Does the Golden Spike Still Glitter?
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Journal of the Geological Society, London, Vol. 143, 1986, pp. 3-21, 17 figs. Printed in Northern Ireland Does the golden spike still glitter? CHARLESHEPWORTH HOLLAND Department of Geology, Trinity College, Dublin 2, Ireland President’s anniversary address 1985 CONTENTS Geochronometryand Precambrianstratigraphical classification ............... 3 Development of Phanerozoicstratigraphical classification .................. 5 First or pioneering phase .............................. 6 Secondphase: fossils andtime. ........................... 6 Third phase: rock as distinct fromtime-rock ..................... 7 Fourth phase:advent of the goldenspike ....................... 7 The Silurian-Devonianboundary .......................... 8 The Ordovician-Silurian boundary .......................... 9 Is there value in the golden spike? ............................ 10 Is there value in chronostratigraphy? ........................... 11 Stratigraphicalprocedures ............................... 13 Event stratigraphy ................................. 14 Back to biostratigraphy ................................. 15 References ...................................... 20 Abstract: A modus operandi forPrecambrian stratigraphy is suggested which combines the geo- chronometrical scale andlate Proterozoicchronostratigraphy. The development of Phanerozoic stratigraphical classification is followed from the pioneering days of Murchison, through the growing understanding of fossils in relation to time and the later distinction of rock and time-rock, to the advent of the goldenspike. Criticisms of chronostratigraphy (global standard stratigraphy) are assessed and its value upheld. Stratigraphical procedures such as those of event stratigraphy are useful in particular situations, but correlative methods in the Phanerozoic are dominated by biostratigraphy. Examples are taken of its use and potential in North American Cordilleran Palaeozoic stratigraphy, in the Triassic System as a whole, and in the Caledonides of Britain and Ireland. Geochronometry and Precambrian stratigraphical that the individual duration of these must be <2Ma, and classification the tentatively suggested standard zones have themselves Apart from providing a time scale against which physical, been achieved throughgroupings of a list of 41 more traditional graptolite biozones, each of which must have an chemical, and organic processes can beassessed, the important function of geochronometry is to give quantitative average duration of substantially <l Ma. temporal significance to the stratigraphical time scale. The As noted by Cowie et al. (1984), the International Union point is seldom explicitly made that the broadly consistent, of Geological Sciences’ Subcommission on Precambrian but everchanging, geochronometric scale is somewhat Stratigraphy has in the past few years proposed thatthe loosely attached to the stratigraphical scale within ranges of standard divisions of the Precambrian should be defined on dates in years. In most of the Phanerozoic, geochronometry the geochronometric scale, with the Archaean-Proterozoic is not a useful correlative tool, asbiostratigraphy can boundary already agreed at 2500 Ma. This is contrary, these authors argue, to the orderof procedure in the Phanerozoic, achieve a much greater precision (Holland 1985a). The notion of an elaborately subdivided segment of the wherea chronostratigraphical scale was established first, stratigraphical scale ‘floating’ against the geochronometric with later geochronometricalcalibration so far as this is scale is presented as an illustration (Fig. 1) of the still possible. Since the 1984 International Geological Congress uncertain time range of the rocks of the Silurian System, set in Moscow, the final choice of a boundary stratotype section against biostratigraphicallya discriminated subdivisional for the Precambrian-Cambrian boundary remains unsettled. There are lingering doubts about correlation of other areas arrangement of what may reasonably be regarded as 20 with the proposed Chinese section in Yunnan Province. ‘standard zones’ (Koren’ & Karpinsky 1984).