Original article

Calcareous nannofossil biostratigraphy of the Paleocene sedimentsof the Odessa Gas Field (NW Black Sea)§

Biostratigraphie à nannofossiles calcaires des sédiments d’âge paléocènedes gisements de gaz de la région d’Odessa (N-O de la Mer Noire)

Dennis Daniel Waga a,*, Aida Sergeivna Andreeva-Grigorovich b,Ninel Volodymirivna Maslun b

a National Enterprise ‘‘NAUKANAFTOGAZ’’, Volodymyra Velykogo street, 4, Lviv-79026, Ukraineb Institute of Geosciences, Academy of Science of Ukraine, 01054, Gonchara street, 55b, Kiev-54, Ukraine

Received 11 December 2008; accepted 30 October 2009

Available online 14 November 2009

Geobios 43 (2010) 33–43

Abstract

Calcareous nannofossils from Paleocene sediments of two boreholes (Odeska-6 and 20) from the north-western shelf of the Black Sea areexamined. Five nannofossil Zones are identified according to the standard zonations of Martini (1971) and Quillévéré et al. (2002): the Chiasmolithusdanicus Zone (NP3), the upper part of Ellipsolithus Macellus Zone (NP4b), the Fasciculithus tympaniformis Zone (NP5), the Heliolithus kleinpelliZone (NP6) and the Heliolithus riedelii Zone (NP8). This biostratigraphical work allows us to correlate the Bilokamian and Kachian regional stages ofthe Stratigraphic Scheme of Southern Ukraine (Zernetskiy et al., 1993) to the standard nannofossil zonations and, therefore, to the InternationalChronostratigraphic scheme. The presence of an unconformity between the Bilokamian and Kachian regional stages in the borehole section of Odeska-6 is suggested by Linear Sedimentary Rates estimated for the two boreholes. This unconformity corresponds to the upper part of the Chiasmolithusdanicus nannofossil Zone (NP3) and the lower part of Ellipsolithus Macellus (NP4a), and is estimated to last nearly 1.94 Ma.# 2009 Elsevier Masson SAS. All rights reserved.

Keywords: Calcareous nannofossils; Biostratigraphy; Paleocene; Odessa Gas Field; NW Black Sea Shelf

Résumé

Cette étude porte sur les nannofossiles calcaires des sédiments d’âge paléocène, récupérés dans deux forages effectués (Odeska-6 et 20) sur lamarge nord-ouest de la Mer Noire. Cinq zones à nannofossiles calcaires sont identifiées sur la base des travaux de référence de Martini (1971) etQuillévéré et al. (2002) : la Zone à Chiasmolithus danicus (NP3), la partie supérieure de la Zone à Ellipsolithus Macellus (NP4b), la Zone àFasciculithus tympaniformis (NP5), la Zone à Heliolithus kleinpelli (NP6) et la Zone à Heliolithus riedelii (NP8). La reconnaissance de ces cinqZones permet de corréler les étages régionaux Bilokamien et Kachien du schéma stratigraphique de l’Ukraine du Sud (Zernetskiy et al., 1993) auxzonations standard à nannofossiles et, in fine, au schéma Chronostratigraphique International. Nos données sur les taux de sédimentations linéairesestimés pour les deux forages suggèrent la présence d’une discordance entre les deux étages régionaux Bilokamien et Kachien dans le forageOdeska-6. Cette discordance, correspondant à la partie supérieure de la Zone à nannofossiles Chiasmolithus danicus (NP3) et à la partie inférieurede la Zone à Ellipsolithus Macellus (NP4a), a une durée estimée d’environ 1,94 Ma.# 2009 Elsevier Masson SAS. Tous droits réservés.

Mots clés : Nannofossiles calcaires ; Biostratigraphie ; Paléocène ; Gisement d’Odessa ; Marge N-O de la Mer Noire

§ Corresponding editor: Emanuela Mattioli.* Corresponding author.

E-mail addresses: wagadennis@rambler.ru, dendani@ukr.net (D.D. Waga).

0016-6995/$ – see front matter # 2009 Elsevier Masson SAS. All rights reserveddoi:10.1016/j.geobios.2009.10.001

1. Introduction

The NW shelf of the Black Sea is an important area for oiland gas exploration. It belongs to the Black Sea-Crimea oil andgas province (Gozhyk et al., 2006). In the past 30 years, this partof the Black Sea has been the object of intense prospecting and

.

