Stratigraphy

Discussions about the Jurassic-Cretaceous boundary (Part II)

2008-02-17T17:49:00.006+01:00

Discussions about the Jurassic-Cretaceous boundary (Part II)

by Ph.J. Hoedemaeker

"The arrogance of ammonitologists"

Please be aware of the fact that Oppel (1856) considered his zones as subdivisions of stages, i.e. as what nowadays are called chronostratigraphic units. He proudly wrote that he was able to subdivide the stages of d'Orbigny into smaller parts, which he proposed to give the names of fossil species that occur in these zones, are easy to identify, have a short range and have an extensive distribution, i.e. have the qualifications of a guide species. Most, but not all, of his zonal names were derived from ammonite species, because they have these qualifications. Most, but not all, of Oppel's zones begin with the appearance of the characteristic ammonites. The main wish of most modern stratigraphers is to subdivide d'Orbigny's stages into smaller parts, to refine the units of chronostratigraphy and correlation. That is why they make zones. That is what Oppel did in 1856, and all geologists used Oppel's zones as small chronostratigraphic units. One may regard ammonite zones as biochronozones. This concept survived for a whole century (though not always in the currently defined sense as I put it here) and is still not dead, for ammonitologists still maintain the zonal name in localities where the guide species is absent, which is of course impossible if they were strictly considered biozones. Of course the concept of Oppel's zones evolved; they were subdivided or corrected in accordance to better known fossil ranges and to new taxonomic concepts of the fossils or better index species. Moreover, it became a custom to use only ammonite species to characterize the zones, and to define all zones by the first appearances of the index ammonites. The appearance of the index ammonite, or, if the index species is rare or absent, the base of the zone, was normally calibrated by the appearances and disappearances of as many other ammonite species as available in the stratigraphic neighbourhood of the zonal base. Other fossil groups can be used to subdivide statigraphical successions, but the relative age of their biozones was commonly given in terms of ammonite zones. All stages of the Triassic, Jurassic and Cretaceous were defined in terms of ammonite zones. All these changes did not change the chronostratigraphic nature of the original ammonite zones, and the Mesozoic stratigraphers have found it convenient to group the ammonite zones into stages. The succession of ammonite zones for the Upper Palaeozoic and Mesozoic became more or less standardized.

Hedberg (1954) made a big historical mistake by considering the zones of Oppel to be biostratiraphic units instead of chronostratigraphic units. With this he deprived stratigraphers from the long cherished, fine, chronostratigraphic subdivisions of stages based on one single kind of objects. Ammonitologists, after a dogged struggle, had to yield, and humbly follow the new trend and mainstream in biostratigraphic thinking; they began to use datum planes in defining ammonite appearances, a procedure that for ammonites is only possible in very specific cases. They draw the base of ammonite zones by merely considering the datum plane of the index species, and disregard the ranges of the other ammonite species. Ammonitologists currently 'say' that they consider ammonite zones biostratigraphic units, but, unconsciously, they still use ammonite zones in the sense of chronostratigraphic units; they often draw the lower boundary of the ammonite zone not at the first appearance of the index species, but lower on account of the appearances and disappearances of other ammonites, or they determine a zone even when the index species is absent. Non-ammonitologists still tend to correlate their biozones with ammonite zones, because they want to correlate their biozones with a 'kind of chronostratigraphical' standard, which they still find in ammonite zones. There is still not a fossil group that could replace the qualifications, especially the historical qualifications, of ammonite zones as subdivision of Mesozoic stages.

In a global urge among palaeontologists and stratigraphers to be fair; they began to consider all fossil groups of equal stratigraphical importance. However, by currently defining the first stage boundary with the first appearance of a nannoplankton species, the second with an ammonite species, the third with a crinoid species, the fourth with a planktonic foraminifer, the fifth with a dinoflagellate cyst, the sixth with a magnetostratigraphic level, the seventh with a sequence boundary, etc., stratigraphy sinks into a chaos and chronostratigraphic correlation does not become easier, nor more practical. Indeed, the zonations of most fossil groups have, notwithstanding Hedberg's (1954) ideas, still been calibrated with the ammonite zones, and that is apparently the main reason why ammonite zones are still considered the best subdivision to calibrate with. Ammonites were nectonic and therfore only little influenced by facies. So, it is not the ammonitologists, but rather the non-ammonitologists who maintain the ammonite zones as the finest and most preferable subdivisions of stratigraphy to calibrate their zonations with, as if they were chronostratigraphic units.

What to do?

Every stage of the Mesozoic Era should be subdivided by one single kind of objects into a succession of smaller chronostratigraphic units, with which every biozonation can be calibrated. The most appropriate seem to be ammonites, because they have been used satisfactorily as these objects for more than a century, and because they allow to recognizing biostratigraphic units of only 100.1000 years. Don't throw away what has proven its workability for more than 100 years. The best thing to do is to define all ammonite zones with golden spikes (GSSPs) by which they become chronostratigraphic units in the modern sense, biochronozones. This should of course be done with the greatest care by carefully selecting those ammonite rich sections that have thoroughly been searched for ammonites, so that one can reasonably be sure that the lowest occurrence of the index species is as close as possible to its real first appearance. If ammonite zones are defined by golden spikes, stages could be defined in terms of ammonite zones, as before, and systems in terms of stages, etc. This is not arrogant, but uniform and therefore practical; it has been practised for more than a century. Ammonites are still the best time-indicating macro-invertebrate. Only micropalaeontological die-hards yelled: "We want to establish stage boundaries once and for all and we don't want to wait for ammonitologists to arrive at a decision". To give these biochronozones an aureole of mathematical exactness one may use matrices as a mathematical tool (Guex 1977, 1979, 1987).

A still better procedure is to define stages in terms of Vailian depositional sequences, and characterize every sequence by its fossil content, not only ammonites but also all other fossils. Every stage would then be defined by a sequence boundary. Every depositional sequence can be identified all over the world and their correlative conformities can be well marked off on account of the characteristic fauna of the adjacent sequences. In this case one replaces the old biozones, abandones the rather arbitrary choice of index species and index groups, and all phanerozoic sequences can be characterized. Everyone is happy because everyone can use his own favorite fossil group to indentify the depositional sequences; nobody is dependent on ammonitologists any more. This proposal is perhaps too progressive for the rather conservative geologists, but they have their well-correlatable subdivision of stages. Good stratigraphers should be proficient in sedimentology (sequence stratigraphy) as well as in palaeontology in order to be able to perform well-founded time correlations.

