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Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 The pre-Pleistocene Phanerozoic time-scale- a review RICHARD ST JOHN LAMBERT CONTENTS x Introduction IO 2 The Cenozoic era II 3 The Mesozoic era . I4 (A) The Cretaceous period I4 (B) The Jurassic period 16 (c) The Triassic period x8 4 The Palaeozoic era I9 (A) The Permian period I9 (B) The Carboniferous period 20 (c) The Devonian period . 22 (D) The Lower Palaeozoic era 23 5 Conclusions 24 6 References 26 Appendix I : The half-life'of S~Rb 27 Appendix 2: List of critical points 29 (A) Most reliable (pre-Tertiary) determinatxons 29 (B) Most reliable (pre-Tertiary) determinations without duplicate or sup- porting analyses . 3 ~ SUMMARY The data in The Phanerozoic Time-scale (Geo- knowledge of the decay rate of STRb. The cur- logical Society, I964) are reviewed, together rent six per cent range in commonly-used with data relevant to the Phanerozoic time- values of this constant is now of over-riding scale published up to mid-I968. Quaternary importance in causing uncertainty throughout problems are not considered, while the Tertiary the Palaeozoic scale; Rb-Sr determinations is considered to be on a comparatively well- now provide the majority of critical Palaeozoic established basis. The use of glauconite ages to ages. Even without this uncertainty, increases establish any part of the time-scale prior to the in the age of the Westphalian, the base of the Oligocene is not to be recommended, as early Carboniferous (to 36o m.y.) and the base of the glauconites can provide anomalously high Devonian (to ?4o5 m.y.) seem to be indicated apparent ages in addition to the occurrence of by new data (using the 4"7 • xol~ half-life low ages. No criteria for distinguishing anomal- for STRb: if 5"0 • IO 10 y proves correct, or any ously high or low glauconite ages have yet been other figure higher than 4"7, then there will established. The definition of inter-system and be correspondingly greater increases in the age within-system boundaries in the Mesozoic of these boundaries). The Devonian data are provides an acute problem, to which there is at particularly tantalizing: imprecision in strati- present no solution. Reconsideration of the graphical correlations is allied to spreads in Cretaceous data shows reasons for preferring radiometric ages from individual complexes to 95 m.y. as the age of the base of the Ceno- cause considerable uncertainty. There are still manian, but neither of the boundaries of the no useful Silurian data, nor is any part of the Jurassic are clearly defined. New data suggest Ordovician other than the Caradocian dated that there should be an increase in the accepted at all. The Caradocian data can be questioned age of the base of the Triassic to 235 :t= 5 m.y., and, if a half-life of STRb > 4"7 • I~176 9is but this is in part dependent upon a firm adopted, would need reconsideration. No part The Phanerozoic Time-scale- a supplement, London (Geological Society), 1971. Part x, pp. 9-31, 3 figs. Printed in Northern Ireland. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 R. St J. Lambert of the Cambrian is satisfactorily dated, but its American data as being at not more than base (as far as it cart be defined palaeontologic- 57~ 4- Io m.y. (4"7 • I~176Y 87Rb half-life), ally) appears to be now controlled by North confirming the most recent estimates. I. Introduction TrlE FIVE YEARS which have elapsed since the publication of The Phanerozoic Time-scale by the Geological Society (Harland, Smith & Wilcock 1964)1, enable some of its suggestions and conclusions to be placed in a new perspective. Some relevant data have since appeared, and a major contribution by the Russians (Afanasyev & Rubinshtein 1964) on the same subject can be taken into consider- ation, Ill this article some of the advances will be discussed and some of the deficiencies of the present scale noted, each primarily for discussion and not in an attempt to provide a revised, quotable scale. The problem of constructing a time-scale is at least two-sided, with uncertainty arising equally from stratigraphical as from radiometric considerations. There is little discussion in P TS of the latter; a single determination is generally relied upon, analytical errors are too often only estimated and the degree of confidence is rarely stated in the original papers, while the assumptions made concerning the reten- tivity of radiogenic daughter products are either refreshingly simple or distressingly naive depending on the cynicism of the observer. Apart from the general problems just mentioned, some outstanding specific problems exist, notably the six per cent uncertainty in the rate of decay of 87Rb (see Appendix I), the retentivity of glauconite (discussed in sections 2 and 