25

GEOLOGICA BALCANICA, 44. 1–3, Sofia, Dec. 2015, p. 25–38.

Uppermost Albian–lower Cenomanian calcareous nannofossils from the Shishentsi-1 and Makresh-1 boreholes, Kula tectonic unit (NW Bulgaria)

Georgi GranchovskiGeological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 24, 1113 Sofia, Bulgaria; e-mail: georgi2801@geology.bas.bg(Accepted in revised form October 2015)

Abstract. Cenomanian sediments, and Albian/Cenomanian boundary strata, are not largely exposed in NW Bulgaria. Many of the boreholes that have penetrated them have poor core recovery for this stratigraphic interval, and this has greatly hampered investigations. Most of the previously published data are based on sporadic, often poorly preserved, macro- and microfossil finds and some biostratigraphic interpretations are outdated. Sometimes, age assumptions have been made only by means of lithologic comparisons and/or stratigraphic position, without fossil evidence. This paper presents the first calcareous nannofossil study on the available (limited and discontinuous) cores from the Upper Cretaceous of the Shishentsi-1 and Makresh-1 boreholes drilled in the Kula tectonic unit (Vidin District, NW Bulgaria). The Albian–Cenomanian sediments of the Rabisha Formation in both boreholes yielded relatively diverse, generally moderately well-preserved nannofloras; samples from higher stratigraphic levels, which have been assigned to the Kula Formation, proved to be barren. As a result, the presence of the uppermost Albian–lower Cenomanian subzone UC0c (in Shishentsi-1 and Makresh-1) and the lower Cenomanian UC2a (in Makresh-1) has been indicated. The con-secutive bases of Gartnerago theta and Prediscosphaera cretacea were both noted within UC0c. The former event has been used to divide UC0c into uppermost Albian–lower Cenomanian and lower Cenomanian inter-vals. The presence of Gartnerago theta from 1529 m to 1555.15 m in the Shishentsi-1 borehole also suggests that the Albian/Cenomanian boundary lies stratigraphically lower than previously thought.

Granchovski, G. 2015. Uppermost Albian–lower Cenomanian calcareous nannofossils from the Shishentsi-1 and Makresh-1 boreholes, Kula tectonic unit (NW Bulgaria). Geologica Balcanica 44 (1–3), 25–38.

Keywords: calcareous nannofossils, biostratigraphy, uppermost Albian–lower Cenomanian, Kula Unit, NW Bulgaria.

INTRODUCTION

Albian/Cenomanian boundary strata, as well as Ceno-manian sediments, are poorly exposed in NW Bulgaria; a few limited Albian–Cenomanian outcrops are known, primarily in the vicinity of the town of Kula and to the west and south of Lake Rabisha. Most of the information about their distribution, lithology and stratigraphy comes from subsurface data gathered via numerous exploratory oil-wells that were drilled during the 1960s to 1980s. It is noteworthy, however, that Upper Cretaceous deposits were drilled almost entirely without core recovery.

In Bulgarian geologic literature, the Albian–Ceno-manian in this part of the country is largely associated with clay-carbonate deposits that are transgressively overlain by “Turonian–Senonian” thick, flysch-like suc-

cessions (Tzankov et al., 1960, 1963; Tzankov, 1961, 1963, 1972; Vrablyanski, Popov, 1962; Tzankov, 1968; Nachev, Sultanov, 1980; Nachev, Yanev, 1991; Decheva et al., 1995; Filipov et al., 1995; Angelov, Dobrev, 2006; Dobrev, 2006; Dabovski, 2009; Dabovski et al., 2009; Sinnyovsky, 2009). Tzankov (1972) included the clay-carbonate sediments in his Albian–Cenomanian “Rabiša Komplex” (later formalized as the Rabisha Formation by Nikolov, Ruskova, 1993) and described the over-lying “Turonian–Senonian” flysch-like rocks as the “Kula Komplex” (formalized as the Kula Formation by Filipov et al., 1995). He based his age assignments on rare ammonites, inoceramid bivalves and planktonic for-aminifera (Tzankov et al., 1960; Tzankov, 1961, 1972; Vrablyanski, Popov, 1962; Tzankov, 1968). Recent studies have shown, however, that the stratigraphic ex-

26

tent of the clay-carbonate successions (i.e., the Rabisha Formation) reaches up to at least the uppermost Turonian (Granchovski, 2013), which implies the possible exist-ence of lateral relations between the Rabisha and Kula formations in some areas of their distributions.

Vaptzarova (1963, 1965a, b, 1974, 1975a, b, 1976a, b, 1978, 1980) examined the Upper Cretaceous for-aminifera of central N and NW Bulgaria, from bore-holes and exposures. Her investigation resulted in two local zonation schemes for the Upper Cretaceous in NW (Vaptzarova, 1976a) and central N Bulgaria (Vaptzarova, 1980), in both of which the Albian/Cenomanian bound-ary is placed at the base of the Thalmanninella appen-ninica appenninica Zone. However, it is now generally accepted that the T. appenninica Zone falls entirely within the upper Albian (e.g., Gale et al., 1996, 2011; Kennedy et al., 2004; Premoli Silva, Verga, 2004; Petrizzo et al., 2008; Ando et al., 2010; Giorgioni et al., 2015) and the base of the Cenomanian is defined by the base of Rotalipora globotruncanoides (Kennedy et al., 2004). Thus, some of the presumably Cenomanian rocks in NW Bulgaria should be referred to the upper Albian. More recent studies of the Albian–Cenomanian suc-cessions in the region, based on calcareous nannofossils (Granchovski, 2012; Sinnyovsky, 2013; Sinnyovsky, Pavlishina, 2014) and organic-walled dinoflagellate cysts (Sinnyovsky, Pavlishina, 2014), concerned only a limited number of sections.