Fig. 1. (A) Map showing the position of the study area in Europe and the IR bathymetry of the Odessa structure. The position of the two studied boreholes, Odeska-6and 20, is also shown. (B) Structural and tectonic map of the Black Sea region (modified after Tugolesov et al., 1985) and location of the Odessa structure. 1,Predobrogean Trough; 2, Karkinit Trough; 3, North-Azov Trough; 4, Azov Ridge; 5, Kanevsko-Berezansky Ridge; 6, Temashev Step; 7, North Dobrogean FoldingZone; 8, Central Dobrogean Folding Zone; 9, Gubkin Ridge; 10, Krayeva Step; 11, Kalmitsky Ridge; 12, Almin Basin; 13, Central Crime Arch; 14, Mountain CrimeaFolding Zone; 15, Feodosiysky Projection; 16, Indolo-Kuban Through; 17, Sorocin Trough; 18, Kerch-Taman Trough; 19, Anapsky Projection; 20, Pallas Rise; 21,Shatsky Rise; 22, Western Caucasus Folding Zone; 23, Tuapse Trough; 24, Moesian Platform; 25, Istryisky Trough; 26, Bulgurian Step; 27, West Black Sea Basin; 28,Tetyaeva Rose; 29, Andrusov Ridge; 30, East Black Sea Basin; 31, Shatsky Ridge; 32, Guduata Arch; 33, Escher Trough; 34, Ochamchira Arch; 35, StrandzhaFolding Zone; 36, Burgass Trough; 37, Balcan Folding Zone; 38, Low Camchian Trough; 39, Zonguldak Step; 40, Sinop Trough; 41, Arkhangelsky Ridge; 42,Giresun Basin; 43, Muratov Ridge; 44, Trabzon Projection; 45, Adzharo-Trialet Folding Zone; 46, West Pontide Folding Zone; 47, East Pontide Folding Zone; 47,East Pontide Folding Zone; A, Krylov Trough; B, Kiliysko-Zmeiynoe High.

D.D. Waga et al. / Geobios 43 (2010) 33–4334

deep drilling. The high-potential oil and gas horizons areassociated with sediments of Lower Cretaceous, Paleogene and,partly, Miocene. The most productive area of the shelf is locatedto the west, near the Kiliysko-Zmeinoye high and GubkinRidge (Fig. 1(B)). In this area, 12 gas and gas-condensate fieldsare recorded in Paleocene sediments (Gozhyk et al., 2006). Theoil and gas potential has also been proven in the Krayova Step,where the Lower Paleocene sediments are the main oilreservoirs.

The Odessa Gas Field is located in the western part of theKarkinit-Northern Crimean trough (Fig. 1). Siliciclastic andcarbonate sediments of Cretaceous, Paleogene and Neogeneages are present in the geological structure of the gas field.Among them, the Paleogene sediments are the most importantin gas exploration. They are widely distributed and well-developed in this area. These sediments have been recovered indeep boreholes, in proximity of the structures of WestOlenivska, Crymska, Schtormova, Selskogo, Gamburceva,Golitsenska and Schmidt (Krayeva and Lyulyeva, 1984).According to Benton (1997), the Odessa gas field has gasreserves of about 426.7 billion cubic feet (Bcf).

A number of studies were performed in this area onstructural geology, stratigraphy and lithology. Despite all these

studies, the geology of the Paleogene and Neogene sedimentsof the NW shelf of the Black Sea still remains poorly known.The present paper, focusing on Paleocene nannofossilbiostratigraphy of two boreholes in the Odessa structure, hasboth a scientific and an applied interest, as this area is one of themost important oil and gas regions in Ukraine.

2. Stratigraphic framework

A biostratigraphical scheme of the Paleocene sediments ofthe NW shelf of the Black Sea has been recently presented(Gozhyk et al., 2006). The biozonation is based on planktonicforaminifera and calcareous nannofossils (Fig. 2). ThePaleocene sediments unconformably lay above Upper Cretac-eous rocks, and are overlaid by Lower Eocene sediments.According to the stratigraphic scheme in Fig. 2, two sub-seriesand stages are distinguished within the Paleocene. The LowerPaleocene sub-series is characterized by the local Bilokamianstage, which comprises the Gromov Formation, subdivided intoa lower and upper part. The Lower Gromov subformationconsists of two members. The lower member is represented bylimy clays and marls (Schtormove, Karkinitska, Selskogohighs). This part of the section corresponds to the Eoglobiger-

Fig. 2. Stratigraphy of the Paleocene of the Crimea-Caucasus region and regional stratigraphic divisions compared to the Paleocene chronostratigraphic scheme ofBerggren et al. (1995). Lithological characteristics of the studied sediments are also provided. After Gozhyk et al. (2006).

