BIOESTATIGRAFIA

Ed on an undertanding of systems within which the rocks were laid down lithostratigraphy is a descriptive, empirical science ,but correlations can be made more comprehensive and are likely to be more reliable if they are erected using genetic, sedimentological principles.

3.5

Biostrastigraphy is the study of the relative arrangement of strata base don their fossil contet . descriptive or empirical biostratigraphy is used in erecting zones for local or regional stratigraphic correlation and forms the basis for a global system of chronostratigraphic subdivisión .

Fossil content varies throgt a stratigraphic succession for two main reasons: evolutionary changes and ecological differences , such as changes in climate or depositional environment .biostratigraphy should be based only on evolutionary changes ,but it s always.difficult to distinguish these from changes that take place in a biostratigraphic assemblage as a result of ecological modifications ,and this problema is a cause of continuing controversy for many fossil groups

Obviously ,biostratigraphy can only be studied and a classification erected where fossils are present.this rules out much of the precambrian strata, although some use has been made of stromatolites for correlation, particularly by russian geologists ,and fossilmicrobe communities hold promise (sect 3.7.9.2)even in the phanerozoic, there are many Rock units for which the fossil record is very sparse , and biostratigraphic subdivisión is correspondingly crude .this is particularly the case in nonmarine strata or those (particularly carbonates) in which fossil remains have been destroyed by diagénesis.

Biostratigraphy is a study for specialists .refined work requires intímate knowledge of the phylony of a large number of fossil groups and their regional or globaldistribution. Toaccumulate this knowledge maytake half a lifetime ,and the subject is an Excellent example of a science in which the practitioner seems to spend inordinate amounts of time learning more and more about less and less.some of the leading authorities in a particular fossil group may be able to discuss the cutting edge of their research with

Only half a dozen other colleagues around the world this gives them considerable value if one happens to tnd their kind of fossil ,but it may somewhat restrict their scientific scope. Geologists engaged in basin analysis of phanerozoic strata are very rarely such specialists.biostratigraphers are , therefore , either employed by many organizations to provide these specialized service skills or they function independently as consultants. They may be engaged , much of the time ,in pursuung paleontological research but are able to provide biostratigraphic diagnoses for selected fossil types over a specified age range.

Profesional biostratigraphic work may take a great deal of field and laboratory time .sections that a sedimentologist may dismiss as

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