Difference between ammonite biochronozones and biozones based on planktonic fossil groups

The nectonic ammonites exhibit a rapid evolution and a great sensitivity for sea-level fluctuations. This makes them the best guide fossils and time indicators among the fossils. In the Jurassic one can mark out biostratigraphic units of 100.000 years with ammonites.

The only problem with an ammonite biochronozones is, (1) that ammonites are relatively rare (averageing 1 billion planctonic fossils to 1 ammonite) and (2) that the index species generally does not occur in the basal bed of the zone, except in its type locality and in ammonite rich successions. Therefore, if one defines an ammonite biochronozone, one has to name all the containing ammonite species, and to note whether they are restricted to the zone, or in common with the underlying or overlying zone. Knowing the ranges of all ammonites of the zonal association, one can correlate the base of the zone with sufficient precision by using the appearances and disappearances of all fossils. This procedure has generally been done, let us say, until the Second World War. After the war one considered this too time consuming, and ammonitologists more and more switched over to the procedure recommended by the ISSN (dominated by of planktonic fossil biostratigraphers), which is used only by those studying planktonic microfossils, and they did not bother to mention all ammonites appearing and disappearing in the zone. Ammonite biochronozones should not be treated as the biozones currently used for planktonic fossils, viz. by merely indicating the first appearance of the index fossil (datum plane), because the presence of the index ammonite species is too rare and may only occur in a small part of the zone. Ammonite biochronozones has the closest similarities with multiconcurrent range zones, i.e. the biozones with the best correlating possibilities.

The base of the S. subalpina Subzone can, according to the current state of the ammonite biostratigraphy, be recognized by the first appearance of either Strambergella subalpina, or Berriasella privasensis, or Neocosmoceras sayni, or Negreliceras paranegreli, or 'Delphinella' boisseti, or Subthurmannia clareti, or Mazenoticeras malbosiforme, or Mazenoticeras curelense, or Mazenoticeras broussei, or Malbosiceras pouyannei (M. pictetiforme), or Malbosiceras malbosi, or Retowskiceras andrussowi, or 'D.' ellenica, or 'D.' sevenieri, or 'Tirnovella' occitanica, or Spiticeras praegratianopolitense.

Sequence boundaries as chronostratigraphic boundaries

Chronostratigaphic boundaries have, by preference, been chosen at lithological changes along hiatusses. These hiatusses could generally be ascribed to sea level falls connected with sequence boundaries. However, the GSSP should not be chosen at a hiatus, because it is unknown how much time/rock has disappeared by erosion or non-deposition; continuously deposited successions are preferred. However, every sequence boundary has its correlative conformity in the basin. This extension of the sequence boundary should be determined in the basin and be studied for correlatable features of lithostratigraphic, biostratigraphic, sequence stratigraphic magnetostratigraphic and chemostratigraphic nature. Every depositional sequence should be searched for its fossil content so that it can be recognized and identified. This procedure would make biozones obsolete; it would free biostratigraphers from endless discussions on boundaries of fossil zones, and on correlating, calibrating, naming and defining fossil zones. If necessary, one could subdivide a sequence into a lower trangressive and an upper regressive part. The isochrony of the boundaries is garanteed and one could correlate all over the world irrespective of palaeobiogeographic realms, regions and provinces. However, all palaeontologists should learn how to recognize sequence stratigraphy in the field, which means that they should have a thorough knowledge of sedimentology. Stratigraphy leans on two legs: sedimentology and palaeontology. A good stratigrapher should be thoroughly trained in these two disciplines; without one of these two he is severly handicapped, which is currently the case with biostratigraphers.

The non-Berriasian, but Tithonian character of the fossil fauna of the H. jacobi Zone

Ammonites:

Perisphictidae

Pseudosubplanites

Himalayitidae

Himalayites

Protacanthodiscus/li>

Neocomitidae, Berriaselinae

Hegaratella (range ends in S. subalpina Subzone)

Dalmasiceras (up to end Occitanica Zone)

Delphinella ( range ends in S. subalpina Subzone)

Chapericeras

Strambergella (range ends in S. subalpina Subzone)

Substeueroceras

Busnardoiceras

Retowskiceras

Pseudoneocomites (up to Valanginian)

Olcostephanidae

Proniceras

Spiticeras (up to Valanginian)

Kilianiceras (up to Valanginian)

Aspidoceratidae

Aspidoceras

Schaireria (range ends in S. subalpina Subzone)

Haploceratidae

Haploceras

Streblitinae

Cyrtosiceras

References

Allemann et al., 1975. Mém. Bur. Rech. Geol. Min, 86: 14-22.

Cecca et al., 1989. Doc. Lab. Géol. Lyon, 107: 1-115.

Clavel et al., 1986. Eclogae geol. Helvetiae, 79, 2: 319-341.

Colloque sur la limite Jurassique-Crétacé, 1975. Mém. B.R.G.M., 86.

Cope et al., 1980. Geol. Soc. London Spec. Report, 15: 1-109.

Coquand, 1869. Bull. Soc. Géol. France, 2, 26: 100-131

Coquand, 1870. Bull. Soc. Géol. France, 2, 27: 73-106.

Coquand, 1871. Bull. Soc. Géol. France, 2, 28:208-234.

Coquand, 1875. Bull. Soc. Géol. France, 3, 3: 670-686.

Desor & Gressly, 1859. Mém. Soc. Sci. Nat. 4: 1-159. Neuchâtel

Détraz & Mojon, 1989. Eclogae geol. Helv., 82, 1:37-112.

Enay & Geyssant (1975). Mém. B.R.G.M., 8: 39-55.

Gradstein et al., 2004. A Geologic Time Scale, University Press, Cambridge: 358.

Guex, 1977. Bull. Soc. Vaud. Sc. Nat., No. 351, Vol. 73: 309-322.

Guex, 1979. Bull. Soc. Vaud. Sc. Nat., No. 355, Vol. 74, 3: 169-316.

Guex, 1987. Corrélations biochronologiques, Presses Polytechniques Romandes: 1-244

Hedberg, 1954.19th Int. Geol. Congr. (Algiers), fasc. 13: 205-233.

Hoedemaeker & Bulot, 1990. Géol. Alpine, 66:123-127.