3(A) below) and the uncertainty to be attached to many of the Rb-Sr age-determinations on individual minerals quoted in PTS (see section 4(B) below). On the stratigraphical and general geological side of the time-scale problem there is also considerable uncertainty: the following series of quotations from PTS may be allowed to speak for themselves concerning the problems of erecting a time-scale. "In spite of these advances no truly international stages have so far been estab- lished for the Tertiary, although frequent attempts to extend the rather unsatis- factory Tertiary stage nomenclature of Europe have been made" (Funnell, p. 18 I). "... the general rule that in the Cretaceous (as in some other systems) radio- metric methods of dating are at present least successful in those regions where palaeontological control is most rigid" (Casey, p. 194 ). "Few dates [actually three at the most] are available for rocks that are well established on stratigraphical grounds, as Triassic" (Tozer, p. 207). "In common with other systems there is considerable difficulty in fixing the limits of the Carboniferous'" (Francis & Woodland, p. 22I). "Before considering the radiometric evidence for the Ordovician period it is desirable to discuss the boundaries of the system and its sub-divisions, which have 1 Hereafter referred to as P TS. IO Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 The pre-Pleistocene Phanerozoic time-scale- a review been the subject of dispute in the past and are still matters of debate" (Whitting- ton & Williams, p. 241). "A review of about a hundred age-determinations which fall within the range 45 ~ to 680 m.y. has shown that their correlation with stratigraphical units is, even at the best, imprecise and often inconclusive" (Cowie, p. 255 ). 2. The Cenozoic era The problems associated with the stratigraphical and radiometric aspects of establishing a Cenozoic scale have been very clearly stated in P TS (Funnell, pp. 179-91 ) and by Evernden and his associates (Evernden, Savage, Curtis & James 1964; Evernden & James, 1964; Evernden & Curtis 1965). Analytical uncertainty on high-potassium minerals does not exceed I m.y. within the Tertiary Period, giving a radiometric accuracy comparable with palaeontological accuracy in the case of the mammalian faunas of North America, or even exceeding palaeo- botanical accuracy, according to Evernden & James (I 964). The problems associ- ated with the measurement of low potassium-argon ages were most clearly stated (and discussed by other writers) in Evernden & Curtis (1965) who gave cogent reasons for believing that the very young ages produced by their techniques from biotites, sanidines and many types of volcanic whole rocks are indeed the ages of formation (eruption and cooling). As yet, it may be noted, few other laboratories have ventured into this particular variety of chronological research, so the great internal consistency of the Berkeley results has not been thoroughly verified. It would be extremely interesting to see some other variety of analysis applied to this age range, particularly fission-track dating. The excellence of the Tertiary scale is, however, not uniform in character, but is confined to those sections of the Tertiary where biotite or sanidine analyses are available in quantity. In Figs. I and 3 stratigraphical ranges of those pre- Pliocene biotites, sanidines and glauconites used by Funnell in constructing his version of the Cenozoic scale are plotted; of twenty-six biotites and sanidines, only four lie off the preferred scale, three appearing to be too old, and one too young. Glauconites are, however, another matter: within the Tertiary four appear to be too old, ten satisfactory and ten too young (Figs. I and 3, see also section 3 (A) below). The data from low-potassium feldspar and whole rocks in the Tertiary are also not entirely satisfactory. As far back as I I m.y. those examples of plagio- clases and whole rocks which are quoted by Funnell were in agreement with the biotite-sanidine scale, but divergence appears in the Middle Miocene (Barstovian) in which the mean age of one biotite and three sanidines (one possibly early Barstovian) is 15. 3 m.y., [PTS p. 186] but three low-potassium samples average 14. 7 m.y. Agreement between biotites and whole-rocks again returns for the Lower Miocene, but low-potassium whole rocks near the Eocene-Oligocene boundary appear to be giving ages which are too low (29. 7 and 31.6 for early Oligocene instead of 32 to 38 m.y.). Even with the rigorous checks advocated by Evernden (1964 papers) it is clear that K-Ar whole-rock studies are not to be given priority in the construction of a pre-Tertiary scale, although care in selection II Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 R.
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