| mhorn | Дата: Friday, 26.08.2011, 23:31 | Сообщение # 1 |
Admin
Group: Администраторы
Posts: 1060
Reputation: 0
Status: Offline
| Nikitenko B.L., Knyazev V.G., Lebedeva N.K., Peshchevitskaya E.B., Kutygin R.V. (2011) Problems of Oxfordian and Kimmeridgian stratigraphy in northern Central Siberia (Nordvik Peninsula section) // Russian Geology and Geophysics. Vol.52. P.963–978.
pdf: http://rghost.ru/19432781
This article is mainly devoted to criticism of the recent article by Rogov & Wierzbowski (2009) which provides new biostratigraphical subdivision of the Kimmeridgian and Oxfordian of the Nordvik reference section (N.Siberia) on the base of new rich ammonite collection. This is the strange criticism, because it is based on the single misinterpreted ammonite from authors’ collection (see below) and long discussion concerning of ranges of foraminiferal zones against ammonite zones, i.e. here ammonite zonation is “improved” on the base of the foraminiferal data.
The Nordvik section is most known as excellent section with continuous succession around the J/K boundary, from Mid Volgian to the lowermost Hauterivian. Its Oxfordian and Kimmeridgian parts were studied only tentatively in terms of ammonite biostratigraphy until recent. With exception of single new record, described by Nikitenko et al. as “Amoeboceras ex gr. alternoides” which is in my opinion without doubts belongs to Middle Oxfordian Maltoniceras, such as M. kokeni or M. maltonense, other discussed ammonites came mainly from the paper by Voronets (1962), based upon field studies by Emeliantsev and Erschova and Elemiantsev in 30th (few other records were mentioned by Sachs et al., 1976 without any data concerning their positions within members). Only 4 ammonites from the Nordvik section were figured by Voronets, 3 as Kimmeridgian and 1 as Oxfordian. Among the Kimmeridgian ammonites most important are “A. ex gr. kitchini” (=A. cf. subkitchini), showed in the very low stratigraphical position and “A. cf. aldingeri” which was found above. But labels of these ammonites possibly were misinterpreted, because latter “A. cf. aldingeri” by its smooth outer whorls and character of ribbing belongs to the Upper Oxfordian Amoeboceras (such as A. regulare), while “A. ex gr. kitchini” is typical Lower Kimmeridgian Amoebites. Voronets also mentioned unfigured “Cardioceras cf. zenaidae” from the lower member of the section (~6 m above the base of the section), but this ammonite (CNIGR, sample 26, without museum no.) is very poorly preserved and could be assigned to either Cardioceras or Amoeboceras.
Nikitenko et al. criticized some ammonite determinations made by Rogov & Wierzbowski (2009): C. ex gr. blakei (l.c., pl.1,f.1) assigned by Nikitenko et al. to Amoeboceras because this ammonite “has distinct dense ribs, not typical of this species, at the beginning of the later whorl”. C. blakei is poorly known species, and except the Nordvik amnmonite only its holotype was figured in the published papers. But it has the same type of ribbing as our specimen – and such kind of ribbing with very high rib ratio in small-sized ammonite is entirely unknown in Amoeboceras!
Other criticized ammonite is Amoeboceras cf. serratum (l.c.pl.2,f.4), which should be, after Nikitenko et al., “marked by smoothed early whorls (D ≈ 30 mm)”, but, for example, Mesezhnikov et al. (1989) indicates presence of fine dense ribs at the diameter of the 10-20 mm in this species.
Fig. 4 includes some out-of-date elements, especially in the Kimmeridgian part of the “Boreal standard”: it is includes Aspidoceras acanthicum Zone (Submediterranean zone) and started from the Kitchini Zone, albeit now there is agreement within the Kimmeridgian WG that Ox/Km boundary shpuld be placed at the base of the Baylei/Bauhini Zone. Coincidens of the base of the JF40 Zone with the base of the Regulare Zone could not be proved by published data. Correlation of different zonation schemes with succession given by Rogov and Wierzbowski (2009) could confuse readers of this paper, because Ox/Km boundary here is placed within the Kitchini Zone of the Boreal Standard!
Amoeboceras ex gr. alternoides (Nikitenko et al., p.975, pl.1, fig.1-3) showing many features typical for Maltoniceras: inner whorls of this ammonite are smooth, and rare secondaries appeared first. In the true A. alternoides (see Mesezhnikov et al., 1989, pl.6-7) well-recognized primaries appeared in the very early whorls.
Using of dinocysts for correcting of the ammonite zonation seems also very strange: both index-species of the Aldorfia dictyota–Nannoceratopsis pellucida d-Zone has very wide ranges including Middle Jurassic and bulk of the Upper Jurassic (see Smelror & Lominadze, 1988 and correlation chart by Ogg et al. (https://engineering.purdue.edu/Stratigraphy/charts/Timeslices/5_JurCret.pdf) for details)
Zones showed in the fig. 3 with references to Basov et al., 1970; Zakharov et al., 1983 are missing in the cited papers and came from nowhere (from re-interpretation of old records by authors of the article)
Middle Jurassic - Lower Cretaceous ammonites & aptychi
|
| |
| |
