The Pre-Pleistocene Phanerozoic Time-Scale- a Review

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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 supporting 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 consideration, 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 retentivity 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 established 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) radiometric 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.

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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 associated 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

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R. St J. Lambert of samples can give reasonable results on occasion [see, for example, the study of the Whin Sill (Fitch & Miller i967) ]. Some sections of the Tertiary scale are very well established, especially the Pliocene, Miocene and Lower Oligocene 1, but data are lacking for the rest of the Oligocene. The earlier divisions of the Tertiary present more of a problem and it would be advantageous to have more ages in the Upper and Lower Eocene and in the Palaeocene. One particular problem is raised by the Upper Eocene biotite ages of 45-o and 45"4 m.y. from the Bridgerian or Uintan of Wyoming (items 3o2 and

,,i I ' 1 ' I ' I J I [' J:l'~

~.~ L

~M LF ~u

L L

~ glcuconite

J biotite or sonid/ne 0

, I ~ I i I , 0 I0 20 30 40 50 dO R#diometrir ogr /n m.y. i o. i. Relationship of radiometric age and stratigraphical level for the biotites, sanidines and glauconites quoted by Funnell (PTS, pp. I79-9I), excepting those with unusually inexact stratigraphical position and those with radiometric age > xo m.y. removed from the probable true age. Errors in ages are not indicated. The plotted data are confined to the age-range 7 to 6o m.y. and show deviations from the preferred time-scale (solid line). All biotites and sanidines lie on the preferred scale except those near 45 m.y. and the anomalous 52 m.y. biotite.

303 ofPTS): these lie a little below the line on Fig. z, suggesting either that the estimate of the scale is locally wrong or that the stratigraphical correlation needs to be re-evaluated. If the correlation is correct, it would be preferable to place the Upper-Middle Eocene boundary at 46 or 47 m.y. rather than 45 m.y. as suggested by Funnell. It could be argued that it is stratigraphically impossible to define the European divisions as closely as this from the mammalian-linked age-data, but as there appears to be no prospect of dating the European sequences directly, the correlations must be established and the dating carried out indirectly. The fifteen m.y. which elapsed prior to the Middle Eocene are not at all well documented, the available evidence from the Rocky Bay carbonatite, Montana x The exceptional figure of 33"I m.y. for sanidine from a supposed Chadronian tuff (PTS, item 84) should be disregarded (Wilson, Twiss, DeFord & Clabaugh (I968)).

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The pre-Pleistocene Phanerozoic time-scale - a review

(52 4-2 m.y., item 37, PTS) and a dacite from Fakhralo, USSR (54"5 m.y., item lO3, PTS) being almost mutually incompatible (see Fig. 1). There is scope for considerable research at this level in the time-scale, particularly as the data relating to the Tertiary-Cretaceous boundary show a considerable spread of values (64 to 68 m.y.). In view of this spread of figures, the standardization of the data used since the scales of Holmes (i959) and Kulp (i96I), and the general uncertainty in long-distance stratigraphical correlations at this particular age-range, there is no point in suggesting any revision of the figure of 65 m.y. for the base of Tertiary. Afanasyev & Rubinshtein (1964) preferred to state 64 + 3 m.y. (using the same set of constants), their conclusion being weighted in favour of data from Russia and the Zion Hill granodiorite, Jamaica, as against the supposedly late-Cretaceous bentonites (64 to 68 m.y.) of Alberta. The data from the Zion granodiorite are, however, not easy to interpret (see comment by Smith, P TS, p. 385) and there is no reason for adopting any particular figure in the range 64 to 73 m.y. (or more) as the age of intrusion. The only new contribution known to the writer which concerns the Palaeocene- Cretaceous boundary is that of Shafiqullah, Folinsbee, Baadsgaard, Cummings & Lerbekmo (1968---originally read at the 1964 International Geological Congress). Their data (Items 363-5) related to three groups of bentonite horizons, one within the Palaeocene (Paskapoo formation), one close to the boundary (uppermost Triceratops zone) and one within the Cretaceous (Kneehills Tuff, Upper Edmonton formation). They concluded that the three horizons should be dated at 55 -4- 1.5, 63 -1- I and 65. I 4- I .o m.y. respectively, the middle date being effectively that of the boundary. In discussion (p. 2o, op. cir.) it was stated that the "date of 63.1 m.y .... was not an average but a best value which was believed to take into account possible minor argon leakage in biotites and other effects which tend to minimize the potassium-argon dates. Slightly greater weight was assigned to the sanidine runs in arriving at this best value"; also that "Additional runs ... tend to increase the best value ... to 64.o m.y." There is some doubt, however, whether this is in fact the best analysis of the data, as Fig. 2 shows. This figure is a simple cumulative plot for the biotites and sanidines separately, each analysis being weighted evenly, so far as the visual presentation is concerned. The conclusion of Shafiqullah et al. (1968) that sanidine dates are more reliable than biotite dates is clearly illustrated, so much so that little reliance can be placed on any of the biotite figures. The reduction of age is presumably due to alteration effects rather than simple diffusion loss in most of the examples given. The writer does not, however, consider that the method of weighted averages (given above) is justified in this case and would prefer a close scrutiny of the sanidine ages alone to find the closest approximation to real age of these bentonites. On this basis, from Fig. 2, we draw the following conclusions: I. Palaeocene bentonite of Colt Creek: 62 m.y. (ignoring the exceptional figure of 67.6 m.y.). 2. Ardley/Pembina coal seam horizon bentonite, uppermost Cretaceous, close to boundary: 64 m.y. (ignoring 61.1 m.y. determination). 3- Kneehills Tuff: 65 m.y. (4-2 ?) (a simple arithmetic mean).

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R. St J. Lambert

I I I I I I I i I I

9 Paleocene x o _....Paleocene-Oreeaceous x x x 9 boundary x x x Cretaceous x x ' o x l x 9 x o x 9 o to 9 o 9"",,..,. 9 o

""0 9 8 X ~ 9 ~x 9 .~'~; X E 9 DO 9 COO 9 0

0 9 O 0 I I I I I I I I I I 48 50 52 54 56 5B 60 62 64 66 68 70 m.y. Fio. 2. Data from Shafiqullah et al. (I968) for biotite and sanidine from Albertan bentonites, plotted as two separate cumulative curves.

These conclusions suggest that a revision to 64 4- I m.y., rather than 65 m.y., is indicated, but it is recommended that further data are required to settle this point.

