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Paleogene Geochronology∷ an Integrated PALEOCEANOGRAPHY,VOL. 3, NO. 6, PAGES707-742, DECEMBER1988 PALEOGENE GEOCHRONOLOGY: AN INTEGRATED APPROACH M.-P. Aubry,•ßz W. A. Berggren,Zß3 D. ¾. Kent, ¸ J. J. Flynn, • K. D. Klitgord, ? J. D. Obradovich,8 and D. R. Pro thero 9 Abstract. Geochronology is the magnetostratigraphy, seafloor spreading conceptual division of continuous time magnetic lineation patterns, as measured (geochronometry) by the biostratigraphy, and isotopic ages. progression in an ordinal series of This integrated approach results in an events. This is best achieved by an ordinal framework which can measure time approach which integrates four with greater resolution, though perhaps independent data sets: less accuracy, than a radiometric approach alone. A comparative analysis of two recently proposed Paleogene •Centre de Pal•ontologie geochronologic time scales is presented. Stratigraphique et Pal•o•cologie de l'Universit• Claude Bernard, INTRODUCTION Villeurbanne, France ZDepartmentof Geologyand Geophysics, A geochronologic framework is Woods Hole Oceanographic Institution, essential to understanding and Woods Hole, MA estimating rates of geological 3Departmentof Geology, Brown processes. However, as Blow [1979, p. University, Providence, RI 1399] observed, "Geochronology is a 4Lamont-DohertyGeological conceptual division of absolutely Observatory, Palisades, NY continuous past time, related to the SDepartmentof Geological Sciences, geostratigraphical sequence. Columbia University, New York, NY Geochronology is not to be regarded as •Field Museumof Natural History, merely a scale of absolute dates Department of Geology, Roosevelt Road at dependent solely on radiometric Lake Shore Drive, Chicago, Illinois dating". Various components are ?U.S. Geological Survey, WoodsHole, MA fundamental to a truly integrated time 8U.S. Geological Survey, Denver, CO scale (for example, biochronology, SDepartmentof Geology, Occidental radiometric dating, biostratigraphy, College, Los Angeles, CA magnetostratigraphy). A number of schemes for the Cenozoic incorporating Copyright 1988 some or all of these components have by the American Geophysical Union. been developed over the past two decades, spurred to a large degree by Paper number 88PA03676. the need of the Deep Sea Drilling 0883-8 305 / 88/8SPA-0 36 76510.00 Project (DSDP) and successors for a ?08 Aubryet al.: PaleogeneGeochronology reliable chronology for estimating rates may be improved by properly merging the of sedimentary processes as well as data sets. The first three data sets basement/sediment contact ages, among provide relative sequence information, other things. independent of any isotopic ages, by Berggren et al. [1985a] recently using conventional stratigraphic revised Paleogene geochronology techniques [e.g., Miller, 1977; Shaw, utilizing an integrated methodology. 1964]. Standard reference sections are Odin and Curry [1985] presented a studied and then combined to form a detailed critique of their paper and, at composite sequence that best represents the same time, raised several questions the relative location of features in the regarding its validity as a "standard" data set. Although identification of for Paleogene geochronology. We address reference sections is usually discussed their critique here. with regard to magnetostratigraphic or The critical comments by Odin and biostratigraphic studies of vertical Curry [1985] can be separated into three (often tilted by later deformation) parts' comparison of the structure of geologic units, it also applies to the time scales submitted by Curry and horizontal patterns provided by seafloor Odin [1982] and by Berggren et al. spreading magnetic lineations. [1985a], the value of the dates based on The combination of standard reference good quality glaucony dating, and sections into a standard composite comments on certain disputable sequence represents the primary correlations. They also question the procedure for converting the three sets temporal resolution and the overall of relative sequence information into a accuracy of the numerical time scale for form in which they can be compared with the Paleogene presented by Berggren et each other and with isotopic age data. al. [1985a]. We address these comments A standard composite sequence represents by first reviewing the role of the our most precise statement of the geomagnetic polarity sequence in the overall succession (relative spacing) construction of an integrated numerical of, in this case, geomagnetic polarities geologic time scale; then, by answering for a defined time interval. the specific points raised concerning Identification of type sections is a "disputable correlations"; and finally crucial first step in establishing a by reviewing major problems which we