From the review above, it is apparent that new palae-ontological and stratigraphic studies on the Upper Cret-aceous deposits in NW Bulgaria are much needed. Most of the previous data are based on sporadic fossil finds, and some biostratigraphic interpretations are obsolete. Also, the scarcity of exposures and the poor core recovery in boreholes greatly hinder investigations. That requires en-deavours to be made to better understand the stratigraphy of the Upper Cretaceous successions in this region. This paper presents the first calcareous nannofossil data on the uppermost Albian–lower Cenomanian sediments of the Shishentsi-1 and Makresh-1 boreholes (NW Bulgaria) (Fig. 1). These boreholes were specifically chosen be-cause: (1) they provide, albeit limited, core material from the uppermost Albian–Upper Cretaceous interval; and (2) there is no published palaeontological or biostratig-raphic information about the uppermost Albian–Upper Cretaceous sediments they penetrated.

GEOLOGICAL BACKGROUND AND SECTIONS

In the sense of current geodynamic models, the Albian–Cenomanian clay-carbonate sediments in NW Bulgaria are assumed to have been deposited on the upper contin-ental slope of a deep-water back-arc basin in the west-ern periphery of the Moesian Platform (Dabovski, 2009;

Fig. 1. Location of Shishentsi-1 and Makresh-1 drill sites. Tectonic map after Dabovski, Zagorchev (2009).

27

Dabovski et al., 2009). They lie on Upper Jurassic–Lower Cretaceous successions, either with a gradual tran sition or transgressively, and are covered, again either with a gradual lithologic transition or transgressively, by younger Upper Cretaceous deposits (Dabovski et al., 2009; Sinnyovsky, 2009, and references therein). In tec-tonic terms, the Shishentsi-1 and Makresh-1 boreholes fall within the Kula Unit of Dabovski, Zagorchev (2009) (equivalent to the Kula Zone of Ivanov, 1983, 1998) (Fig. 1). This para-autochthonous unit is regarded as a part of the South Carpathian orogenic system (Dabovski, Zagorchev, 2009).

Shishentsi-1 borehole

The Shishentsi-1 borehole was drilled ~3 km WNW of the village of Shishentsi (Vidin District, NW Bulgaria) (Fig. 1). Primary description of the penetrated sediments was given in the unpublished report of Monova, Ivanov (1982). According to these authors, the Upper Cretaceous section comprises the interval from 1073 m to 1570 m. It lies, with a gradual transition, upon Aptian lime-stones with Orbitolina spp. and is tectonically covered by a Berriasian–Valanginian succession of limestones, marls, siltstones, sandstones and polymict conglomer-ates and breccias. The Upper Cretaceous interval is subdivided into “Senonian–Turonian” (1073–1510 m; alternation of sandstones, siltstones, limestones, marls and argillites) and Cenomanian–Albian (1510–1570 m; marls, marly limestones and rare siltstones) by means of mainly lithologic comparisons with adjacent boreholes and rare, poorly preserved, pyritized foraminifera, iden-tified only to generic level (Monova, Ivanov, 1982). The Albian/Cenomanian boundary was tentatively placed at 1543 m without fossil evidence, but based on an increase in clay content from this level upwards (Monova, Ivanov, 1982). Core recovery throughout the Upper Cretaceous interval is poor (~11%). Decheva et al. (1995) and Dobrev (2006) referred the Albian–Cenomanian successions to the Rabisha Formation and the “Turonian–Senonian” was attributed to the Kula Formation. This is the litho-stratigraphic framework adopted herein.

Makresh-1 borehole

The Makresh-1 borehole is situated 6.1 km to the NW of the village of Makresh (Vidin District, NW Bulgaria) (Fig. 1). Primary information about lithology and strati g-raphy is presented in the unpublished report of Valkov (1977). The Upper Cretaceous deposits (586–1472 m) cover, with a sharp lithologic boundary, a thick carbon-ate succession of very thickly-bedded limestones and do-lostones; they are overlain, with a stratigraphic gap, by Miocene clays interbedded with siltstones, sandstones and limestones. The Upper Cretaceous interval is subdivided into the Campanian (586–634 m), Santonian–Coniacian (634–1293 m) and Turonian–Cenomanian (1293–1472 m), based solely on lithologic comparison with adjacent bore-holes, and with no palaeontological data (Valkov, 1977). Core recovery for the Upper Cretaceous interval is rela-tively poor (17.5%). According to the lithostratigraphic framework of Filipov et al. (1995) and Angelov, Dobrev

(2006) adopted herein, the Cenomanian–Turonian silty marls and infrequent marly limestones are part of the Rabisha Formation and the Coniacian–Campanian flysch-like sediments belong to the Kula Formation.

MATERIAL AND METHODS

A total of 110 samples (85 from the Shishentsi-1 borehole and 25 from the Makresh-1 borehole) have been investi-gated for their nannofossil content. They were taken pref-erentially from marls that were characterized by lower terrigenous content; where marls were rare, marly lime-stones were also sampled. The average sampling density is one sample/50 cm.

Calcareous nannofossils were examined in simple smear-slides, made following the methodology described by Bown, Young (1998). These slides were viewed at 1250× magnification, using an oil-immersion objective lens (100×) on a Zeiss Axioskop 40 transmitting light-microscope. Micrographs were taken with a ProgRes GT3 digital camera in both cross-polarized light (XPL) and plane-polarized light (PPL) and subsequently uni-formly enlarged at 2000× magnification.

Relative abundances of the species were estimated semi-quantitatively over four traverses (three central and one random, lest rarer taxa be overlooked) of each slide and the data are presented as range-charts. On the charts, abundances are indicated as: common (C = 1–10 speci-mens/field of view), few (F = 1 specimen/2–50 fields of view), rare (R = 1 specimen/51–100 fields of view), very rare (VR = 1 specimen/sample), and questionable (? = identification uncertain).

