Abstract Geologic understanding depends on interpreting earth history using relative time scales derived from the study of positional relationships of rock units and mineral bodies. Age determinations of specific rocks and events are used to calibrate a relative time scale. The fundamental problem in geochronology is correlation. Scientists date fossils, rocks, minerals, and remanent magnetism by inferring their position in a geologic succession and correlating them to a relative time scale. The only real evidence for an interval of time is relative position which is extended by hypotheses of correlation. Fossils and radiometric specimens, by their succession, may indicate the direction of time because of evolutionary changes or relative decay of elements. In magnetostra- tigraphy, apparent polar wandering may similarly indicate time direction but successive paleomag- netic reversals show no such direction and must therefore be "dated" by some other method before accurate magnetic correlation can be used. The biochronological time scale is an empirical artifact that has grown haphazardly over the last 200 years in areas determined by extraneous factors in regard to development of geologic concepts. The pitfalls of biochronology are known, but the vast and increasing body of empirical knowledge continues to reduce major possibilities of error. Although paleontology provides the "empirical basis for evolutionary theory" it must not be forgotten that the concept of evolution is derived from superposition and correlation. Within the Phanerozoic, accuracy of biochronology does not decrease with distance in time, although application may be strongly facies controlled. In radio- metric dating, the percentage error may remain constant, therefore decreasing resolution with increasing age. The difficulty in applying radiometric systems is in knowing what event has been dated; the materials may have undergone successive changes subsequent to their formation. Similar considerations apply to paleomagnetic evidence. Since the first relative time scales were constructed, new information and insights have emerged from both continental areas and the ocean floor. New methods of correlation now allow hope for an accurate time scale unifying biochronology, radiometry, and magnetostratigraphy. Multiple systems must be used within each major discipline. Thus correlation of many animal or plant groups may achieve greater precision in biochronology than single zonal indices. Similarly, several independent radiometric dating systems may increase confidence or help distinguish successive events that may be detectable by such methods. The most important single task of geochronology is to construct a unified time scale using the strengths of all methods and to test the time scale repeatedly by stratigraphic reproducibility. The potential accuracy of a unified time scale has scarcely been realized.

Abstract The word geochronology is derived from H. S. Williams (1893), use of geochrone which was a unit of time (equivalent to the duration of the Eocene period) whereby estimated ages were quantified. His estimate of ages back to Cambrian time, done before the discovery of radioactivity, were remarkably accurate. Since the use of radiometric methods, the term geochronology, for most scientists, has referred to the measurement of time in units of a year, generally measured radio- metrically but interpolated by thickness and fossil content of strata. However, another scale was conceived for the natural chapters of earth history, and was calibrated by successive creations of life with intervening revolutions. That was refined into a bio- stratigraphic scale until, in recent years, it became clear that standardization for international use was impossible without stratotype boundaries. So a new scale is in the process of definition. The International Subcommission of Stratigraphic Classification has referred to the time element of these global chronostratigraphic divisions as geochronologic. Geochronology might in any case be thought to signify geologic time. Several scales, different in their construction, that apply to geologic time have been proposed. Consequently, geochronology is now a general term referring to the whole science of geologic time, and it is an essential discipline in stratigraphy. Time correlation is the primary activity to improve the time-space framework. To achieve international correlation a degree of standardization is necessary so that divisions shall have agreed names and definitions. Two kinds of scales are used to provide a time frame to support, and be supported by geological events. One is the obvious chronometric scale of the physicist in caesium seconds or of the astronomer (and seemingly geologist) in earth-years. The other will be the chronostratic scale. Each provides a common language to express the time relationships of geological phenomena. The symposium was our effort to calibrate the chronostratic scale in boundary stratotypes against the chronometric scale.

