Cretaceous/Tertiary Boundary in the Area of Bjala, Eastern Bulgaria

GEOLOGICA BALCANICA, 24, 6, Sofia, Decemb., 1994, p. 3—22

Cretaceous/Tertiary boundary in the area of Bjala, eastern Bulgaria — biostratigraphical results

Marin I. Ivanov, Kristalina H. Stoykova

Geological Institute of Bulgarian Academy of Sciences, ! 113 Sofia

(Received 12. 09. 1994; accepted 21. 09. 1994) Марин И. Иванов, Красти.шна V. Стойнови — Граница мел1палеоген в районе г. Бяла, Восточ- ная Болгария — биостритигрифические результапш. Граница мел.'палеоген устаноплена и иепере- рмвноп седнисптаинонной последоватсльмостм в окрссности г. Бяла. на берегу Черното моря. Обиажснньш интервал отноептея к Белепекоп евнте, которая изучена по четирем разрезам на морском берегу в отдельньгх днслоцнрованншх блоках. Граница мел/палеоген маркируется ли- тологическн оди им граничньтм глннистш.ч пластом толщнной от 1 до 6 сш. Бмостратиграфически она проводится по исчезновеншо мелового наниопланктопа п появлепию Bianlholithus sparsus. В неперерьвнои последовательностн установлепш се.мь наннофоссильних зон — от зонн Micula munis верхнего маастрихта до зони Fascicnlithus tympanifonnis верхнего палеоцепа. Разрез Бяла 2в одни из не.многих разрезов в мире, в когорьх беспозвоночпне макрофоссилии часго встречают- ся. Вьделень три аммонитовне зони на основе нредставнтелей Pachydiscidae: зона А па р achy disc us freisvillensis, зона Pachydiscus gollevillensis arméniens н зона Anapachydiscus terminus. Данние из разреза Бяла 2в показьвают. что аммоннтш постепенно теряют видовое разнообразие в позднем маастрн.хте. „Последние“ аммонитм найдепи в 40 cm ниже граници мел/палеоген. Извсстковьш нанпопланктон нзменяется значительпо над грапнцей, прмчем смени таксономического состава пропеходмт в ра.мках первой палеоценовой зони NP 1. Abstract. The Cretaceous/Tertiary boundary is idenii Tied in the vicinity of the town of Bjala on the Black Sea coast, within an continuous sedimentary succession. It is located in the Bjala For mation. The latter has been studied in four sections oil the coast in separate, tectonically dislocated blocks. The K/T boundary is lithologically marked by a boundary clay bed, from 1 to 6 cm thick. BiostratigraphicalIv, it is defined by the disapperance of Cretaceous nannoplankton and appearance of Bianlholithus sparsus and Cyclagclosphaera alia. Seven nannofossil zones Ггот CC25C (Micula munis Zone) in the Upper Maastrichifan to NP 5 (Fasciculiihus tympanifonnis Zone) in the Upper Palaeo- cene, have been identified in an uninterrupted succession. The Bjala 2b section is one of the few Upper Maastrichtian sections in the world in which macro-invertebrate fossils occur commonly. Three ammonite zones are recognized based on the representatives of Pachydiscidae: Anapachydiscus freisvillensis Zone, P. gollevillensis gollevillensis Zone and Anapachydiscus terminus Zone. Evidence from ihe Bjala 2b section indicates that ammonites gradually became less diverse during the Late Maastrichtian. The “youngest” ammonites were found 40cm below the K/T boundary. The calcare ous nannoplankton assemblages vary considerably across the K/T boundary: a rapid turnover of taxa occurred wit hi n the limits of the first Palaeocene zone NP 1.

Introduction

The Cretaceous/Tertiary boundary and its related biological events has, for a long time, been a subject of discussion. The discovery of the geochemical anomaly in the boundary bed (Alvarez et al., 1980) started a rejuvenated phase of discussions

3 in which a large variety of specialists have taken part. This interest has fueled inve stigations aimed at finding and analysing complete, uninterrupted sections across the K'T boundary interval. The K/T boundary in Bulgaria has not been studied so far in terms of event stra tigraphy. Therefore, we have addressed the following in our study: — finding complete sections of uninterrupted sedimentation between the Cre taceous and Tertiary; — establishing the position of the I\/T boundary; — studying, in detail, the nannofossil assemblages; — observing the succession of macro- and nannofossils across the boundary. Regional geological and stratigraphical features of Cretaceous and Tertiary in Bulgaria indicated the East Balkan region to be the most promising area to investi gate. Stoykova, Ivanov (1992) gave a detailed chronology of the investiga tions, which, between 1991 —1993 have been partly carried out within the framework of the East-West Project of the Bulgarian and Austrian Academy of Sciences. In the present paper, the stratigraphical results are documented independently in order to present the raw data (i. e. fossil stratigraphical distributions) and to compa re extinction between the different fossil groups across the K/T boundary. This is im portant because there are only few sections in the world, from which similar observa tions can be obtained.

Methods The precise location of the K/T boundary was established by using calcareous nanno fossil biostratigraphy. Sample density differed between the section investigated. The lower horizon of the boundary clay bed was designated 0 m. Within the boundary in terval (—0,1 m to -)-0.25 m) samples were taken at every 1-2 cm; from —1 m to —0,1 m and from —0,25 m to —4 m samples were taken at every 20 cm; elsewhere, samples were taken at every 0,5—1 m. Smear slides, mounted with Canada balsam, were pre pared. Leitz Orthoplan Pol and Jenapolj light microscopes (1200 X magnification) were used for determination of the nannofossil taxa.

Geological setting The sections around the town of Bjala lie within the Luda Kamcia zone, a partly ex ternal zone of the East Balkan, situated between the Sredna Gora zone and the Bal kan— Fore Balkan zone (Z. Ivanov, 1983, 1988) (Fig. 1A). The Luda Kamcia zone comprises Late Alpine sinclinorium, complicated by inner nappes and reverse faults. Palaeogeographically, the zone was a well differentiated basin (Emine lowe ring— Начев, 1977) which was being filled with predominantly flysch sediments during the Turonian to Palaeocene. The Cretaceous — Palaeocene sediment complexes are strongly tectonized around Bjala, cropping out in blocks of different size or incorporated in nappe lamellae (Fig. 3).