A qualitative estimate of preservation is also given on the range-charts: moderate (M = all specimens are easily identifiable, but secondary calcite overgrowth and/or cal-cite dissolution have modified the appearance of certain taxa and/or features); poor (P = calcite dissolution has re-sulted in depleted assemblage and/or taxonomic identifi-cation of an appreciable proportion of specimens is ham-pered by calcite dissolution or secondary overgrowth); very poor (VP = much of the assemblage is dissolved and/or a large proportion of specimens are difficult/im-possible to identify).

Taxonomic concepts largely follow those of Perch-Nielsen (1985) and Burnett (1998), but some recent works have also been taken into account (Bown, 2005; Lees, Bown, 2005; Lees, 2007). For biostratigraphic subdivision of the studied sediments, the UC zonation of Burnett (1998) was applied.

Sample material and slides are stored in the Department of Palaeontology, Stratigraphy and Sedimentology, Geo-logical Institute, Bulgarian Academy of Sciences.

RESULTS AND DISCUSSION

Shishentsi-1 borehole

Seven core intervals were studied from this borehole (Fig. 2). Those associated with the sediments of the Kula Formation proved to be barren and thus are not further discussed. On the other hand, the rocks of the Rabisha

28

Fig. 2. Lithostratigraphy, lithology, biostratigraphy, sample levels and nannofossil events (shaded section barren) from the uppermost Albian–lower Cenomanian of the Shishentsi-1 borehole. Lithostratigraphy after Decheva et al. (1995) and Dobrev (2006).

←

Formation (1529–1559 m) yielded predominantly mod-erately well-preserved nannofossils and, as a result, a total of 49 species have been indentified (Fig. 3). Biostratigraphically significant and some rarely/never previously reported from Bulgaria taxa are illustrated in Fig. 4.

The assemblages are dominated by Watznaueria barnesiae (Fig. 4.23), Tranolithus orionatus (Fig. 4.3), Rhagodiscus achlyostaurion (Fig. 4.13), Biscutum con-stans (Fig. 4.17), Zeugrhabdotus spp., Eiffellithus spp. and Retecapsa spp. Nannoconids are also a consider-able constituent. However, most of them were observed only in top views, so the author has refrained from as-signing them to particular species; they are recorded in Fig. 3 as Nannoconus spp. (top view). Albeit not com-mon, Calculites anfractus (Fig. 4.29–30), whose base defines the base of subzone UC0c, is characterized by a reasonably consistent stratigraphical range through-out the studied sediments (absent only in three samples with very poor preservation). Base Gartnerago theta (Fig. 4.28a–b) was detected at 1555.15 m. The first reli-ably identifiable specimens of Prediscosphaera cretacea (Fig. 4.19) appear in sample Sh 63 at 1534.50 m; up-section, this taxon is consistently present. Whether that reflects its true base or not, however, is uncertain since: (1) preservation in samples Sh 64 and Sh 65 is very poor and poor, respectively, and that has made differentiating Prediscosphaera cretacea from Prediscosphaera spino-sa difficult/impossible when the central area cross is not visible/completely etched; (2) samples Sh 65 and Sh 66 are separated by a 14-m coreless interval. A single cocco-lith of Corollithion kennedyi was observed at 1554.80 m. Burnett (1998) used the base of this species to draw the base of subzone UC1a; however, due to the virtual ab-sence of Corollithion kennedyi in Shishentsi-1, the pres-ence of UC1a could not be unambiguously identified. Thus, the studied sediments of the Rabisha Formation are assigned to UC0c. It should be mentioned that the first consistent occurrence of Corollithion kennedyi has been recognized in surface exposures of the same lithostratig-raphic unit near Tolovishko vrelo karst spring, Vidin District, NW Bulgaria (Granchovski, 2012; Sinnyovsky,

Fig. 3. Range-chart of calcareous nannofossils, uppermost Albian–lower Cenomanian, Shishentsi-1 borehole.

←

29

30

31

2013; Sinnyovsky, Pavlishina, 2014), and it post-dates base Prediscosphaera cretacea (Sinnyovsky, 2013; Sinnyovsky, Pavlishina, 2014). Hence, hypothetically, this stratigraphic event may also be identifiable in the Shishentsi-1 borehole, above the 1529-m level.

Stratigraphically, UC0c (= subzone BC27c of Bown et al., 1998) spans the interval from the uppermost Albian to the lower Cenomanian (pars.) (Bown et al., 1998; Burnett, 1998; Kennedy et al., 2004; Jiménez Berrocoso et al., 2010). According to Gale et al. (1996) and Kennedy et al. (2004), base Calculites anfractus (i.e., the lower boundary of UC0c) in the Global Stratotype Section and Point for the base of the Cenomanian Stage lies in the uppermost Albian, only 4 m below base Rotalipora glo-botruncanoides (planktonic foraminifera), which makes it the nannofossil event closest to the Albian/Cenomanian boundary. Base Gartnerago theta, although allegedly diachronous in the Cenomanian (see Burnett, 1998), has been dated as early Cenomanian in SE France (e.g., Gale et al., 1996; Burnett, 1998; Kennedy et al., 2004), the Czech Republic (e.g., Svobodová et al., 2004), southern

Fig. 4. Calcareous nannofossils from the Shishentsi-1 borehole.