Abstract An international geochronologic scale is needed in order to provide a single universal standard of reference for dating rock strata, or events recorded in rock strata, anywhere in the world. There are many possible means of relating rock strata and the geologic events they record to the passage of timephysical relations of strata (law of superposition), degree of isotopic decay, stage of organic evolution indicated by fossils, and other methods. Each is useful, but each is fallible under certain circumstances. The ideal standard, therefore, is one which does not depend on any one method but allows and encourages the utilization of all methods of age determination and time correlation. All methods of geologic age determination and time correlation must be based fundamentally on features of the rock strata; and thus the rock strata constitute the best register in which to inscribe the standard reference points stratotypes for whatever units or other horizons we wish to recognize in a global geochronologic scale. Only thus, by the designation of standard unit stratotypes, boundary stratotypes, and other horizon stratotypes, can we establish unequivocal definitions of points on this scale in a manner which lends itself to the utilization of all methods of age determination and time correlation, reinforcing the evidence from each by that from all others, and limited for each only by its capacity to usefully contribute. The practical value and utility of points on a global geochronologic scale is dependent on the extent and accuracy with which chronohorizons coincident with the stratotypes of these points can be traced or identified elsewhere in the world. Therefore, effort should be made to designate these stratotypes at places in the stratigraphic sequence which, through their coincidence with, or relation to time-significant features (isotopic dates, fossils, magnetic reversals, etc), particularly lend themselves to reliable widespread time correlation. Individual chronohorizons are no less important than chronostratigraphic units and their boundaries in the reference base for a geochronologic scale. Some geologists have questioned a basic tenet of chronostratigraphytheoretical boundaries of a chronostratigraphic unit should be everywhere isochronouson the grounds that erosion or nondeposition has locally altered the position of these boundaries from those seen at the stratotypes. However in such cases, the boundary is still theoretically present at the same time level, although included within the time value of the hiatus or unconformity. The stratotype concept should be applied to the global geochronologic scale, including both Quaternary and Precambrian.

Abstract Biochronology is the organization of geologic time according to the irreversible process of evolution in the organic continuum. It is an ordinal framework which measures all but youngest Phanerozoic time with greater resolution (ca 1 m.y., the average age range of species in rapidly evolving lineages), if with less accuracy, than radiochronology. Both are aspects of geochronology. In contrast, biostratigraphy (strictly speaking) is iterative, consisting of observed (not predicted) superpositional sequences of fossils without inherent chronologic significance. (This is to say that an upside-down biostratigraphy or one with huge time gaps is perfectly useful as long as it is consistent.) It is the arrangement and correlation in time of biostratigraphies that constitute the often unappreciated role of biochronology. The basis of biochronologic correlation is any notable singular occurrence, or "datum event, in the fossil record which has a geographic range overlapping the spatial limits of coeval but disjunct biostratigraphical zones. However, biostratigraphic sequences are the milieu in which datum events in the biochronology of different fossil lineages are compared and radiometrically calibrated. The concept of biochronology is illustrated by reference to the essentially isochronous First Appearance Datum (FAD) of the planktonic foraminiferal species, Globigerina nepenthes, and the three-toed horse, Hipparion, in the marine and continental strati- graphic record, respectively, during the late middle Miocene, ca 12.5 m.y.

Abstract The pattern of geomagnetic reversals for the past 150 m.y. has been well established from potassium-argon and polarity measurements on lava sequences, polarity measurements on continuous sequences in deep-sea sediment cores, and the interpretation of marine magnetic anomalies. Interest in the wider use of magnetostratigraphic methods has increased through the development of the extremely sensitive SQUID magnetometer, capable of fast measurement of virtually all rock types. The frequency of reversals appears generally to have been lower in pre-Cenozoic times when the magnetic field exhibited a cyclic behavior in polarity bias. Analysis of land-based paleomagnetic data suggests the existence of quiet (no reversals) and disturbed (many reversals) intervals throughout the Phanerozoic. The delineation of the quiet intervals offers the most encouraging prospect for their use in intercontinental correlation. The discovery of a quiet interval just below the PrecambrianCambrian boundary suggests a possible method for arriving at a definition of this boundary.

Abstract On August 24, 1976 the IUGS Subcommission on Geochronology 4 met in Sydney, Australia, during the 25th International Geological Congress. They unanimously agreed to recommend the adoption of a standard set of decay constants and isotopic abundances in isotope geology. Values have been selected, based on current information and usage, to provide for uniform international use in published communications. The Subcommission urges that all isotopic data be reported using the recommended values (see appendix). The recommendation represents a convention for the sole purpose of achieving inter laboratory standardization. The Subcommission does not intend to endorse specific methods of investigation or to specifically select the works of individual authors, institutions, or publications. All selected values are open to and should be the subjects of continuing critical scrutinizing and laboratory investigation. Recommendations will be reviewed by the Subcommission from time to time to bring the adopted conventional values in line with significant new research data.