Hoedemaeker, Company et. al., 1993. Revista Española Paleontol., 8: 117-120.

Hoedemaeker & Leereveld, 1995. Cretaceous Res. 16, 195-230.

Hoedemaeker et al. 1998. Geologica carpathica, 49, 1: 15-32.

Hoedemaeker, 1987. Scripta Geologica, 84: 1-55.

Hoedemaeker, 1991. Newslett. Stratigr., 25: 37-60.

Hoedemaeker, 1998. SEPM (Soc. Sed. Geol.) Special Publication, 60: 423-441.

Hoedemaeker, 2002. In Michalik (Ed) Tethyan/Boreal Correlation, VEDA Publishing House of the Slovak Acad. Sciences, Bratislava: 235-284.

Hoedemaeker, 2003. Cretaceous Res. 24: 253-275.

Housa et al., 2007. Stratigraphy and Geol. Correlation, 15, 3: 297-309.

Jan du Chêne et al.,1993. Bull. Centr. Rech. Explor.-Product. Elf Aquit., 17: 151-181.

Kilian, 1896. Bull. Soc. Géol. France, 3, 23 (1895): 659-803.

Le Hégarat, 1973. Doc. Lab. Géol. Fac. Sc. Lyon, 43, part 1 and 2: 1-576.

Mazenot, 1939. Mém. Soc. Géol. France, N.S., 41: 5-303.

Nikolov, 1977.

Oppel, 1856. Die Juraformation, Verlag von Ebner & Seubert, Stuttgart

Oppel 1865. Zeitschr. Deutsche geol. Ges., 17:535-558.

Pictet, 1867. Mém. Soc. Phys. Hist. Nat. Genève, 7 (Mélanges palënt.), 2: 43-130.

Tavera, 1985. Thesis doctoral Univ. Granada: 1-381.

Toucas, 1890. Bull. Soc. Géol. France, 3, 18:560-629.

Discussions about the Jurassic-Cretaceous boundary (Part I)

2008-02-15T14:50:00.006+01:00

Discussions about the Jurassic-Cretaceous boundary (Part I)

by Ph.J. Hoedemaeker

Let me try to make a few things clear to you by elucidating my opinion on the matter. I am at one with you about the stratigraphic position of the Jurassic-Cretaceous boundary at the base of the Neocomian (= boundary between the ammonite zones of 'Berriasella' jacobi and 'Tirnovella' subalpina), because this boundary practically correlates with the top Portlandian, the top Volgian and the original base of the Berriasian. If we want to speed up the procedure of the Working Group on the the Jurassic-Cretaceous Boundary, we should not waste time on finding a level for the GSSP at the base of the Berriasella jacobi Zone, but start as quick as possible to find an appropriate level at or close to the base of the Neocomian. (...)

Basic assumptions

First: the stratigraphic position of the J/K boundary should be as close as possible to the original boundary. Priority should be respected, and disrespected only when absolutely necessary. Slight deviations are tolerated on account of modern achievements in science.

Second: The GSSP should be defined in the Tethyan realm, not in the boreal realm. This is because of the continuous and easy accessibility of Tethyan GSSP locations.

Third: The GSSP should be located in open marine sediments deposited in not too shallow and not too deep environment.

Fourth: The objects or features used to determine and correlate the boundary should preferably be visible objects or features, i.e. of lithostratigraphical (sedimentological) or biostratigraphical (palaeontological) nature. Chemostratigraphical or magnetostratigraphical features in the rock are not visible and require long and expensive field and laboratory work to detect them; they can, however, be used to controle and support the correlation by visible features.

Argumentation, history, priority

1. The stratigraphic position of the J-K boundary has not been established yet. There is only a recommendation of the former 'IUGS Working group for the Jurassic-Cretaceous Boundary' (Meeting in Sümeg, Hungary, 18-21 September, 1984), that this boundary should, for the time being, be in accordance with the outcome of the vote among the participants of the 'Colloque sur la limite Jurassique-Crétacé, 1975.' This outcome endorses the proposal of Raymond Enay and his group that the stratigraphic position of the J-K boundary should be at the base of the Berriasella jacobi Zone. This species belongs to the generic group Hegaratella Nikolov, 1977, type species Berriasella paramacilenta Mazenot, 1939.

2. The Berriasian Stage defines the Cretaceous System, and the Berriasian Stage is defined by its base, which is by definition also the base of the Cretaceous System. According to the recommendation worded above, the base of the Berriasian is at this moment defined by the base of the H. jacobi Zone, which is on its turn defined by the first appearance of H. jacobi. This means that the Cretaceous is recommended to be defined by the first appearance of H. jacobi.

3. I reject this recommended definition, because it is unacceptably far away from the original stratigraphic position being the base of the Neocomian in the Neuchâtel region (Desor & Gressly, 1859; Oppel 1865), i.e. between the Goldberg Formation and the 'Unité oolitique inférieur' of the Marbre Bâtard. (This boundary is the original one, and is, according to the ammonites found and translated into current ammonite zones, situated between the H. jacobi and S. occitanica zones (Clavel et al., 1986; Détraz & Mojon, 1989; Hoedemaeker, 1987, 1991). This boundary has been defined in sediments deposited in a very shallow sea. The Maximum Flooding Interval in the upper part of the S. subalpina Subzone presumably correlates with the basal part of the Pierre Châtel Formation. This stratigraphic position lasted about 30 years, from 1865 to1895 and with a small shift up to 1973, thus more than 100 years.

4. 4. The base of the H. jacobi Zone as the Jurassic-Cretaceous Boundary is also unacceptably far from the base of the Berriasian Stage as it was originally defined. about two (Gradstein et al., 2004) to three (about 150 precessions of the equinoxes earlier) million years earlier. According to the original definition of Coquand (1869, 1870, 1871, 1875) the Berriasian comprises the 'Calcaires marneux de Berrias' containing the ammonite fauna described by Pictet (1867), which embraces, translated into current ammonite zones, the S. occitanica and S. boissieri zones. The important thing is that this original base of the Berriasian unintendedly correlates with the original base of the Cretaceous, i.e. the base of the Neocomian.

