Discussions about the Jurassic-Cretaceous boundary (Part II)

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

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