Stratigraphic Boundaries - A Reply

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In reply to a recent paper by Dr. Jost Wiedmann criticizing the position of the International Subcommission on Stratigraphie Classification (ISSO with respect to principles of stratigraphie classification, it seems necessary to re-emphasize the following points: 1. Stratigraphy is a broad subject covering all properties of rocks as strata. Important as are fossils in stratigraphie classification (biostratigraphic units), there are still many other kinds of stratigraphie units which it is also desirable to recognize (lithostratigraphic units, chronostratigraphic units, etc.) in portraying most usefully the Earth's stratigraphie sequence and the history of its origin. 2. Chronostratigraphic units, representing the rocks formed during specific timespans of earth history, are basic to grasping earth history. Biostratigraphy has been our most valuable aid to identifying and correlating chronostratigraphic units in much of the Earth's stratigraphie sequence; but it is not the only means and there are many parts of the sequence where fossil evidence is very inadequate. We need to utilize all available kinds of evidence in attempting to extend isochronous horizons and units as widely and with as great precision as possible. 3. A biostratigraphic unit (delimited by the limits of occurrence of certain fossils) is fundamentally different from a chronostratigraphic unit which is delimited strictly by isochronous horizons. The limits of a biostratigraphic zone, due to changes in depositional facies, variations in conditions for preservation of fossils, or vagaries of fossil discovery, cannot everywhere coincide with the same time horizons, whereas the boundaries of a chronostratigraphic zone must by definition be isochronous. Hence, while we can and do utilize biostratigraphic evidence to the greatest extent, we cannot correctly say that a biostratigraphic unit is necessarily a chronostratigraphic unit. In fact, a biostratigraphic marker rarely, if ever, constitutes a synchronous horizon worldwide through all kinds of strata. 4. Since our only record of the passage of geologic time lies in the rock strata, it is appropriate that our standards for division points of geologic time should be referred to designated points in the sequence of rock strata - boundary-stratotypes. Internationally agreed stratotypes appear essential to effectively define the scope of most chronostratigraphic units; they may be helpful for some kinds of biostratigraphic units, but are not applicable to other kinds of biostratigraphic units. The choice of stratotype limits for chronostratigraphic units must inevitably be somewhat arbitrary. In the standard chronostratigraphic hierarchy, the stratotypes of units of major rank should be the composites of stratotypes of units of lesser rank. 5. The ISSC is a widely representative international body (including in its membership not only individuals but almost all national stratigraphie committees) which attempts to study principles of stratigraphie classification and terminology and to formulate recommendations for their application. It welcomes constructive criticism from all geologists. There is nothing binding in its conclusions other than as they appeal to geologists in general as being worthy of following in the interests of international geological understanding and progress. 12 pp., 2 figs, stapled gr. 8.…

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Détails bibliographiques

Titre
Stratigraphic Boundaries - A Reply
Auteur
Hedberg, H. D.
Année de publication
1970

ConchBooks

Harxheim, Allemagne

Vendeur avec une évaluation de 4 étoiles

Vendeur AbeBooks depuis 1 mars 2007

Frais d'expédition de Allemagne vers Etats-Unis

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