5. Finally Hoedemaeker (1987, 2002, 2003) showed by interrealmal correlation that the boundary between the B. jacobi and S. occitanica zones practically correlates with the modern top of the Portlandian Stage (since Cope et al., 1980) between the Subcraspedites lamplughi and Runctonia runctoni zones) and the top of the Volgian Stage in Siberia (between the Chetaites chetae and Praesurites sibericus zones) and European Russia (between the Subcraspedites nodiger Zone and the base of the Ryazanian Stage). So, the tops of the Boreal Portlandian and Volgian stages correlate with the original base of the Cretaceous System and with the original base of the Berriasian stage. Also the American Parodontoceras-Substeueroceras-Proniceras beds and the Argentine Substeueroceras koeneni Zone remain in the Tithonian. Only non-typical 'Parodontoceras' reedi and 'Substeueroceras' disputabile occur in younger beds. Hoedemaeker's correlations were confirmed by magnetostratigraphy (Housa et al., 2007); actually he traced major sequence boundary Be3 all over the world. I cannot find any legitimate and compelling reason to deviate from this boundary. What Enay and his group did is unacceptable and solely based on fossil taxonomy; he disturbed the already established good correlation. I cannot find any legitimate and compelling reason to deviate from this boundary. What Enay and his group did is unacceptable and solely based on fossil taxonomy; he disturbed the already established good correlation.

6. The first small deviation from the original J-K boundary was done by Kilian (1895), who respected the original definition of the Berriasian all his life. He found directly below the Berriasian sensu stricto of Berrias a few beds with a mixed Berriasian-Tithonian ammonite fauna, which he regarded as a transitional fauna between the Tithonian and Berriasian. Translated into current ammonite zones these beds, which he called Niveau de Jansiac (or Gensiac) and can be studied in the Montagne de Lure, approximately represent a part of the former 'Pseudosubplanites grandis Zone.' He surprisingly united these transitional beds and the Berriasian sensu stricto' into what he called the 'Infravalanginien.' He considered the base of the Infravalanginian the base of the Cretaceous, but he keeps the base of the Berriasian where it always has been (Coquand 1871), i.e. a few beds higher. This would be an unacceptable situation nowadays. The base of the Infravalanginian (only a few beds below the base of the original Berriasian) became known as the 'classical boundary' of the Cretaceous System, which lasted 80 years: from 1895 to 1975.

7. Mazenot (1939) corrected this unacceptable condition by considering the 'Infravalanginian' equivalent to the Berriasian and the base of the Berriasian equivalent to the base of the Cretaceous System. The 'Pseudosubplanites grandis Zone' became the basal ammonite zone of an extended Berriasian and therefore also of the Cretaceous. This boundary was respected by Le Hégarat (1973) as the J-K boundary; he introduced the H. jacobi Zone as the uppermost Tithonian ammonite zone. This zone covers the greater part of the "Calcaire blanc d'Ardèche," which represents the Ardescian Substage (Toucas, 1890), the uppermost substage of the Tithonian Stage (Cecca et al., 1989). In the Berriasian stratotype the 'Calcaires blancs de l'Ardèche' include even the P. grandis Subzone. The minor lower part of the 'Calcaire Blanc' is occupied by the M. microcanthum and Durangites zones.

8. It appears difficult to distinguish the P. grandis zone form the H. jacobi zone, because nearly all ammonite, belemnite, echinoid and calpionellid species of P. grandis Zone occur also in the H. jacobi Zone. The former zone is hardly recognizable without the presence of the latter and it is not considered workable to define the base of the Cretaceous at the base of the P. grandis Zone. During the Colloque sur la limite Jurassique-Crétacé (1975) both zones were, therefore, united into the 'jacobi-grandis zone'. In 1990 the IGCP (projects 262 and 362) Lower Cretaceous Cephalopod Team (= the predecessor of the IUGS Lower Cretaceous Ammonite Working Group, the "Kilian Group,"of the Subcommission on Cretaceous Stratigraphy) regarded the two zones as subzones of an extended H. jacobi Zone (Hoedemaeker & Bulot, 1990). In 1993 the Lower Cretaceous Ammonite Team abandoned the P. grandis Subzone as unworkable, and only kept the extended H. jacobi Zone (Hoedemaeker & Company, 1993).

9. It therefore became desirable to shift the Jurassic-Cretaceous boundary and the base of the Berriasian Stage to the top of the extended H. jacobi Subzone again, i.e. to their original position. However, Enay and his students studied the uppermost Jurassic succession in Andalusia (Spain) where the top of the P. grandis Subzone (=) cannot be well defined; it is probably a level within the Retowskiceras andrussowi Zone of J. M. Tavera, 1985, the totality of which he correlated with the S. occitanica Zone, but which probably correlates only with the top part of the former 'P. grandis Zone' and the S. subalpina Subzone, because it represents the total range of Hegaratella paramacilenta. Enay & Geyssant (1975) proposed to place the Jurassic-Cretaceous boundary either at the base of the H. jacobi Zone where the genus Berriasella suddenly becomes dominant. This level correlates with the base of Calpionellid zone B and with major sequence boundary Be1. According to Enay & Geyssant the only alternative would be a level within or at the top of the Berriasian Stage. This procedure would make the way free for every stratigrapher to indicate at random the events that he likes to be a boundary between stages or systems This cannot be. Every indication of a boundary should take account of already existing boundaries and priority should be violated only when absolutely necessary.

10. After an inquiry among the attendants of the Colloque sur la Limite Jurassique-Crétacé, the base of the H. jacobi zone was accepted by the majority of the attendants, presumably because of the perfect and convincing presentation of Enay and Geyssant, and because Enay and Geyssant did not give an acceptable alternative. There were only two dissidents mentioned: Jost Wiedmann and Raymond Casey, who are actually working around the J/K boundary. In 1984 during the meeting of the IUGS Working Group on the Berriasian Stage in Sümeg (Hungary) the members decided to respect for the time being the outcome of the iquiry until a final definition in the future (see point 1).

11. The base of the H. jacobi Zone is too far from the original Jurassic-Cretaceous boundary of Oppel (1865). It cannot be indicated in the stratotype of the Berriasian near Berrias, where only the uppermost bed of the former P. grandis Zone yielded a few ammonites. Also a large part of the original Ardescian Substage of the Tithonian was severely truncated and reduced to an insignificant part of the upper Tithonian. Nevertheless the base of the H. jacobi Zone has all qualification to be an important stratigraphic boundary: major sequence boundary attended by a clear change in the calpionellid fauna and a clear change in ammonite fauna (Tavera, 1985 found only three ammonite species (= 15%) crossing the boundary and occurring in the basalmost part of the H. jacobi Zone, viz. Corongoceras köllikeri, Protacanthodiscus heterocosmus and Durangites sutneroides), which makes its recognition and identification easy.