Previous studies on the K/T boundary interval in the region of the town of Bjala Stratigraphical data on the rock in the region of the town of Bjala were first publi shed by 3 л a t a p c к и (1905, 1907). He assigned the sandstones of Sveti Atanas Cape to the Cenomanian, and the carbonate rock in the vicinity of Bjala to the Upper

4 Fig. /. Location of lhe investigated area. A: tectonic subdivision of Bulgaria (after Z. I v a n o v, 1983. 1988): 1 — Moesian platform: 2 — Balkan — Fore Balkan Zone: 3 — Luda Kamchia Zone; 4 — Sredna gora Zone; 5 — Rhodopes: B: coast-line of Black Sea between the town of Varna and Emilie Cape; C: location of the studied sections in the vicinity of Bjala

Senonian (Aturien) on the basis of the presence of Echinocorys vulgaris. Бончев (1926, p. 17) described these rocks as “Bjaia clayey marls” of Senonian age. G е 1 I e r t (1929) spoke about the Senonian in the vicinity of Bjala and about the Eocene (flysch molasse) at Sv. Atanas Cape, giving a sketchy characterization. Гочев (1932) ref- fered the marls around Bjala to the Senonian. Poliak (1933) assumed the marls in the vicinity of Bjala to be Late Senonian in age, citing the presence of Echinocorys gibbus, Inoceramus ballicus, Parapachydiscus cgerloni, Scaphiies sp. and Hamiles sp. Ботев (1953) reported a number of fossils which corroborate an Upper Senonian age for the “marly limestones” near the town of Bjala, including Echinocorys ovaius mar- garilaius, Inoceramus sp. and Pachydiscus neubergicus. T p и ф o и o в a (1960) proved for the first time the presence of Maastrichtian, Danian and Palaeocene sediments in a well (R-l) near the town of Bjala using forami-

5 Fig. 2 Relationships between the lithostratigraphical units within the K/T boundary interval

nifers. Juranov (1983). Джурапов (1989) distinguished and characterized zones based on planktonic foraminifers for the Palaeocene — Lower Eocene interval in a drill sections around the villages of Gorica and Bjala. The rocks of the boundary interval were included within the range of the limestone-marl suite (Д ж у p а н o в, 1983, 1989) or within the Bjala Formation (Д ж у p а п o в, 1991). The authors have established an uninterrupted section across the K T boundary in the vicinity of Bjala (S t o y k o v a & I v a n o v, 1992). The neutron activation analysis of samples from the clay bed has shown an abnormal content of Ir of the or der of 7 ppb (P r e i s i n g е r et al., 1993a, 1993b).

Lithostratigraphy In the coastal part of the East Balkan Mountain the Elpper Cretaceous and Palaeocene consist of the Emine, Velikovo and Bjala Formations. The K/T boundary occurs in the Emine and Bjala Formation and probably in the Velikovo Formation. The mar ker boundary clay bed and the /г-anomaly have been established so far only within the Bjala Formation (Fig. 2).

Emine Formation (Late Campanian — Late Palaeocene) Consists of an alternation of thin- to medium-bedded sandstones, siltstones, argillites and marls, which form the complete rhythms of a typical flysch.

Velikovo Formation [Maastrichtian (partly) — Palaeocene (partly)] Distinguished on the basis of a characteristic alternation of aleurolites, marls and marly limestones, forming the rhythms of a flysch of increased carbonate content.

Bjala Formation (Maastrichtian — Palaeocene) Characterized by marls and marly limestones. The marls dominate in the lower part of the formation. They are dark to light-grey, indistinctly bedded, and include hori

6 zons of varying carbonate content. In the upper part of the formation, the marls al ternate with light-grey to whitisch marly limestones. A tendency toward increased carbonate content has been established in the sec tions of the formation north of the town of Bjala. The Bjala Formation is restricted to the area around Bjala and the village of Gorica, Varna district. The clay bed, cha racteristic of the K T boundary and found in the Bjala Formation, is invaluable as a lithological marker. Its thickness is more commonly 1-3 cm, but at certain places, it reaches 5-6 cm. The bed consists of dark-grey, dark-brown, black, pure (almost non- calcareous) clay, and is discernible in the field. Although it is difficult find, once lo cated, it is easily traced.

Studied sections

Studies have been carried out on four sections in the vicinity of Bjala, exposed on the coastline, situated at about 1-1,5 km from one another. Their exact location is shown on Fig. 1 В, C. The structural environment was taken into consideration when selecting sections studied. Some of the sections are next to the fault dislocations or are included

BJALA 2b ____ N

S- BJALA 2c -» N

Fig. 3. Tectonic situation of the studied sections (legend as in Fig. 5). in nappe structures. For that reason, separate tectonic situations for each section are shown (Fig. 3). The Bjala 1 section

