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October 2001 discussions are related to the Cenomanian stratigraphy based on a report on SW Crimea (a contribution by L. Kopaevich). March 2001 discussions were related to the Lower Cretaceous stratigraphy of the Middle East (a report by B. Granier): Forum Nº 3. March - April 2000 discussions were related to Aptian bentonites, that is « altered volcanic ash » (a report by G. Friès et alii), and Aptian-Albian stratigraphic issues: Forum Nº 2 . January - February 2000 discussions were related to the Jurassic - Cretaceous boundary position, and the Valanginian relative sea level curve (a report by B. Granier): Forum Nº 1.
September 24, 2001 October 2001 by L. Kopaevich The description of the Cenomanian succession in SW Crimea will be the starting point for further discussion on the Cenomanian stratigraphy: The Cenomanian succession in south-western Crimea Introduction The Cenomanian of the south-western Crimea comprises 50-60 metres of rhythmically bedded marly chalks, chalky marls and limestones. The succession includes several region-wide erosion surfaces. Sedimentary sequences have been dated with ammonites, inoceramids, planktonic and partly benthic foraminifers.
Fig. 1a: Tectonic sketch-map of the Crimea.
Sedimentary units The Sebukhra section is a classic and almost continuous outcrop of the thickest Albian - Cenomanian succession in the south-western part of Crimea (see Fig. 1). The terminal Albian sequence ends with calcareous glauconitic sandstones of dark-green or rusty-brown colour. They contain thin intercalations of quartz gravel and tuffaceous material. The thickness of the terminal Albian ranges from 1.8 m to several centimetres, and wedges out completely in some places. The sandstones are known as the beds with Stoliczkaia and denoted as Alb33. These sandstones are of a condensed nature and their upper boundary represents the Lower/Upper Cretaceous boundary with a minor erosion surface.
Fig. 1b: The Cenomanian succession of the Sebukhra Mountain.
The Cenomanian succession consists of two sedimentary sequences, which are correlated with global cycles of sea-level change. The Lower Cenomanian contains of four lithological units (I-IV-1), while the Middle Cenomanian consists of three units (IV-2, V-VI). Unit IV is separated into two parts: IV-1 and IV-2. The silty glauconitic bed of the basal Cenomanian (unit I) passes upwards into alternating marls and marly limestones of units II and III. The highest few metres of the Lower Cenomanian contain layers of limestones (IV-1). The submarine erosion surface with the distinctive Thalassinoides-type burrows represents a Mid-Cenomanian nonsequence. This erosion surface is overlain by thin grey-green glauconite marl with pebbles of limestones and sandstones of the Late Triassic to Liassic Tavria Formation and pebbles of magmatic rocks of the Middle Jurassic. The Middle - Upper Cenomanian of the Crimea contains three lithological units - IV-2, V and VI. The beds above the erosion surface comprise an alternation of marls and hard limestones (IV-2). Unit V consists of rhythmically bedded grey marls and white limestones. These are bioclastic sediments (mudstones and wackestones) with a very small clastic content. The marls are usually strongly bioturbated. Both limestones and marls reveal cyclic vertical variation, controlled by orbitally forced Milankovitch-type climatic oscillations. Unit VI is a white chalk containing marls at 0.5-1.0 m intervals. Unit VI contains small erosion surfaces in all examined sections. There is considerable lateral variation in the succession immediately overlying the Late Cenomanian erosion surface in the Crimea. The completeness of sections increases from north-east to south-west in this area. In the Mender and Kizil-Chigir sections, a highly condensed glauconitic marl resting directly upon the erosion surface is overlain by flaggy chalks containing siliceous nodules. The erosion surface is overlain by 1 m of sandy and silty chalks with intraclasts and chalk conglomerates, debris flows, and with dark-grey lenses of thin diffused organic matter at Selbukhra Mountain. In the Aksudere and Belaya sections, 1 to 1.5 metres of organic-rich, brown and dark-grey variably laminated marls are present with silt-size quartz, glauconite and volcanic material, and with fish scales and Chondrites. This interval is a local representation of the Late Cenomanian "Oceanic Anoxic Event". Overlying the boundary "black shale" is the succession of thinly bedded, grey marls passing gradually upwards into light grey marls and marly limestones of the Lower Turonian (unit VII on Fig. 2).
Fig. 2: Intrazonal correlation of the Upper Cenomanian - Lower Turonian successions.