1. Amphizygus cf. A. brooksii, XPL, Sh 80.2. Staurolithites gausorhethium, XPL, Sh 70.3. Tranolithus orionatus, XPL, Sh 85.4. Zeugrhabdotus bicrescenticus, XPL, Sh 57.5. Zeugrhabdotus diplogrammus, XPL, Sh 85.6. Zeugrhabdotus embergeri, XPL, Sh 57.7. Zeugrhabdotus noeliae, XPL, Sh 74.8. Zeugrhabdotus xenotus, XPL, Sh 85.9. Eiffellithus gorkae, XPL, Sh 57.10. Eiffellithus cf. E. monechiae, XPL, Sh 83.11. Eiffellithus turriseiffelii, XPL, Sh 85.12. Helicolithus compactus, XPL, Sh 57.13. Rhagodiscus achlyostaurion, XPL, Sh 57.14. Rhagodiscus angustus, XPL, Sh 85.15. Rhagodiscus asper, XPL, Sh 71.16. Axopodorhabdus albianus, XPL, Sh 85.17. Biscutum constans, XPL, Sh 70.18. Prediscosphaera columnata, XPL, Sh 70.19. Prediscosphaera cretacea, XPL, Sh 57.20. Cretarhabdus cf. C. striatus (fragment), XPL, Sh 80.21. Retecapsa angustiforata, XPL, Sh 57.22. Retecapsa crenulata, XPL, Sh 74.23. Watznaueria barnesiae, XPL, Sh 85.24. Watznaueria britannica, XPL, Sh 57.25. Watznaueria fossacincta, XPL, Sh 74.26. Broinsonia enormis, XPL, Sh 57.27. Broinsonia matalosa, XPL, Sh 84.28a–b. Gartnerago theta, XPL (a), PPL (b), Sh 78.29–30. Calculites anfractus, XPL (29), PPL (30), Sh 74.31. Calculites percernis, XPL, Sh 85.32. Owenia hillii? (side view), XPL, Sh 83.33. Braarudosphaera africana, XPL, Sh 70.34. Nannoconus truittii subsp. frequens, XPL, Sh 85.35. Radiolithus planus, XPL, Sh 57.

←

England (e.g., Burnett, 1998), the NW Pacific Ocean (e.g., Bown, 2005), and the southern Indian Ocean (Lees, 2002). Based on these data, the studied sediments of the Rabisha Formation, which fall above base Gartnerago theta (i.e., 1529–1555.15 m), are referred to the lower Cenomanian; for those below (i.e., 1555.15–1559 m), due to the lack of a reliable stratigraphic framework for Shishentsi-1, a latest Albian–early Cenomanian age is adopted. The presence of Gartnerago theta in the inter-val 1529–1555.15 m also suggests that the Albian/Cenomanian boundary lies stratigraphically lower than previously interpreted by Monova, Ivanov (1982).

Makresh-1 borehole

Three core intervals were examined from this borehole (Fig. 5). The uppermost interval (1156–1290 m), attrib-uted to the Kula Formation, comprises almost entirely sandstones and matrix-supported conglomerates with a silty to fine-sandy matrix. Sampling of these sediments was greatly hindered by the very poor preservation state

32

Fig. 6. Calcareous nannofossil data and biostratigraphic sum-mary, Makresh-1 borehole.

Fig. 5. Lithostratigraphy, lithology, biostratigraphy, sample levels and nannofossil events (shaded section barren) from the upper Albian–lower Cenomanian of the Makresh-1 bore-hole. Lithostratigraphy after Filipov et al. (1995) and Angelov, Dobrev (2006).

of most of the core. Instead, samples were taken from the only three marl levels in the uppermost part of the interval; however, they did not yield nannofossils and therefore are not further discussed. In contrast, gener-ally moderately well-preserved nannofloras were re-covered from the deposits of the Rabisha Formation (1323–1328 m; 1433–1438 m). Fifty-one species have been identified in total (Fig. 6), some of which are de-picted in Fig. 7.

In the interval 1323–1328 m, Watznaueria barnesiae (Fig. 7.25) and Tranolithus orionatus (Fig. 7.2) are most common. Although less (some much less) abundant Biscutum constans (Fig. 7.18), Zeugrhabdotus bi cres-centicus (Fig. 7.3), Corollithion kennedyi (Fig. 7.15), Gartnerago theta, Zeugrhabdotus xenotus (Fig. 7.7), Eiffellithus turriseiffelii (Fig. 7.10), Rhagodiscus spp., Retecapsa spp. and Gartnerago segmentatum (Fig. 7.29) are consistently present. The co-occurrence of Gart-nerago segmentatum and Zeugrhab dotus xenotus indi-cates subzone UC2a, which is of early Cenomanian age (Burnett, 1998; Reboulet et al., 2013).

The stratigraphically lower interval (1433–1438 m) yielded similar, if not identical, nannofloral associ-ations to those observed in the Shishentsi-1 borehole. Again, Watznaueria barnesiae, Tranolithus orionatus and Rhagodis cus achlyostaurion (Fig. 7.12) are the dominant taxa, closely followed by Biscutum constans, Eiffellithus gorkae (Fig. 7.9), Eiffellithus turriseiffelii, Zeugrhabdotus spp., Eprolithus floralis (Fig. 7.34) and Radiolithus planus (Fig. 7.35). Calculites anfractus (Fig. 7.30) is consistently present throughout the entire inter val (absent in only one sample with very poor preserv-ation); it is even preserved commonly in sample Mkr 25 (at 1437.70 m). The consecutive bases of Gartnerago theta and Prediscosphaera cretacea (Fig. 7.20) were noted at 1436.24 m and 1433.48 m, respectively. The presence of Calculites anfractus and absence of Corollithion kennedyi indicates subzone UC0c and thus the studied core inter-val belongs to the uppermost Albian–lower Cenomanian (pars.). The sediments above base Gartnerago theta (1433–1436.24 m) are referred to the lower Cenomanian and those below (1436.24–1438 m) are regarded as upper-most Albian–lower Cenomanian (see discussion above).

