Troublesome definition of the LowerIMiddle Jurassic boundary

G. E. G. WESTERMANN Department of Geology, McMaster University, Hamilton, Ont., Canada L8S 4Ll

Received April 24, 1979

Revision accepted June 22, 1979

Recent North American usage of the series boundary is divided: (1) following English tradition it is placed between the Toarcian and the Bajocian s.1. stages; (2) following central European tradition it is placed at the same position, but with the distinction of the Aalenian stage = Lower Bajocian s.1. ; (3) following the resolution of the 1st Luxembourg Colloquium it is placed at the AalenianlBajocian s.str. boundary. Subsequent resolutions of the Cassis Meeting and, equivo- cally, of the 2nd Luxembourg Colloquium, however, rescinded the third usage because it conflicts with the original series definition and with the LiaslDogger boundary; they also supported the Aalenian stage. Although no decision has as yet been taken by an internationally recognized body, the second alternative is strongly recommended in the meantime.

En Amerique du Nord, I'usage recent pour la limite des series se partage comme suit: (1) suivant la tradition anglaise, on place la limite entre les etages du Toarcien et du Bajocien s.1. ; (2) suivant la tradition de I'Europe centrale, on la place dans la meme position, mais avec la difference que I'etage de I'Aalenien = le Bajocien inferieur s.1.; (3) suivant une resolution du Premier Colloque de Luxembourg, on la place i la limite AalenienlBajocien s.str. Toutefois, des resolutions ulterieures a la Rencontre de Cassis et, de manitre equivoque, au Deuxieme Colloque de Luxembourg, ont annule le troisieme usage parce qu'il est en conflit avec ladefinition originale de la serie et avec la limite LiaslDogger; on a aussi supporte I'etage de I'Aalenien. Bien qu'aucune decision n'ait ete encore prise par un organisme reconnu internationalement, on recommande fortement la deuxieme solution pour I'instant.

[Traduit par le journal] Can. J . Earth Sci., 16,2060-2062 (1979)

In a recent issue of the Geological Association of Canada's newsletter Geolog, F. M. Gradstein (1978) gave a very useful summary of the Jurassic-to-Cenozoic radiometric time scale. For the Jurassic and Cretaceous, he followed the up- to-date scale by Van Hinte (1976, 1978). The Hinte scale assumes for its chronostratigraphic units the equal duration (1 Ma) of the ammonite (chrono) zones of Arkell (1956). Although a few supra- regional zones have since been added, deleted, or substituted here and there in the last 22 years, the number of such zones per stage has increased only moderately and more or less proportionately in the stages. The only major discrepancy with Arkell's scheme is the recognition of the Aalenian stage in place of the Lower Bajocian substage and the placing of the LowerlMiddle Jurassic boundary at the top of the Aalenian, i.e., one (small) stage above the conventional series boundary. The same strati- graphic scheme was used in the magnificent atlas for North and Central America (Cook and Bally 1975), but I know of no recent European liter- ature placing the Aalenian in the Lower Jurassic (Table 1) (except for the popular chart by van Eysinga 1975).

The recognition of the Aalenian stage conforms with the recommendations of the 1st and 2nd

Luxembourg Colloquia in 1962 and 1967 of the Jurassic Subcommission, Stratigraphic Commis- sion under the presidency of P. L. Maubeuge; and also with the recommendation of the Cassis, 1964, meeting of the Committee of the Mediterranean Mesozoic. I participated in both Luxembourg Col- loquia, and have been involved in the problem as writer of an International Stratigraphic Lexicon volume on Germany (Westermann 1967), as past member of the Subcommission and as current member of its Middle Jurassic Working Group. The 1st Colloquium indeed resulted in the unfortunate recommendation, based on a close vote, to place the Aalenian in the Lower Jurassic (Commission de Stratigraphie du Congrks Geologique International 1964, p. 78), while the base of the Dogger, i.e., the common substitute for the Middle Jurassic of Ger- many, remained coincident with the base of the Aalenian. A number of North American authors, e.g., Cook and Bally (1975), Van Hinte (1976, 1978) followed these recommendations, placing the series boundary above the Aalenian stage.

