Minor body of rock in many different categories of stratigraphic classification. The type of zone indicated is made clear by a prefix, e.g., lithozone, biozone, chronozone.

- Horizon. An interface indicative of a particular position in a stratigraphic sequence. The type of horizon is indicated by a prefix, e.g., lithohorizon, biohorizon, chronohorizon.

- Correlation. A demonstration of correspondence in character and/or stratigraphic position. The type of correlation is indicated by a prefix, e.g., lithocorrelation, biocorrelation, chronocorrelation.

- Geochronology. The science of dating and determining the time sequence of the events in the history of the Earth.

- Geochronologic unit. A subdivision of geologic time.

- Geochronometry. A branch of geochronology that deals with the quantitative (numerical)measurement of geologic time. The abbreviations ka for thousand (103), Ma for million (106), and Ga for billion (milliard of thousand million, 109) years are used.

- Facies. The term “facies” originally meant the lateral change in lithologic aspect of a stratigraphic unit. Its meaning has been broadened to express a wide range of geologic concepts: environment of deposition, lithologic composition, geographic, climatic or tectonic association, etc.

- Caution against preempting general terms for special meanings. The preempting of general terms for special restricted meanings has been a source of much confusion.

Trace fossils occur in sediments deposited in environments ranging from sub aerial continental to deep marine. Trace fossil in freshwater, continental deposits are group into what is called the ‘Scoyenia ichnofossils’. This ichnofacies is rather non-distinctive, consisting of a low-diversity suite of invertebrate and vertebrate tracks, trails, and burrows (Ekdale, et al., 1984). Trace fossil are not true bodily preserved fossils, but are silly biogenic structures that originated through the locomotion, feeding, burrowing or resting activities of organisms.

Two types of contact: Conformable and Unconformable and Conformable.

Unconformable: Period of erosion/non-deposition. The surface stratum resulting is called an unconformity.

Unconformity. A surface of erosion between rock bodies that represents a significant hiatus or gap in the stratigraphic succession. Therefore, unconformity-bounded units are bodies of rocks bounded above and below by significant unconformities. They are composed of diverse types of any kind or kinds of rocks, but the lithologic properties of these rocks, their fossil content, or the chronostratigraphic span of the rocks on either side of the bounding unconformities are significant only to the extent that they serve to recognize the bounding unconformities

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 stata: Abrupt contacts ( Directly separate beds of distinctly different lithology, minor depositional break, called Diastems) Gradational contact (Gradual change in deposition, mixing zone).

Four types of unconformity

Angular unconformity . An unconformity in which the bedding planes above and below the unconformity are at an angle to each other.

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

Many investigations indicated that the benthonic foraminifera very useful in dating the sediment locally, beside the major use in the recognition of paleoenvironments. The ability to accurately determine environment is of fundamental importance in the oil industry since both hydrocarbon source rocks and reservoir rocks accumulate under rather restricted environmental condition.