The sequence crops out in a cliff-section to the south of Bjala, 210 m south of the wa ter-slide. The K T boundary in this section is discontinuous and bears the characteris tic of a “hardground”. The uppermost Maastrichtian (CC 26B, Micula prinsii Zone) and the base of the Danian (zones NP 1, NP 2 and part of NP 3) are missing. Thus, the age оГ the lowermost 5,70 m of the Bjala 1 section is Late Maastrichtian (CC 25C Micula munis Zone), whereas the age of the upper part from 0 to 27,70 m above the clay bed is Early to Late Palaeocene (NP 3 — NP 5) with respect to nannofossil zones. A total of 36 samples were examined for their nannofloral contents, thirty-three in Danian and three in Maastrichtian. Nannofossil diversity and abundance was high in the Palaeocene samples; 29 species were recorded. In the Maastrichtian, however, the nannofossils were rare and poorly preserved, 8 species were identified. The nannofloras are Tethyan in aspect. Nannofossil (sub-) zones CC 25 C, NP 3, NP 4 and NP 5 were recognized on the first occurrences (F. O.) of Micula murus, Chiasmolilhus clanicus, Ellipsoliihus bollii and Fasciculithus tympaniformis, respectively. Bjala limestone-marl Formation (No. 6-1. Upper Maastrichtian-Palaeocene) 6. (10 m) Alternation of the marls ant! marly limestones. The marls are light grey to dark grey, and the limestones are grev-heige with grey spots. The limestone beds are 0,3 m thick. In the upper most parts of this section at 10 m, a rich nannofloral association has been established. It consists of: Chiasmoli thus bidens, C. consuetus, C. dunictts, Cruciplacolithus tenuis, Prinsius marlinii, P ■ bisulcus, Fasciculithus sp.. Neochiastozygus perfectus. In the middle part of the packet (m. 4,50) an association has been found, which comprises the following representatives of genus Fasciculithus: F. tympani formis, F. janii, F. ulii, F. biteclus, F. billii, F. pileaius. At the same level Chiasmolilhus danicus, C. bidens, Neochiastozygus perfectus, N. modestus, Ellipsoliihus bollii, Prinsius marlinii, P. bisulcus, Placozygus sigmoides have been found as well. In the lowest 2,30 m the following species have been determined: Cruciplacolithus asymmetricus, C. tenuis, Chiasmolilhus danicus, C. edentu- lus, C. inconspicuus, Prinsius bisulcus, Thoracosphaera. operculata, Coccolithus cavus, Placozygus sig moides, Neochiastozygus eosuepes, N. saepes, N. modestus, N. perjectus, Markalius inversus. 5. (10 m) Marls — grev-heige to whitish, dark grey in the upper part. The following species have been recorded from 7 to 4m: Neochiastozygus modestus, N. eosaepes. N. perfectus, Chiasmolilhus danicus, Cruciplacolithus tenuis, Prinsius martinii, P. bisulcus, Placozygus sigmoides, Coccolithus cavus, Tho racosphaera operculata, T. saxea. 4. (7.70 m) An alternation of marls with thin beds of marly limestones. The marly limestones vary in thi ckness (between 0,2 and 0,4m); approximately they occur at every lm across the section. In the up permost part of that packet, at 7,70 m, the first appearance of the species Ellipsoli/hus bolliihas been recorded. At the same level there have also been found: Neochiastozygus modestus, N. eosaepes, Prin sius bisulcus, Cruciplacolithus tenuis. The following species have been determined within the interval from 7 to 4 m: Cruciplacolithus asymmetricus, C. intermedins, C. tenuis, Prinsius martinii, P. bisulcus Braarudosphaera lurbinca, Neochistozygus modestus, Thoracosphaera operculata. The nannofossil asso ciation recovered between the 4th and the 0,35th m consists of the following species: Chiasmolilhus danicus, C. inconspicuus, C.edwardsii, Cruciplacolithus tenuis, C. asymmetricus, C. intermedins, Prinsius martinii, P. bisulcuus, Placozygus sigmoides, Neochiastozygus modestus, Coccolithus, cavus, Markalius inversus. In the lower part of the packet, at 0,15 m from its base upwards certain Cretaceous reworked species are common: Arkhangclskiella cymbiformis, Microrhabdulus decoratus, Eiffellilus turriseiffeli, Walzaucria barnesae. They associate togerther with Chiasmolilhus danicus, Coccolithus cavus, Markalius apertus, M. inversus, Braarudosphaera turbineu, Prinsius martinii, P. bisulcus. 3. (0,02-0.04 m) Clay bed — dark grey, dark brown to black clay. The clay bed overlies unequal surface. The species established are: Chiasmolilhus danicus, Cruciplacolithus ienuis, C. intermedins, C. asymmetri cus, Coccolithus cavus, Markalius inversus, Prinsius bisulcus, Placozygus sigmoides, Thoracosphaera operculata, T. saxea. ------wash-out K/T boundary------2. (0,56 m) Light grey to light beige limestone. In the lower 20-30 cm it is bioturbated. The upper surface of that bed is unequal, bearing marks of wash-out and “hard ground” . Calcite lenses, filling in crack

8 can be seen in i(. No nannofossiIs have been found. Several specimens of irregular Echinoidea have been established. 1. (4,70 m) Grey, indistinctly bedded marls of varying carbonate content. There arc lenses or layers of lno- cerainus-shell detrite and remains of Echinoidea in some of the beds. In the upper pari of the packet the following species occur: Micula munis, M. decussaia, Arhhangelskiella cymbiformis, Thoracosphacra operculata, Eijfellithus lurriseiffeli, Cyclagelosphaera margerelii, Zygodiscus spiralis. Underground — tectonic contact with marls strongly cleaved.

The Bjala 2b seel ion The section is located at the coast-line, about 800 m north of the central entrance to the beach (Fig. 1C). This is the most complex and most representative of the section studied. The Maastrichtian to Palaeocene succession crops out to the north of the town, where an southwestward dipping sequence (210"—235°) complicated by faulting, rep resented the best exposures of the Cretaceous/Tertiary boundary. The sequence can be followed in several adjacent, tectonically dislocated blocks (Fig. 3). There are some recent investigations of bio-, magneto- and event stratigraphy on this section, and the presence of Ir-anomaly at the K/T boundary is documented by Prei singer et al. (1993a, 1993b). The results of the present study are shown in Fig. 4, where occurrences of the nannofossils are plotted against the composite log and subdivided into the standard sche mes of S i s s i n g h (1977) and Martini (1971). A total of 31 samples were exa mined for their nannofloral contents. The nannofloras were diverse and abundant (68 species were recorded) in all samples, but were poorly preserved throughout the section. Specimen ex i b i tec! both etching and overgrowth; there was a high proportion of bro ken specimens. Nannofloras have a Tethyan aspect, as indicated by the presence of rare Micula prinsii and Nephrolithus frequens. The Bjala 2b section extend from Late Maastrichtian to Early Danian in age. The bases of nannofossil zones CC 26, NP 1, NP 2 and NP 3 were recognized by the first occurrences of Micula prinsii, Biantholilhus sparsus (or Cyclagelosphaera alia), Crucip- lacoliihus lenuis and Chiasmolithus danicus, respectively. The base of nannofossil zone CC 25C Micula mums could not be drawn due to the missing of exposures, but it is inferred to be present below-45 m. Micula prinsii first occurs-17 m below the K/T boundary, although Ncphrolilhus frequens, the index species for CC 26 zone, occurs -14 m below the top of the Maastrichtian. The base of NP 1 zone is drawn immedia tely above boundary clay bed. Nannofossil zone NP 2 is marked by the incoming Cru- ciplacolithus lenuis +4 m above the K T boundary. Chiasmolilhus danicus first occurs +22 m above the boundary and marks the base of the nannofossil zone NP 3.