There are several reasons to object to the bound- ary proposed by the 1st Luxembourg Colloquium, 1962 (Commission de Stratigraphie du Congrks Geologique International 1964):

(1) The Aalenian was originally defined by

0008-4077/79/102060-03$01 .OO/O @ 1979 National Research Council of CanadaIConseil national de recherches du Canada

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Customary 1st Luxembourg Coll. 2nd. English and 1962 (1964); van Hinte Luxembourg N. American (1976, 1978); Cook Cassis Coll. Coll. 1967 This

usage Classic German usage and Bally (1975) 1964 (1971) Paper

1 !$ (Bajocian) Bajocian Bajocian Bajocian Bajocian Bajocian %I rn - "'..----- -------- ----?"-

6 (Aalenian) Aalenian Aalenian Aalenian Aalenian 9 - r r r - - - - r r - - 3 - - 9 m - - - 9 - - - - - - - - - - - - - - r - - r r - m m ----?-I- .---ID-

V1 Toarcian Toarcian . Toarcian Toarcian Toarcian Toarcian 3 cl

Mayer-Eymar to begin with the "Beds of Boll with Ammonites torulosus " . These are about equivalent to the beds with Trigonia navis, and both belong in the Opalinum Zone, which is at the base of the Middle Jurassic, by original definition of Von Buch and A. Oppel (Westermann 1967, pp. 7,8,90, Table 4; Sapunov 1965; Diet1 and Etzold 1977).

(2) The recommended series boundary differs from the LiaslDogger boundary, which is below the Aalenian (references as in (1) above).

(3) North American geologists in particular still use the Bajocian stage in the extended sense of Oppel (and Arkell 1956), regarding the Aalenian synonymous with the Lower Bajocian. This is in fact still the standard usage of the U.S. Geological Survey and the Geological Survey of Canada, and is related to ambiguities surrounding the Aalenian stage (Sapunov 1965). The proposed series bound- ary would thus coincide with the substage bound- ary LowerIMiddle Bajocian, which is incompatible with the hierarchial system of stratigraphy (Hed- berg 1976).

(4) The majority of recent authors who distin- guish the Aalenian place it in the Middle Jurassic.

Significantly, this 1962 (1964) resolution was again discussed at length at the 2nd Luxembourg Colloquium in 1967. While the vote in the general assembly was almost unanimously in favour of re- taining the Aalenian (60:3), the assembly voted by slightly less than a 213 margin (36:25) for placing it in the Middle Jurassic. Nevertheless, accord- ing to the official report, circulated by President Maubeuge (July 29, 1967) and published by the International Commission on Stratigraphy, the Jurassic Subcommission had confirmed the origi- nal, 1st Colloquium resolution. Ager (1968) re- ported in the I.U.G.S. Geologic Newsletter that the Subcommission considered the simple majority vote of the general assembly as insufficient to change the original resolution, and that thus the Aalenian should remain in the Lower Jurassic. Yet

the next sentence of the official report by the Presi- dent says that the Subcommission "maintained the Aalenian at the base of the Middle Jurassic" (translated from French). Zeiss (1969), in a German translation of this official announcement, thus placed the Aalenian in the Middle Jurassic.

Between the two Luxembourg Colloquia, the Cassis (1964) meeting of the Committee of the Mediterranean Mesozoic had already strongly rec- ommended that the series boundary be transferred back to the original ToarcianIAalenian position (Sapunov 1965).

I have contacted the presidents of the Interna- tional Commission on Stratigraphy (A. Martinsson) and its Jurassic Subcommission (A. Zeiss), and the Chairman of the International Subcommission on Stratigraphic Nomenclature (A. Salvidor), re- questing information on the most recent develop- ments and, perhaps, decisions made with regard to these various resolutions.

I have been informed by these different bodies that no official submission has been received as a consequence of either the Luxembourg or the Cas- sis meetings. The matter of the LowerIMiddle Jurassic boundary, therefore, remains unresolved (officially !).

Professor A. Zeiss (President, Jurassic Sub- commission) has added the following statement: "The Subcommission on Jurassic Stratigraphy, newly established in 1978, is fully aware of the ambiguous situation concerning the Aalenian stage (see Newsletter 2). Until a final decision can be reached further investigations and discussions are necessary. In the meantime, it is recommended to follow the general vote of 1967, which resulted in a majority for the inclusion in the Middle Jurassic. This recommendation follows from the decision to base the work of the newly established subcommis- sion on the earlier resolutions (Newsletter I)."

