Biostratigraphy(redirected from biostratigraphically)
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a branch of stratigraphy which studies the distribution of fossil remains of organisms in sedimentary deposits with the object of determining the relative age and the correlations between layers of the same age in different territories. The task of biostratigraphy is the elaboration of scales of the relative age of layers (in various degrees of detail and of various scopes, but in zonal layers in particular). The succession of biostratigraphic zones reflects the change in geological cross section of fossil remains from a group of extinct organisms of different taxonomic classes or their systems. Of special importance in the differentiation of zones, and above all of biozones, are groups of extinct organisms that had relatively short life-spans but that attained widespread distribution, significant abundance, and variety (for example, nummulites, graptolites, and dinosaurs). Zones are frequently based on stages in the evolution of certain rapidly changing groups of extinct organisms (for example, corals of the subclass Rugosa). The study of the remains of ancient microscopic organisms (micropaleontology), the quantity of which can be extremely large even in small specimens (for example, from deep chinks), is important for the goals of biostratigraphy. Remnants of planktonic organisms (foraminifers, algae, and others) that were carried for great distances by currents allow zones of great territorial expanse to be distinguished. Fossil remains of plant spores and pollen, which were carried for great distances by the wind, are important for correlating deposits of maritime or continental origin that are of the same age. Biostratigraphy makes broad use of the methods of paleoecology for reconstructing the conditions under which ancient organisms existed in order to distinguish complexes of organisms of the same age living in different conditions from complexes of organism of different ages living in similar conditions.
REFERENCESMenner, V. V. “Biostratigraficheskie osnovy sopostavleniia morskikh, lagunnykh i kontinental’nykh svit.” Tr. geologicheskogo instituta AN SSSR, 1962, issue 65.
Stratigraficheskaia klassifikalsiia, terminologiia i nomenklatura. Leningrad, 1965.
Stepanov, D. L. Printsipy i metody biostratigraficheskikh issledovanii. Leningrad, 1958.
R. L. MERKLIN