* We can see More work onthe Precambrian-Cambrian small shelly * In the English version of this paper Dr Koren’ is, through no fossils and on magnetostratigraPhY hasbeen called for. fault of her own, provided with a co-author who died rather too However, the relevant Working Group has already decided long ago to have exercised this function. Some confusion with the that this boundary will be defined chronostratigraphically at name (Karpinsky) of her place of work is apparent. such a section. Since present estimates of the radiometric 3 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/1/3/4893405/gsjgs.143.1.0003.pdf by guest on 01 October 2021 4 C. H.HOLLAND A B C D E F G H I Ma. 390 - 395 - 400 - 405 - 410 - 415 420 - 425 - 430 - 435 - 440 - 445 - 450 Fig. 1. Age range of the Silurian System according to (A) Geological Society (1964), (B) Lambert (1971), (C) Boucot (1975), (D) Spjeldnaes (1978), (E) McKerrow et al. (1980), (F) Gale et al. (1980), (G) Odin et al. (1982), (H) Harland et al. (1982), and (I) McKerrow et al. (1985). Each Silurian column is divided into 20 equal parts representing the standard graptolite zones of Koren’ 1984 (see text). age of this boundary range in the extreme over some one theCanadian Shield (Stockwell 1972), itself depends hundred million years,geochronometricallya defined ultimately upon radiometric dates. boundary would be considerably less precise as a correlative The situation is different only in the latest part of the standard. Precambrian, where palaeobiological criteria are already The 1983 revision of the geological time chart, Major plentifully available. There is also palaeoclimatological Geochronologic and Chronostratigraphic Units, prepared by evidence from widespread glacial horizons. Work has been the Geologic Names Committee for the use of the United pursued in different ways in the varying facies of different States Geological Survey (see US Geological Survey 1983), countries. Thus there are rival contenders for the provision divides both the Proterozoic and Archaean Eons into Early, of standards and there is some confusion of nomenclature. Middle and Late Eras, the boundaries between them placed The problem is succinctly put by Harland et al. (1982). Their at 3400,3000,2500,1600, and 900 Ma (Fig. 2). The informal suggestion of a Sinian Era between the Proterozoic and the term pre-Archaean is used for the time between the origin Palaeozoic, and of Sturtian and Vendian Periods within this of theEarth and the date of the oldest rock atabout (Fig. 2), has much to recommend it. 3800Ma.Harland et al. (1982) employ most of the same Sinian (consideredas a period within the Palaeozoic, figures within the Archaean and Proterozoic but prefer the perhaps the first) was originally used in 1922 and thus may rounded 3500 for the first. It is unrealistic to say that there is be said to have priority. There is a splendid section of Sinian any immediate prospect of defining any of these, or similarly rocks along the Yangtze Gorges, which has been taken as placed boundaries, within most of the Precambrian on a providing a standard.The succession of approximately chronostratigraphical basis; that is to say with boundary 1000 m yields many species of microplants. Macroscopic stratotype sections. The use of tectonic divisions, their algae, metazoa, and trace fossils make their appearance in boundaries marked by orogeniesbased upon structural the highest beds. Fossiliferous Sinian rocks are seen in provinces, as done by the Geological Survey of Canada in various other regions of China. The succession includes a Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/1/3/4893405/gsjgs.143.1.0003.pdf by guest on 01 October 2021 DOESTHE GOLDEN SPIKE STILL GLITTER? 5 - PHANEROZOIC EON PALAEOZOICERA PALAEOZOICERA CAMBRIANPERIOD VENDIANPERIOD SINIANERA LATE CTllPTlAN PFPIOD PROTEROZOICERA 800 4 900 MIDDLE PROTEROZOICERA PROTEROZOIC EON 1600 Fig. 2. A suggested geochronometrical and chronostratigraphical classification for the Precambrian. z EARLY 4 PROTEROZOIC ERA z I S 2500 4 LATE Y ARCHAEANERA rrc 3000 n ARCHAEAN MIDDLE EON ARCHAEANERA 3400 EARLY ARCHAEANERA -------- c3801 PRE-ARCHAEAN Ma tillite horizon. A recent review of the Sinian (Xing Yusheng adopt chronostratigraphicallyconventional geographical 1984) puts its base at about 800 Ma. names forthe Sinian and its contents. The base of the The Vendian rocks and fossils of the Soviet Union (first Vendian may be properly defined at a boundary stratotype recognized as such in 1949-50) are now very well known within a short period of years, but the base of the Sinian is throughthe work of Sokolov,Rozanov, andtheir likely to remain at 800Ma, or some other geochronometri- colleagues. Sokolov (1984) provides an up to date review, in cal level. The interface between the properly chronostrati- which thetemporal range of the Vendian is given as graphically defined units with their geographical names and 640 f 10 to 530 f 10Ma.The fossils include shelf and the simply date-defined boundaries of the earlier Precam- pelagic soft-bodied invertebrates as well as many plants. The brian could then move downwards as biostratigraphy (and stratotype for the Vendian System is in the western part of perhaps otherhitherto unknownmethods of correlation) the RussianPlatform. Thereare alternativesections, allows the use of boundary stratotypes in these older rocks, particularly in