D.D. Waga et al. / Geobios 43 (2010) 33–43 35

ina eobulloides-Globoconusa daubjergensis foraminifer Zones,and the Cruciplacolithus tenuis s.s. nannofossil Zone (Gozhyket al., 2006). The upper member is characterized by a decreasein CaCO3 content, and is represented by organic-rich, detriticmarls that correspond to the Acarinina inconstans foraminiferZone. The age of the Lower Gromov subformation is estimatedas Early Paleocene (Danian). The Upper Gromov subformationis widely distributed along the structural highs of the NW BlackSea Shelf. Lithologically, this subformation is mainlyrepresented by limestones and marls and, rarely, by sideriticrocks. The Upper Gromov subformation corresponds to theMorozovella angulata foraminifer Zone, and to the Ellipso-lithus macellus and Fasciculithus tympaniformis (lower part)nannofossil Zones. The age of this subformation is LatePaleocene (lower part of Selandian). Thus, the Bilokamianstage is diachronous as it may range from the Early Paleocene(Danian) to the base of the Late Paleocene (Selandian).

The Upper Paleocene sub-series is locally represented by theKachian regional stage (Fig. 2). This stage corresponds to theLazurnoye Formation, which is recorded in most of thestructural highs. Lithologically, this formation consists of greenand grey marls, limestones, siltstones, and clays with dark-coloured inter-layers. Three layers are distinguished within theLazurnoye Formation: a lower layer is composed of marls, limyclays, and limestones. It corresponds to the Igorina djanensisforaminifer Zone. The number of foraminifer specimens and

the CaCO3 content both decrease in the intermediate layer,which corresponds to the Acarinina subsphaerica Zone. Theupper layer corresponds to the Acarinina acarinata Zone, and isdominated by siliciclastic, carbonate-depleted siltstones andclays (Gozhyk et al., 2006).

3. Material and methods

The Odessa structure is located 130 km far from the Odessaharbour (Ivan’uta, 1998). Recently, two boreholes (Odeska-6and 20), which were drilled in the central and north-east part ofthe Odessa structure (Fig. 1(A)), recovered Paleocene andEocene sediments.

A detailed lithological study was performed in the twoboreholes. Calcareous nannofossils and foraminifera wereanalysed from the Paleocene interval of both boreholes. Theresults obtained from these two groups of micro-organismswere compared and correlated. Previous results from otherboreholes of the North Western shelf area were integrated to theresults of the present work.

The Odeska-20 borehole was drilled for a total depth of1566 m. The Lower Paleocene section consists of grey andlight-grey massive limestones bearing carbonate-siliceousintraclasts. The Upper Paleocene consists of an alternationof grey and dark-grey sandstones and silts, and organic-richbiogenic marls. Fifty-five core samples were collected from the

Fig. 3. Nannofossil zonal markers recorded in the two studied boreholes, and biostratigraphy of Paleocene sediments from Odeska-6 and 20 boreholes. Thebiostratigraphical framework was established based on comparison with standard biozonation schemes (Martini, 1971; Okada and Bukry, 1980; Quillévéré et al.,2002).

D.D. Waga et al. / Geobios 43 (2010) 33–4336

interval comprised between a depth of 1452 and 1566 m. Thesamples were taken every 1.5–2 m: sampling space was lesseraround lithological changes.

The total depth of borehole Odeska-6 is 1635 m. The LowerPaleocene is characterized by marls, organic-rich and detriticlimestones, and spongoliths. The Upper Paleocene comprisescarbonate-poor siltstones, marls and silty, organic-rich, detriticlimestones. Thirteen samples were collected from thePaleocene interval (1419.2–1625.2 m).

Smear-slides of 68 samples were prepared following thestandard technique described in Bown and Young (1998).Slides were analyzed using a Polam-211 light-microscope at�840–1200 magnification, in natural and crossed-polarizedlight. The standard Paleogene nannofossil Zones of Martini(1971), Okada and Bukry (1980), and Quillévéré et al. (2002)were used. The taxa used for biozonation are listed in Fig. 3.

