AUBRY, MARIE-PIERRE, Laboratoire de Geologie du Quaternaire, CNRS-Luminy, Marseille, France

ABSTRACT: From Chronology to Stratigraphy: Assessing the Continuity of the Stratigraphic Record.

The elusive concept of geological time and our understanding of its passage as recorded in the stratigraphic record, have developed in parallel fashion as best attested to by the nature of early biochronologic scales. Today, our appreciation of geological time remains narrowly dependent upon our interpretation of the stratigraphic record. That is to say, stratigraphic sections which are apparently continuous are regarded as fully representative of the geologic time they span. Yet we have become aware that the stratigraphic record in epicontinental settings is not a faithful recorder of geologic time. For instance, although it has dictated the bases for Paleogene chronology for over a century (and still does), the Paleogene stratigraphic record of northwestern Europe represents less th n half of the 43 m.y. of Paleogene time. As for the deep sea record, it is still regarded by some as essentially continuous, changes in thickness between particular horizons being interpreted in terms of changes in sedimentation rates rather than in terms of stratigraphic gaps.

A quantum leap has been achieved in geochronology with the advent of integrated magneto-biochronologic scales about a decade ago. The strength of this generation of scales is to have recognized that the sea floor magnetic anomaly pattern is another expression of the geologic time, independent of, but linked to, the stratigraphic record. In deriving a bio/magneto/isotopic chronology, the sea-floor anomaly pattern serves as the primary reference (template) for time, while the stratigraphic record serves mainly as a substrate for correlation. Based on the use of well-selected chronologic tie-points (paleontologic datums and magnetic reversals) I reassess the temporal continuity of the Cenozoic deep-sea record of the Atlantic Ocean and show that, contrary to what is currently assumed, it s highly discontinuous. In doing so, I place strong emphasis on the various problems encountered when interpreting stratigraphic sections, difficulties which relate to diachrony, stratigraphic gaps, changes in sedimentation rates and condensed sections.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.