Bjala limestone-marl Formation (No 5—1, Upper Mnastrichlian-Pnlncocenc) 5. (over 25 m) Alternation of limestones and marls, represented by beds of approximately equal thickness (0,3-0,4 m), The limestones are light beige to whitish, in certain cases forming beds up to 2-2,5 m thick. Fossil remains of Echinoidea have been established. The following nannofossil species have been distinguished 5m above the base of the packet: Chiasmolilhus danicus, C. edwardsii, Coccolilhus cavus, C. crassus, Marhalius inversus, M . aperlus, Cruciplacolilhus primus, C. lenuis, Ncochiastozygus modestus, Thoracosphacra operculata, T. saxea, Braarudosphaera bigelowii. 4. (13 m) Regular alternation of marls and clayey limestones. Both lithologic types arc in equal propor tion and form beds 0,3 to 0,5 m thick. The marls arc greyish-green, the limestones — light yellow to beige, compact. The following species have been found at 10 m above the base: Coccolilhus cavus, Cruciplacolilhus tenuis, C. inlermedius, Cyclagelosphaera reinhardtii, C. alia, Marhalius inversus, Pla- cozygus sigmoides, Thoracosphacra operculata-, at 5,00 m — Walznaueria barnesac, Cruciplacoiihus inlermedius, Braartulosphaera bigelourii, B. Iurbinea, Cyclagelnsphaera reinhardtii, Coccolilhus cavus, Thoracosphacra operculata. The species established lm above the base are Thoracosphacra operculata and Braarudosphaera bigelowii only. 3, (12 m) Marls, interbedded by thin layers of clayey limestones. The marls are grey to light beige, indis tinctly bedded. The marly limestones vary in thickness (0,2-0,4 m) and occur almost regularly at about every lm across the section. In the uppermost levels of Ihe packet the following species have been found from the 12th to the 10th m: Cruciplacolithus tenuis, C. asymmetricus, Coccolithus cavus, Cqclage- losphaera reinhardtii, Wainaueria harnesae, Thoracosphaera nperculata, T. saxea, Braarudosphcicra bigelowii, B. turbinea, Markalius inversus. The nannofossil association in the middle and lower parts of the packet have been studied in details cm by cm, starting from the bottom upwards. In the inter val from the 8th to the 4th m have been recorded: Cruciplacolithus tenuis (-|-4m), Markalius inversus, Coccolithus cavus (+5m), Prinsius dimorphosus (+7,50 m), Micrantholithus crenulatus, Placozygus sig- nioides. The following species have been determined to occur within the 4-1,8 m interval: Cruciplaco lithus primus (+1,80 m), C, intermedins (+ 3 m), Futyania pctalosa (2 m), Markalius inversus: from 1,8 to 0,7 m — Biantholithus sparsus, Cyclagelosphaera reinhardtii, C. alia, Micula decussata, Predis- cosphaera crctacea, Thoracosphaera operctilala, T. saxea; Ггот 0,7 to 0,1 m — Markalius inversus, Cyc- lagelosphaera reinhardtii, C. alia, Placozi/gus sigmoides, Braarudosphaera turbinea. B. discuta, Biantho lithus sparsus, Thoracosphaera operculata; from 10 cm to 0 — Biuntholilhus sparsus (+1 cm), Cyclage- losphaera alia, C. reinhardtii, Thoracosphaera operctilala, Micula munis, Arkhangelskiella cymbiformis. 2. (0,02—0,04 m) Clay bed — dark grey to black clay, plastic when moistened. Rare fragments of poorly preser ved nannofossils occur in the bed: Thoracosphaera operculata, T. saxea, Micula mums, Cqclagelos- phaera reinhardtii, Braarudosphaera bigelowii. ------Cretaceous/Tertiary boundary------1. (over 45 m) Marls — light to dark grey, in some levels — whitish, indistinctly bedded, bioturhated, stron gly cleaved. The “last” ammonites were recorded at —0,4 m, represented by: Pseudophyllitcs indra, Anagaudryceras pollitissimum. Pachydiscus sp. indet., Anagaudryceras sp. indet. At —1 m a few frag ments of Anagaudryceras and Pseudophyllitcs were documented only, and at —7 m — Hoploscaphites constrictus. Between 0 and —10 m the following nannofossils have been occurred: Arkhangelskiella cymbiformis, Watznaueria barnesae, Micula prinsii, M. murus, M. decussata, M. swasiica, Lithraphi- dites quadraius, Prediscosphuera cretacea, P. grandis, P. spinosa, Markalius inversus, Eiffeliihus turriseiffeli, Nephrolilhus frequens, Microrhabdulus decoratus, Cyclagelosphaera reinhardtii, Thoracosphaera operculata, Braarudosphaera turbinea, B. bigelowii, Ahmuelerella octoradiaia, Stradneria crenulala, Cribrosphaera ehrenbergii, C. mimerosa. Between —8 m and —14 m have been determined Anapa- chydiscus terminus, Pachydiscus jacquoti and undetermined ammonite — gen. et sp. indet. It is a frag ment, probably of Gaudryceratidae. The following nannofossils have been recorded in the same in terval: Micula prinsii, M. murus, Cyclagelosphaera reinhardtii, Arkhangelskiella cymbiformis, Predis- cosphaera grandis, Thoracosphaera operculum, Markalius inversus. The ammonites established from the —15th to the —25th m are: Phylloceras (Neophylloceras) rumnsum, Pseudophyllitcs cf. indra, Anagaudryceras poliiissimum, A. sublilincatum, Verlebrites kayei, Saghalinites wrighti, Dyplamoce- ras cylindraceum, Baculitcs anceps, Pachydiscus gollcvillensis gollevillensis, P. jacquoti, Anapachy- discus terminus (first appearance at the —18 m). Other species occurred within the same interval of nannofossils are: Micula prinsii (first appearance at the —17 m), M. murus, M. swastica, M. decussata, Nephrolilhus frequens, Ahmuelerella octoradiaia, Lithraphidites quadratus, Eiffellithus turriseiffeli, Stradneria crenulala, Arkhangelskiella cymbiformis, Zygodiscus spiralis, Braarudosphaera bi gelowii, B. turbinea, rare Thoracosphaera operculata, Chiasozygus liierarius. The ammonites found from the —25 th to the —35 th m are: Phylloceras ( Neophylloceras) ramosum, Anagaudryceras poliiissimum, Verlebrites kayei, Pseudopliyllites indra, Saghalinites wrighti, Hoploscaphites constrictus, Dyplomo- ceras cylindraceum, Baculitcs anceps, B. vertebralis, Glyptoxoceras cf. subcompressum, Glyptoxoceras sp., Desmophyllites larleli, D. diphylloides, Pachydiscus gollevillensis gollevillensis (first appearance at —33 m), P. jacquoti (first appearance at —27 m). P. gollcvillensis armenicus. From the same interval the distinguished nannofossils are: Nephrolilhus frequens (first appearance at —14 m), Micula murus, M. decussata, M. swastica, Lithraphidites quadratus, L. carniolensis, Markalius inversus, Cyclagelos phaera reinhardtii, C. margarelli, Arkhangelskiella cymbiformis, Stradneria crenulala, Prediscosphaera cretacea, P. spinosa, Microrhabdulus decoratus, Cribrosphaera ehrenbergii, Zygodiscus spiralis, Braaru dosphaera turbinea, Watznaueria barnesae, Ahmuelerella octoradiaia. Ammonites occurring from the —35th to —451 li m are: Phylloceras ( Neophylloceras) ramosum, Ph. (N .) surya, Anagaudryceras poliiissimum, A. subtilineatum, Verlebrites kayei, Desmophyllites larleli, D. diphylllodes, Pseudophylli- tes indra, P. cf. latus, Hoploscaphites constrictus, Phylloplychoceras sp. indet., Dyplomoceras cylindra ceum, Baculitcs anceps, B. vertebralis, Anapaehydiscus freisviltensis, Pachydiscus gollevillensis armeni cus (Tirst appearance at —43 m), P. neubergicus neubergicus. P. neubergicus rarccosiatus. The nanno fossils Tound here are: Lithraphidites quadratus, Micula murus, M. decussata, M. concava, M. swasiica, Arkhangelskiella ci/mbiformis, Cribrosphaera ehrenbergii, C. numerosa, C. elliplica, Eiffellithus turriseiffeli, E. purallelus, Microrhabdulus decoratus, Stradneria crenulala, Watznaueria barnesae, Prediscos phaera grandis, P. cretacea, P. spinosa, Zygodiscus spiralis, Cyclagelosphaera mnrgerelii, Rcinliardiles levis (reworked? a t—45 m), Braarudosphaera bigelowii, Ahmuelerella octoradiaia.