Abstract A computer file of K-Ar, Rb-Sr, and U-Pb dates that provide constraints on the pre- Cenozoic Phanerozoic time scale has been created. New data appear slowly and thus the file size grows at the rate of only a few percent per year. The time scale presented at the 1974 International Meeting for Geochronology, Cosmochronology, and Isotope Geology in Paris is not in conflict with the data added since then. Precise chronologic subdivision of the Cretaceous is difficult because even the most optimistic uncertainties in the dates are greater than the duration of some stages. Nevertheless the stages of the Upper Cretaceous have been calibrated reasonably well. Subdivision of the remainder of the Mesozoic and Paleozoic systems cannot be done precisely and objectively from geochronometric data. Important boundary dates must be derived from interpolation between points with experimental and geologic uncertainties of at least a few percent. Efforts to obtain additional data for Lower Cretaceous to Upper Permian and Devonian and older rocks should be given special priority. One potential source of confusion in time-scale calibration, and conversely in the assignment of geologic age to rocks that have been dated, is the use of different decay constants by different laboratories and by the same laboratory at different times. Recently adopted values for uranium decay constants have the effect of reducing previously published U-Pb dates by about 1%. Proposed new decay constants for potassium (K) would increase previously published Phanerozoic K-Ar dates by about 2% in the case of western literature, and would reduce dates published by the eastern European countries by about 2.5%. The suggested new decay constant for rhubidium (Rb) is a compromise between values previously used. Most western and all eastern European Rb-Sr dates would be reduced by about 2%. Dates from western labs that have used the 47 x 10 9 -year half life would be increased about 3.5%. Revised time scales will reflect these changes. Because most calibration points are K-Ar dates the net effect is a 1 to 2% increase in the ages assigned to time-scale boundaries for scales published by geologists from western countries, and an approximate 2% reduction for scales published in eastern Europe. A discrepancy exists between time scales used in the two groups of countries. The eastern European scale is younger than the western one because of the greater use of glauconite K-Ar dates by the eastern Europeans. In contrast western emphasis is on dated volcanic rocks coupled with skepticism toward glauconite and whole-rock K-Ar dates.

Abstract From the critical interpretation of many examples of rubidium-strontium (Rb-Sr) dating of shales and related rocks, two main conclusions can be drawn: (1) Isochron diagrams of whole rock samples of pelitic sediments, originated from the same depositional environment, in many cases allow the determination of the age of sedimentation. In such cases, a uniform dispersion of clastic material within the basin must be inferred. (2) Isochron diagrams for subsystems of the fine fraction (insoluble and soluble materials) can indicate almost always the time of the diagenesis, anchi- or epi-metamorphism suffered by each sample. In such cases, Sr isotopic homogenization among clay minerals and soluble material occur. Several examples for the above referred concepts were found in the literature, and others were produced in the Sao Paulo Geochronology Laboratory on more than 300 samples from the following sedimentary units: Trombetas Formation (Silurian, marine); Botucatu Formation (Jurassic, continental); Rio do Rasto Formation (Permian, continental); Irati Formation (Permian, marine); Rio Bonito Formation (Permian, marine); Sepotuba Formation (Cambrian, marine); Estancia Formation (Cambro-Ordovician, marine); and Bambui Group (late Precambrian, marine).

Abstract Advantages and disadvantages of the potassium-argon (K-Ar) isotopic dating method are discussed in relation to its application to the accurate calibration of the physical time scale for the Phanerozoic. The method is best applied to the dating of igneous rocks but authigenic glauconite in sedimentary rocks also is amenable to dating. In physical time-scale studies, the ideal situation is that of volcanic rocks datable from sequences for which there is good stratigraphic and biostrati- graphic control. Ages determined on intrusive rocks in a biostratigraphically well-controlled sequence are useful in providing a younger limit to the age of the sediments. As with all dating methods, it is of utmost importance to measure ages on several samples in known stratigraphic relation in order to evaluate and test the assumptions underlying the K-Ar method, in particular those involving closed-system behavior and the possible effect of later events.