3. The Mesozoic era (A) T~ CR~T.C~OOS P~R~OD The difficulties inherent in securing a good time-scale for the Cretaceous are concisely summarized by Casey in PTS. The solution "for discussion" which he proposed, however, does not appear to be the best, as may be seen from Fig. 3- Good biotite or sanidine ages, demonstrated as consistent and valuable in the Tertiary, are restricted to three horizons (sensu lato) in the Cretaceous: Upper (?) Maestrichtian, Campanian and Upper Albian/Cenomanian. On a basis of the average figures for each group, and using the most probable stratigraphical assignments as set out on Fig. 3, the following scale emerges: base of Maestrichtian: 72 m.y. base of Campanian: 77 m.y. base of Cenomanian" 95 m.y. The ages for the first two boundaries above do not differ greatly from those suggested by Kulp (I96 I) or Casey (P TS) but the base of the Cenomanian is much younger (Kulp i io m.y., Casey ioo m.y.). Casey did in fact state that the age of I4 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

The pre-Pleistocene Phanero zoic time-scale- a review the base of the C enomanian which he suggested might be a little high. Kulp's higher figure pre-dated the Mowry Shale work (Item 204, P TS) and depended heavily on a I 15 m.y. post-Albian monazite from the Southern California batholith, on glauconites (see below) and on the i 17 m.y. biotite from the Hudson Hope Shale (item 203, PTS). The latter age is suspect in that the palaeontological control gives the younger limit of age only, while the K20 content of the chloritized biotite is only 3 per cent, near the generally accepted lower limit of K~O content for reliable retention of original 4~176 ratio. The significance of the monazite age is not known, but the fairly consistent data from the Mowry Shale appear preferable to the higher group of figures (cf. Dodson, Rex, Casey & Allen: PTS, p. I52 ).

H ' , , , , , , u, Dill El' / ,, o D / / B Apt ~ / ~ Apt

/ [l --

c. I c. , / 7": T

Co 0 Co

$ S

I biotite or Ca co ~ J I'1 , I , [] =anidineslo.~o~it, M M o.I / I , I I ,o,7/ 60 70 80 90 I00 llO 120 Radiometrir agts in m.u

FIG. 3. Relationship of radiometric age and stratigraphical level for the biotites, sanidines and glauconites quoted by FuaneU (PTS, x79-9I) and Casey (PTS, I93- 202) and which lie in the age range 60 to x20 m.y. Examples with unusually inexact stratigraphical position or with radiometric age > 20 m.y. removed from the probable true age are not included. The dashed line indicates the scale preferred by Funnell and Casey; the solid line indicates a possible modification based on the limited data available.

The further sub-division of the Cretaceous depends entirely upon the use of glauconites. These have already been criticized (section 2 above) and can be criticized further from the Cretaceous data. In PTS, Baadsgaard & Dodson noted the common occurrence of 'too young' glauconites and considered that great caution should be used in theinterpretation of glauconite ages. Figure 3 gives rise to the suggestion that it is not possible to rely at all on unsupported glauconite ages in the Cretaceous, as, using the stage boundaries suggested above, taken with a modification of Casey's linear interpolation, we find that, among the already selected samples, seven glauconites are too old, five about right and eleven too young. It might be possible to select a very small number of the 'incorrect' glauconites and x5 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

R. St or. Lambert produce other stage boundaries and thereby reduce the number of anomalies, but any such selection would be very arbitrary. Consider, for example, the problem posed by the seven Campanian-to-Turonian (inclusive) glauconites plotted in Fig. 3: it could be that these show an argon-loss pattern alone, giving an age of about 813 m.y. for the base of the Coniacian. A total length of 7 m.y. for Turonian and Cenomanian together might not be acceptable to geologists generally, Kulp (I96I) proposed the same base for the Coniacian, but a total of 22 m.y. for these two stages. The Lower Cretaceous is therefore effectively devoid of ages of good definition, while the base of the Cretaceous is equally ill-defined. Although P TS and Afanas- yev & Rubinshtein (1964) more or less agree (I36 m.y. and 128-133 m.y. respectively) the actual data on which these conclusions are based are not of the highest class. The Shasta Bally batholith, with one biotite K-Ar age of I27 m.y. (Item 75, PTS), is pre-Barremian according to Casey's analysis of the palaeontological information, while ages ranging up to I36 m.y. are reported from 'Cretaceous' batholiths of Mongolia. Reliance on the highest among a spread of ages from a slowly-cooled igneous complex is not to be recommended and considerable uncertainty must remain attached to the age of the base of the Cretaceous. Glauconite data given in PTS confirm the necessity of leaving the question of the age of the base of the Cretaceous open: from the three lowest Cretaceous stages and the Tithonian/Volgian we find glauconites giving (stratigraphical order, youngest first) IO8, II4, I3I , I3I , II7, 119, 139 , I32, I25, 129 and 129 m.y., ignoring possibly metamorphosed examples (data from Table 5, Dodson et al., PTS). The stratigraphical inter-period boundary comes half-way along this list (i 17/I 19 m.y. figures). At the very best, the boundary may fall in the range I25 to I45 m.y. and there is no sound reason for taking the mean figure as the most likely.

(B) THE JURASSIC PERIOD It is unfortunate, but not surprising, that it is possible to criticize every determination which has been quoted in all recent time-scales as being relevant to the Jurassic. The twelve dates quoted by Howarth (P TS, pp. 2o3-5) include all known reliable figures and apparenty form a fairly consistent pattern: details, however, show otherwise. There is no point in considering the five glauconites quoted by Howarth further: if any one is correct, then on the basis of the evidence cited in sections 2 and 3 (A) above, it might be concluded that an interesting coincidence has occurred, but no more. The biotites at I28 (127 ?) m.y. from the Shasta Bally batholith, and 136 m.y. from the Horseshoe Bar quartz-diorite (Item 76, PTS) were allocated to the Jurassic on the basis of alleged positions in orogenies, but the former assignment is not consistent with all the available data (see above, also Casey, PTS, item 75) while the latter is only the highest of a group of mica ages from the Sierra Nevada batholith, which range from 124 to 136 m.y. The geological evidence indicates a post-Lower Kimmeridgian age (Curtis, Evernden & Lipson I958) but 136 m.y. is i6 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

The pre-Pleistocene Phanerozoic time-scale- a review itself only a minimum figure, as it is from part of a large batholithic complex with an unknown cooling history. In any case the precise stratigraphical position of the Horseshoe Bar intrusion is not known, being itself some distance from any sedi- mentary rocks. The Guadalupe Mountain quartz-monzonite, biotite age 136 m.y. (Curtis et al. I958; not itemized in PTS) which was said to cut the Mariposa formation ("Oxfordian-(early) Kimmeridgian") is better defined stratigraphically than the Horseshoe Bar quartz-diorite, so is perhaps more valuable. The middle and Lower Jurassic ages quoted by Howarth and also by Afanasyev & Rubinshtein (1964) are all from plutonic intrusions and, as seems to be the case with nearly all plutons ever dated and used for time-scale purposes, some element of vagueness is associated with each. The British Columbia intrusions (Topley and Hotailuh) are said to be post-Norian or Karnian with rather vaguer upper age limits, but the individual biotite ages of 178 and 193 m.y. belong to a large group of figures for which it is stated that 4~ (radiogenic) = ioo per cent of all 4~ in the sample, a possible state of affairs in argon analysis, but one which gives rise to suspicion that the age quoted must be regarded as a maximum. It may be observed that the discrepant Guichon Creek batholith data (item Io, PTS) belong to this group. There is also an age of 185 4- I o m.y. on the same batholith, but it is from a low-K biotite and therefore slightly suspect. The Billiton granite ("post-Triassic-pre-Jurassic") is also subject to radiometric uncertainty, the quoted age of 18o 4- 5 m.y. being the highest of a widely discordant range1: the stratigraphical assignment is unaccompanied by sufficient detail to support its narrow range. The stratigraphy of the environment of the Talkeetna diorite, Alaska, is likewise a little uncertain and all that can be said is that its single low-K (3.63 per cent) biotite age of 169 m.y. is post-Pliensbachian and pre- Oxfordian. Finally, there is the apparently reliable Kelasury granite of Georgia with five concordant ages (I6I to I67 m.y.) and one higher age (I76 m.y.) from a hornblende-biotite mixture; the best figure probably being the average of the concordant group--i63 m.y. The accepted Bathonian stratigraphical position (PTS, item 9 ~ Kulp 196 I) has, however, been attacked by Afanasyev, who also cast doubt on the radiometric age and recommended that further work be done (Afanasyev & Rubinshtein I964). It is tempting to conclude that the uncertainty indicated in the above discussion is based on an over-pessimistic view of the stratigraphical and radiometric difficulties, using as a prime counter-argument the general agreement of the I6o to I9o m.y. figures from the Middle or Lower Jurassic, but it is felt that such agreement is accidental at the present time. A few duplicate K-Ar determinations on hornblende, where possible with co-existing biotite as well, new Rb-Sr analyses and a fresh look at the stratigraphical position of each of the six plutons mentioned above might well provide some very interesting information.