Stratigraphy and bioevents The terminal part of Albian (Alb33) is characterized by an assemblage of planktonic foraminifers of the Rotalipora appenninica Zone. The beginning of the Cenomanian coincides with the appearance of Rotalipora globotruncanoides - the zonal species for the lower part of the Cenomanian. Poorly preserved specimens of Mantelliceras sp. and the first appearance of Inoceramus virgatus are also common in the basal part of the Cenomanian stage (unit I). Units II and III contain poorly preserved inoceramids, Mantelliceras cf. dixoni and Pusozia planulata. The co-occurrence of abundant Orbyrinchia mantelliana and Sciponoceras baculoide, coincides with the first occurrences of Rotalipora cushmani and Lingulogavelinella globosa (40 cm above erosion surface between IV-1 and IV-2), and confirms a Middle Cenomanian age. This level is closely comparable with the northern Anglo-Paris Basin and Northern Germany. The boundary of the Middle and Upper Cenomanian cannot be fixed precisely, but two levels with the oyster Amphidonte obliquatum in the Selbukhra succession correspond with abundance of the same species in the Münster and Lower Saxony Basins in northern Germany and in southern England (Fig. 1, GALE et alii, 1999). Two planktonic foraminiferal Zones recognised around the Cenomanian/Turonian boundary are: (1) - Whiteinella archaeocretacea -with a low diversity or a complete absence of Foraminifera, this interval coincides with "black shale" facies; (2) - Praeglobotruncana oraviensis Zone (analogous to the Helvetoglobotrucana helvetica Zone), again shows a diversify of foraminifers which become very abundant (Fig. 2).  The second erosion surface within the upper part of unit VI coincides with a sub-disconformity erosion surface as in the Anglo-Paris Basin (JARVIS et alii, 1988; GALE et alii, 1999). However, the duration of the sub-disconformity hiatus was longer in the Crimean sections than in the Anglo-Paris Basin, based on foraminiferal data. The appearance of Mytiloides cf. labiatus, and foraminifers Dicarinella hagni, Whiteinella aprica, nannofossils - Quadrum gartneri mark the Cenomanian/Turonian boundary within the Whiteinella archaeocretacea Zone. The lower part of the Whiteinella archaeocretacea Zone coincides with the "black shale" facies and quite corresponds to the Oceanic Anoxic Event 2, widely known at the K/T boundary. The Crimean "black shale" has a relatively high content of organic Carbon - up to 7.2%. This interval is characterised by a positive shift of δ13C +4‰ - 5‰ (NAIDIN & KIYASHKO, 1994). The existence of "black shale" facies, with impoverished benthos, together with increased values of the Corg indicates the existence of an oxygen-minimum zone and the increase of anaerobic bottom conditions. Conclusions Lithological successions, facies and faunas are very similar in the Crimea, southern England Basin and Münster Basin of north-west Germany. The major hiatuses are very similar extent in the Crimea, Anglo-Paris and Münster Basins having a Mid-Cenomanian nonsequence and sub-disconformity erosion surface within the terminal part of the Cenomanian.
Acknowledgements Special thanks are due to Prof. H.S. EDGELL for reviewing the English manuscript and to Dr. B. FERRÉ. This paper corresponds to the abstract of an oral presentation that will be given at the "Colloque sur le Cénomanien" (October 18th- 24th, 2001) in Rouen - France. References GALE A.S., HANCOCK, J.M. and KENNEDY W.J. (1999): Biostratigraphical and sequence correlation of the Cenomanian successions in Mangyshlak (W Kazakhstan) and Crimea (Ukraine) with those in southern England. Bull. Inst. Royal des Science. Natur. de Belgique, Sciences de la Terre, vol. 69, suppl. A, p. 67-86. NAIDIN D.P. and KIYASHKO S.I. (1994): Geochemistry of Cenomanian/Turonian boundary deposits in the Crimean highlands. 2. Isotopic composition of carbon and oxygen; environment of organic carbon accumulation. Bull. of the Moscow Soc. of Natur., Geological Series, vol. 69, p. 59-74. [In Russian]. NIKISHIN A. M., ALEKSEEV A. S., KOPAEVICH L.F., YANIN B.T., BARABOSHKIN E. YU. and YUTSIS V.V. (1993): Cretaceous-Eocene sedimentation in the Shelf Alma Basin of Cimmerian mobile belt (Crimea): eustatic and tectonic influences. - In: P.R. VAIL (Ed.). Book 4. Sequence Stratigraphy Workshop, p. 1-74. May 20-May 30. Crimea. Ukraine. Vrije Universiteit Amsterdam - Moscow State University. Discussion Do you agree / disagree ? Let us debate on this topic and any related issues in the Guestbook (Read / Write) with the following reference: « Cenomanian »! Previous Forum Forum Nº 1 was dealing with the Jurassic - Cretaceous boundary position, and the Valanginian relative sea level curve (with a report by B. Granier). Forum Nº 2 was dealing with Aptian bentonites, that is « altered volcanic ash » (with a report by G. Friès et alii). Forum Nº 3 was related to the Lower Cretaceous Stratigraphy of the Middle East (a report by B. Granier).