Sample material, smear slides and foraminifera are housedat the Institute of Geological Sciences of the National Academyof Science of Ukraine. Some of the best-preserved specimenswere photographed under scanning electronic microscopeJEOL-JSM-T220A (Fig. 4). The slides for scanning microscope

analysis were coated by copper and studied at a magnificationof �10000–20000.

4. Results

The nannofossils in the studied samples are characterized byvariable diversity, abundance and preservation. They are fewand rare in compact siltstones, and are absent in grey,micaceous siltstones and sandstones. In general, preservationof nannofossils throughout the borehole sections is poor.Specimens showing moderate to poor preservation dominate inmassive limestones and micaceous shales. Generally, speciesrichness varies between 6 and 14; the highest species richness(22 taxa) is observed in a sample at the depth of 1424.2 m ofborehole Odeska-6.

4.1. Borehole Odeska-6

The stratigraphic distribution of nannofossils from theborehole Odeska-6 is reported in Fig. 5. Calcareous nanno-fossils are present in all the studied samples. Preservation is

Fig. 4. Pictures of selected nannofossil taxa taken in Scanning Electron Microscope. 1. Eprolithus sp., Odeska-20 (1489 m). 2. Cylindralithus sp., Odeska-6(1424.2 m). 3. Toweius tovae, Odeska-6 (1424.2 m). 4. Prinsius bisulcus?, Odeska-6 (1424.2 m). 5. Fasciculithus sp., Odeska-20 (1489 m). 6. Placozygus sigmoides,Odeska-6 (1424.2 m). Scale bar: 1 mm.

D.D. Waga et al. / Geobios 43 (2010) 33–43 37

poor for the first two samples and moderate for the rest of thesamples. The Chiasmolithus danicus Zone (NP3/CP2) corre-sponds to the interval 1625.2–1544 m. The Zone is definedbased on the presence of Chiasmolithus aff. Ch. danicus. The

most common species in the assemblage are Coccolithuspelagicus, Neochiastozygus sp., Toweius sp., Thoracosphaerasp., Sphenolithus sp., Zygodiscus sp., disaggregated pentalithsof Braarudosphaera, and Coccolithus sp. Reworked and

Fig. 5. Range chart of calcareous nannofossils in the Odeska-6 borehole plotted against lithostratigraphy. Standard nannofossil zonations are shown on the right side.Underlined occurrences indicate zonal markers.

D.D. Waga et al. / Geobios 43 (2010) 33–4338

overgrown Cretaceous species are also typical of this interval:Watznaueria barnesiae, Arkhangelskiella cymbiformis, Miculastaurophora, Broinsonia parca, Kamptnerius? sp., Eiffelithussp., and Gartnerago sp.

Although the zonal marker Ellipsolithus macellus ismissing, the Subzone NP4b (named here Sphenolithus primusSubzone) defined by Quillévéré et al. (2002) can be recognizedin the 1544.0–1521.4 m interval. This Subzone is defined as theinterval comprised between the first occurrence (FO) ofSphenolithus cf. S. primus (1544.0 m) and the FO ofFasciculithus tympaniformis (1521.4 m). The lower limit ofthis Subzone is, however, doubtful since only one specimen ofSphenolithus cf. S. primus has been recorded. The absence ofthe two marker species E. macellus and S. primus from theLower Danian sediments may be explained either by thepossible presence of an unconformity, or by a poor nannofossilpreservation. One specimen of Ellipsolithus macellus wasrecognized in a sample at 1515.4 m that, according to ourzonation, belongs to the NP5 Zone. The species commonlyrecorded in the Subzone NP4b are: Thoracosphaera sp.,

Toweius sp., Chiasmolithus aff. C. danicus, Cruciplacolithustenuis, Zygodiscus sp., Coccolithus sp., Fasciculithus sp.,Prinsius sp., Braarudosphaera bigelowi, Neochiastozygus sp.,Ericsonia cf. E. robusta, and Thoracosphaera saxea. ThisSubzone is also characterized by a certain number of reworkedCretaceous species: Watznaueria barnesiae, Arkhangelskiellacymbiformis, Micula staurophora, Broinsonia parca, Gartner-ago sp., and Rhagodiscus sp.