1 Similar granites in Malaya range from 175 to 2 IO m.y. (N. J. Snelling, personal communication) Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

R. St J. Lambert

C) THE TRIASSIC PERIOD The weaknesses so far outlined in the Mesozoic scale continue unabated into the Triassic. It is interesting to observe that the same situation as recorded above for the Jurassic holds good in the Triassic: that there are actually no completely reliable ages, with the probable exception of the new Australian data (Webb & McDougall 1967) and possibly that from Italy (Borsi & Ferrara 1967). The Palisades Sill figure of 193 m.y. was strongly criticized by Afanasyev (op. cit.) who considered that excess Ar might be present in the analysed micas from the contact zone, citing as evidence the wide range of ages obtained from the sill. It may also be criticized as being the mean of two biotite and one whole-rock determination; it would be preferable to use 19o m.y., the biotite mean. In any case the stratigraphical assignment contains an element of uncertainty, that is, the correlation of the sill with the Watchung basalt. The other Upper Triassic dated horizon, the Chinle Formation, is subject to the uncertainty which has been referred to above in all cases where a spread of ages are observed, that the highest age must be regarded as a minimum, except that any individual figure is itself liable to random or systematic error in either direction, the 'minimum rule' therefore being a misleading simplification. Finally of the dates quoted in P TS the Yatyrgvart intrusion is given the revised figure of 238 4- io m.y. and was stated to be pre-Scythian by Afanasyev & Rubin- shtein (1964) (cf. PTS, p. 208). The latter authors also gave an age of 228 • 3 for a Permian intrusion at Mt Zakan, northern Caucasus, so the latter figure is the one which most closely defined the base of the Trias, but is in conflict with the data of Webb & McDougall (1967). The Russian data are, however, in accord with those from the probably Upper Permian granites of New South Wales (Items 67 and 69, PTS) with dates of 223 m.y. and 238 m.y. The new data of Webb & McDougall (i967: Items 357 and 358, this volume) provide an indirect minimum age for at least part of the Middle Triassic, if it is accepted that the Brooweena Formation of Queensland is of Middle Triassic age. Their data, on granites belonging to a suite of which some members cut the Brooweena Formation (although none of the analysed samples are specifically said to be from such geologically-dated intrusions) appear to be very satisfactory, eleven biotites and two hornblendes giving a spread of K-Ar ages from 207 to 226 m.y. (mean, 217 4- 2.5), in agreement with a Rb-Sr whole-rock isochron of 218 4- 16 m.y. (t~ = 4"7 • lO1~Y). These figures are supported by K-Ar data for two hornblendes (218 m.y.) on intrusions cutting the Ipswich Coal Measures, the stratigraphical assignment of which has variously been from low Middle Triassic to Upper Triassic. Webb & McDougall regarded 235 4- 7 m.y. as the best estimate of the base of the Triassic, using the Upper Permian biotite K-Ar age of 239 m.y. (their own analysis) as support, together with maximum ages (K-Ar) of 21o 4- IO m.y. for the Middle Upper Triassic and 238 4- 2o m.y. for the Middle Triassic (quoted from Russian work on Vietna- mese intrusions). The Russian and Queensland data appear to be at variance with the data from the Predazzo granite (Borsi & Ferrara 1967; Item 36I, this volume). This granite, i8 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

The pre-Pleistocene Phanerozoic time-scale- a review which is stated as being of post-Karnian age, gives a mineral and whole-rock Rb-Sr isochron of 229 + 3 m.y. (original data recalculated using the least- squares-cubic method). This is higher than any other well-established figure from any igneous complex occurring so late in the Triassic and makes some of the potassium-argon figures for this age range look low. The isochron itself seems satisfactory, but a little doubt must be cast upon its reliance; there is a scatter of figures from nearby rocks (see Item 36 I, this volume). If correct, it would necessi- tate a substantial revision of ideas about the Permian-Jurassic age-range. It does not appear likely that all these figures in the range 2 IO to 240 m.y. can in fact be reconciled on the basis of the data as at present stated, the most anomalous figures being those for the Predazzo granite and for the Stanthorpe granite (Item 69, PTS) at 223 m.y. The latter granite, though regarded as Permian, could be Triassic (see P TS, stratigraphical comment) and it is perhaps best to suggest that the Webb & McDougall estimate of 235 4- 5 m.y. for the base of the Triassic be accepted, the data of Borsi & Ferrara perhaps indicating that the upper limit of this range is more nearly correct.