stratigraphic succession that is perceive with the methodological consistent over a large region or that approach of Curry and Odin [1982] in the is of global extent. Standard reference construction of their Paleogene time sections are chosen from type sections scale purportedly based solely on using, usually, subjective criteria such isotopic data. as the most characteristic, most complete, and/or highest resolution COMPARISON BETWEEN THE CONSTRUCTION record of a segment of the overall OF A GEOMAGNETIC POLARITY TIME SCALE sequence pattern. The "most AND THAT OF AN ISOTOPIC TIME SCALE characteristic" criteria are usually made less subjective by comparing A Geomaõnetic Time Scale sequence patterns from different locales and identifying sections which were The construction of a geomagnetic formed during periods of relatively polarity time scale is based on the constant deposition rates or seafloor integration of four independent data spreading rates. It is usually possible sets: (1) seafloor spreading magnetic to identify part or all of the better lineation patterns; (2) local type sections which were formed magnetostratigraphy from sedimentary and during a period of constant deposition igneous rocks; (3) biostratigraphic or spreading rate, assuming a specific assemblage correlations; and (4) amount of data scatter. These are radiometric, or rather, isotopic, ages. standard correlation techniques with There are precision and resolution best fit criteria determined by the limits associated with each of these desired resolution for polarity data sets, but because these data sets sequences. A composite standard are largely independent, these limits sequence does not necessarily, if at Aubry et al.: Paleogene Geochronology ?09 OBSœ2VœZ)A4ANœT/C ANOA4AL 24 23 22 2I 20 I9 I2' I716 I5 I3 I2 NOI•TH ATLANTICOCEAN X I I I I X • I I I I I I ••.••••...... ....:•SOUTH28øS- VEMAATLANTIC 20 OCEAN X I I I I I I I I I I X I X 41øN-COMPOSITE I I I I I I I I I I I I ....... •••••i• SOUTH54øS - ELTANINPACIFIC OCEAN19 I I I I I I I I I I I I 2625 2423 22 2I 2019.I.•,,\fI• . EASTElaN127øw- ELTANININDIAN :55OCEAN I I I I I I I X-RATELOCATIONCHANGEOF SPREADING • ..... CENTRAL83øW-CORE INDIAN2 OCEAN Fig. 1. Magnetic anomalies 12 to 26 on profiles from the North Atlantic (north of Charlie Gibbs Fracture Zone), South Atlantic (south of the equator), North Pacific (off California coast), South Pacific (near Eltanin Fracture Zone), eastern Indian Ocean (north side of Australian-Antarctic Ridge), and central Indian Ocean. Constant spreading rates can be shown for the segments between crosses and these segments have been expanded by a constant amount to match the anomaly spacing on the South Pacific profile. all, correspond to the entire succession Alvarez, 1981; Harland et al., 1982; recorded in any given magnetic anomaly Berggren et al., 1985b]. The primary profile or magnetostratigraphic differences for the Paleogene for all section. The reliance on an abstract these time scales are the calibration composite in magnetic polarity points selected and the techniques used correlation is identical with the to apply them to the standard reference technique of many other types of sections. Lowrie and Alvarez [1981] and stratigraphic correlations which are Harland et al. [1982] assumed that every rooted in type specimens (for example, calibration point was exact and biostratigraphic zones) or type sections. interpolated the polarity pattern of On the basis of the study of seafloor LaBrecque et al. [1977] between each spreading magnetic patterns in different point. The other three time scales ocean basins, Heirtzler et al. [1968] assumed periods of constant spreading chose the Vema-20 (west flank) magnetic between anomaly 5o and 24ø (23 for profile in the South Atlantic as their LaBrecque et al. [1977]). All five of reference section. At that time, it was the above mentioned time scales are the longest and most complete record of based on the Vema-20 profile from Chrons the seafloor spreading magnetic patterns 5ø to 24ø (or 23) and on either the consistent with the patterns in other North Pacific profile or Vema-20 profile ocean basins. Subsequent studies have for 24ø (or 23) to 34. These studies identified seafloor magnetic anomaly do not assume (or support) constant reference sections that span shorter spreading rates for any ocean basin time intervals [e.g., Blakely, 1974; during the entire Cenozoic. However, it Klitgord et al., 1975; Cande and is possible to identify periods of Kristoffersen, 1977] but which are more relative constant spreading that lasted complete, have better resolution, and for 5 to 20 m.y. [Klitgord et al., 1975; better fit the constant spreading rate LaBrecque et al., 1977; Klitgord and assumption than the Vema-20 profile. Schouten, 1986]. Otherwise
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