Abstract:
The rock record is the only support for reconstructing earth history. Although it is increasingly recognized that not all intervals of geological time are equally represented in sedimentary stratigraphic sections, it is commonly assumed that deep sea sections are essentially complete over long intervals (>10 my) of time. Consequently, stratigraphic records of isotopic and faunal changes are converted to temporal records mostly through linear interpolation/extrapolation using magnetic properties (reversals and susceptibility), sometimes complemented by characteristic isotopic signatures. With the help of theoretical cartoons I show how undeciphered unconformities in sections may blur the true patterns of temporal changes and how they affect temporal resolution in these sections. Temporal analysis of stratigraphic sections must be conducted to help distinguish between true events and pseudo-events. The latest Paleocene-earliest Eocene (Magnetic Chron C24r) interval is one of the poorest represented in the Cenozoic deep sea record. Yet, most studies have ignored this fact and have assumed temporal continuity of upper Paleocene-lower Eocene sections. Based on two examples I discuss errors that incur from inappropriate conversion of a stratigraphic sequence into a temporal (numerical) record. In the first example, I examine the consequences of a shortcoming of the current Geomagnetic Polarity Time Scale (GPTS), which stems from the use of an age calibration tie-point on a level in DSDP Site 550, which was assumed to correspond to a biochronal boundary but in fact dates the upper surface of an unconformity. In the second example, I revise the stratigraphic and temporal significance of a 5.5 m thick upper Paleocene-lower Eocene section recovered from DSDP Site 577 and show that paleoceanographic reconstructions and biodiversity changes that have been inferred from high-resolution isotopic records and diversity patterns are illusory due to temporal misinterpretation of the section and incorrect (temporal) correlation with other sections. The accuracy of geological interpretations is intimately dependent upon the correctness of the temporal interpretation of stratigraphic sections. For parts of the stratigraphic record that are riddled with unconformities, such as the upper Paleocene-lower Eocene interval, composite (virtual) reference sections provide the relative chronology of events and constitute the means of temporal correlations through stratigraphic analysis, not only of marine sections but also of sections in the continental record. Ultimately, the composite reference section assists in determining the best criterion(a)/event(s) to characterize epoch/series boundaries, and to select the most suitable Global Stratotype Section and Point (GSSP) to place the "golden spike."