4. The Palaeozoic era (A) THE PERMIAN PERIOD The documentation of the Upper Palaeozoic is of an entirely different and more satisfactory order from that of the Mesozoic, but unfortunately, with tcchniques as at present used, analytical error begins to loom large and become progressively more important at about this level in the stratigraphical column. This results in a general blurring of all boundaries and makes the interpretation and correlation of any results, except the most definitive, vague. Simultaneously, problems of long- term geochemical effects and the increasing probability of subsequent thermal events become important, usually evidenced in discordant patterns from single complexes. It is to be expected, therefore, that the Palaeozoic scale will be in- herently uncertain and incapable of refinement. Another factor at present important, but one day, it is to be hoped, resolvable, is the decay constant of 87Rb. The uncertainty attached to this constant is discussed in Appendix I: the 6 per cent range in values at present in use corresponds to 13 m.y. at the end, and 34 m.y. at the beginning, of the Palaeozoic, causing a significant problem in the correlation of Rb-Sr and K-Ar data. Another difficulty attached to Rb-Sr results on minerals, especially those stated in P TS, is that unless the initial sTSr/S6Sr is independently determined, there may be an uncertainty in the age, depending on the Rb/Sr ratio in the mineral. In the calculation of an Rb-Sr age, sTSr/S6Sr is usually assumed to be o.711 unless directly determined otherwise, but values of o'7o5-o'72I have been found in the course of studies at Oxford on British igneous rocks. In a 300 m.y. old rock this uncertainty of 4-0.005 represents an uncertainty of 4 per cent in the age of a mineral with Rb/Sr = I o, a not uncommon ratio. In evaluating the uncertainty in the data in PTS due to this effect, simple proportion will generally suffice. The Permian data are fortunately free from the Rb-Sr problem and those which are available are moderately consistent. Before the granites which occur near the I9 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Permo-Triassic boundary, referred to in 3(c) above, the first useful age is 238 m.y., that of the Solikamsk sylvite (Item 53, PTS), which may be low by reason of recrystallization and loss of radiogenic calcium. It sets, however, a useful minimum for the upper part of the Lower Permian (Kungurian) and helps confirm the general Permian pattern. The remaining Lower Permian determinations are all moderately suspect in one way or another: the Berkley latite determination (252 m.y. on low-K feldspar) is inconsistent with the data from an underlying, radiometrically younger, latite, which is automatically dubious on account of argon loss from feldspars; the Drammen granite age (259 4- 7 m.y.) is suspected by its authors and the age of the 'Oslo nordmarkite' (259 4- 5 m.y. by 93su/2~ is also regarded as a minimum. Russian evidence is of little help, for apart from whole-rock K-Ar determinations, Permian results quoted by Afanasyev & Rubinshtein (i964) included: from the Upper Carboniferous-Lower Permian Kalbinian complex 271 m.y.; the post-Lower Permian Kyzylkian massif 284 m.y. ; and 304 m.y. for the Upper(?) Permian "trachyliparitic porphyry of Eastern Kazakhstan". They also quoted ages up to 320 -t- IO m.y. for biotite from the Bektanat massif, part of the group of leucocratic granites of Central Kazakhstan, but this may be an Upper Carboniferous intrusion. A more important figure is that of Rosenkranz, Monich & Kovleva (i964) for biotite from the riebeckite granite porphyry of the Kyzylkiy Mountains (276 4- IO m.y.), a minimum age for the Permo-Carboniferous boundary. There appears to be a general lack of precision in all the evidence concerning the Lower Permian, extending in part into the Upper Carboniferous: a definitive age for the Oslo igneous suite would be very welcome in helping eradicate the confusion 1. The problem of the base of" the Permian is considered below in the light of the Carboniferous data.

(B) THE CARBONIFEROUS PERIOD The absolute chronology of Carboniferous rocks is better known than that of any other pre-Tertiary system, due to the close relationship of igneous activity of Hercynian/Appalachian affinity with the Coal Measures and related strata. The main problem (apart from the difficulties attached to experimental error) concerns the palaeontological correlation of the main Carboniferous terrestrial facies. The younger limit of the Carboniferous was set by Francis (PTS) as 280 m.y. on the basis of data from the Castro Daire granite, the Sande essexite and the Brassac tufts. Another useful figure has been published by Bonhomme (1966) who quoted 308 4- 7 m.y. as the age of the Bilstein and Br6zouard granites, of which veins from the latter cut Westphalian C or D sediments, [the writer considers that Bonhomme's data indicate an age of 3o6 -+- 7 m.y. rather than the above figure (see Item 356, this volume) using the same half-life (4"7 • IO1~Y)]. The definition of the stratigraphical position and the correlation of the veins with the granite are both a little weak, but otherwise this minimum age seems of high quality. If the

1 S. Moorbath (personal communication) reports a K-Ar age of 274 4- 8 m.y. for biotite from a Larvik foyaite: this figure is significantly higher than the above results.

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87Rb half-life is correct the data confirm the general level of the Permo-Carbonif- erous boundary at 28o m.y., though it could be argued that the Stephanian plus a small part of the Westphalian is hardly likely to be 26 m.y. long, a subjective matter, as thicknesses of sediments are not of much use in dividing the Carbonif- erous. Reliance on the three ages mentioned at the beginning of the last paragraph depends, as is often the case, on the general agreement of three individually unreliable sets of figures. The objections to acceptance of these three figures are as follows: (i) Sande 'essexite': biotite from lava hornfelsed by the essexite 284 m.y., supposed Lower Permian age; a single determination on material correlated (and not directly related) with stratigraphically controlled equivalents. (ii) Brassac tufts: three (one partly) duplicated biotite Rb-Sr ages (273, 286, 297, weighted mean 288 m.y.) ; half-life problem; initial 87Sr/S6Sr not known and significant (Rb/Sr ___ 5) ; and poor replication of the age. (iii) Castro Daire granite: three duplicated biotite Rb-Sr ages (278, 274, 290 m.y., mean 281 m.y. (given as 282 in PTS); indirect stratigraphical assignment; half-life problem; and poor replication of age. The initial 87SrS6/Sr is not a difficulty in this case, but the data are incorrectly stated in PTS, p. 35 o. The quoted ages are not in the most likely stratigraphical order, but would become so if a half-life of 5.0 • lO1~ y is used for 87Rb: In such a case the arguments for the age of the Permo-Carboniferous boundary would rest on:

Sande biotite hornfels, 284 m.y., Lower Permian Castro Daire granite, 299 m.y., post-Stephanian B (?C) Brassac tufts, 3o6 m.y., Stephanian C (?) Br6zouard granite, 326 m.y., post-Westphalian C (?D)

--in which case it would be tempting to quote the probable age of the boundary as 295 to 3o0 m.y., bearing in mind the fundamental inaccuracy to be ascribed to the Brassac tuff age. The abundance of data for the Carboniferous in general is not accompanied by proportional certainty concerning any of its internal boundaries or its base. Indeed, Afanasyev preferred 34 ~ m.y. and Rubinshtein 36o m.y. for the base (Afanasyev & Rubinshtein 1964) basing their arguments on both K-Ar data using constants giving ~4 per cent higher ages than in PTS and Rb-Sr data using the 4"7 • 1~176Y half-life. The published Carboniferous data suffer from a wide range of draw- backs, ranging from analytical uncertainty [as for the Gien-sur-Cure 'granite', which is weighted favourably by Francis (PTS, p. 229) ] to the usual stratigraphical problems. The great majority of known Carboniferous intrusions and volcanics give ages in the range 3oo to 34 ~ m.y., so the broad picture seems clear, but no single determination is really good enough to define any stage boundaries. The age of the base of the Carboniferous presents an interesting problem, on which new data (Cormier & Kelly 1964; McDougall, Compton & Bofinger 1966 ) cast some light. The data from the Fisset Brook Formation, Nova Scotia (Cormier