At a special organizational meeting held in Frankfurt on March 15, 1979, the Subcommission

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2062 CAN. J. EARTH

again discussed the matter of the Aalenian (Inter- national Subcommittee on Jurassic Stratigraphy, Newsletter No. 3). It was agreed to place this stage, for the mesent. in the Middle Jurassic. and to have a deciding vote at the next ordinary meeting of the Subcommission, during the 26th International Geological Congress (Paris) in July, 1980.

In the meantime, I will certainly continue to place the boundary at the base of the Aalenian (or of the Bajocian s.1. if the Aalenian is not recognized), i.e., at 174 f Ma, instead of 171 * Ma (megayears), of the Hinte scale.

AGER, D. V. 1968. Commission on Stratigraphy, Subcommis- sion on Jurassic Stratigraphy: 2nd Colloquium on Jurassic Stratigraphy, Luxembourg, July 17-22, 1967. Geological Newsletter, 1967,4, pp. 14-17.

ARKELL, W. J. 1956. Jurassic geology of the world. Hafner Publishing Company, New York, NY. 806 p.

Commission de Stratigraphie du Congres Geologique Interna- tional. 1964. Colloque du Jurassique a Luxembourg, 1962 (recommendations in French, German, English and Italian). In Colloque du Jurassique B Luxembourg 1962. Edited b y P. C. Maubeuge. Institut Grand-ducal, Section des Sciences Naturelles Physiques et Mathematiques, pp. 77,88.

Commission International de Stratigraphic, Sous Commission du Jurassique. 1970. Colloque du Jurassique a Luxembourg, 1967. Union International des Sciences Geologiques, Grand-Duche Luxembourg, Musee Histoire Naturelle.

COOK, T. D., and BALLY, A. W. (Editors). 1975. Stratigraphic atlas of North and Central America. Shell Oil Company and Princeton University Press, 272 p.

SCI. VOL. 16. 1979

DIETL, G., and ETZOLD, A. 1977. The Aalenian at the type locality. Stuttgarter Beitrage zur Naturkunde, B, 30, pp. 1-13.

GRADSTEIN, F. M. 1978. Jurassic, Cretaceous and Cenozoic radiometric time scale and standard stages. Geolog, 7(3), pp. 34-36.

HEDBERG. H. D. (Editor). 1976. International stratigraphic guide, a guide to stratigraphic classifications, terminology, and procedure (International Subcommission on Strati- graphic Classification). Wiley-Interscience Publication, John Wiley and Sons, New York, NY. 200 p.

SAPUNOV, I. G. 1965. The boundary between Lower and Middle Series of the Jurassic System. Carpatho-Balkan Geological Association, VII Congress Sofia 1965, Reports 11, 1, pp. 43-46.

VAN EYSINGA, F. W. B. 1975. Geologic time table. 3rd ed. Elsevier Scientific Publishing Company, Amsterdam, The Netherlands.

VAN HINTE, J. F. 1976. A Jurassic timescale. American Associ- ation of Petroleum Geologists Bulletin, 60(4), pp. 489-497.

1978. A Jurassic timescale. In Contributions to the geologic time scale (papers given at the Geologic Timescale symposium 106.6, 25th International Geological Congress, Sydney, Australia, August, 1976. Edited b y G. V. Cohee, M. F. Glaesner, and H. D. Hedberg. American Association of Petroleum Geologists, Studies in Geology No. 6, pp. 289-297.

WESTERMANN, G. E. G. 1967. Allemagne, fascicule 3 2 , Juras- sique Moyen (Alpes exclues). Lexique Stratigraphique Inter- national 1, Europe, fascicule 5, 197 p. (Centre National de la Recherche Scientifique, Paris).

ZEISS, A. 1969. Beschliisse des 2. Internationalen Jura- Kolloquiums (Luxembourg, 17-22 Juli, 1967). Geologisches Blatt Nordost Bayerns, 19(3), pp. 144-146.

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