Wimbledon W.A.P. The Jurassic-Cretaceous boundary: an age-old correlative enigma // Episodes. 2008. Vol.31. No.4. P.423-428.
Latest issue of “Episodes” for year 2008 includes, among the other articles, paper by W.A.P. Wimbledon devoted to long-ranging controversies in correlation of the J/K boundary strata. Albeit Wimbledon gave more or less full review of recent advances in such correlation, there are many points in his paper which could be easily criticized.
Firstly he writes “That datum must be readily correlatable, traceable as much as possible around the world-utility being the major consideration” and when concluded that this boundary should be traced at the base of Jacobi zone. But this level not a best-traced among other proposals concentrated aroung J/K boundary: it well correlates only through Tethyan ammonite-rich sections.
Unfortunately many notions about J/K boundary beds of Russia, including used zonal scheme, correlation chart, gaps etc seems to be out-of date.
Thus I again emphasize most important points:
1. W.A.P.Wimbledon [WW] (p.426): “Though this seems far too early, if the approximately basal Berriasian Kachpurites fulgens zone is indeed conformable on the nikitini zone on the Volga: sediments suggest otherwise…”
M.Rogov [MR]: how lithological features can tell about biostratigraphic completeness of the section??? We have strong biostratigraphical evidences about gradual transition from Nikitini to Fulgens Zone in Russian Platform and from Exoticus Zone (which is really corresponds to uppermost Nikitini Zone nor Fulgens Zone as shown in fig.5) to Okensis Zone.
Thus gaps shown at the fig 5 within the Volgian succession strictly contradicts with many lines of biostratigraphical evidences
Correlation of J/K boundary beds of England and other regions (fig.5 in Wimbledon's article)
Why Berriasian not Purbeckian used here and as a stage which could be recognized in England? Palynology is a poor base for such a conclusions while paleomagnetics permits different interpretations of its black and white stripes
2. WW (p.426): “…how is it possible to correlate all or even a substantial amount of the Portlandian-early Berriasian with the Volgian of Russia, if 70 m of Portland beds is being equated with solely the nikitini biozone (30-40 cm at Gorodishche and Kashpir and 50 cm at Moscow)”
MR: lets take for comparison Volgian sections of the Russian North-East and Novosibirsk Islands! In these regions thickness of the Upper Volgian only attains more than 1000 m. Are these sections more complete in comparison with those of the Russian Platform (U.Volgian ~2-7 m)? Those of Subpolar Urals (~17 m)? Of Northern Siberia (~13-15 m)? No, if we considering their biostratigraphical completeness. Thickness is not the same as a geological time! We compare geological time by using of geological events, no more.
The same or very close zonal successions of ammonites, buchiids, forams etc was traced through hundreds of sections and many thousands of drill cores in Arctic regions from Barents sea to NE Russia, Alsaka and California. But nobody found new zones which were represented in other Boreal areas. Many sections and successions were carefully described, for example, by Mesezhnikov (1984; Kimmeridgian and Volgian Stages of north of the USSR. Leningrad: Nedra. 224 p. pdf: http://rogov.zwz.ru/Mesezhnikov,1984_KmvlgN_USSR.pdf ), Zakharov (1981; Buchiidae and biostratigraphy of Boreal Upper Jurassic and Neocomian // Trans. Inst. Geol. Geophys. Sib. Branch Ac. Sci. USSR. Vol. 458. 271 p. Pdf: http://rogov.zwz.ru/Zakharov, 1981_Buchiidae.pdf) and others
3. WW (p.427): “As is the identification of Boreal ammonites in the fittoni and albani biozones of Dorset, which is seemingly the only level where species similar to those of Russia, distinctive virgatotome and polygyrate ribbed forms, commonly occur. It is difficult to see a match for these anywhere on the Russian platform, except in the nikitini zone (or lower).”
MR: Why??? Ammonites from Nikitini Zone of Russian Platform usually lacking vigratotome ribbing, which is, instead, usually in their forerunners from Virgatus and Panderi zones
4. WW (p.427): “The recent identification of Subcraspedites sowerbyi also in the Epivirgatites nikitini zone on the Volga is a fascinating record. That species in eastern England has been described only from beds (the S. preplicomPhalus zone: Casey, 1973) which have been consistently correlated with the Craspedites nodiger zone. But its use (Zakharov et al., 2006) to suggest correlation of this nikitini zone with the preplicomphalus zone would mean that the nikitini zone would then have horizons equivalent to perhaps 5-6 ammonite biozones as defined in England”
MR: Why not? Ammonite zones are far from to be equal in time. Recent data show that true duration of successive zones can differ in ten times and even more. From the other hand, Nikitini Zone (which, as the Rjazanensis Zone, really is a sum of few zones) includes at least 6 distinct faunal horizons in its type region. 6 horizons = 6 “zones”. Good arithmetics, isn’t it?
References concerning duration of successive ammonite zones: McArthur et al. (2000) Strontium isotope profile of the Early Toarcian (Jurassic) Oceanic Anoxic Event, the duration of ammonite biozones, and belemnite palaeotemperatures // Earth Planet. Sci. Letters, 179, 269-285. pdf: http://www.es.ucl.ac.uk/people/mcarthur/McArthur%20Donovan%202000.pdf
5. WW (p.427): “A number of lines of evidence have led more consistently to acceptance that the rjasanensis zone correlates approximately with the paramimounum subzone, around magnetochron M16r, that is, at about the Praetollia (Runctonia) runctoni zone of E. England…”
MR: range of the “Rjasanensis Zone” and its contents changes significantly during last years, and last contributions to it detailed stratigraphy by Mitta proved that only upper part of this “zone” (which is really superzone) corresponds to lower part of the Upper Berriassian
A way forward.
6. WW (p.427): "but there are a number of datums which approximate, at least, to the base of the grandis zone, and provide potential proxies for its recognition and definition: for instance, the base of the Kachpurites fulgens ammonite zone, the FADS for Apiculatisporis verbitskaya, Amphorula monteilii and Warrenia californica, perhaps Gochteodinia villosa, and LADS for Dichadogonyaulax culmula and Sennonisphaera jurassica, the base of calpionellid zone B and so on"
MR: I am afraid many palynological "levels" listed above have not true correlative potential, and base of the Fulgens Zone lies at leat two zones below equeivalents of the basal Jacobi Zone in Boreal succession
But additionally we can add more well-traced level:
base of the Buchia okensis Zone, which is roughly corresponds to base of the Kochi Zone and Occitanica Zone
top of the Nodiger ammonite zone, which lies close to base of the zone "B" and well-traceable through Arctic
paleomagnetic reversals (especially distinct Kysuca and Brodno)... etc