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& Kelly I964) consist of Rb-Sr whole-rock measurements on two groups of volcanics, both giving the same age (358 m.y., 4"7 • Iol~ Y half-life), though only one group is directly stratigraphically controlled. The isochron plots are, however, by no means perfect and some failure of the closed-system assumption must be suspected. As the stratigraphical position is only known as pre-Mississippian (? lowest Mississippian) it does not resolve the 34 ~ to 360 m.y. problem, taken by itself. The other set of new data, from the volcanics of the Cerberean Cauldron of Victoria, Australia (McDougall et al. 1966 ) strongly reinforces the view that Cormier and Kelly's data give something closely approximating to the age of the Devonian-Carboniferous boundary. McDougall, Compston & Bofinger reported K-Ar mineral and Rb-Sr whole-rock and mineral ages, summarizable as follows (see Item 354, this volume): Cerberean Volcanics K-Ar mineral ages 345 m.y., 358 and 362 m.y. Rb-Sr whole-rock and feldspar isochrons 357 4- IO, 365 4- 9 ~ m.y. Snobs Creek Volcanics K-Ar mineral age 348 m.y. Rb-Sr whole-rock isochron 367 :k 22 m.y. The Snobs Creek Volcanics form a continuous sequence via the Taggarty Group (with U. Devonian fishes) with the overlying Cerberean Volcanics. The discrepant mineral ages from the Cerberean Volcanics were regarded by the authors as due to argon loss from fine-grained biotites, but no explanation is available for the older Snobs Creek Volcanics discrepancy. The authors recommended that the age of the whole group be taken as 362 -+- 6 m.y. and that an 87Rb half-fife of 4.85 • IO 10 m.y. would give the best agreement between their K-Ar and Rb-Sr results. The age of 362 4- 6 replaced the 348 m.y. figure hitherto accepted for the Snobs Creek Volcanics and the suggestion was made that the age of the base of the Carbonifer- ous should be at least 36o m.y. This suggestion is, however, subject to acceptance of the long-range fish-fauna correlation and the value of the half-life used in these cal- culations: it does, however, seem reasonable and therefore, as a round figure, 36o m.y. seems better than 34 ~ m.y.

(c) D voN A PERIOD Despite (or possibly because of) replicate analyses on most of the samples critical to the age of the Devonian, the increasing absolute value of analytical error and the peculiar geological circumstance that much of the dateable material is associated with continental sediments, combine to make the final picture of the Devonian (excepting the Upper) very indistinct. Of the earlier evidence, that from the Calais granite, Maine (Item 5, PTS) would appear to be the most reliable for fixing the base of the Devonian: if taken by itself it would indicate a minimum age of4o 4 4- 8 for that boundary, a figure which is not contradicted by any other evidence. In P TS Friend & House have noted some uncertainty in the stratigraphical evidence concerning the Calais granite and prefer to take the mean figure for the other probably post-Silurian granites (Shap, Creetown, Snowy River). Of these only Snowy River is closely placed with any certainty in the succession, but its age is

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only unfortunately known by one K-Ar biotite determination. There would appear to be a very good case for further investigation of both the Calais and Snowy River granites: the information from them should prove most useful, within the limitations imposed by their plutonic character. The adoption of 36o m.y. as the base of the Carboniferous would make it less likely that 395 m.y. is the correct figure for the base of the Devonian, on grounds of the sediment thickness and sedimentation rates as compared with the Carboni- ferous (see Hudson, PTS p. 4o). The otherwise anomalous Kap Franklin granite age of 393 4- IO m.y. for the Givetian (PTS item I) would fit well into an older Devonian period, as would the evidence from Spitsbergen (Gayer, Gee, Harland, Miller, Spall, Wallis & Winsnes 1966) where pre-Devonian igneous and metamor- phic events, excepting chloritization of biotite, seem to have terminated in the range 42o to 4oo m.y. Any increase in the 87Rb half-life would further increase the likelihood of such a conclusion being correct. The half-life uncertainty also prevents the drawing of any firm conclusions from the work of Bottino & Fullagar (1966), who established whole-rock isochrons of 388 • 5 and 389 • IO m.y. for probably Lower Devonian Volcanics from Maine (Item 355, this volume). The stratigraphical assignment of these volcanic rocks is not as precise as could be wished, appearing to be within the Lower Devonian. The Devonian data as a whole are consistent with the suggestion that the Kulp (i96i) figure of4o 5 m.y. is preferable to any younger age for the base of the Devonian. None of the intra-Devonian ages appear to be particularly critical, except the Bear River and Nictaux (Nova Scotia) and Jackman (Maine) intrusions. Un- happily many of the results from these possess high individual errors and together show a fairly wide spread. Further investigation of these might tie down the base of 9of the Middle Devonian more closely, but the most promising course would be study of new material (Friend & House, PTS, p. 235).

(D) THE LOWER PALAEOZOIC ERA Little can be written about the poverty of the situation in the Lower Palaeozoic which has not already been stated. There have been three interesting developments since PTS which clarify the situation a little, but none is conclusive. The work on the Ballantrae igneous complex, Scotland by Harris, Farrar, MacIntyre, York & Miller (i965) (Item 35 o, this volume) confirms the general view that the Arenig probably lies in the range 475 to 500 m.y. Their data however show a slight scatter and there is the further difficulty that the complex is badly exposed and the stratigraphical relationships exceedingly difficult to deter- mine. The contribution by McCartney, Poole, Wanless, Williams & Loveridge (1966) (Item 352, this volume) is extremely valuable in providing excellent data on the Holyrood granite, Newfoundland, which is certainly Pre-Cambrian, is unmeta- morphosed and of comparatively low age. Their figure, 574 4- I I m.y. 1 seems very a The authors suggest that 15 m.y. may have elapsed between the intrusion of the Holyrood granite and the beginning of the Cambrian. This unknown interval weakens the usefulness of the data. 23 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

R. St J. Lambert good as a maximum for the base of the Cambrian, remembering the problem of the half-life of SFRb. Rose (1967) , however, cast doubt on the reliability of the stratigraphical and structural evidence concerning the Holyrood granite. The base of the Cambrian at Hoppin Hill, Massachusetts, has been investigated by Fairbairn, Moorbath, Ramo, Pinson & Hurley (i967, Item 353, this volume) who concluded that the base of the local Cambrian (within the Lower Cambrian) should be at 54o • 20 m.y. (recalculated using t89= 4"7 • lO1~Y). Their data, consisting of three whole-rock isochrons from possibly comagmatic igneous bodies, are not internally consistent (the isochrons give 486 :k 17, 539 4- 4 and 56o 4- 28 m.y.) : the high error of the 56o m.y. isochron, allied with a belief that weathering has produced a low age for the 486 m.y. body, led them to suggest that 54 ~ 4- 2o m.y. is the most reliable estimate for the age of the whole group and, therefore, a maximum for the base of the Cambrian. This is in general agreement with the Holyrood granite data, but sets a closer limit than the latter, the half-life restric- tion, of course, applying to this 54 ~ m.y. figure also. Within the Lower Palaeozoic the data previously available are generally so poor that it is difficult or impossible to propose any divisions. It is commonly agreed that the upper Ordovician (Caradocian) is fairly well fixed at 447 m.y., but the data show more spread than is desirable. The two puzzling features which emerge are low K-Ar biotite ages from Carters River Limestone (419 m.y.) and the spread in the Rb-Sr ages on biotite from the same horizon. It is not possible to assess the latter spread, as the necessary data have not been published, except for the biotite from the Wenonah no. 8 mine, which has low common Sr and an age, therefore, which is not critically dependent on the sTSr/SeSr (initial), There is close agreement between the *3sU/*~ ages of zircons from bentonites in the Carters River Lime- stone, Rb-Sr ages on biotites from the same horizon (using the 4"7 half-life) and the K-Ar ages on sanidine and biotite from close stratigraphical equivalents in Sweden. In the event of adoption of a higher SVRb half-life, the question of the real age of these strata would have to be re-opened, with loss of radiogenic argon or lead in mind, as there is a slight scatter in the relevant ages which may indicate such a possibility. Other Lower Palaeozoic ages are of little relevance.