12. From 1981 onward I proposed with ample argumentation to put the J-K boundary at the base of the S. occitanica Zone, which is defined by the base of the S. subalpina Subzone, and which can be recognized by the first appearance of at least Strambergella subalpina, Berriasella privasensis, Neocosmoceras sayni, Negreliceras paranegreli, 'Delphinella' boisseti, Subthurmannia clareti, Mazenoticeras malbosiforme, Mazenoticeras curelense, Mazenoticeras broussei, Malbosiceras pouyannei (M. pictetiforme), Malbosiceras malbosi, Retowskiceras andrussowi, 'D.' ellenica, 'D.' sevenieri, 'Tirnovella' occitanica, and Spiticeras praegratianopolitense. None of these ammonite species that characterize the Berriasian S. occitanica and S. boissieri zones, does occur in the extended H. jacobi Zone, whereas only eight of the 62 species (13%) that occur in the H. jacobi zone, viz. Hegaratella paramacilenta, Hegaratella subcallisto, Strambergella shipkovensis, Strambergella floquinenesis, Subalpinites aristides, Dalmasiceras gigas and Dalmasiceras djanelidzei, and remarkably Schaireria longaeva, occur in the S. subalpina subzone. This means that the change in the ammonite fauna is very great and the measure of extinction surpasses the normal background extinction to a large extent, which greatly enhances the biostratigraphic recognizability of the boundary. This is interpreted to be due to the extra deep fall of the sea level connected with major sequence boundary Be3.

13. The most preferable locality for the GSSP (Global stratotype section and point) should of course be sought for in the stratotype of the Berriasian Stage near Berrias, because of the thorough pluridisciplinar investigations performed in this stratotype section (e.g. Jan du Chêne et al.,1993). According to Le Hégarat (1973) the S. subalpina Subzone begins at the base of bed 147 (= base of bed 147/12 in Jan du Chêne et al., 1993). However, the only S. subalpina mentioned from the Berriasian stratotype occurs in bed 148 (= bed 148/16 in Jan du Chêne et al., 1993), and from beds 146 and 147 no ammonites were reported. The first and last ammonites of the extended B. jacobi Zone occur in bed 145.

14. Halfway up bed 147 there is a major sequence boundary (Be3) at the erosive base of bed 147/14A. This sequence boundary coincides with the base of magnetostrigraphic chron M17r. Also the base of the 'Calcaires marneux de Berrias' virtually coincides with this sequence boundary. It is the base of the Berriasian defined by Coquand (1869, 1870, 1871, and 1875). The presence of about 20 % kaoliniet begins at the base of the first non reworked sediments above this sequence boundary. The expression of this major sequence boundary is lithostratigraphically as well as biostratigraphically discernable and recognizable in most successions in the Tethyan and Boreal realms. It is biostratigraphically well recognizable by ammonites, belemnites and calpionellids. The faunas of all these groups show a marked change-over at this sequence boundary. The GSSP may be placed at the major sequence boundary Be3. The only problem is the hiatus along this sequence boundary. The Berriasian near Angles is very incomplete around the B. jacobi-S. subalpina zonal boundary (very large hiatus between the B. jacobi and S. occitanica zones) and the magnetostratigraphy has not been studied.

15. The perfect boundary would be at a locality where the hiatus produced by sequence boundary Be3 does not exist, and the beds are conformable. The Lower Cretaceous succession along the Río Argos west of Caravaca (Province of Murcia, SE Spain), for instance, is such a locality. The sediments were deposited at a depth of about 200 to 300 m (upper slope) (Hoedemaeker & Leereveld, 1995; Hoedemaeker, 1998). In this locality the correlative conformity of sequence boundary Be3 separates the last bed with Pseudosubplanites of the extended H. jacobi Zone, bed Z204, from the first bed with Strambergella subalpina of the S. subalpina Zone, bed Z206. The first Calpionella elliptica appears in bed Z200, which is120.000 years (6 precessions of the equinonoxes) before the top of the last bed of the H. jacobi Zone. According to Allemann in Allemann et al. (1975) the base of the C. elliptica Zone characterizes "one of the most easily definable breaks in the calpionellid faunas" (viz. the appearance of many well developed Calpionella elliptica; Tintinnopsella carpathica becomes large and frequent and develops subspecies/varieties; within the C. elliptica Zone the T. carpathica-T. longa transition sets in, and the first T. longa appears; the appearance of the forerunner of Calpionellopsis simplex. Crassicolaria parvula and Calpionella alpina disappear within the C. elliptica Zone; C. alpina is still prent and develops subspecies/varieties). Unfortunately magnetostratigraphy is not possible in the Río Argos succession because of remagnetization during the orogenic movements in Miocene times (Hoedemaeker et al. 1998), and pollen and dinoflagellate cysts are absent around this boundary, because they are all carbonized by geothermal heat. However the magnetic susceptibility has been measured and can be used in magnotostratigraphic correlation.

16. A section for the GSSP of the base of the Cretaceous System that is perfect in all aspects, has not been described yet for the Mediterranean Faunal Province.

J/K-markers of the top of the extended H. jacobi Zone

There are several levels that are suitable to function as boundary planes for the base of the Berriasian/Cretaceous:

1. The entry of Calpionella elliptica, i.e. the base of the C. elliptica Zone, which according to Allemann in Allemann et al. (1975) characterizes "one of the most easily definable breaks in the calpionellid faunas." Why? Because it heralds a new association which begins with the presence of C. elliptica s.s., the increase in the size and abundance of Tintinnopsella carpathica, the appearance of Tintinnopsella longa preceded by transitional forms between T. carpathica and T. longa, the entry of the forerunner of Calpionellopsis simplex; Crassicolaria parvula and Calpionella alpina disappear within the C. elliptica Zone. (Tropical Realm)