←←

33

34

35

CONCLUSIONS

The calcareous nannofloral associations from the upper-most Albian–lower Cenomanian of the Shishentsi-1 and Makresh-1 boreholes, NW Bulgaria, have been examined for the first time. They are predominantly moderately well-preserved and allowed biostratigraphical interpret-ations to be made. As a result, the presence of two sub-zones from Burnett’s (1998) UC zonation has been indi-cated: UC0c (in Shishentsi-1 and Makresh-1) and UC2a (in Makresh-1). For the sediments referred to UC0c, a latest Albian–early Cenomanian age is interpreted; the sediments that fall in UC2a correspond to the lower Cenomanian. The consecutive bases of Gartnerago theta and Prediscosphaera cretacea have been recognized within UC0c. Base Gartnerago theta, early Cenomanian in age elsewhere (e.g., in SE France, the Czech Republic, southern England, Indian Ocean, NW Pacific Ocean), at 1555.15 m in the Shishentsi-1 borehole, indicates that the Albian/Cenomanian boundary lies stratigraphically lower than previously suggested by Monova, Ivanov (1982). However, further investigation using other micro- and/or macrofossils is needed to precisely place the base of the Cenomanian Stage in NW Bulgaria.

AcknowledgementsWholehearted thanks are expressed to Dr Jacqueline A. Lees (University College London) for highly construct-ive suggestions and comments on an earlier version of the manuscript, and for her linguistic help. I am also im-mensely grateful to Profs Kristalina Stoykova and Iskra Lakova (Geological Institute, Bulgarian Academy of Sciences) for fruitful discussions, constructive criticism and their encouragement to write this paper.

APPENDIX. TAXONOMIC LIST

A full alphabetic list of all taxa mentioned in text and figures is given below. Bibliographic references that do not appear in Perch-Nielsen (1985) and Bown (1998) are listed in the references herein.

Amphizygus Bukry, 1969A. cf. A. brooksii Bukry, 1969Axopodorhabdus Wind & Wise in: Wise & Wind, 1977A. albianus (Black, 1967) Wind & Wise in: Wise & Wind, 1977Biscutum Black in: Black & Barnes, 1959

Fig. 7. Calcareous nannofossils from the Makresh-1 borehole. All images taken in XPL.1. Staurolithites gausorhethium, Mkr 16.2. Tranolithus orionatus, Mkr 25.3. Zeugrhabdotus bicrescenticus, Mkr 25.4. Zeugrhabdotus diplogrammus, Mkr 25.5. Zeugrhabdotus embergeri, Mkr 25.6. Zeugrhabdotus noeliae, Mkr 19.7. Zeugrhabdotus xenotus, Mkr 5.8. Chiastozygus platyrhethus, Mkr 25.9. Eiffellithus gorkae, Mkr 20.10. Eiffellithus turriseiffelii, Mkr 25.11. Calcicalathina alta (side view), Mkr 25.12. Rhagodiscus achlyostaurion, Mkr 25.13. Rhagodiscus angustus, Mkr 16.14. Rhagodiscus asper, Mkr 22.15. Corollithion kennedyi, Mkr 5.16. Stoverius achylosus, Mkr 7.17. Axopodorhabdus albianus, Mkr 25.18. Biscutum constans, Mkr 16.19. Prediscosphaera columnata, Mkr 25.20. Prediscosphaera cretacea, Mkr 16.21. Cretarhabdus striatus, Mkr 5.22. Retecapsa angustiforata, Mkr 19.23. Retecapsa crenulata, Mkr 19.24. Manivitella pemmatoidea, Mkr 25.25. Watznaueria barnesiae, Mkr 25.26. Watznaueria britannica, Mkr 19.27. Watznaueria ovata, Mkr 25.28. Broinsonia enormis, Mkr 16.29. Gartnerago segmentatum, Mkr 14.30. Calculites anfractus, Mkr 25.31. Calculites percernis, Mkr 16.32. Owenia hillii (plan view), Mkr 16.33. Lithraphidites carniolensis, Mkr 25.34. Eprolithus floralis, Mkr 25.35. Radiolithus planus, Mkr 25.

←

36

B. constans (Górka, 1957) Black, 1967Braarudosphaera Deflandre, 1947B. africana Stradner, 1961Broinsonia Bukry, 1969B. enormis (Shumenko, 1968) Manivit, 1971B. matalosa (Stover, 1966) Lyulyeva in: Lyulyeva & Permyakov, 1980B. signata (Noël, 1969) Noël, 1970Calcicalathina Thierstein, 1971C. alta Perch-Nielsen, 1979Calculites Prins & Sissingh in: Sissingh, 1977C. anfractus (Jakubowski, 1986) Varol & Jakubowski, 1989C. percernis Jeremiah, 1996Chiastozygus Gartner, 1968C. bifarius Bukry, 1969C. litterarius (Górka, 1957) Manivit, 1971C. platyrhethus Hill, 1976Corollithion Stradner, 1961C. kennedyi Crux, 1981C. signum Stradner, 1963Cretarhabdus Bramlette & Martini, 1964C. striatus (Stradner, 1963) Black, 1973Cribrosphaerella Deflandre in: Piveteau, 1952C. ehrenbergii (Arkhangelsky, 1912) Deflandre in: Pive-teau, 1952Discorhabdus Noël, 1965D. ignotus (Górka, 1957) Perch-Nielsen, 1968Eiffellithus Reinhardt, 1965E. gorkae Reinhardt, 1965E. cf. E. monechiae Crux, 1991E. turriseiffelii (Deflandre in: Deflandre & Fert, 1954) Reinhardt, 1965Eprolithus Stover, 1966E. floralis (Stradner, 1962) Stover, 1966Gartnerago Bukry, 1969G. segmentatum (Stover, 1966) Thierstein, 1974G. theta (Black in: Black & Barnes, 1959) Jakubowski, 1986Grantarhabdus Black, 1971G. coronadventis (Reinhardt, 1966) Grün in: Grün & Allemann, 1975Helicolithus Noël, 1970H. compactus (Bukry, 1969) Varol & Girgis, 1994H. trabeculatus (Górka, 1957) Verbeek, 1977Lithraphidites Deflandre, 1963L. carniolensis Deflandre, 1963Manivitella Thierstein, 1971M. pemmatoidea (Deflandre in: Manivit, 1965) Thier-stein, 1971

