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Concept
Lithostratigraphie
Résumé
Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology.
In general, strata are primarily igneous or sedimentary relating to how the rock was formed. Sedimentary layers are laid down by deposition of sediment associated with weathering processes, decaying organic matter (biogenic) or through chemical precipitation. These layers are often distinguishable as having many fossils and are important for the study of biostratigraphy. Igneous layers occur as stacks of lava flows, layers of lava fragments (called tephra) both erupted onto the Earth's surface by volcanoes, and in layered intrusions formed deep underground. Igneous layers are generally devoid of fossils and represent magmatic or volcanic activity that occurred during the geologic history of an 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.
The principles of lithostratigraphy were first established by the Danish naturalist, Nicolas Steno, in his 1669 Dissertationis prodromus. A lithostratigraphic unit conforms to the law of superposition, which in its modern form states that in any succession of strata, not disturbed or overturned since deposition, younger rocks lies above older rocks. The principle of lateral 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 physical rock characteristics (lithology).
Source officielle
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