10 The Bjala 2c section

The section is situated about 2 km north of the central entrance to the beach at Bjala and about 1,2-1,3 km north of Bjala 2b section (Fig. 1C). Within this locality a repetition of the rock sequences has been established. The latter are contained in a thrust of N dipping (Fig. 3). The successions in the K/T boundary interval (Plate I), which belong to the nappe autochthon, are described in the present paper. Twenty-four samples were examined for their nannofloral content; nannofossil diversity and abundance was high in this section. However, preservation was predi ctably poor throughout. Thirty-five species were identified, ranging from Late Maast- richtian to Early banian in age. Nannofossil zones CC 26, NP 1 and NP 2 were recogni zed on the first occurrence of Micula prinsii, Biantholiihus sparsus and Cruetplacoli- thus tenuis, respectively. The base of zone CC 26 could not be ascertained; it is infer red to be present below —7m. Biantholithus sparsus, a zonal marker for NP 1, occur red at + 10 cm above the the boundary clay bed. The base of nannofossil zone NP 2, marked by the first occurrence of Cruciplacolithus tenuis, lies at +3,50 m above the K/T boundary. Bjala Formation (No. 4-1, Upper Maastrichlian — Lower Danian) ------thrust plain------4. (8 m) Alternation of marls and clayey limestones. The marls are light grey to whitish; Torm beds from 0. 8 io lm in thickness. The clayey limestones are light grey to whitish, of varying thickness (From 1,5 to 0,3 m). A rich nannoflora has been Found in the packet, which has been investigated in metres and centimetres. The species distinguished within the interval of 8—6 m are: Cruciplacolithus tenuis, C. intermedins, C. asymmctricus, Placozygus sigmoides, Markalius inversus, M. apertus, Biantholithus sparsus, Micrantholithus crenulatus, M. entaster, Braarudosphaera bigelowii, B. turbinea, Cyclagelos- pliaera reinhardtii, Arkhangelskiella ctymbiformis, Eiffelithus turriseiffeli, Wa/znaueria barnesae, Pre- discosphaera cretacea. The same association, supplemented by species Cruciplacolithus primus, Cyc- lagelosphaera alia, Braarudosphaera discuta, Micula mums, M. decussata, Cribrosphaera ehrenbergii, Thoracosphaera operculata, T. saxea is situated between the 6th and 4th m. The species found at +3.50 m from the packet base are: Cruciplacolithus tenuis, C. primus, C. intermedins, Placozygus sigmoides, Thoracosphaera operculata, Kcphrolithus frequens. Arkhangelskiella cymbijnrmis, Micula prinsii, Cri brosphaera ehrenbergii, Braarudosphaera bigelowii, B. discute., Markalius inversus, Watznaueria bar nesae. Reworked Cretaceous species are abundant at —3 m in nannofossil association: Slradneria crenulata, Alnnueierella octoradiato, Micula decussata, Waiznaueria barnesae, I.ithraphidites praequad- ratus, Arkhangelskiella cymbi/ormis, Micula prinsii. At -|-2,60 m the following species were round: Cru ciplacolithus intermedins, Neobisculum romeinii, Staurolilhiles crux, Placozygus sigmoides, Braarudos- phacra turbinea, Markalius inversus, Micrantholithus crenulatus. The first occurrence of the species Cruciplacolithus primus is at +1.60 m from the base of that packet: in addition numerous cocco- sphaeres of Thoracosphaera operculata, as well as a lot of “Cretaceous” species have been observed. The following species have been distinguished Ггот +1 to +0,1 m: Biantholithus sparsus, Cyclagelosphae- ra alla, C. reinhardtii, C. margerelii, Neochiasiozygus primitivus, Prediscosphaera cretacea, Braarudos phaera turbinea, B. bigelowii, Placozygus sigmoides, Markalius inversus, Micula decussata, Thoracos phaera operculata, Watznaueria barnesae, Ahmuclerella octoradiato. 3. (0,01-0,06 m) Boundary clay bed — dark brown to black clay varying in thickness, usually from 1 to 3 cm, at certain places up to 6 cm. The clay is filling in the cracks of the underlaying limestone bed. Nanno- fossils are rare in that clay; there are only established Micula decussata and Thoracosphaera opercu lata. ------Cretaceous/Tertiarv boundary------2. (0,5-0,6 m) Clayey limestone — light beige to whitish, unequal upper surface. The species established in that packet are: Micula mums, Nephrolithus frequens, Markalius inversus, Cyclagelosphaera margere lii, Cribrosphaera ehrenbergii, Arkhangelskiella cymbiformis, Alnnueierella octoradiato, Prediscosphaera spinosa, P. cretacea, Braarudosphaera turbinea, Thoracosphaera operculata. 1. (7 m) Marls — grey to light grey, of varying carbonate content. At certain levels they are banded, slightly aleuritic, thin-bedded. The species distinguished 6 m from the base are: Micula murus, M. prinsii, M. decussata, Arkhangelskiella cymbiformi, Lithraphidites quadra lus, Cribrosphaera ehrenbergii, Prediscosphaera cretacea, Thoracosphaera operculata, Watznaueria barnesae.