Nannoconus Kamptner, 1931N. truittii Brönnimann, 1955 subsp. frequens Deres & Achéritéguy, 1980Owenia Crux, 1991O. hillii Crux, 1991Placozygus Hoffmann, 1970P. fibuliformis (Reinhardt, 1964) Hoffmann, 1970Prediscosphaera Vekshina, 1959P. columnata (Stover, 1966) Manivit, 1971P. cretacea (Arkhangelsky, 1912) Gartner, 1968P. ponticula (Bukry, 1969) Perch-Nielsen, 1984P. spinosa (Bramlette & Martini, 1964) Gartner, 1968Radiolithus Stover, 1966R. planus Stover, 1966Retecapsa Black, 1971R. angustiforata Black, 1971R. crenulata (Bramlette & Martini, 1964) Grün in: Grün & Allemann, 1975R. surirella (Deflandre in: Deflandre & Fert, 1954) Grün in: Grün & Allemann, 1975Rhagodiscus Reinhardt, 1967R. achlyostaurion (Hill, 1976) Doeven, 1983R. angustus (Stradner, 1963) Reinhardt, 1971R. asper (Stradner, 1963) Reinhardt, 1967Rotelapillus Noël, 1973R. crenulatus (Stover, 1966) Perch-Nielsen, 1984Staurolithites Caratini, 1963S. gausorhethium (Hill, 1976) Varol & Girgis, 1994S. laffittei Caratini, 1963Stoverius Perch-Nielsen, 1986S. achylosus (Stover, 1966) Perch-Nielsen, 1986Tranolithus Stover, 1966T. orionatus (Reinhardt, 1966) Reinhardt, 1966Watznaueria Reinhardt, 1964W. barnesiae (Black in: Black & Barnes, 1959) Perch-Nielsen, 1968W. britannica (Stradner, 1963) Reinhardt, 1964W. fossacincta (Black, 1971) Bown in: Bown & Cooper, 1989W. ovata Bukry, 1969Zeugrhabdotus Reinhardt, 1965Z. bicrescenticus (Stover, 1966) Burnett in: Gale et al., 1996Z. diplogrammus (Deflandre in: Deflandre & Fert, 1954) Burnett in: Gale et al., 1996Z. embergeri (Noël, 1959) Perch-Nielsen, 1984Z. noeliae Rood et al., 1971Z. scutula (Bergen, 1994) Rutledge & Bown, 1996Z. xenotus (Stover, 1966) Burnett in: Gale et al., 1996

REFERENCES

Ando, A., Huber, B.T., MacLeod, K.G. 2010. Depth-habitat re-organization of planktonic foraminifera across the Albian/Cenomanian boundary. Paleobiology 36 (3), 357–373.

Angelov, V., Dobrev, G. 2006. Rabisha Formation. In: Angelov, V., Khrischev, Kh. (Eds), Explanatory notes for the geo-logic map of Bulgaria in scale 1:50 000, Dimovo map sheet. Bulgarian National Geological Survey, Sofia, 39–40.

Bown, P.R. (Ed.). 1998. Calcareous nannofossil biostratig-raphy. British Micropalaeontological Society Publications Series. Chapman & Hall/Kluwer Academic Publishers, London, 315 pp.

Bown, P.R. 2005. Early to mid-Cretaceous calcareous nan-noplankton from the northwest Pacific Ocean, ODP Leg 198, Shatsky Rise. Proceedings of the Ocean Drilling

37

Program, Scientific Results 198, 1–82 (online). http://www-odp.tamu.edu/publications/198_SR/VOLUME/CHAPTERS/103.PDF

Bown, P.R., Rutledge, D.C., Crux, J.A., Gallagher, L.T. 1998. Lower Cretaceous. In: Bown, P.R. (Ed.), Calcareous nan-nofossil biostratigraphy. British Micropalaeontological Society Publications Series. Chapman & Hall/Kluwer Academic Publishers, London, 86–131.

Bown, P.R., Young, J.R. 1998. Techniques. In: Bown, P.R. (Ed.), Calcareous nannofossil biostratigraphy. British Micro-palaeontological Society Publications Series. Chapman & Hall/Kluwer Academic Publishers, London, 16–28.

Burnett, J.A. 1998. Upper Cretaceous. In: Bown, P.R. (Ed.), Calcareous nannofossil biostratigraphy. British Micropalaeontological Society Publications Series. Chapman & Hall/Kluwer Academic Publishers, London, 132–199.

Dabovski, H. 2009. Upper Cretaceous geology. Introduction. In: Zagorchev, I., Dabovski, H., Nikolov, T. (Eds), Geology of Bulgaria. Vol. II. Mesozoic geology. “Prof. M. Drinov” Academic Press, Sofia, 305–307 (in Bulgarian, with English abstract).

Dabovski, H., Sinnyovsky, D., Vasilev, E. 2009. Upper Cret-aceous geology. Geological evolution. In: Zagorchev, I., Dabovski, H., Nikolov, T. (Eds), Geology of Bulgaria. Vol. II. Mesozoic geology. “Prof. M. Drinov” Academic Press, Sofia, 554–589 (in Bulgarian, with English abstract).

Dabovski, H., Zagorchev, I. 2009. Introduction: Mesozoic evo-lution and Alpine structure. Alpine structure. In: Zagorchev, I., Dabovski, H., Nikolov, T. (Eds), Geology of Bulgaria. Vol. II. Mesozoic geology. “Prof. M. Drinov” Academic Press, Sofia, 30–37 (in Bulgarian, with English abstract).

Decheva, A., Filipov, L., Haydutov, I., Tzankov, Tz., Kojumdgieva, E., Popov, N. 1995. Explanatory notes for the geologic map of the Republic of Bulgaria in scale 1:100 000, Zaechar and Bor map sheets. Avers ST, Sofia, 43 pp. (in Bulgarian, with English abstract).