5. Conclusions The application of fairly stringent criteria to the selection of the most reliably dated pre-Tertiary horizons reduces the number to a mere handful (Appendix 2). In the above discussion reasons were advanced for disregarding all pre-Oligocene glauconite ages; whole-rock and low-K-feldspar K-Ar ages were also found to be generally too low. The proportion of such cases which agreed with the most probable time-scale was too low (about one-third) for use to be made of the figures in constructing a scale. Many of the early Rb-Sr determinations on minerals are too imprecise through lack of knowledge of the isotopic composition of the initial strontium, while all Rb-Sr ages are at present affected by the uncertainty in the half-life. It is possible to draw up a list of the apparently most reliable ages using ~4 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

The pre-Pleistocene Phanerozoic time-scale- a review the above criteria for rejection together with elimination of all single (not duplicated) analyses. The reason for rejecting unsupported single figures, even where fitting in with other evidence, lies in the frequently occurring spread between replicate figures, particularly found in the Palaeozoic examples considered in P TS. The peril of accepting unsupported analyses is clearly illustrated by the recent change in date of the Cerberean Volcanics from 348 to 362 m.y. Accordingly, in Appendix 2, the 'most reliable' list is supplemented by a second list of otherwise apparently reliable single determinations.

TABLE I : Recommended version of the time-scale Age to basO (x) (2) (3) Tertiary: Pliocene 7 7 7 Miocene 26 26 26 Oligocene 38 38 38 Eocene 54 54 54 Palaeocene 65 65 65 Cretaceous: Upper 95 95 95 Lower 135 ? 135 ? 135 ? Jurassic 195 ? 200 ? 2o 5 ? Triassic 235 240 ? 245 ? Permian 275 280 29o Carboniferous 360 370 38o Devonian 405 415 ? 43o ? Silurian 435 ? 445 ? 460 ? Ordovician 5 ~ ? 515 ? 53 ~ ? Cambrian 57 ~ ? 590 ? 61 o ?

1 Rounded off to 5 m.y. for all pre-Tertiary ages. Columns (I), (2) and (3) are for STRb half-lives of 4"7, 4"85 and 5"0 • Iol~ respectively. Scale 2 is recommended by the writer. ? indicates substantial uncertainty on the basis of existing figures. See Appendix 2 for a different Tertiary scale.

The shortness of the most reliable list does not carry the implication that the 1964 time-scale is itself unreliable. The limitations of the time-scale have long been recognized and the approximations and assumptions necessary to produce a scale are realized. In the above discussion, some suggestions have been made concerning modifications to the adopted boundaries, and these are adopted in Table I which gives a preferred scale with variations for various half-lives of STRb. The present situation regarding the development of the scale is clear enough: there is a shortage of raw material suitable for its further refinement, but progress could be made by wider application of Rb-Sr analysis, particularly using refined techniques on whole-rock systems. Comparatively little use has been made of K-Ar measurements on hornblende, which can provide good information, particularly where several samples are analysed and compared with mica ages. The present scale and that of the Geological Society, 1964 (P TS), are essentially K-Ar biotite time-scales.

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ACKNOWLEDGEMENT.The author wishes to acknowledge, with thanks, useful discussions with his colleagues at Oxford, in particular S. Moorbath and N. J. Snelling, who contributed materially to revision of the text; also N. Rast for providing translations of relevant Russian material and W. B. Harland and E. H. Francis for abstracts and references.

6. References AYANASVSV, G. D. & RUBINSHTEIN, M. M. 1964. Geochronological scale of absolute age determination based on data of the U.S.S.R. laboratories up to April 1964 with a consideration of foreign data. [Scale given in Afanasyev, Rubinshtein and others (1964), q.v.] , -- & others, 1964. Absolute age of geological formations. ~2nd Int. geol. Congr. DoM. soy. geol. no. 3, PP. 287-324. BONHOMm~, M. 1966. Age, par la methode au strontium, de quelques granites des Vosges moy- ennes. Geochronologie Absolu. Actes du 15 Ie Golloque International du G.N.R.S., Nancy 1965 (published 1966), pp. 387--94 . BORSI, S. & F~RRAR~, G. I967. Determinazione dell'eta delle rocce intrusive di Predazzo con i metodi del Rb/Sr e K/Ar. Mineralog. petrogr. Acta 13, 45-66. BOTTINO, M. L. & FULLAGAR, P. D. 1966. Whole-rock rubidium-strontium age of the Silurian- Devonian boundary in Northeastern North America. Bull. geol. Soc. Am. 77, I 167-76. CORMIER, R. F. & KELLY, A. M. 1964. Absolute age of the Fisset Brook Formation and the Devonian-Mississippian boundary, Cape Breton Island, Nova Scotia. Can. Jl Earth Sci. x, 159-66. CURTIS, G. H., EVERNDEN, J. F. & LIPSON, J. i958. Age determination of some granitic rocks in California by the potassium-argon method. Spec. Rep. Calif. St. Min. Bur. 54. EVERNDEN, J. F. & CURTIS, G. H. 1965. The potassium-argon dating of late Cenozoic rocks in East Africa and Italy. Curt. Anthrop. 6, 343-85. & JAMES, G. T. 1964 . Potassium-argon dates and the Tertiary floras of North America. Am. J. Sci. 262~ 945-74. SAVAGE, D. E., CURTIS, G. H. & JAMEs, G. T. 1964. Potassium-argon dates and the Cenozoic mammalian chronology of North America. Am. J. Sci. ",62, 145-98. FAIRBAIRN, H. W., MOORBATH, S., RAMO, A. 0., PINSON, W. H. Jr. & HURLEY, P. M. 1967. Rb- Sr age of granitic rocks of southeastern Massachusetts and the age of the Lower Cambrian of Hoppin Hill. Earth & planet. Sci. Lett. 2, 321-8. FITCH, F. J. & MILLER, J. A. i967. The age of the Whin Sill. Geou J. 5, 233-5 ~ GAYER, R. A., GEE, D. G., HARLAND, W. B., MILLER, J. A., SPALL, H. R., WALLIB, R. H. & WINSNES, T. S. 1966. Radiometric age determinations on rocks from Spitsbergen. Skr. norsk Polarinst. no. 37- HARLAND, W. B., SMITH, A. G. & WILCOCK, B. (Eds) I964. The Phanerozoic Time-scale. London (Geological Society). HARRIS, P. M., FARRAR, P. M., MACINTYRE, R. R., YORK, D. & MILLER, J. A. I965. Potassium- argon age measurements on two igneous rocks from the Ordovician system of Scotland. Nature, Lond. 2o5, 352-3. HOLMES, A. I959. A revised geological time-scale. Trans. Edinb. geol. Soc. 17, I83-~I6. KULP, J. L. 1961. Geologic timescale. Science, N.Y. 133, I Io5-I 4. & ENGLES, J. 1963. Discordances in K-Ar and Rb-Sr isotopic ages. Symposium on Radio- active Dating, International Atomic Energy Agency, Vienna, ~963, pp.219-38. MeCARTNEY, W. D., POOLE, W. H., WANLESS, R. K., WILL~AraS, H. & LOWRIDGE, W. D. 1966. Rb/Sr age and geological setting of the Holyrood granite, southeast Newfoundland. Gan. Jl Earth Sci. 3, 947-57. McDoUGALL, I., COMPSTO~, W. & BOFINGER, V. M. 1966. Isotopic age determinations on Upper Devonian rocks from Victoria, Australia: a revised estimate for the age of the Devonian- Carboniferous boundary. Bull. geol. Soc. Am. 77, lO75-88. MCMULLEN, C. C., FRITZE, K. & TOMLINSON, R. H. I966. The half-life of rubidium-87. Can. d. Phys. 44, 3o33 -8. 26 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