Abstract Since 1970, a new set of experiments and research on the dating of glauconites has been underway in different laboratories of Europe. The quality of the results seems to be due to a rigorous preselection of the outcrops, levels, and pellets before dating. This work, preliminary to an isotopic analysis, permits an easier interpretation of the apparent ages obtained. From the glauconites chosen in Europe it is possible to test the local horizontal and vertical reproducibility and accuracy of the chronometer. After interlaboratory analysis and internal verification (Rb-Sr and K-Ar isochrons with 10 points or more) it appears that a carefully selected sediment sample gives an apparent age equivalent to other samples of the same stratigraphic level. The accuracy is lower with the Rb-Sr method. Comparison of results obtained by both radiometric methods or with different chronometers (high temperature - low temperature) does not show an important systematic difference. As far as it can be concluded with the present dates, it is clear that most boundaries proposed since 1964 must be changed for Cretaceous and Tertiary stages. There are three reasons: (1) we know that some of the oldest results and reasoning taken into account were not adequate; (2) we have more results on better known samples; and (3) the decay constants are nearer their probable value today. Apparent ages proposed are essentially obtained from glauconites and must be completed and compared with more apparent age data on known high-temperature minerals. For more than a century, the stratigraphic position of European basin sediments has been studied. These investigations have led to the construction of the stratigraphic column. A maximum number of isotopic dates on these stratigraphically well-determined horizons is needed to correlate the stratigraphic column with a numeric time scale. Glauconite as a geochronometer is available in many sediments. Specific precautions have been determined and are observed. For a few years, different laboratories have undertaken preliminary isotopic studies for a better understanding of glauconite.

Abstract Mesozoic igneous rocks from Southwest Japan are critically reviewed in terms of isotopic and stratigraphic ages, and new K-Ar and Rb-Sr age results on the Cretaceous granitic and volcanic rocks from the Kitakami Mountains and some other areas are presented and discussed. Cretaceous granites from the western half of the Inner Zone of Southwest Japan are bounded between the Albian Shimonoseki Subgroup and the Campanian and Maestrichtian Izumi Group. The K-Ar ages of the granites generally agree with the stratigraphic evidence, and indicate that the granites were uplifted and eroded soon after the intrusion. Granites and related Cretaceous formations become younger eastwards, suggesting that a site of plutonism, associated with a site of volcanism and sedimentation, migrated eastwards at a rate of 2.6 cmyear. The isotopic ages of granites from Amami-oshima and those of the Funatsu granite also indicate a rapid uplift and erosion subsequent to the granite intrusion. The Miyako and Taro granites from the Kitakami Mountains, which are stratigraphically bracketed between the Neocomian and the upper Aptian, give Rb-Sr whole-rock isochron ages of 121 ±6 m.y. and 128 ± 12 m.y. respectively, and agree with the stratigraphic evidence. Seven mineral samples of the Miyako granite give an average K-Ar age of 113 ± 3 m.y., suggesting that the Aptian-Albian boundary may be slightly younger than 113 m.y. Cretaceous volcanic rocks from the Kitakami Mountains give K-Ar whole rock ages of 93-119 m.y., all slightly younger than stratigraphically estimated. Ages of the Harachiyama and Kanaigaura Formations are equal to those of the Miyako granite, and indicate either the contact effect of the granite or contemporaneity with the granite. Lower ages of the Niitsuki and Yamadori Formations may be attributed to the alteration. Volcanic rocks of the Sennan acid pyroclastic rocks in Kinki and those of the Mifune Group in Kyushu yield much younger K-Ar whole rock ages than estimated. A biotite from tuff in the Middle Yezo Group in northwest Hokkaido is dated at 91.4 ± 2.4 m.y., which agrees with the stratigraphically assigned Turonian age.