Dobrev, G. 2006. Rabisha Formation. In: Angelov, V., Khrischev, Kh. (Eds), Explanatory notes for the geologic map of the Republic of Bulgaria in scale 1:50 000, Zaechar and Bor map sheets. Bulgarian National Geological Survey, Sofia, 15–16.

Filipov, L., Decheva, A., Kojumdgieva, E., Popov, N., Tzankov, Tz., Haydutov, I. 1995. Explanatory notes for the geologic map of the Republic of Bulgaria in scale 1:100 000, Vidin map sheet. Avers ST, Sofia, 60 pp. (in Bulgarian, with Eng-lish abstract).

Gale, A.S., Bown, P., Caron, M., Crampton, J., Crowhurst, S.J., Kennedy, W.J., Petrizzo, M.R., Wray, D.S. 2011. The up-permost Middle and Upper Albian succession at the Col de Palluel, Hautes-Alpes, France: An integrated study (ammon-ites, inoceramid bivalves, planktonic foraminifera, nanno-fossils, geochemistry, stable oxygen and carbon isotopes, cyclostratigraphy). Cretaceous Research 32 (2), 59–130.

Gale, A.S., Kennedy, W.J., Burnett, J.A., Caron, M., Kidd, B.E. 1996. The Late Albian to Early Cenomanian suc-cession at Mont Risou, near Rosans (Drôme, SE France): an inte grated study (ammonites, inoceramids, planktonic foraminifera, nannofossils, oxygen and carbon isotopes). Cretaceous Research 17 (5), 515–606.

Giorgioni, M., Weissert, H., Bernasconi, S.M., Hochuli, P.A., Keller, C.E., Coccioni, R., Petrizzo, M.R., Lukeneder, A., Garcia, T.I. 2015. Paleoceanographic changes during the Albian–Cenomanian in the Tethys and North Atlantic and the onset of the Cretaceous chalk. Global and Planetary Change 126, 46–61.

Granchovski, G. 2012. Calcareous nannofossils from the type section for the Rabisha Formation (NW Bulgaria).

Bulgarian Geological Society, National Conference with international participation “Geosciences 2012”, Abstracts, 81–82.

Granchovski, G. 2013. New data on the age of the Rabisha Formation from borehole Belo Pole 1 (NW Bulgaria). Bulgarian Geological Society, National Conference with international participation “Geosciences 2013”, Abstracts, 71–72 (in Bulgarian).

Ivanov, Ž. 1983. Aperçu général sur l’évolution géologique et structurale des Balkanides. In: Ivanov, Ž., Nikolov, T. (Eds), Guide de l’excursion géologique de la Société Géologique de France en Bulgarie, Sofia, 3–26.

Ivanov, Ž. 1998. Tectonics of Bulgaria. Professorship thesis, Sofia University “St. Kliment Ohridski”, 579 pp. (in Bulgarian, unpublished).

Jiménez Berrocoso, Á., MacLeod, K.G., Huber, B.T., Lees, J.A., Wendler, I., Bown, P.R., Mweneinda, A.K., Londoño, C.I., Singano, J.M. 2010. Lithostratigraphy, biostratigraphy and chemostratigraphy of Upper Cret-aceous sediments from southern Tanzania: Tanzania drilling project sites 21–26. Journal of African Earth Sciences 57 (1–2), 47–69.

Kennedy, W.J., Gale, A.S., Lees, J.A., Caron, M. 2004. The Global Boundary Stratotype Section and Point (GSSP) for the base of the Cenomanian Stage, Mont Risou, Hautes Alpes, France. Episodes 27 (1), 21–32.

Lees, J.A. 2002. Calcareous nannofossil biostratigraphy illus-trates palaeoclimate change in the Late Cretaceous Indian Ocean. Cretaceous Research 23 (5), 537–634.

Lees, J.A. 2007. New and rarely reported calcareous nannofos-sils from the Late Cretaceous of coastal Tanzania: outcrop samples and Tanzania Drilling Project Sites 5, 9 and 15. Journal of Nannoplankton Research 29 (1), 39–65.

Lees, J.A., Bown, P.R. 2005. Upper Cretaceous calcareous nan-nofossil biostratigraphy, ODP Leg 198 (Shatsky Rise, north-west Pacific Ocean). Proceedings of the Ocean Drilling Program, Scientific Results 198, 1–60 (online). http://www-odp.tamu.edu/publications/198_SR/VOLUME/CHAPTERS/114.PDF

Lyulyeva, S.A., Permyakov, V.V. 1980. Coccolithophorids and corals from the Mesozoic of Ukraine. “Naukova dumka” Publishing House, Kiev, 172 pp. (in Russian).

Monova, D., Ivanov, V. 1982. Report on the results of the Makresh-1 borehole conducted in 1979–1980. Ministry of Economy, National Geological Fund, 187 pp. (in Bulgarian, unpublished).

Nachev, I., Sultanov, Y. 1991. Sedimentology of the Upper Cret aceous in Bulgaria. Bulgarian Academy of Sciences Academic Press, Sofia, 80 pp. (in Bulgarian).

Nachev, I., Yanev, S. 1980. Sedimentary geocomplexes in Bulgaria. “Nauka i izkustvo” Publishing House, Sofia, 203 pp. (in Bulgarian).

Nikolov, T., Ruskova, N. 1993. The Lower Cretaceous in the wells of Northwestern Bulgaria. Geologica Balcanica 23 (5), 47–67.

Perch-Nielsen, K. 1985. Mesozoic calcareous nannofossils. In: Bolli, H.M., Saunders, J.B., Perch-Nielsen, K. (Eds), Plankton stratigraphy. Cambridge University Press, Cambridge, 329–426.

Petrizzo, M.R., Huber, B.T., Wilson, P.A., MacLeod, K.G. 2008. Late Albian paleoceanography of the western sub-tropical North Atlantic. Paleoceanography 23, PA1213, doi:10.1029/2007PA001517

Premoli Silva, I., Verga, D. 2004. Practical manual of Cret-aceous planktonic foraminifera. In: Verga, D., Rettori, R. (Eds), International School on Planktonic Foraminifera: Universities of Perugia and Milano. Tipografia di Pontefelcino, Perugia, 283 pp.