The pre-Pleistocene Phanero zoic time-scale - a review RosE, E. R. I967. Discussion: Rb/Sr age and geological setting of the Holyrood granite, southeast Newfoundland. Can. Jl Earth Sci. 4, 746. ROS~NKRANZ, MOmCH & KOVL~VA, 1964 [Quoted by Afanasyev and others (1964), q.v.] SHAFIQULLAH, M., FOLINSBEE, R. E., BA.ADSGAARD,H., GUMMING,G. L. & LERBEKMO, J. F. I968. Geochronology of Cretaceous-Tertiary boundary, Alberta, Canada. Int. geol. Congr. 22, (3), 1--20. WEBB, A. W. & McDoUGALL, I. 1967. Isotopic dating evidence on the age of the Upper Permian and Middle Triassic. Earth & planet. Sci. Lett. 2~ 483-8. WILSON,J. A., TwIss, P. C., Dt~FORD, R. K. & CLABAUGH, S. E. 1968. Stratigraphic succession, potassium-argon dates, and vertebrate faunas, Vieja Group, Rim-Rock Country, Trans- Pecos Texas. Am. J. Sci. 266, 59o-6o4. Revised manuscript received 16 January 1969. R. St J. Lambert, PH.D.F.G.S. Department of Geology and Mineralogy, Parks Road, Oxford. Present address: Department of Geology, University of Alberta, Edmonton, Alberta, Canada.

Appendix i" The half-life ofa7Rb It has become clear in the recent past that there continues to be a strong division of opinion concerning the correct half-life of 87Rb (see, for example, the proceedings of the Edmonton conference on the Geochronology of Precambrian Stratified Rocks, Can. Jl Earth Sci. 59 I968). The half-lives in general use are 4"7 and 5-o x io 1~ y; the evidence relating to these is summarized by Moorbath (PTS, pp. 87- 89). Since 1964 there have been two developments of relevance, the one the increasing use by geochronologists of the higher half-life and the other the publication of the results of a further and original direct physical measurement (McMullen, Fritze & Tomlinson I966 ) which gave 4"72 4-0"0 4 • Iol~ These experiments, in which the 87Sr produced in pure Rb over a period of seven years was measured using an 84Sr-enriched internal standard, gave results which were in very good internal agreement, supporting the lower figure. Their data, however, cannot be regarded as conclusive, as in the measurement of 87Sr*, only the s4Sr, 87Sr and 88Sr were measured, enabling only two of the three unknowns (instrumen- tal isotope fractionation, common Sr and 87Sr*) to be found. Despite the efforts of the experimenters to eliminate common Sr from the system and provision by them of an allowance for mass discrimination, their published data are not sufficient to eliminate the possibility of substantial isotope fractionation in the analysed samples. Such fractionation would lead to incorrect estimation of Sr; any excess contaminant Sr would lead to an over-estimate of the quantity of 87Sr* produced and hence to too low a value of the half-life. Any such bias may well be small, though, as the occurrence of variable fractionation would have been revealed by inconsistent results for the half-life. It is not possible to estimate the magnitude of such effects from the statement in the paper, which does not give the necessary data, or their precision. The increasing use of 5.o is based on experience in the Pre-Cambrian and Lower Palaeozoic, where examples requiring 5.o for concordance of Rb-Sr whole rock and K-Ar mineral ages and sometimes U-Pb zircon ages are beginning to be reported. Published evidence on this point is meagre, but McDougall et al. (I 966) ~7 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

R. St 3". Lambert suggested, as noted above, a value of 4.85 • Io 1~ y. This is in conflict with the evidence collected by Kulp & Engles (I963) on ages of minerals which strongly favoured 4"7. A few of the examples quoted by Kulp & Engles suffer from some uncertainty because of lack of knowledge of initial STSr/SeSr, but not enough are defective to warrant rejecting their argument entirely. A similar pattern emerges from the data given in the symposium proceedings mentioned above (Can. Jl Earth Sci. 5, 1968), which are summarized in Table 2. The figures quoted in Table 2

TABLE 2 : Recent data comparing Rb-Sr and K-Ar ages (source: Can. Jl Earth Sci. 5, 555-772, 1968) Authors Page ref. Rb-Sr datal(m.y.) K-Ar data(m.y.) Rb-Sr/K-Ar Compston & Arriens 571 183o isochron 2 1695 biotite I.O8 571 182o isochron 1645 biotite I. I I 571 1725 isochron 1650 biotite I "05 571 1825 isochron 1760 biotite 1.04 and biotite x76o biotite I-o4 573 I8IO biotite 1775 biotite I-o2 Purdy & York 7o2 239~ isochron 248o hornblende o-96 703 173~ isochron 183o hornblende o-95 7o3 173o isochron 166o muscovite i .o4 Green, Baadsgaard & 73~ 261o isochron 247~ biotite 1.o6 Cummings 731 2575 isochron 253o muscovite I .o2 731 2575 isochron 242o biotite I-O6 Peterman & Hedge 753 I42o isochron 138o biotite 1-o3