The Fasciculithus tympaniformis Zone (NP5/CP4) isrecognized in the interval 1521.4–1475.55 m. The upper limitof this Zone is based on the FO of Heliolithus cf. H. kleinpelli.The assemblage of this Zone is represented by moderatelypreserved species, such as Coccolithus pelagicus, Ericsoniasubpertusa?, Toweius sp., Fasciculithus sp., Toweius cf. T.rotundus, Zygodiscus sp., Ericsonia robusta, Thoracosphaerasp., Fasciculithus involutus, Prinsius sp., Fasciculithus cf. F.involutus, Braarudosphaera bigelowi, Toweius pertusus (cra-ticulus), Coccolithus eopelagicus, and Fasciculithus pileatus?The only reworked species recorded in this interval isWatznaueria barnesiae.

D.D. Waga et al. / Geobios 43 (2010) 33–43 39

The interval 1475.55–1424.2 m corresponds to the Helio-lithus kleinpelli Zone (NP6/CP5), which is identified by thepresence of the marker species. Apart from the zonal marker,the assemblage of this Zone is represented by Fasciculithustympaniformis, Thoracosphaera sp., Toweius pertusus (crati-culus), Fasciculithus sp., Fasciculithus cf. F. ulii, and Prinsiussp. The only reworked Cretaceous species recorded in thisinterval is Watznaueria barnesiae.

The Heliolithus riedelii Zone (NP8/CP7) is recognized in theinterval 1424.2–1419.2 m. It is defined on the basis of the FO ofthe zonal marker; the upper limit of this Zone is not identified.The highest diversity of 22 species is observed in sample 3259(at 1424.2 m). The common species recovered in theassemblage are: Pontosphaera sp., Heliolithus conicus,Sphenolithus cf. S. anarrhopus, Heliolithus sp., Prinsiusbisulcus, Fasciculithus tympaniformis, Coccolithus eopelagi-cus, Fasciculithus sp., Heliolithus kleinpelli, Discoaster cf. D.elegans, and Heliolithus cf. H. cantabriae. Five reworkedspecies are also recorded: Watznaueria barnesiae, Cretarhab-dus cf. C. conicus, Micula staurophora, Eiffellithus sp., andGartnerago sp.

The base of the studied borehole, namely the intervalbetween 1510.0 and 1635.0 m is characterized by the followingforaminifer taxa: Oridorsalis plummerae, Eoglobigerina

Fig. 6. Range chart of calcareous nannofossils in the Odeska-20 borehole plotted aside. Underlined occurrences indicate zonal markers.

microcellulosa, Subbotina triloculinoides, Subbotina nana,Subbotina trivialis, and Anomalina danica amongst others.These taxa indicate an Early Paleocene age, corresponding tothe local Bilokamian stage. The upper interval (1420.25–

1496.6 m) contains the following foraminifer taxa: Morozo-vella velascoensis, Globorotalia pseudomenardii, Subbotinavarianta, etc., that are typical for the Late Paleocene, Kachianstage.

4.2. Borehole Odeska-20

Compared to the previous borehole, Odeska-20 recovered ashorter portion of the Paleocene. Calcareous nannofossils areabundant throughout the section; they are well-preserved andmostly common in silty limestones or marls, and rare insiltstones. They are totally absent in layers of grey, carbonate-depleted, micaceous siltstones. The ranges of calcareousnannofossils are shown in Fig. 6.

The Fasciculithus tympaniformis Zone (NP5/CP4) isrecognized in the interval 1563.1–1544.0 m, based on thepresence of the marker species. The upper limit of this Zone isplaced according to the FO of Heliolithus kleinpelli. Theassemblage of the NP5 consists of moderate-to-poorlypreserved specimens of Coccolithus sp., Toweius sp., Spheno-

gainst lithostratigraphy. Standard nannofossil zonations are shown on the right

D.D. Waga et al. / Geobios 43 (2010) 33–4340

lithus sp., Fasciculithus cf. F. involutus, Prinsius cf. P. martini,Prinsius bisulcus, Ericsonia subpertusa, Campylosphaera dela,Neochiastozygus sp., Fasciculithus sp., Toweius pertusus(craticulus), Coccolithus pelagicus (small), Thoracosphaerasp., Ericsonia cf. E. robusta (small), Coccolithus pelagicus,Chiasmolithus danicus, Prinsius sp., Sphenolithus cf. S. primus,Chiasmolithus sp., Toweius eminens, and Heliolithus conicus.Reworked specimens of Hornibrookina sp., Arkhangelskiellacymbiformis, Micula staurophora and Watznaueria barnesiaeare also present.

