Strata

INTRODUCTION The word Stratigraphy, originates from two Greek words (strati) strata meaning layers and (graphy) graphion meaning description. Theologically stratigraphy means description of layers. DEFINTION OF STRATIGRAPHY Stratigraphy, in geology is the study of rock layers, or strata, particularly their ages, compositions, and relationships to other rock layers. Stratigraphy provides geologists with clues about the earth’s past. Stratigraphy relies on four principles which are used to explain and unveil these Geologic histories of the earth. PRINCIPLES 0F STRATI GRAPHY These are principles in which Stratigraphy operate. These principles include: - The principle of original horizontality: This states that sediments deposited in water form a horizontal or nearly horizontal layer. This principle is applied in the identification of deformed rock i.e. in terms of faults and folds etc. - The principle of super position: This states that in an undisturbed sequence of underling sedimentary or volcanic rock, the rock strata gets younger from bottom to the top. Strata are groups of sedimentary rocks which are distinct from the rocks above and below them. This principle aids geologist to determine the time series of the deposition of the rock. - The principle of crosscutting relation: This states that when a rock body is cut or intruded by a fault or another body, the faults or intrusion is younger than the rock body.   - The principle of faunal and floral succession: This states that because plants and animal evolve into new species, the sedimentary rocks of different age must be of different species. Knowing the age of fossils helps to date the rocks which the where found. These four principles of Stratigraphy can be used to unravel the geologic history of a given area. HISTORIC IMPORTANCE OF STRAIGRAPY Earlier before the Stratigraphy was into being, the earth was generally considered to be younger than a million years old. Stratigraphy advanced in England in the early 1800’s through the work s of an English land surveyor William Smith. It was in the 19th century that Geologist could prove that earth was more than a million years old. This was done with the aid of Stratigraphy. FEILDS OF STRATIGRAPHY There are feilds of stratigraphy is classified namely: a) Biostratigraphy: Biostratigraphy is the use of fossils for age determination and correlation of rock layers b) Magnetostratigraphy : Magnetostratigraphy is the use of magnetic properties in rocks for similar purposes. c) Chemostraigraphy: Chemostratigraphy uses chemical properties of strata for age determination and correlation as well as for recognizing events in the geologic record. For example, oxygen isotopes (forms of oxygen that contain a different number of neutrons in the nuclei of atoms) may provide evidence of an ancient paleoclimate. Carbon isotopes may identify biologic events, such as extinctions. d) Seismic stratigrapy: Seismic stratigraphy is the subsurface study of stratified rocks using seismic reflection techniques. This field has revolutionized stratigraphic studies since the late 1970s and is now used extensively both on land and offshore. Seismic stratigraphy is used for economic reasons, such as finding oil, and for scientific studies. An offshoot of seismic stratigraphy is sequence stratigraphy, which helps geologists reconstruct sea level changes throughout time. The rocks used in sequence stratigraphy are bounded by, or surrounded by, surfaces of erosion called unconformities. e) Lithostratigrapy: This uses the chemical and mineral components of rock strata to determine age and past actives the happen within and the rock unit or area. This is useful in correlation because rocks of similar kind with similar composition are formed at given time in different but not very far of places. f) Chronostratigraphy: Here rock strata are dated according to their system of formation. STRATIGRAPHIC COLUMN Fossils preserved in rock strata provide scientists with clues to evolutionary history. This stratigraphic column is based on paleontological evidence, and shows the order in which organisms appeared in the fossil-rich Paleozoic Era. Each layer represents a particular time frame and shows a representative organism which flourished during that time. Although fossils are rarely found in the idealized and localized fashion shown here, they are often in more or less chronological order. Generally, the oldest fossils appear in lower layers, and the most recent fossils at the top, so that placement may be used as an aid in dating the specimens.