11 SI

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o The Bjala 3 section The section is located on the Black Sea coast, about 2,5 km north of Bjala, 250 m south of the mouth of the Bjala Reka River (Fig. 1C). The section lies immediately next to a fault dislocation (Fig. 3). There are indications of sliding and slipping along the clay bedding plane. Bjala limestone-marl 1'omialion (No. 3-1, Upper Maaslriclitiaii-Lower Daniaii) 3. (3 m) Marls with interbeds of clayey limestones. The marls arc light beige, indistinctly bedded, at certain places — aleuritic. Numerous specimens of the following species have been found 0,5 m from the base of that packet: Biantholiihus sparsus, Neobiscutuni romeinii, Cyclagellosphaera alla, C. rein- hardiii, Markalius inversus, Thoracosphacra operculata, Braarudosphacra bigelowi, B. turbinea, B. discuta, Micula decussata, M. prinsii, Arkhangelshiellu cijmbijonnis, Cribrosphucra chrcnbergii, Watz- naueria barnesae. 2. (0,01-0.02 m). Clay bed — light brown to brown, at certain places the clay is in grey shades, plastic when mcis- tured. XannofossiIs rarely occur and poorly preserved. The species found are: Tkoracosphaera operculata, T. saxca, Micula dccussala, M. munis, Prediscosphacra crctacea, Braarudosphacra bigeloivii, Watznaucria barnesae. ------Crelaceous/Tertiary boundary------1. (47 m) Marls — light to dove-grey in the upper part of the packet — beige, indistinctly bedded, sligh tly aleuritic. There have been found: at —1,5 m — Arkhangclskiella q/mbiformis, Micula decussata, M. murus, Zygodiscus spiralis, Prediscosphacra crclacea, P. spinosa, Eiffellilhus turriseijjeli, Cribros- phaera ehrenbergii, Stradneria crenulata; at —4m — Microrhabdulus dccoralus, Micula concava, M. dccussala, Л4. prinsii, Prediscosphaera spinosa, Lithraphidiles quadratus, Walznaueria barnesae, Ark- hangelskiclla cymbijormis. Ammonites have been recovered in this section as well: at —10 m — Phylloccras (X cnphylloceras) surua, P. (N.)ramosum, Phyllopachyceras forbesianum, Saghalinites cf. iz’righ/i, Saghalinites sp. indet.. Pachydiscus sp. indet.; at — 20—25 m — Phylloccras (Xeophylloce- ras) ramosum, Desmophylli/cs cf. larteli, Pscudophyllites cf. indra, Sagnaliniles sp. indet., Baculiics sp. indet., Pacliydiscus gollevillcnsis gollcvillensis, P. jucqu.oti.

Biostratigraphy In most of the K/T boundary sections in the world the zonal subdivision of the Maas- trichtian and Palaeocene has been based on nannofossil and/or planktonic foraminiferal biostratigraphy. Only in a few sections has the Upper Maasirichtian been subdi vided using macro-invertebrates as well. These sections are discussed in detail in the parers of \V i e d m a n n (1988), W a r d (1990), W a r d & K е n n c d v (1993), K en n e d y (1993). The sections around the town of Bjala (2c, 3 and particularly 2b) arc extremely suitable for biostratigraphic observations and interpretations on account of: — an uninterrupted clayey-carbonate sedimentation across the K/T boundary; — the abundance and diversity of nannofossils and the frequent occurrence of macro-invertebrates (ammonites, echinoids, inoceramids). The integrated study [carried out on faunal and floral sequences allows a direct connection and comparison of the events of the first occurrence of biostratigraphically significant ammonites and nannoplankton taxa (Fig. 6).

Calcareous nannoplankton The standard zonal scheme of M a r t i n i (1971) was applied to the Palaeocene sedi ments. The standard zonation of S i s s i n g h (1977) and Perc h-N i е 1 s e n (1979, 1985a) was applied to the Upper Cretaceous sediments. The rare occurrence of the ge nus Ellipsolilhus in the upper parts of the Danian and the abundance of Neochiastozygus spp. within the same stratigraphic interval have allowed also the application of vau H e c k & P r i n s ’s (1987) zonation. Other works on nannoplankton at the K/T

13 CRETACEOUS Т Е: ГГГ i A R Y Y S T Е M U. MAASTRICH7IAN D A N I A M m . MPI IM P 2 NP3 ÎM P A ?rirv Nannofossil Zone B. sparsu? C. tenuif- . danicus mncellus Species Arkhangelskiella cymbiformis Micula decussaia Micula murus Micula prinsii Cribrosphaera ehrenbergii Prediscosphaem cretacea Liihraphidites quadraius Eiffeliihus îurriseiffeü Mierorbabdulus éecoratus Nephroliihus frequens Thoracosphaem opercuUda Thoracosphaera saxea Braarudosphaera bigeloKÏi Braarudosphaera turbinea Cyclagellosphaera reinhardtii Cyclagellosphaera alia Markalius inversus Placozygus sigmoïdes Biantholithus sparsus Crueiplacoliihus primus Cruciplacoliihus intermedins Crueiplacoliihus tenuis Cruciplacoliihus asymmctricus Coccoliihus cavus Neobiscutum romeinii Chiasmolithus danicus Chiasmolithus inconspicuus Chiasmolithus edvardsii Chiasmolithus bidens Chiasmolithus consuetus Prinsitts martinii Prinsius bisulcus Meochiasîozygus modestus Neochiastozygus eosaepes Neochiasiozygus saepes Neochiastozygus perfeclus Ellipsolithus bcllii Fasciculithus jatiii Fasciculithus ulii Fasciculithus tympauifennis