2. The first appearence of either Subthurmannia (Strambergella) subalpina, or Berriasella privasensis, or Neocosmoceras sayni, or Negreliceras paranegreli, or or Subthurmannia clareti, or Mazenoticeras malbosiforme, or Mz. curelense, or Mz. broussei, or Malbosiceras pouyannei, or M. pictetiforme, or M. malbosi, or Retowskiceras andrussowi, or 'Delphinella' boisseti, or 'D.' ellenica, or 'D.' sevenieri, or 'Tirnovella' occitanica, or Spiticeras praegratianopolitense, which start their ranges in the S. subalpina Subzone. (Mediterranean Faunal Province)

3. The base of magnetostratigraphic zone M17r. In the type section of the Berriasian the base of M17r coincides with the sequence boundary Be3. Magnetostratigraphic boundaries are not visible in the field and therefore not as apparent as the sedimentological and palaeontological characteristics of the sediments. They can only be detected by expensive and time-consuming laboratorium work. Many rocks are not suitable to do magnetostratigraphic studies. (Mondial)

4. Sequence boundary Be3, which is a major (type 1) sequence boundary, which means that the sea level falls below the platform edge, and erosion of the platform is severe. This causes a severe turn-over in the ammonite fauna. 87% of the ammonite fauna of the subjacent H. jacobi Zone becomes extinct, whereas the overlying S. subalpina Subzone is characterized by a great number of new ammonite species, none of which occur in the H. jacobi Zone. 13% of the ammonite fauna of the H. jacobi Zone becomes extinct in the S. subalpina Zone. (Mondial)

5. The entry of kaolinite in France, England and Germany at correlatable levels. The appearance of kaolinite at the base of the S. subalpina Zone is interpreted as a change in climat, viz. from semiarid to semihumid. (Europe)

All these markers of the Jurassic-Cretaceous boundary have the same importance, but I prefer the base of the S. subalpina Subzone or sequence boundary Be3.

2007-08-21T09:50:00.000+02:00

On the trail of a major trans-Tethyan discontinuity

[Sur la piste d'une discontinuité majeure trans-Téthys]

Bruno Granier

Département des Sciences de la Terre, UFR Sciences et Techniques, Université de Bretagne Occidentale, 6 avenue Le Gorgeu - CS 93837, F-29238 Brest Cedex 3 (France)

Manuscript online since May 15, 2007

Citation

Granier B. (2007).- On the trail of a major trans-Tethyan discontinuity. In: Bulot L.G., Ferry S. & Grosheny D. (eds.), Relations entre les marges septentrionale et méridionale de la Téthys au Crétacé [Relations between the northern and southern margins of the Tethys ocean during the Cretaceous period].- Carnets de Géologie / Notebooks on Geology, Brest, Memoir 2007/02, Abstract 07 (CG2007_M02/07)

Abstract

At the base of the Valanginian a major discontinuity associated with one of the major transgressions of the Cretaceous is nominated as a candidate for the boundary between the Cretaceous and Jurassic systems. This surface has been identified in both platform and basinal domains, at the eastern edge of the North American continent and in the Middle East, as well as through the western edge of Africa and Europe.

Key Words

Jurassic; Cretaceous; Tithonian; Berriasian; Valanginian; boundary; stages; systems; periods.

Résumé

À la base du Valanginien, une discontinuité majeure associée à une des plus grandes transgressions du Crétacé est désignée comme candidate pour le titre de limite des systèmes Jurassique et Crétacé. Cette surface a été identifiée aussi bien en domaine de plate-forme qu'en domaine de bassin, de la bordure orientale du continent nord-américain au Moyen-Orient, en passant par la bordure occidentale du continent africain et l'Europe.

Mots-Clefs

Jurassique ; Crétacé ; Tithonien ; Berriasien ; Valanginien ; limite ; étages ; systèmes ; périodes.

I - Introduction

While working in the field in southeastern Spain in connection with the preparation of my PhD thesis (Granier, 1987) I made my first contact with a particular surface that for years I was to continue to track in the literature, or encounter by chance during exploratory trips (Iraq, Oman, Ukraine) and while living abroad (United Arab Emirates). The surface involves a major discontinuity that in a chronostratigraphic sense is situated at the transition from the Berriasian to the Valanginian stages (more precisely in the basal levels of the Valanginian).

II - A trans-Tethyan Tour

II.1 - Spain (Fig. 1)

I first recognized this discontinuity in southeastern Spain within the platform series of the Prebetic of Alicante, at Puig Campana (Granier, 1987). One of the first problems was its stratigraphic position. In facies of this type the absence of the ammonites and calpionellids that are the classic markers of the Early Cretaceous led me to make use of larger foraminifera and calcareous algae that allowed me to attribute a Late Berriasian to Early Valanginian age to the beds that bound it.

Figure 1: Correlations of lithostratigraphic successions on a platform (Cabezon de Oro) to basin (Sierra de Fontcalent) profile in SE Spain (modified from Granier et alii, 1995). Dark blue: Berriasian marls and calcareous limestones; light blue: Berriasian limestones; yellow: Valanginian silty limestones and calcarenites; light green: Valanginian-Hauterivian strata; dark green: Hauterivian-Barremian strata (including a ferruginous oolite).

Using lithostratigraphic correlation it was then possible to establish a correlation between the beds laid down immediately above this stratigraphic unconformity at Puig Campana, the "Calcarenites with Pseudocyclammina", and a sequence of strata rich in fine-grained siliciclastics accumulated in the Sierra de Fontcalent at the edge of the citrabetic basin, dated by ammonites as Early Valanginian (Rasplus & Fourcade (eds.) et alii, 1987). The stratigraphic position of the event that created this sedimentary discontinuity could then be refined from an initially relatively broad interval involving the Upper Berriasian-Lower Valanginian to the extreme base of the Lower Valanginian (in the Thurmanniceras pertransiens Zone). A return to the sites between the two localities, Cabezon de Oro (a platform margin domain) and the village of Busot (a talus/slope area), resulted in new collections of ammonites and calpionellids that complete and make more precise the age assignments of Azéma (1977) and of Estevez et alii (1984), and validated my earlier correlation.

In the platform domain (at Puig Campana) we saw no indication of emergence at the top of the underlying unit, the "Limestones with Trocholina" and at the edge of this platform (at Cabezon de Oro) the same stratigraphic unit contains indications (presence of calpionellids and of the foraminifer Protopeneropolis ultragranulata) of a slightly more open-marine environment.

