8Nov2000

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8 November 2000

Dear colleagues,

I beg to not agree with interpretation of early occurrence of P.semichativae in Shishkat section, which have offered in their letter from 6.11 much-esteemed by me Charlie Sandberg and Willi Ziegler, in spite of the fact that in Lower hassi zone of Shishkat really occur listed by them taxa (except P.nasuta). In Shishkat are allocated all 11 zones of Ziegler and Sandberg, but not all Frasnian species have in it the same distribution, what was defined for them by these colleagues. The level of transgression coincides with the basis of Upper hassi Zone(Bardashev, Bardasheva, 2000). Anywhere above the similar sharp change of facies in Frasnian of Shishkat is not observed . Isn't it that these facts should be taken into account during definition of subdivisions?

As you can see from translation of the summary of my article (Bardashev, Sabirov, 1997) in Shishkat there are also other discrepancies with the prevalent points of view on some phenomena in Frasnian. However I don't consider that it's necessary to continue this discussion "on fingers" before the ending of the large monography on Frasnian of Shishkat, the work under which lasts almost 10 years and co-author of which, as well as in previous works on Shishkat, I hope will be my Teacher and Friend Papa Willi.

Bardashev, Sabirov, 1997, p.32. "Summarizing the above stated, it should be noted that:

1. The Shishkat section, being accessible, complete, and rich in well preserved fossils, is worth to be a global reference section for the Devonian. Despite the difficult economic situation in Tajikistan, it is necessary to establish here a National Geological reserve with all relevant functions (protection, research, demonstration, etc.).

2. Lithologically, the section is a classical example of pelagic deposits, in which the fine-grained carbonate sediments formed in situ are alternated with those of turbiditic flows (mud, grain, fragment). The latter are represented by organogenic-detrital (crinoid-brachiopod) limestone, variously graded, and, as the analysis of their fossils shows, are h o m o g e n o u s intraformational breccia.

3. In the Frasnian part of the section, all standard conodont zones and all variants of the Frasnian boundaries are distinguished. We agree with some investigators [Sandberg et al., 1989] in that the choice of the GSSP of the Middle/Upper Devonian boundary was not very good, because it could be correlated by the only species - the shallow-water Ancyrodella pristina Khal. & Chern. The best and closest to the base of the former Lower asymmetricus Zone are the levels coinciding with the bases of the present Upper falsiovalis and transitans zones. The latter is preferable, because it is only here that the three main taxa, which characterize the Lower asymmetrica Zone, occur together [3]: Mesotaxis (= Polygnathus) asymmetria (Bischoff & Ziegler), Palmatolepis transitans Muller, and Ancyodella rotundiloba (Bryant). As for the upper boundary of the Frasnian, we consider the level at the base of the Upper triangularis Zone most significant; above this in our section, many Frasnian conodonts disappear and the Famennian species radiation begins.

4. In recent years, the mass extinction of conodonts in the late Frasnian, associated with the Kellwasser event at the end of the linguiformis time, is generally recognized [Sandberg et al., 1988]. However, this event is not observed in the Shishkat Section. Many Frasnian conodonts, including manticolepid palmatolepids, that were extinct at the top of the linguiformis Zone in other regions, occur up to the base of the Upper triangularis Zone in Shishkat. The Frasnian/Famennian boundary deposits are mostly represented by clastic rocks, that certainly might make those, who support the sharp mass extinction in the late Frasnian, doubt that their fossils are all of the same age. However, the analysis of faunas from similar deposits common in Shishkat both in the Lower-Middle and Upper Devonian that we assign to turbidites, shows their stratigraphic homogeneity. The same is indicated by the disappearance of the Frasnian many-chambered foraminifers at the older stratigraphic level (the base of the Middle triangularis Zone), the lack of manticolepids in the Upper triangularis Zone, and the presence of transitional forms from Frasnian to Fammenian conodonts. The latter points to the invalidity of the almost globally accepted opinion that the Frasnian extinction was survived only by one species of the genus Palmatolepis - Palm. praetriangularis Ziegler & Sandberg -, which was the ancestor of all Famennian palmatolepids. In our opinion, the Frasnian conodont extinction occurred in the process of the evolution, and here we quite agree with the point of view formulated by O. Walliser, one of the authors of the eventual stratigraphy theory, in conversation during his visit to and study of he Shishkat Section. According to him, the mass extinction of the organisms, associated with the Kellwasser event, could be a long process lasting from the late linguiformis time up to the beginning of the Early Famennian crepida.

5. The wide development of turbidites, containing conodonts of both neritic (Ancyrodella, Ancyrognathus, Icriodus, Polygnathus) and pelagic (Mesotaxis, Palmatolepis) biofacies with alternative biozonations based on shallow-water assemblages.

6. The positions of the Frasnian boundaries, drawn by conodonts and foraminifers, are very close. Thus, the considerable changes in foraminifer composition, that characterize the Middle/Upper Devonian boundary, begin only at 60 cm below the officially approved GSSP of the boundary. The changes in fossil plant composition occur below. The Upper boundary of the Frasnian drawn by foraminifers is at the base of the Middle triangularis Zone, i. e. at the level, which has been used as the boundary until recently and is located just at 20 cm above the level that is being proposed for the Frasnian/Famennian boundary at present."

Always Yours,

Igor.