Applied Stratigraphy (eBook)

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Chapter 2 Buried Time: Chronostratigraphy as a Research Tool (p. 23)

MARIE-PIERRE AUBRY1 and JOHN A. VAN COUVERING2
1 Department of Geology, Rutgers State University of New Jersey, Piscataway NJ 08854, USA.
2 Micropaleontology Press, 256 Fifth Avenue, New York, NY 10001, USA.
2.1 Introduction

Improvements in stratigraphic dating methods have transformed chronostratigraphy into a practical tool that reveals unexpected realities in the place of earlier conventions. As strata become more precisely ordered in time, applied chronostratigraphy allows us to unravel cause and effect across facies boundaries and gaps, to resolve the context of past changes, and to find relationships between the proxy records of vanished forces found in different lines of evidence.

The advances in stratigraphic timeanalysis have in turn resulted in the reformulation of some of the basic concepts of stratigraphy and geochronology, as for instance the operational assumption that conformable sections are also depositionally continuous until proven otherwise. In this chapter we emphasize the need for integrated stratigraphy as the essential foundation for greater precision in interpretations of regionally extensive stratigraphic sections.

Paradoxically, as temporal interpretations become more easily visualized on first inspection, it is more difficult to hold them intellectually separate from the objective evidence, and a new terminology is required to clarify this basic distinction in discussions. As an example of problems still to be overcome, we review the discrepancies between lower upper Miocene magnetobiostratigraphic correlations in different sections, as reported by Berggren et al. (1995c) that result in an unstable early late Miocene biochronology.

Without conscious effort, every Earth scientist understands Steno’s First Law – that superposed strata represent the passage of time. Beyond this, even the first geological maps irresistibly imply a lengthy history, simply by showing that countless successive strata have accumulated in formations that are themselves seen to be superimposed.

The extrapolated concept of chronostratigraphy – that intervals of geological time could be defined in terms of accumulated strata – was soon grasped by Alcide d’Orbigny, who in his monumental study of stratigraphy only 15 years after the publication of William Smith’s great map of England, proposed the concept of the "stage" to embody the concept of a worldwide synoptic perspective that coincided with deposition of a given body of strata (our translation):

"In summary, rigorous application of the general and specialized principles of geology to the sedimentary layers which constitute the Earth’s crust, lead to the understanding that these layers form distinct superposed stages, characterized by a specific fauna, that each fauna has clear and definite limits, and that the occurrence of a significant number of species that are limited to and characteristic of these stages always permits them to be distinguished, whatever the different mineralogic [i.e. lithologic] compositions that the strata presently show.

Indeed, whereas the study of superposition and concordance of stratification of the geological stages alone often gives excellent results when the stages are superposed without stratigraphic gaps, such [physical] studies cease to provide positive evidence when intermediate stages are missing, as we see on a multitude of points on our planet." (D’Orbigny, 1849, pp. 7, 8.)