In both areas the transition to the overlying beds, the "Calcarenites with Pseudocyclammina" is not the mark of an abrupt downward shift of facies that is there is no indication of a forced regression, but rather the reverse, the installation of a more open-marine environment (as documented by the presence of ammonites, calpionellids and characteristic foraminifera such as Protopeneropolis ultragranulata and Montsalevia salevensis) with an associated change in the hydrodynamics to a wave-dominated regime (as indicated by hummocky and swaley cross-bedding).

The "Calcarenites with Pseudocyclammina" (also known as the "upper member of the Sierra del Pozo Formation" in regional literature) have been identified over most of the Prebetic platform domain: for instance, this unit crops out in the Sierra Mariola, an historical locality (Busnardo & Durand-Delga, 1960: "13. grès calcareux" in text-fig. 2) some 40 km north of the localities studied; and farther northward it almost reaches the southern borders of both the Meseta (to the NW) and the Iberian domain (to the NE).

This major discontinuity at the base of the Valanginian is marked above by the abrupt appearance of siliciclastics of which the clay fraction is either lacking or present in negligible quantities. What was the source of this well-sorted and abundant material (silts at Sierra de Fontcalent, sands and gravels at Cabezon de Oro and Puig Campana)? Among the gravels extraclasts (not endoclasts) predominate, i.e. lithoclasts derived from older calcarenites that consist of allochems and quartz or feldspar grains with a mosaic-calcite cement (Fig. 2); the larger lithoclasts reach the size of cobbles (up to 10 cm in their larger dimension). These older calcarenites were formed by an episode of cementation affecting mixed, calcareous and siliciclastic, sand layers in a meteoric-phreatic zone. Examination of the derived lithoclasts by cathodoluminescence (Fig. 2) indicates a strong resemblance to calcarenites at the base of the Berriasian that are known to me only from outcrops in the Sierra Mariola (see Busnardo & Durand-Delga, 1960: "5. grès calcareux" in text-fig. 2).


	A		B

Figure 2: A.- Lithoclastic floatstone with a grainstone matrix, a mixture of carbonate allochems and terrigenous sand or silt. The large lithoclast has the same composition and the same texture as the matrix. Sample CLUSE 5, "Calcarenites with Pseudocyclammina", Lower Valanginian, Puig Campana. Scale bar = 1 mm; B.- Cathodoluminescence analyses indicate that these lithoclasts are not intraclasts, but genuine extraclasts reworked from older rocks and then transported to the edge of the platform. The colors of the calcite cements and their zonations differ in the lithoclast (bottom) and matrix (top). Sample CAM 8, "Calcarenites with Pseudocyclammina", Lower Valanginian, Puig Campana. Scale bar = 100 µm.

As a whole, the source of the siliciclastics appears to have been an important stock of detritus that accumulated over time in the innermost part of the platform or on the emergent Meseta at the edge of the Iberian microcontinent. Presumably it was a general transgression onto this carbonate platform inherited from Jurassic times that caused remobilization of this material. Surprisingly, deposition of carbonates did not cease, for small bulbous coral colonies occur and carbonate grains (ooids, aggregates, bioclasts) exist in association with the siliciclastic material; it suggests that the turbidity was low, an interpretation supported by the scarcity of very fine-grained siliciclastic particles (clay-size). One cannot call this type of transgression a drowning, although it involved a major change in hydrodynamics with the change to a wave- and storm- dominated regime as documented by hummocky and swaley cross-bedding. A large part of the detritus was carried over the edge of the platform to accumulate beyond it, both on the slope and below it in the basin, thus constituting an enormous transgressive, prograding prism (a special case in which the rate of sedimentation, a variable supposedly insignificant and treated as a constant in classic models of sequence stratigraphy, is by far more important than accomodation).

II.2 - Westward, oceanic sites DSDP 392A and 416A

Site DSDP 416A is in the open sea off the Atlantic coast of Morocco at the foot of the continental plateau. Its situation is analogous to that of Sierra of Foncalent with a thick accumulation of Lower Valanginian beds consisting mainly of deepwater turbite deposits. Farther West on the other side of the North Atlantic, site DSDP 392A is located well off the Florida coast at the edge of the continental plateau which is bounded by the Blake Nose submarine escarpment. The limestone series of the platform of which the uppermost layers are attributed to the Berriasian (Fourcade & Granier, 1989) is capped by a pelagic series, Barremian or younger; strata representing the Valanginian and Hauterivian, as well as a part of the Barremian, are either condensed or absent. In this distal portion of the platform, one can truly speak of drowning; however carbonate sedimentation was not interrupted in its proximal portion.

II.3 - Eastward, the United Arab Emirates and Oman

The author (Granier, 2000) recently presented a revision of the stratigraphy of the Kahmah Group (better known regionally as the Thamama Group, although the two entities are not strictly synonymous). Here, relationships between platform and basin domains are discussed in print for the first time.

In the platform domain a succession of distinctive lithostratigraphic units is encountered; as they are bounded by regional or supra-regional discontinuities they were treated as alloformations. From bottom to top (see a part of the sequence in Figure 3, Zakum 1) they are named: Habshan Fm (Tithonian), Bu Haseer Fm (Tithonian and Berriasian), Belbazem Fm (Berriasian), and Zakum Fm (Lower Valanginian).

In the basinal domain, above a tectonic discontinuity that includes a more or less important stratigraphic hiatus, the series begins with "porcellanites", sometimes preceded by a polygenic conglomerate; together they make up the Rayda Fm; from it Late Tithonian ammonites and Berriasian calpionellids have been collected. These dense white limestones are replaced abruptly by the marls and argillaceous limestones of the Salil Fm. The upper portion of this unit has a few more calcareous strata that contain Pseudocyclammina lituus; this more limy subunit has been called erroneously "Habshan" (see Fig. 3, Dhulaima 4).

Biostratigraphic arguments can be evinced to establish:

the contemporaneity of the Bu Haseer (lower unit) and Belbazem (upper unit) formations of the platform domain with the Rayda Fm of the basin domain, and
the equivalence in time of the Zakum Fm on the platform with the Salil Fm and the so-called "Habshan Fm" in the basin.

A major discontinuity separates the Belbazem and Zakum formations on the platform and the Rayda and Salil facies in the pelagic sequence. In these lithologic successions the distinction is reinforced by the abrupt appearance of an important argillaceous fraction in the upper unit.

As is the case in southern Spain lithostratigraphic arguments (here the argillaceous content) may be used to supplement biostratigraphic information.