All using 8TRb t89~ 5"o • I o TM y. 2 All isochrons whole-rock. are selected from the symposium volume, excluding glauconites, sediments and whole-rock K-Ar samples and all problematic cases (e.g. from polymetamorphic environments with obvious evidence of incomplete retention of one or the other apparent age). Excluding the two hornblendes given in Table 2 and the one exceptional ratio of i.i i, the remainder give an average value for Rb-Sr age/K- Ar age of I-o 4 using 87R.b t89= 5.0 • IO 1~ y, suggesting that one or both half- lives used are in error or that there is always a general lowering of K-Ar mineral ages vis-a-vis Rb-Sr ages. These data tend to confirm the conclusion of Kulp & Engles (I963). It may be observed, however, that this evidence could be held to indicate regular argon loss from micas, up to about 5 per cent. The evidence from natural sources therefore remains conflicting and the evidence from experiments subject to some uncertainty. The author is informed by his colleagues (N. H. Gale and R. D. Beckinsale) that some reasons are likely to emerge for altering the half-life of 4~ in such a manner as to reduce the inconsis- tencies (by I to 2 per cent) between the K-Ar and Rb-Sr ages reported above. Further geological evidence is also provided in the Scottish Dalradian complex, where R. J. Pankhurst (in press) finds it impossible to combine satisfactorily all K-Ar and Rb-Sr data if any half-life value for 87Rb less than 4.85 is used. Taking all factors into consideration, it would seem that a value of 4.8 5 • IO 1~ y for the half-life of 87Rb would give amenable results, with no necessity for artificial 28 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

The pre-Pleistocene Phanerozoic time-scale- a review interpretations of unexpected discordances between ages obtained by different methods. The effect of adopting a higher half-life on The Phanerozoic Time-scale (1964) would be to raise the age of the Carboniferous and Devonian, to confuse the Ordovician situation and to raise the maximum age of the base of the Cambrian. In Appendix 2 (A) the effect of a 4.85 • Io 1~ y half-life is noted in brackets against each Rb-Sr age. Appendix 2: List of critical points

Cretaceous Pembina bentonite, Alberta; Maestrichtian; K-Ar sanidine, 6 4 -4- I m.y. Kneehills tuff, Alberta; Maestrichtian; K-Ar sanidine, 65 4- 2 Bearpaw shale bentonite, Alberta; Campanian; K-Ar biotite, 74 4- 2 sanidine, 76"5 4- 3 Crowsnest Volcanics, Alberta; ? U. Albian; K-Ar K-feldspar, 9 ~ 4- 5 Coleman bentonite, Alberta; ? U. Albian; K-Ar sanidine, 92 4- 3 Mill Creek bentonite, Alberta; ? U. Albian; K-Ar biotite, 99 4- 5 sanidine, IOI 4- 5 Mowry Shale bentonite, Wyoming and Montana; ? U. Albian; K-Ar biotite, 91 4- ? sanidine, 95 4- ? Jurassic Kelasury granite, Georgia; post-Bajocian; K-Ar biotite, 165 4- 41

Triassic Palisades sill, N.Y. ; Upper Trias ?; K-Ar biotite, 186 + 5, 194 4- 5 Various intrusions, Queensland; post-(part of) Middle Trias; K-Ar biotite and hornblende, 2 I 7 4- 2"5 Rb-Sr whole-rock 218 -t- 16 [225] 2

Triassic or Permian Stanthorpe granite, N.S.W.; in range U. Permian to M. Trias; K-Ar, biotite 225 4- ? hornblende, 221 4- ? 1 Weighted mean. 2 [ ] age using 4"85 • IOa~ Y half-life. 29 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

R. St 3". Lambert

Permian Gyranda volcanics, Australia; uppermost Permian; K-Ar biotite, 239 4- ? m.y. Carboniferous Br6zouard and Bilstein granites, France; Westphalian D; Rb-Sr whole-rock and minerals, 3o6 4- 7 [3 I5] 2 Fisset Brook Formation, Nova Scotia; pre-base of, or lowest, Mississippian; Rb-Sr whole rock, 358 4- 15 [369] ~ Devonian Cerberean Volcanics, Australia; Famennian ? K-Ar biotite, 362 4- 6 Rb-Sr whole-rock and minerals, 359 4- 15 [37 ~ ~ 367 4- 22 [378 ] Shiphead bentonite, Quebec; Siegenian; K-Ar biotite, 38I 4- I8 sanidine, 388 4- 12 Calais granite, Maine; post-lowest Devonian ?; K-Ar biotite, 4o4 + 8 Eastport and Hedgehog Formations, Maine; L. Devonian ?; Rb-Sr whole-rock, 388 4- 5, 389 4- IO [400, 401] 2 Ordovician Carters R. Limestone bentonite, Tennessee and Alabama; Caradocian; U-Pb zircon, 447 4- 5 Rb-Sr biotite, 447 4- 7 [46~ ~ (K-Ar biotite, 419 4- 5) Kinnekulle bentonite, Sweden; Caradocian; K-Ar biotite, 44 ~ 4- ? sanidine, 442 4- ? (?447) Pre-Cambrian Holyrood granite, Newfoundland; Late Pre-Cambrian; Rb-Sr whole-rock, 574 4- ii [591] 2

(B) MOST RELIABLE (PRE-TERTIARY) DETERMINATIONS WITHOUT DUPLICATE OR SUPPORTING ANALYSES Cretaceous Cache Creek bentonite, California; ? Cenomanian; K-Ar biotite, 96 4- ? m.y. Jurassic Horseshoe Bar diorite, California; post L. Kimmeridgian; K-Ar biotite, 136 4- 4 Hotailuh batholith, British Columbia; near Jurassic-Triassic boundary; K-Ar biotite, I93 ? 4- ? 3~ Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

The pre-Pleistocene Phanero zoic time-scale- a review

Permian 'Essexite', Sande, Norway; ? L. Permian; K-Ar biotite, 284 4- ? Granite porphyry, Kyzylkiy, Kazakhstan; post-Carboniferous; K-Ar biotite, 276 4- ? Devonian Snowy River granite, Australia; L. Devonian or Upper Silurian; K-Ar biotite, 394 4- ?

NOTE ADDED IN PROOF

The Siluro-Devonian boundary

The remarks on lack of data for the Silurian (above) are to be modified in the light of the work of Bofinger, Compston & Gulson (197o). Their data indicate an Rb-Sr age (5.o • I O1~ y half-life) of 438 4-4 m.y. for post-Lower Ludlovian intrusives and extrusives at Mount Painter, near Canberra. There is, unfortunately, a slightly greater scatter of the analytical data than that due to analytical error alone, but the authors believe that the figure of 438 4- 4 m.y. is nevertheless a better minimum age for the Lower Ludlovian than any other. Quoting the work of Fullagar & Bottino (1968) they assign an age of 445 4- 7 m.y. to the Upper Llandoverian Stage Circle shale, utilizing their own new data in conjunction with Fullagar & Bottino's estimate of the duration of the Silurian. On the basis of these data, the Siluro-Devonian boundary should not be far from 430 m.y. (5.0 4- io 10 y half-life), 415 m.y. (4.85 half-life) or 405 m.y. (4"7 half-life), to the nearest 5 m.y.

References BoFmoER, V. M., COMPSTON,W. & GULSON,B. L. I97o. A Rb-Sr study of the Lower Silurian Stage Circle Shale, Canberra. Geochim. Cosmochim. Acta 34, 433-46. FULLAOAR,P. D. & BOTTmO, M. L. x968. Rb-Sr whole-rock ages of Silurian-Devonian volcanics from Eastern Maine. Trans. Am. geophys. Un. 49, P- 346.