Product Code Database
Stratigraphy is a branch of geology concerned with the study of rock layers (Stratum) and layering (stratification). It is primarily used in the study of sedimentary rock and layered . Stratigraphy has three related subfields: lithostratigraphy (lithologic stratigraphy), biostratigraphy (biologic stratigraphy), and chronostratigraphy (stratigraphy by age).
The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Known as the "Father of English geology", Smith recognized the significance of Stratum or rock layering and the importance of fossil markers for correlating strata; he created the first geological map of England. Other influential applications of stratigraphy in the early 19th century were by Georges Cuvier and Alexandre Brongniart, who studied the geology of the region around Paris.
Chemostratigraphy studies the changes in the relative proportions of trace elements and within and between lithologic units. Carbon and oxygen vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to the specialized field of isotopic stratigraphy.
Cyclostratigraphy documents the often cyclic changes in the relative proportions of (particularly carbonates), grain size, thickness of sediment layers () and fossil diversity with time, related to seasonal or longer term changes in paleoclimatology.
One important development is the Vail curve, which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir rocks, seals, and traps of petroleum geology.
A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively, the gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity.Martinsen, O. J. et al. (1999) "Cenozoic development of the Norwegian margin 60–64N: sequences and sedimentary response to variable basin physiography and tectonic setting" pp. 293–304 In Fleet, A. J. and Boldy, S. A. R. (editors) (1999) Petroleum Geology of Northwest Europe Geological Society, London, page 295, It is called a hiatus because deposition was on hold for a period of time.Kearey, Philip (2001). Dictionary of Geology (2nd ed.) London, New York, etc.: Penguin Reference, London, p. 123. . A physical gap may represent both a period of non-deposition and a period of erosion. A geologic fault may cause the appearance of a hiatus.Chapman, Richard E. (1983) Petroleum Geology Elsevier Scientific, Amsterdam, p. 33, .
Oriented paleomagnetic core samples are collected in the field; , , and very fine-grained are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition. If the ancient magnetic field were oriented similar to today's field (North Magnetic Pole near the North Pole), the strata would retain a normal polarity. If the data indicate that the North Magnetic Pole were near the South Pole, the strata would exhibit reversed polarity.
Results of the individual samples are analyzed by removing the natural remanent magnetization (NRM) to reveal the DRM. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale.
This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for the estimation of sediment-accumulation rates.
|
|