Abstract Results achieved in different Quaternary sediments by different methods are discussed, especially with regard to the worldwide correlation of Quaternary sediments. The necessity of a mutual checking of isotopic dating results by other geologic, paleontologic, and geophysical methods is stressed. There is danger of overestimating some methods of absolute dating (in Quaternary geology) which has led in many cases to false stratigraphic conclusions. There is danger especially in conditions of continental sedimentation, where the Quaternary is characterized by the alternation of different genetic and lithologic types of deposits. Isotopic methods must be regarded only as one of the existing methods leading to the establishment of a geochronologic scale, considering the many existing breaks of sedimentations.

Abstract The boundary between the Neogene and Quaternary (Anthropogene) Systems has been the subject of discussion for many years. Its position has been discussed at almost all INQUA congresses since 1932 (2d International Conference, Association for Study of Quaternary Period in Europe, Leningrad), as well as at international geological congresses. At the 28th Session of IGC held at London (1948) it was proposed to draw the boundary between the Pliocene and Pleistocene at the first signs of climatic cooling in the marine section of the Italian Neogene. Calabrian marine deposits include the first appearance of northern immigrants in their faunas and their stratigraphical continental analogue, the Villafranchian, was adopted as the basal member of the Pleistocene. The 29th Session of IGC held in Algeria (1952) approved the resolutions of the 28th IGC. At the 5th Inqua Congress at Madrid and Barcelona (1957) the Subcommission on the Plio- cene-Pleistocene boundary was organized under the Commission on Nomenclature and Stratigraphy of the Quaternary. The Subcommission has submitted recommendations to every subsequent INQUA Congress but these were not approved by the Sessions of the International Geological Congress. In 1972, the International Colloquium on the lower boundary of the Quaternary organized by the INQUA Subcommission was held in the Soviet Union. Decisions made at this Colloquium were approved at the 24th International Geological Congress held in Canada in 1972. These recommendations confirmed a proposal submitted to the 28th International Congress that Italy be the strato- type area for the boundary between the Neogene and Quaternary Systems, and the original definition that the base of the Pleistocene should be drawn in marine deposits at the lowermost level in the section at La Castella, Catanzaro, where Hyalinea baltica is recognized. In addition, the 1972 InternationalColloquiumchanged the correlation of the Calabrian with the subdivisions or biozones of Villafranchian continental deposits because, as proved by subsequent studies, the lower Villafranchian corresponds to the Astian, not the Calabrian. Where Calabrian analogues cannot be established easily, it was decided to use local subdivisions based on established stratotypes.

Abstract The radiometric time scale of Paratethys Neogene is presented. Fifty-four radiometric ages of the volcanic rocks are taken into consideration. The majority of them are controlled mostly by marine biostratigraphy. The Egerian (Oligo-Miocene) and Eggenburgian (lower Miocene) are dated only by glauconite ages contradictory among themselves and the volcanic rock ages. As a result, glauconite ages are omitted in the time scale. The only existing volcanic rock age (21.9 m.y.) and three ages of the lower Karpathian (between 19.4 and 20.7 m.y.) are considered as the maximum age of the Ottnangian and the Karpathian. The base of the lower Badenian (Moravian) is estimated to 16.5 ± 0.5 m.y., the base of the upper Badenian (Kosovian) 15 ± 0.5 m.y., the base of the Sarmatian to 13.3 ± 0.5 m.y., and the base of the Pannonian to 10.5 to 11.0 ± 0.5 m.y. Radiometric calibration of the Pontian, Dacian, and Rumanian has not been estimated because reliable radiometric dates with biostratigraphic control are not available. Two ages from the top of the Eastern Paratethys Pliocene are of 0.95 and 2.26 m.y. Probably the older one is closer to the actual age of the Pliocene-Pleistocene boundary.

Abstract Differences in data reported by various authors raise questions on the absolute dating of the main Paleogene boundaries. Ignoring established geochronometric and biostratigraphic principles only complicates the task of correlating stratigraphic units and establishing the precision of the absolute geochronologic scale. Absolute ages of younger, radiologically dated samples provides a lesser absolute error but greater relative error. It is suggested that age determinations on effusives are closer to true values (or the error is minimized), whereas interpretation on glauconites is more complicated because characteristically high argon loss in authigenic minerals seems to yield younger trending dates. With new data becoming available on continental rock units, biostratigraphic and geochronometric correlation with coeval marine facies appears to be increasingly important.