Phylloecras (Neophvlloceras) ramosttni Anagaudryceras politissimum Vertebrites kayei Pscudophyllites indra Saghaliniier, wrighti Hoptoscaphiics constriclus Phylloptychoceras sp. indet Diphmoceras cylindraceum Haculifes anceps Glvntoxoceras cf. subcomprcssum Л napnebydiscus frcrsvi/frnsis Paciiydiscus neubergicus Pachydiscus gollevillcnsis unnmkus Paciiydiscus gollenllensis gd'c•■ii:cn\;\ Pach vdiscus jacqu oti tlnapaebydisrus terrain .\cutilcids Jnoceramids Fchiuocoiys Stegaster

Ammonite Zone

Fig. 6. Range-chart of the important nannofossils and inacroinvertebrales across the K/T boundary in the sections studied — zonal subdivision 1 — Anupachydiscus /reisvillensis Zone; 2 — Pachydiscus go l lev i lie nsi s gollevillcnsis Zone; 3 — Anupachydiscus terminus Zone boundary (Perc h-N i е 1 s e n, 1979, 1981, 1985b; Perc h-N i е 1 s e n et al 1982; R o m e i n, 1979; S e y v e, 1984, 1990; V a r o 1, 1989; L a m o 1 d a et al 1983; Burnet et al., 1992 etc.) were consulted for purposes of comparison.

14 M i c u 1 a mu r u s Zone The base of lhat zone could not be distinguished in the sections studied (see above). The nannofloral association is rich and species diversity is high. The zone has been recognised in the sections of Bjala 1 and Bjala 2b with a thickness of 30 m.

M i c и 1 a p r i n s i i Zone The base of the zone was recognised by the first occurence of Micula prinsii; it is drawn in the upper part of packet No 1 of Bjala 2b section. We detected M. prinsii in the top 17 m, and Nephroiithus frequens — in the top 14 m of the Maastrichtian of Bjala 2b. The presence of M. prinsii Zone proves the copmleteness of the Maastrichtian stage. Thickness — 15-17 in in Bjala 2b section; 5-7 m in sections Bjala 2c and 3.

Biantholithus sparsus Zone The base of the zone coincides with the К/T boundary. The first occurence of the index species Biantholithus sparsus is immediately above the boundary clay bed. Only within that zone the species Cyclagelosphaera alia has been found. The zone is characterized by the dominance of Thorasphacra operculala — a species, which amount of 2-3% in the last centimetres of the Upper Maastrichtian reaches over 80%, immediately above the K T boundary (S t o y k o v a in Preisinger et al., 1993a, Fig. 4). In the base of that zone together with Biantholilhus sparsus and Cyclagellosphaera alia, almost all “Upper Cretaceous” nannofossil taxa from the underlying zones (M. prinsii Z., M. murus Z.) were found. In the middle and upper part of that zone the first occurrence of the genus Cruciplacolithus — C. primus, C. intermedius, as well as the species Neobis cutum romeinii, Futyania petalosa, has been recorded. Thickness — 3,50 m in the Bja la 2c section; 4m in the Bjala 2b section; over 3m in the Bjala 3 section.

Cruciplacolithus tenuis Zone Within that zone a gradual quantitative decrease of Thoracosphaera operculata down to 10-20% has been mentioned (S t o y k o v a in Preisinger et al., 1993, Fig. 4). On can registeres here the first occurrence of the species Coccolithus cavus (appea ring slightly after the first occurrence of C. tenuis), Prinsius dimorphosus and Crucip lacolithus asymmetricus. Thickness — 18 m in the Bjala 2b section.

Chiasmolithus danicus Zone The base of the zone is drawn by the first occurrence of the genus Chiasmolithus (Ch. danicus and Ch. edwardsii). The first occurrence of Chiasmolithus inconspicuus, Prinsius martini, P. bisulcus, Neochiastozygus modestus has been recorded in that zone as well. The species, considered to be Cretaceous occur here much more rarely comparing with two perceding zones. Thickness: about 35 m in the section Bjala 2b; reduced thickness — 7,70 m in Bjala 1 section.

Ellipsolithus m a ce 1 1 us Zone There are some problems associated with the recognition of the base of E. macellus zone, which we have been unable to recognize because of the poor presence or absence of the genus Ellipsolithus used in the standart zonal scheme of Marti n i (1971). The first occurrence of genus Ellipsolithus approximately coincides with that of Neo chiastozygus eosaepes. That fact allowed to use N. eosaepes for drawing the base of the

15 explanations to t h e p l a t e s PLATE I Cretaceous/Tertiary boundary in section Bjala 2c A-B — the boundary interval, packet No 1-3; C — the boundary clay bed, detail