38

Reboulet, S., Giraud, F., Colombié, C., Carpentier, A. 2013. Integrated stratigraphy of the Lower and Middle Ceno-manian in a Tethyan section (Blieux, southeast France) and correlations with Boreal basins. Cretaceous Research 40, 170–189.

Sinnyovsky, D. 2009. Upper Cretaceous geology. Carpathian type Upper Cretaceous. In: Zagorchev, I., Dabovski, H., Nikolov, T. (Eds), Geology of Bulgaria. Vol. II. Mesozoic geology. “Prof. M. Drinov” Academic Press, Sofia, 389–394 (in Bulgarian, with English abstract).

Sinnyovsky, D. 2013. Upper Cretaceous calcareous nanno-plankton biostratigraphy in Bulgaria. “St. Ivan Rilski” Publishing House, Sofia, 80 pp.

Sinnyovsky, D., Pavlishina, P. 2014. Nannoplankton and paly-nological evidence for the Albian–Cenomanian boundary in Northwest Bulgaria. Comptes rendus de l’Académie bul-gare des Sciences 53 (11), 41–44.

Svobodová, M., Hradecká, L., Scupien, P., Švábenická, L. 2004. Microfossils of the Albian and Cenomanian shales from the Štramberg area (Silesian Unit, Outer Western Carpathians, Czech Republic). Geologica Carpathica 55 (5), 371–388.

Tzankov, Tz. 1961. Notes on the relations between the South Carpathians and the Balkanides in Northwest Bulgaria. Travaux sur la Géologie de Bulgarie, Série Stratigraphie et tectonique 3, 173–251 (in Bulgarian).

Tzankov, Tz. 1963. Stratigraphy of the Carpathian type Upper Cretaceous in the vicinity of Kula (Northwest Bulgaria). Bulletin de l’Association Géologique Carpato-Balkanique, V-ème Congrès, 1961, Bucarest, Communications Scientifiques, II-ème section: Stratigraphie 3 (2), 207–219 (in Russian).

Tzankov, Tz. 1972. Jungalpidische Deformationen im Krajna-Gebiet (Nordwestbulgarien). Geologie 21 (1), 24–60.

Tzankov, V. 1968. Upper Cretaceous. In: Tzankov, V., Spasov, H. (Eds), Stratigraphy of Bulgaria. “Nauka i izkustvo” Publishing House, Sofia, 253–294 (in Bulgarian).

Tzankov, V., Cheshitev, G., Dimitrova, N. 1963. Summary of the Upper Cretaceous stratigraphy in North Bulgaria. Bulletin de l’Association Géologique Carpato-Balkanique, V-ème Congrès, 1961, Bucarest, Communications Scientifiques, II-ème section: Stratigraphie 3 (2), 221–243 (in Russian).

Tzankov, V., Dimitrova, N., Stefanov, J., Vrablyanski, B. 1960. Stratigraphic studies on the Jurassic and Cret a ceous Series in Northwest Bulgaria. Travaux sur la Géo logie

de Bulgarie, Série Stratigraphie et tectonique 1, 203–241 (in Bulgarian).

Valkov, I. 1977. Report on the results of the Shishentsi-1 borehole conducted in 1974–1976. Ministry of Economy, National Geological Fund, 104 pp. (in Bulgarian, un pub-lished).

Vaptzarova, J. 1963. On the presence of the Danian Stage near the villages of Novo selo and Koshava (Vidin District). Proceedings of the Bulgarian Geological Society 24 (1), 84–86 (in Bulgarian).

Vaptzarova, J. 1965a. Stratigraphy of the Upper Cretaceous in North and Northwest Bulgaria based on micropalaeon-tological data. Annual of the Head Office of Geology 15, 125–161 (in Bulgarian).

Vaptzarova, J. 1965b. Upper Cretaceous. In: Tzankov, V. (Ed.), Microfossil assemblages in Bulgaria. Principal Directorate of Geology, Sofia, 255–281 (in Bulgarian).

Vaptzarova, J. 1974. Foraminifera from the Carpathian type Maastrichtian of Northwest Bulgaria. Proceedings of the Geological Institute, Series Palaeontology 23, 31–56 (in Bulgarian).

Vaptzarova, J. 1975a. Planktonic foraminifera and zonation of the Campanian in Northwest Bulgaria. Palaeontology, Stratigraphy and Lithology 1, 19–32 (in Bulgarian).

Vaptzarova, J. 1975b. Benthic foraminifera from the Campanian in Northwest Bulgaria. Palaeontology, Stratigraphy and Lithology 2, 57–72 (in Bulgarian).

Vaptzarova, J. 1976a. Zonation du Crétacé supérieur du type carpathique en Bulgarie du Nord-Ouest d’après les fo-raminifères planctoniques. Geologica Balcanica 6 (3), 93–109.

Vaptzarova, J. 1976b. Planktonic foraminifera from the Co-nia c ian–Santonian of Northwest Bulgaria. Palaeontology, Stratigraphy and Lithology 4, 47–62 (in Bulgarian).

Vaptzarova, J. 1978. Foraminifera from the Coniacian–Santon-ian of Northwest Bulgaria. Palaeontology, Stratigraphy and Lithology 8, 55–68 (in Bulgarian).

Vaptzarova, J. 1980. Zonation du Crétacé supérieur du type plate-forme en Bulgarie du Nord d’après les foraminifères. Geologica Balcanica 10 (3), 45–68.

Vrablyanski, V., Popov, N. 1962. Explanatory notes for the geo-logic map of the Republic of Bulgaria in scale 1:200 000, Vidin map sheet. Principal Directorate of Geology, Sofia, 58 pp. (in Bulgarian).