Abstract New data and a reinterpretation of existing data are the bases for this emended Paleogene numerical time scale. Biostratigraphy, radiometric dating, stratotypes, and paleomagnetic stratigraphy have been independently evaluated and subsequently integrated into a single numerical time scale framework. In the three Paleogene series, Oligocene, Eocene, and Paleocene, we have recognized eight commonly used stages. The corresponding age-units vary in duration from 3 to 8 million years (average 5 Ma). As a result of the improvement in establishing the relationships between the stratotypes of these stages and plankton biostratigraphy, the position of the stages within the series has been modified from earlier integrated chronostratigraphic schemes. Stratotypes (type sections) represent the formal basis for relating rock and time. However, the stratotypes of the western European Paleogene stages were not originally established on the basis of planktonic microfossils, and it was therefore not possible to recognize these stages worldwide. Subsequently, independent plankton zonations were established that could not be properly related to the stratotypes of the standard European chronostratigraphic units. Nevertheless, tentative relationships between these planktonic zonations and the relative chronostratigraphic units have been suggested and have been followed for practical reasons by nearly all plankton biostratigraphers. Calcareous nannoplankton have recently provided an indirect means of relating the stratotypes of the northwestern European Paleogene stages to the planktonic foraminiferal biostratigraphy and have revealed discrepancies between these two stratigraphic systems. The most significant of these is in the Eocene, where the type section of the traditional late Eocene Bartonian contains planktonic microfossils generally considered indicative of middle Eocene age according to most biostratigraphers. This situation is corrected by placing the middle-late Eocene boundary between the Priabonian and the Bartonian. The alternative solution, moving planktonic foraminiferal zones P13 and P14 from middle to late Eocene, would cause far greater confusion in the worldwide correlation framework. An evaluation of published radiometric dates provided us with the following age ranges for the Paleogene geochronologic units: Oligocene: 24 to 37 Ma Late: 24 to 32 Ma (Chattian) Early: 32 to 37 Ma (Rupelian)

Abstract With respect to the uncertainties of the method, measurements on glauconites of the Paleogene of western Europe effected during the last decade provide isotopic dates within a margin of error of 5%, that is about 2 m.y. (Table 2). A discussion of the age of the main limits: Cretaceous- Paleocene (63 to 65 m.y.), Paleocene-Eocene (53 to 55 m.y.), Cuisian-Lutetian (47 to 49 m.y.), Lutetian-Bartonian (42 to 44 m.y.), Eocene-OIigocene (34 to 36 m.y.), Oligocene-Miocene (22 to 24 m.y.) is presented. The middle-late Eocene boundary may be placed either between the Lutetian and the Bartonian (42 to 44 m.y.) or between the Bartonian and the Priabonian (39 to 41 m.y.). During the last decade several hundred datings have been made in Europe on Paleogene glauconites, principally from France, England, Belgium, Germany, and the U.S.S.R. Others, fewer in number, have been obtained in the Mediterranean region and in central Europe. Before a presentation of the results, the inherent uncertainties of the method are briefly discussed. These originate either in the specimen, in the mode of operation, or in the choice of physical "constants."

Abstract Numerous results obtained since 1964 are presented. A scheme of the reliability and accuracy of the different chronometers used for numerical dating is given. All the rocks and minerals are not of identical value. Some comparisons between Rb-Sr and K-Ar data are mentioned and comparisons between different chronometers are presented. The probable numerical age of the boundaries between the stages are discussed, taking into account the new suggestions on the best position of these stratigraphic subjective boundaries. The Paleogene has a duration of 42 million years65 to 23 m.y. The Paleocene-Eocene boundary is at about 51 m.y. The appearance of Pseudohastigerina is a little more than 52 m.y. ago. The Lutetian stage is between 45.5 and 40 m.y. The base of the Ericsonia subdisticha zone is younger than 37 m.y. while the Priabonian-Rupelian boundary is between 34.5 and 36.5 m.y. All the apparent ages are calculated with the new standardized constants proposed during this symposium.