The interval 1544.0-1511.22 m represents the Heliolithuskleinpelli Zone (NP6/CP5). The base of this Zone ischaracterized by the presence of overgrown, large specimensof the zonal marker. Most of the species recorded in theassemblage are already present in the previous NP5 zone:Coccolithus sp., Sphenolithus sp., Fasciculithus tympaniformis,Neochiastozygus sp., Thoracosphaera sp., Ericsonia subper-tusa (small), Coccolithus pelagicus, Markalius inversus,Chiasmolithus danicus, Prinsius sp., Sphenolithus sp., Chias-molithus sp., Toweius eminens, and Heliolithus cf. H. conicus.Some other species, namely Chiasmolithus consuetus, Helio-lithus sp., Chiasmolithus bidens, Fasciculithus cf. F. ulii,Fasciculithus cf. F. shaubii, and Cruciplacolithus tenuis arealso typically recorded in this Zone. Reworked Cretaceousspecies consist of Micula staurophora and Watznaueriabarnesiae.

The interval between 1492.8 and 1510.0 m was not sampled.Possibly the Discoaster mohleri (NP7) Zone corresponds to thisinterval, although the presence of an unconformity is alsopossible.

The interval 1492.0–1457.75 m represents the Heliolithusriedelii Zone (NP8/CP7). The upper limit of the zone isdoubtful. It is based on the presence of Toweius sp.,Thoracosphaera sp. and Coccolithus pelagicus in an intervalthat is very poor in nannofossils (see Fig. 6). Most probably theupper limit of the NP8 Zone may be placed at a depth of1470.8 m, because the overlaying 1470.8–1450.0 m intervalrepresents the upper layer of the Lazurnaya Formation, whichconsists of siliciclastic carbonate-depleted siltstones and clays.As mentioned earlier, this interval corresponds to the Acarininaacarinata foraminifer Zone and to the Discoaster multiradiatus(NP9) nannofossil Zone. The species commonly recorded in theHeliolithus riedelii (NP8) Zone are: Coccolithus sp., Toweiussp., Fasciculithus tympaniformis, Neochiastozygus sp., Fasci-culithus sp., Toweius pertusus, Ericsonia robusta (small),Markalius inversus, Coccolithus pelagicus, Prinsius sp.,Heliolithus sp., and Chiasmolithus bidens. The FOs ofPlacozygus cf. P. sigmoides, Discoaster sp., Fasciculithus cf.F. billii, Heliolithus cf. H. cantabriae, and Pontosphaera sp. arealso recorded in the NP8 Zone. Watznaueria barnesiae, is theonly reworked Cretaceous species.

The foraminifer record of the interval comprised between1554.0 and 1566.0 m consists of the following commonspecies: Eoglobigerina microcellulosa, Globigerina varianta,Subbotina nana, Subbotina trivialis, and Brotzenella praeacutathat are all typical of the Early Paleocene, Bilokamian regionalstage. Late Paleocene foraminifer species (Kachian regional

stage) were found in the interval 1453.4–1519.05. These are,among others: Karreriella zolkaensis, Acarinina acarinata,Morozovella angulata, and Acarinina subsphaerica.

5. Discussion

5.1. Correlations

The nannofossil Zones recognized in this work are definedby Martini (1971), Okada and Bukry (1980) and Quillévéréet al. (2002). These Zones allowed us:

� T

o date the recovered sediments with respect to astratigraphical standard scheme; � T o correlate them with the regional stages of the Paleogene of

the Black Sea Shelf (Gozhyk et al., 2006), to thestratigraphical chart of adjacent areas (Zernetskiy et al.,1993), and to the global chronostratigraphic chart ofGradstein et al. (2004).

The NP2-NP3 zones in the NW shelf of the Black Seacorrelates to the lower part of the Gromov formation (LowerGromov subformation) of the Bilokamian regional stage(Fig. 2), and to the Danian stage of Gradstein et al. (2004).With respect to the onshore sections (steppe and mountains ofCrimea, Prechornomorskaya monocline, Northern Caucasus),this interval characterizes the lower parts of the Bilokamian andElburganian regional stages (Waga, 2007).

