1. Hosted by the Institute of Geology the
Berriasian Working Group 8th meeting– Prague, Czech, October 2012
Hosted by the Institute of Geology the
Academy of Science the Czech Republic and the Institute of Geology and Palaeontology, Charles University in Prague
Panboreal correlation of the Jurassic-Cretaceous boundary based on buchias
Victor Zakharov
Geological institute of RAS, Moscow

2. Jurassic and Early Cretaceous deposits of the boreal type cover the total area of about 25 million square kilometers
I can agree with Bill Wimbledon and his co-authors who noted that Boreal realm biogeographically was not homogenous and that within it, "there are at least six separate biozonations: Western Canada, Arctic, East Greenland, North Atlantic, Russian Platform and Siberia». In local biostratigraphic zonal scales, based on ammonites, in the same intervals one can easily see different taxa of specific, sometimes even of generic rank. However, the authors' final conclusion that these differences among certain ammonite scales make the panboreal correlations impossible, is far from the truth.

3. First of all, extensive biostratigraphic studies in ammonites in Arctic realm, which took place in the 60's and 70's of the XXth century, revealed numerous levels of high correlation potential. These levels provided a reliable framework for correlation of stages, substages and zones within the Jurassic and Neocomian

4. Parallel scales for Middle-Upper Jurassic of Northern Siberia
Second, in the 80-th and 90-th numerous parallel biostratigraphic scales were created, based on other groups of fossil – in particularly, belemnites, bivalves, foraminifera, ostracods, dinocysts and palynologic material. These scales allowed to make inter-regional correlations within the Panboreal SuperRealm more precise
(after Shurygin et al., 2011)

5. One of these parallel scales was the scale, based on bivalves belonging to the genus Buchia.
This scale has different resolution for different stratigraphic intervals.
The highest rates of Buchia evolution occur in the Late Volgian and in the Boreal Berriasian, so in this interval Buchia-based scale can provide us with very precise age determinations
(after Shurygin et al., 2011)

6. It’s necessary to mark that the distribution of buchias was not controlled with sedimentary facies
(after Dypvik, Zakharov, 2012, based on data by B. Nikitenko)

7. Distribution of buchias was not controlled by biogeography
Distribution of buchias was not controlled by biogeography. In all provinces of Panboreal Superrealm the biodiversity of Buchias is almost identical. Buchia zones, established in different provinces, often have the same names. This allows direct interregional correlations. Resolution of buchia scale is lower, than that of ammonite scales. But the rate of buchias evolution varies much over the geological time. The highest rate is observed in the upper Volgian and Boreal Berriasian. Two zones are established in the Upper Volgian and 4 - in the Boreal Berriasian

8. Terebratuloides Zone, Upper Volgian
Unschensis Zone, Upper Volgian-lowermost Ryazanian
Buchia terebratuloides, Northern Siberia
Buchia unschensis, Northern Siberia

9. Okensis Zone, Ryazanian Jasikovi Zone, Ryazanian
Buchia okensis, Northern Siberia
Buchia jasikovi, Northern Siberia

10. Tolmatschowi Zone, Ryazanian
Buchia tolmatschowi, Northern Siberia

11. Lower limit of the Cretaceous, placed in the base of ammonite zone Sibiricus/Maynci falls within Unschensis buchiazone, which is followed by Okensis buchiazone. The base of Okensis zone is located just above the base of the Kochi ammonite zone. Kochi zone is well-established all over the Panboreal Superrealm. Okensis buchiazone is traced even geographically wider, than Kochi Zone. This bundle of two zones from parallel scales allows us to consider it to be a nice marker for determining the lower limit of the Cretaceous system within the Panboreal Superrealm. First appearance of the ammonite Hectoroceras kochi would fix the base of the Cretaceous in all sections of the boreal type. New magnetostratigraphic data, obtained by Siberian specialists, well supports this proposal.

13. Upper Jurassic and Lower Cretaceous sections around the Laptev sea
1 – Chernokhrebetnaya and Osipa rivers; 2 – Bolshoi Begichev island; 3 - Nordvik; 4 – Anabar bay; 5 – Anabar river; 6 – Buolkalah river; 7 – cape Chekurovsky; 8 – cape Chucha; 9 – Stolbovoi island

14. Correlation of Upper Jurassic – Valanginian successions around the Laptev Sea

15. Bolshoi Begichev
Nordvik
Stolbovoi island
Cape Chucha
Anabar bay

16. Succession of Buchiazones from Northern Siberia to California
Chekurovka, Chucha (Zakharov,Kuzmi-chev, 2008)
No ammonites
Pacific ammonites

17. Paskenta (Northern California)

18. Grindstone section (Paskenta, Ca)
Base of the Berriasian
Buchia cf. okensis

19. Buchia okensis

20. Panboreal correlation of the Jurassic-Cretaceous boundary beds by buchiids
Direct correlation of Northern Siberia (blue) and Northern California (yellow) by Buchiazones

21. Buchia okensis, Northern Siberia (specimens from the single palaeopopulation)

22. Buchia okensis, Northern Siberia (specimens from the single palaeopopulation)

23. Buchia okensis
Lectotype, Sura river (Volga river basin)
Pechora river basin
Western Siberia

24. Buchia okensis
Lena river basin
Northern Siberia (Nordvik)
North-East Russia
New Siberian Islands

25. Buchia okensis
Northern Alaska
East Greenland
North Greenland (Peary Land)

26. Buchia okensis
Northern California
British Columbia

27. Geographical range of the Buchia okensis

28. Conclusion
The purpose of my presentation is to demonstrate you the opportunities of buchiids in detailed zonal biostratigraphy of Jurassic-Cretaceous boundary beds.
Using of Buchia zones allows correlations of the Upper Volgian and Berriasian sequences all over the huge area of distribution of boreal type deposits, covering the territory from the North Atlantic to the North Pacific.
The highest correlation effect is achieved by the joint use of ammonite zones and buchia zones. It’s clear that multidisciplinary approach involving all the macro- and micropaleontological objects, as well as physical and chemical methods of study, greatly increases the reliability of the stratigraphic results

29. Thanks for your attention!
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