Figure 3: Correlation of platform sequence: a Zakum well in the Abu Dhabi offshore (modified after Hassan et alii, 1975) and basinal facies: Dhulaima No. 4 in the Oman onshore (modified after Mohammed et alii, 1997).

II.4 - In France (Provence and the Jura)

In Provence a discontinuity spanning the same interval of time separates the "Marnes vertes" Formation from the "Calcaires blancs supérieurs" (Virgone, 1997). Here, there was no emergent land-mass to provide detrital siliciclastics. They are replaced by granular limestones called "high energy" that mark the locations occupied by the sandstones and calcarenites in Spain.

Work in progress concerns the Jura and will be the subject of new publications.

III - Discussion

The limits of the stages of the Jurassic and Cretaceous are defined by "good stratigraphic fossils" that classically are ammonites, but the fact that ammonites are "facies fossils" is all too often not considered. Consequently, in platform domains the rarity of these biostratigraphic markers often makes it impossible to identify the limits of stages as they were defined (and amended) in basinal domains.

The problem is even more grave when in addition the stratigrapher is not able to define/map the limit of systems. Consider the very special case of the boundary between the Jurassic and Cretaceous systems; this surface as currently defined must be found "somewhere":

within the mass of the "Calcaires Blancs Inférieurs" in Provence (Virgone, 1997),
in the body of Purbeckian limestones in the French-Swiss Jura,
within the "barre tithonique" of southeastern France (Jan du Chêne et alii, 1993),
in the Bu Haseer Fm of the United Arab Emirates, etc.

Note that if the Boreal domain too is taken into consideration, confusion increases because the boundary between the Jurassic and Cretaceous in the Tethyan province is founded on the same criteria as those used to separate the Tithonian from the Berriasian (for a long time this boundary was located in the middle of an ammonite zone called the Jacobi-Grandis Zone), but according to the several authors, in the Boreal domain it is either intra-Volgian or at the limit between the Volgian and Ryazanian (intra-Berriasian).

In the platform domain, the transition from the Tithonian to the Berriasian is gradual, unmarked by lithologic or faunal distinctions. A break does occur higher in the succession in the vicinity of the limit between the Berriasian and the Valanginian. As we have seen in this rapid review this major discontinuity does not have a unique signature but it can be well identified in both the proximal and distal domains, on the platform and in the basin, for it is contingent on one of the largest Mesozoic transgressions. From a chronostratigraphic viewpoint it is very close to a stage limit defined by ammonites (it is even closer if one accepts that the Thurmanniceras otopeta (Sub-) Zone should be included in the Berriasian (see Bulot et alii, 1996).

IV- Conclusions

The criteria proposed in the several fields (sedimentological, petrographical, biological, geochemical, etc.) to define the Tithonian-Berriasian limit are too tenuous to be of value as a system limit (but have been so employed since Kilian) or even for bounding a stage. therefore, I propose to return to a more practical definition of systems (and stages). Authors - as d'Orbigny did in his time - should focus on the identification of significant events (any record of a global "catastrophy") to set unit limits. Such a concept involves the integration of litho-, bio-, and sequence stratigraphies to delimit chronostratigraphic units defined by physical criteria that will be calibrated as closely as possible with biozone "proxies", and not defined strictly/solely by biozones as has been the practice to date. From sedimentological, petrographical, biological, etc. standpoints the trans-Tethyan discontinuity at the base of the Valanginian (which passes laterally into a continuity in the basinal domain) merits elevation to the rank of a system boundary (an option close to the viewpoints of Toucas and Haug, see Barbier & Thieuloy, 1965).

Acknowledgments

This proposal has benefited from discussions with a number of colleagues whom I thank for having taken the time to discuss counter-arguments or to offer me new keys to my thesis. I hope to be able to associate them in forthcoming publications.

References

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Bulot L.G. et alii (1996).- The Valanginian stage.- In: Rawson P.F. et alii (eds.), Second International Symposium on Cretaceous Stage Boundaries, Brussels (1995).- Bulletin de l'Institut Royal des Sciences Naturelles de Belgique, Bruxelles, 66 (Supplement), p. 11-18.

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Granier B. (2000).- Lower Cretaceous stratigraphy of Abu Dhabi and the United Arab Emirates- A reappraisal.- The 9th Abu Dhabi International Petroleum Exhibition & Conference, October 15th-18th, Conference Proceedings, Abu Dhabi, ADIPEC 0918, p. 526-535.

Granier B., Virgone A., Busnardo R. & Bulot L.G. (1995).- Des calpionelles dans l'Hauterivien supérieur. Découverte exceptionnelle à Busot (Alicante, Espagne).- Comptes-Rendus de l'Académie des Sciences, Paris, (série II, fasc. a), t. 321, p. 1179-1186.

Hassan T.H., Mudd G.C. & Twombley B.N. (1975).- The stratigraphy and sedimentation of the Thamama Group (Lower Cretaceous) of Abu Dhabi.- Ninth Arab Petroleum Congress, Dubai, 107(B-3), 11 p.

Jan du Chêne R., Busnardo R., Charollais J., Clavel B., Deconninck J.-F., Emmanuel L., Gardin S., Gorin G., Manivit H., Monteil E., Raynaud J.-F., Renard M., Steffen D., Steinhauser N., Strasser A., Strohmenger C. & Vail P.R. (1990).- Sequence-stratigraphic interpretation of Upper Tithonian-Berriasian reference sections in South-East France: a multidisciplinary approach.- Bulletin des Centres de Recherches Exploration-Production elf-aquitaine, Pau, vol. 17, n° 1, p. 151-181.

Mohammed A.R., Hussey J.A. & Reindl E. (1997).- Catalogue of Oman lithostratigraphy.- CD-ROM, Petroleum Development Oman, Muscat, 373 p.

Rasplus L. & Fourcade É. (eds.) et alii (1987).- Stratigraphie intégrée du sillon citrabétique (Sierra de Foncalent, Province d'Alicante, Espagne).- Géobios, Lyon, n° 20, fasc. 3, p. 337-387.

Virgone A. (1997).- Stratigraphie, sédimentologie et dynamique d'une plate-forme carbonatée : Le Berriasien supérieur - Valanginien basal de Basse Provence occidentale (S.E. France).- Thèse, Docteur de l'Université de Provence - Aix-Marseille I (nouveau régime), 6 juin 1997, 196 p.