Journal of the Geological Society, London, Vol. 155, 1998, pp. 421–422. Printed in Great Britain.
Discussion on estimates of the amount and rate of sea-level change across the Rhaetian–Hettangian and Pliensbachian–Toarcian boundaries (latest Triassic to Early Jurassic)
Journal, Vol. 154, 1997, pp. 773–779
J. C. W. Cope writes: Hallam’s (1997) paper provides some Further analysis of Howarth’s details of the succession reveals interesting ideas about sea-level change in the Early Jurassic. that the first bed belonging to each of the three ammonite His accordance of equal durations to ammonite zones and horizons (Beds 33, 35 and 37, and additionally Bed 40) subzones may distort the picture somewhat, but I would agree contains calcareous concretions. It could well be that some that it does allow the overall calculations to be in the right non-sequence is represented within these concretionary levels, order of magnitude. as each of the first three coincides with a faunal change and One of the subjects which Hallam discusses is an estimation this could well explain the missing time. In any event, the time of the time taken to deposit the Yorkshire Jet Rock. On the differences involved (a factor of four times) are not major basis of the fact that it lasts one ammonite subzone, equal when the approximation involved in equalization of the time division of the Gradstein et al. (1994) time-scale would give it involved in each subzone is taken into account. a duration of c. 630 000 years, as shown by Hallam (1997), but There is an interesting comparison here with another Hallam’s calculations from laminations suggested 3.5 Ma— a Jurassic laminated organic clay, the Oxford Clay. The factor close to five times the other, as noted by Hallam (1997, Oxford Clay ammonite fauna was studied in great detail by p. 776). Brinkmann (1929) through some 13 m of the clays; he The most recent account of this succession of laminated recognized within this thickness some 27 ammonite horizons. bituminous shales, which total 7.53 m thick in the Whitby Cope (1993, p. 261) calculated by using the time-scales of area, is by Howarth (1992). Hallam (1967) concluded that the Hallam et al. (in Snelling 1985) and Harland et al. (1990) that laminae were probably annual, and in the same paper recorded a Callovian horizon lasted on average <61 500 or <43 000 the organic laminae as about 20 ìm in average thickness and years respectively. If now, for direct comparison with the the interlaminated clays as varying from 20 to 50 ìm. The figures of Hallam (1997), the time-scale of Gradstein et al. figure Hallam (1997) quotes for the average thickness of a (1994) is used, the duration of the Callovian is 5.3 Ma bituminous shale/clay couplet is 50 ìm. In a thickness of the (top=154.1&3.2; base=159.4&3.6). The 17 subzones of the 7 m he calculated that there would be 3 500 000 such couplets Callovian would then each have a mean duration of 311 765 (p. 776), which led him to dismiss the idea that the couplets years, a figure about half that of the Toarcian. Four subzones, could be annual as their number exceeded the time calculated which include the 13 of Brinkmann’s horizons which can be by a factor of five. However, the true figure of couplets of this directly linked to the modern subzones, would then have lasted mean thickness in 7 m would be 140 000, and not 3 500 000. c. 1 247 060 years and each horizon would have lasted Taking Howarth’s (1992) figures for the thickness and c. 54 220 years—a figure remarkably similar to the Toarcian Hallam’s (1997) figure of 50 ìm, a figure of 150 600 results. It ammonite horizons calculated from the laminations, and sug- is thus clear when Hallam’s arithmetical error is corrected that gesting that, for the Jet Rock at least, these may well be more the total number of laminae is less than the time in years, reliable indices than the straight division of time used for allowing the possibility that the laminae could indeed be comparison by Hallam (1997). annual. What about the disparity of over four times when compared 5 September 1997 with the mean duration of an ammonite subzone? It could well be that the exaratum Subzone was of shorter duration, but A. Hallam replies: I am gratified that Cope considers that my there is an alternative explanation. tentative estimates of the amount and rate of sea-level change Howarth (1992, p. 11) lists detail of the Whitby Jet Rock to be of the right order of magnitude. My reinvestigation of the succession (Beds 33–40 of Howarth 1992). It all belongs to the Jet Rock laminae formed only a minor part of my paper, and Cleviceras exaratum Subzone of the Harpoceras falciferum is in no way crucial to the more general age estimates, but is Zone. Examination of Howarth’s ammonite faunal lists shows nevertheless a matter of great interest. Cope prefers my earlier that it is possible to recognize a series of ammonite horizons interpretation, that these laminae are annual, and cites in within this succession. Thus Beds 33–34 (2.75 m) comprise a support Howarth’s recent monograph, which I had over- horizon characterized by Eleganticeras elegantulum; Beds looked. By focusing on Howarth’s ammonite horizons he 35–36 (2.43 m) are characterized by Cleviceras exaratum, makes a plausible case for an annual origin, and I would be whilst Beds 37–40 (2.35 m) comprise the horizon of Cleviceras delighted if this could be confirmed, because the potential elegans. Howarth (1992, p. 6) noted these faunal subdivisions significance for the currently fashionable high-resolution of the subzone, but did not accord them horizon status. It will stratigraphy is immense. The main problem is of course be noticed that each of these horizons is c. 2.5 m thick and on the inadequacy of the radiometric timescale. The quality of the basis of the lamination being annual and an average of the best available timescales improves considerably from 50 ìm thick, each horizon would have lasted c. 50 000 years. Cenomanian times onwards, so studies of laminated shales
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within the last 100 million years, hopefully in conjunction with G, F. M., A, F. P., O, J. G., H, J., V V, P., purported or presumed Milankovitch cycles, could prove very T,J.&H, Z. 1994. A Mesozoic time scale. Journal of Geophysical Research, 99, B12, 24 051–24 074. illuminating. H, A. 1967. An environmental study of the Upper Domerian and Lower 26 September 1997. Toarcian in Great Britain. Philosophical Transactions of the Royal Society, London, B252, 393–445. —— 1997. Estimates of the amount and rate of sea-level change across the Rhaetian–Hettangian and Pliensbachian–Toarcian boundaries (latest References Triassic to early Jurassic). Journal of the Geological Society, London, 154, 773–779. B, R. 1929. Statistisch-biostratigraphische Untersuchungen an H, W. B., A, R. L., C, A. V., C, L. E., S,A.G.& mitteljurassischen Ammoniten über Artebegriff und Stammesentwicklung. S, D. G. 1990. A geologic time scale 1989. Cambridge University Press. Abhandlungen der Gesellschaft der Wissenschaft, Göttingen, H, M. K. 1992. The ammonite family Hildoceratidae in the Lower mathematische und physische Klasse. N. F. 13. Jurassic of Great Britain. Monograph of the Palaeontographical Society, C, J. C. W. 1993. High resolution biostratigraphy. In:H,E.A.& London. K, R. B. (eds) High Resolution Stratigraphy. Geological Society, S, N. J. (ed.) 1985. The chronology of the geological record. Geological London, Special Publications, 70, 257–265. Society of London, Memoir, 10.
Scientific editing by Duncan Pirrie.
J C. W. C, Department of Earth Sciences, University of Wales Cardiff, PO Box 914, Cardiff CF1 3YE, UK (e-mail: copejcw@cardiff.ac.uk). A. H, School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (e-mail:[email protected]).
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