The NP4b Zone of the Quillévéré et al.’s (2002) zonation,named here Sphenolithus primus Zone, and the lower half of theNP5 Zone correspond to the lower half of the Upper Gromovsubformation of the Bilokamian regional stage (Fig. 2) and tothe lower part of Selandian stage of Gradstein et al. (2004). Inthe Prechornomorskaya monocline, zones NP4 and NP5 aredefined within the Inkermanian regional stage, while inNorthern Caucasus sections they are identified in the upperhalf of Elburganian regional stage (Waga, 2007).

Based on foraminifera and lithology, the boundary betweenthe Bilokamian and Kachian regional stages in boreholeOdeska-6 is placed at the level 1510 m. In borehole Odeska-20,this boundary is placed at the depth of 1544 m. The NP6 andNP8 Zones characterize the middle part of the LazurnayaFormation of the Kachian regional stage (Fig. 2), and correlateto the Upper Selandian and Lower Thanetian of Gradstein et al.(2004) timescale. In Southern Ukraine (PrechornomorskayaMonocline, Steppe and Mountains of Crimea), these Zonescorrespond to the Kachian regional stage. In Northern Caucasussections, they are found within the Nalchik or Goryachiy Kluchregional stages (Waga, 2007).

5.2. Linear sedimentary rates and unconformities

In order to compare the sedimentary rates of the twoboreholes studied and to detect possible unconformities, linearsedimentation rates (LSRs) were calculated for each borehole.The sedimentary rates for borehole Odeska-6 are based on fivenannofossil bioevents, while for Odeska-20 they are based on

D.D. Waga et al. / Geobios 43 (2010) 33–43 41

three nannofossil events. The ages of the datums used toreconstruct the sedimentary pattern of the Odeska-6 and 20boreholes Paleocene sections were estimated according tobioevent ages provided by Berggren et al. (1995) andQuillévéré et al. (2002). A comparison of LSRs of the twoboreholes (Figs. 7 and 8) indicates a different depositionalpattern. Sedimentary rates vary between 5 to 70 m/Ma for theOdeska-6 borehole, and from 14.6 to 53.47 m/Ma for theOdeska-20.

Nannofossil events show that the Paleogene interval isincomplete in both boreholes. In fact, at least two unconfor-mities are detected in the Paleocene. A first unconformity ispresent in the borehole Odeska-6 within the Bilokamian stage(Gromov Formation), and corresponds to the absence of thelower part of the Ellipsolithus macellus Zone (NP4a) ofQuillévéré et al. (2002). A second unconformity showing amuch shorter duration corresponds to the Discoaster gemmeus(NP7) Zone. This second unconformity is probably presentwithin the Kachian stage (Lazurnaya Formation) of both

Fig. 7. Depth of nannofossil bioevent datums (mainly FOs; y-axis) in the Odeska-6 bTwo unconformities are indicated by zigzag lines.

boreholes. According to the zonations of Berggren et al. (1995)and Quillévéré et al. (2002), the approximate duration of thefirst unconformity is estimated to about 1.94 Ma, while thesecond one lasted 0.7 Ma.

The presence of unconformities can be deduced from theage-depth plots that were draft for each borehole (Figs. 7 and8). The unconformity within the Bilokamian stage in Odeska-6 (Fig. 4) can be easily recognized. At the base of theBilokamian stage, a low accumulation rate is calculated(12.9 m/Ma). After the hiatus, the sedimentary rate increasedup to 70 m/Ma. This higher accumulation rate may indicatean increase of sediment supply probably related to increasingsubsidence of the basin in the Middle Selandian. Thedifferences in sedimentary rates within the Fasciculithustympaniformis Zone in Odeska-6 and Odeska-20 can beexplained by a potential hiatus at Odeska-20. The lower limitof the NP5 Zone was tentatively dated as 59.2 Ma (Berggrenet al., 1995), and therefore the sedimentary rate is lower(14.6 m/Ma) than at Odeska-6 (35.3 m/Ma).

orehole plotted against absolute ages (x-axis). Sedimentary rates are also shown.

Fig. 8. Depth of nannofossil bioevent datums (mainly FOs; y-axis) in the Odeska-20 borehole plotted against absolute ages (x-axis). Sedimentary rates are alsoshown.

D.D. Waga et al. / Geobios 43 (2010) 33–4342

During the second half of the Paleocene, the two boreholesshow rather similar, relatively high sedimentary rates. Betweenthe FOs of Heliolithus kleinpelli (characterizing the NP6 Zone)and Heliolithus riedelii (marker of the NP8 Zone), sedimentaryrate is higher at Odeska-6 (46.6 m/Ma) than at Odeska-20(29.8 m/Ma), suggesting that a higher sediment supply or amore important accommodation space occurred at Odeska-6.Sedimentary rate of Odeska-6 drops down to 5 m/Ma in theupper part of the borehole, while in the same interval atOdeska-20 it varies from 36.75 to 53.47 m/Ma (Figs. 7 and 8).However, these different accumulation rates may depend onwhere the upper limit of the Heliolithus riedelii Zone is placedat borehole Odeska-20. The exact position of this limit is in factdoubtful.

The unconformity recorded in Odeska-6 within theBilokamian stage is widely distributed in the NW Black Seaarea. In another borehole, Odeska-2, located in the Odessageological structure (Fig. 1), the unconformity has the sameduration (Gozhyk et al., 2006). In the borehole Bezimenna-2,located northward of the Odessa structure, the duration of thishiatus was longer based on the absence of the intervalcorresponding to the Chiasmolithus danicus and Fasciculithustympaniformis Zones (Gozhyk et al., 2006). Onshore Paleocene

sections of the Mountains of Crimea and PrechornomorskayaMonocline also display this unconformity (Waga, 2007).

A correlation with synchronous sediments from other wellsof the same region and with onshore sections of Crimea andPrechornomorskaya Monocline shows that all these boreholeshave common stratigraphical features. None of these Paleocenesections is complete. The first important hiatus occurs at theCretaceous/Tertiary boundary. It is present in most of theboreholes of the NW Black Sea Shelf (Olympiyska-400,Bezimenna-2, Desantna-1, Gamburtseva-2, etc.; Gozhyk et al.,2006), and is also recognized in onshore sections from themountains of Crimea (at the west) through the NorthernPrechornomor monocline, East Precaspian area in the LowerVolga region, to the east until the Southern Turkmenistan(Waga, 2007). The only exception to this widespreadunconformity is represented by the sections in the NorthCaucasus area, which are continuous across the Cretaceous/Tertiary boundary.

6. Conclusions

In spite of the fact that the NW shelf of the Black Sea is animportant area for oil and gas exploration, few works exist on

D.D. Waga et al. / Geobios 43 (2010) 33–43 43

correlation of regional chrono- and lithostratigraphy to theInternational Chronostratigraphic scheme. In this work, 64nannofossil species are identified in the samples of twoboreholes of the NW Black Sea Shelf, namely Odeska-6 and 20.Among these taxa, 10 species are reworked from Cretaceoussediments. The nannofossil biozonation (mainly based on thefirst occurrences of nannofossil markers) allowed us to date thetwo borehole sections and to correlate them to the regionalStages as well as to the International Chronostratigraphicframework. Five nannofossil Zones are identified in the studiedinterval: the Chiasmolithus danicus Zone (NP3), the upper partof the Ellipsolithus macellus Zone (NP4b), named hereSphenolithus primus Subzone, the Fasciculithus tympaniformisZone (NP5), Heliolithus kleinpelli Zone (NP6) and Heliolithusriedelii Zone (NP8).

According to the Paleogene stratigraphic scheme of the NWBlack Sea Shelf (Gozhyk et al., 2006), the range of the NP2-NP3, the upper part of NP4b and the lower part of NP5 Zonescorresponds to the Bilokamian regional stage, correlative to theEarly Paleocene (Gromov formation). The upper part of theNP5, the NP6 and NP8 Zones correlate with the Kachianregional stage (Lazurnaya Formation).

The Gromov Formation at Odeska-6 borehole is incomplete.A hiatus between the lower and upper part of this Formation isdetected on the basis of the absence of the upper part of theChiasmolithus danicus Zone and the lower part of theEllipsolithus macellus Zone (NP4a). The relative duration ofthis hiatus is estimated to about 1.94 Ma. The secondunconformity, with a duration of 0.7 Ma, is present within theKachian regional stage (Lazurnaya Formation). Dated as LatePaleocene, it corresponds to the absence of the Discoastergemmeus Zone (NP7). The duration of these two unconformitiesis calculated for each borehole, based on the five nannofossilbioevents according to standard biochronology presented in theworks of Berggren et al. (1995) and Quillévéré et al. (2002).

Acknowledgements

We wish to thank both reviewers for their critical review ofthis work. Their comments really improved the quality of anearly version of the manuscript. Special thanks are addressed toEmanuela Mattioli for her editorial and linguistic help.

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