PLATE II Upper Maastricht!an — Lower Danian nannofossils from the sections near the town of Bjala. a — Micula munis (M a r t i n i) B u kr y, section Bjala 3, No 3, +3,25 m, revoked specimen in B. sparsus Zone b — Micula prinsii P e r c h-N i е I s e n, section Bjala 2b, No 1, —0,03 m, M. prinsii Zone c — Micula swasiica S t r a d n e r & S t e i п m c l z, section Bjala 2c, No 1, —0,015 m, M. prinsii Zone d — Markalius apertus P er c h-N i c 1 s e n, section Bjala 3, No 3, +3,60 m, B. sparsus Zone e-f — Cyclagelosphaera alia P e r c h-N i е I s е n, e — section Bjala 3, No 3, +0,5 m; f — section Bjala 2c, No 3, +0,97 m, B. sparsus Zone g — Cyclagelosphaera reinhardtii (P e r c h-N i е 1 s e n) R o m c i n, section Bjala 2c, No 3, + 0,5 m, B. sparsius Zone h — Markalius inversus (Dcflandre in Fort) Bramlette & Marti ni, section Bjala 2b, No 3, + 1,25 m, B. sparsus Zone i, n — Bianlholilhus sparsus Bramlette & M a r t i n i, i — section Bjala 2b, No 3, +0,09 m; n — section Bjala 3, No 3, +0,5 m, B. sparsus Zone j — Braarudosphaera lurbinea S t r a d n e r, section Bjala 3, No 3. +0,5 m, B. sparsus Zone k — Nephrolithus frequens G o r k a, section Bjala 2b, No 1, —0,09 m, M. prinsii Zone 1 — Eiffellilhus iurriseiffeli (Dcflandre in F e r t) Reinhard, section Bjala 2b, No 3, + 1,80 m, reworked specimen in B. sparsus Zone m — Arkhangclskiclla cytnbijormis Vekshina, section Bjala 2b, No 1, —0,03 m, M. prinsii Zone o — Cruciplacolilhus asijmtnelricus v. Heck, Pri п s, section Bjala 2c, No 4, +7,50 m, C. tenuis Zone p, r — Braarudosphaera bigelowii (G r a n & B r a a r u d) Dcflandre, p — section Bjala 2b, No 3, +0,78 m, B. sparsus Zone; r — section Bjala 2c, No 2, —0,05 m. M. prinsii Zone q — Thoracosphacra opcrculata Bramlette & M a r t i п i, section Bjala 2b, No 3, +1,25 m, B. sparsus Zone

PLATE III Upper Maastrichtian ammonites Ггот the section Bjala 2b, Bjala Formation, packet No 1 A-B — Hoploscaphiles conslriclus (J. Sower b y); A — small macroconch, MP-US Kx 10208, B — microconch, MP-US 1+ 10209, from —35 to —42* m, A. freisvillensis Zone C — Pachydiscus gollcvillcnsis gollevillensis (d’O r b i g n y), —31 m, MP-US Kx 10210, P. gollcvillensis gollevillensis Zone D — Pachydiscus gollevillensis arméniens Atabckian & Acopian, —42 m, MP-US K110211, Anapachydiscus freisvillensis Zone E- — Pachydiscus ncubcrgicus neubergicus (H a u e r), from —39 to —45 m; E — MP-US 1+ 10212; F —MPUS K110213; A. freisvillensis Zone All figures arc in natural size. Photo B. Jabljanski.

PLATE IV Uppermost Maastrichtian ammonnites from the section Bjala 2b, Bjala Formation, packet No 1 A — Pachydiscus gollevillensis armenicus Atabekian&Akopian, —38 m, MP-US Kx 10214, A. freisvillensis Zone B — Pachydiscus gollevillensis gollevillensis (d’O r b i g n y), — 24 m, MP-US KKj 10215, P. gollev illensis Zone C — Anapachydiscus terminus W a r d & K c n n c d y, —13 m, MP-US K 10216, A. terminus Zone D-E — Pachydiscus iacquoti (S e u n e s), from —22 to —27 m, D -MP-US K;[ 10217; E — MP-US K+0218; P. gollevillensis Zone All figures are in natural size. Photo B. Jabljanski.

16 PLATE I

Geologica Balcanica 24.6 — Ivanov, K. Stoykova — Gretaceous/Tertrary bonnolary in the area of Bjala. . . PLATE II PLATE III PLATE IV zone. In the lower part of that zone the first occurrence of Neochiastozygus perfeclus and Chiasmolilhus bidcns has been recorded. Thickness: about 15 m in the Bjala 1 sec tion; over 20 in in the Bjala 2b section.

F a s c i c u 1 i t h U s t y m p a n i f o r in i s Z o n e The base of the zone is clearly marked by the incoming of the genus Fasciculiihus. The first occurrence of the index species is slightly later after the appearance of the diver se Fasciculiihus janii, F. bitectus, F. billii, F. pileatus, F. ulii. Wiihin the zone the first occurrence of Chiasmolilhus consuelus has been registered. The upper boundary of that zone has been established in none of the sections studied near the town of Bjala. Thickness: over 6 m in the Bjala 1 section.

Ammonite successions Only in few sections, containing ammonite fauna, the K/T boundary has been docu mented (B i r k е 1 u n d, 1979, 1993; Ward et al., 1986; W i e d m a n n, 1986; 1988; I^S tinnesbeck, 1986; M a c е 1 1 a r i, 1986; Kenned y, 1989, 1993; Ward, 1990; Ward, Kennedy, 1993). An attempt for the ammonite subdivision of the Upper Maastrichtian has been made in a few sections only (M a c е 1 1 a r i, 1986; W i e d m a n n, 1988; Ward, Kenned y, 1993). There are no standard ammonite zonation scheme for the Upper Maastrichtian so far. Ammonites, inoceramids and echinoids have been found in the sections near the town of Bjala. A special attention has been paid to the ammonite successions. They have been studied in the most complete section — Bjala 2b, where below the K/T boun dary about 45 m of Maastrichtian are exposed. Additional data were obtained from kVft ’Bjak" 3 section. An amount of 22 ammonite species has been identified belon ging to 13 genera. The following genera are represented: Phylloceratina — Phylloceras (Neophylloccras) (2 species); Gaudryceralidae — Anagaudryceras (2 species), Verlebriies (I species); Telragonitidae — Pseudophylliles (2 species), Saghaliniles (1 species); he- teromorphs — Hoploscaphites (1 species), Diplomoceras (1 species), Baculiles (1 species), Glyploxoceras (1 species), Phyllopiychoceras. The representatives of Desmocerataceae — Pachydiscus (6), Anapachydiscus (2) and Desmophylliles (2) are of greatest diversity. The attempts for dividing the Upper Maastrichtian have been based so far on Pachy- discidae (M a c е 1 1 a r i, 1986; W i e d m a n n, 1986, 1988; Ward, Kenned y, 1993). Our observations have corroborated the concept that Pachydiscus and Anapachy discus have a rapid evolution during the Late Maastrichtian and their filiations could be used for an ammonite zonation. The following species forming a vertical succession and partly overlapping, have been found in the Bjala 2b section: Anapachydiscus freisvillensis, Pachydiscus neubergicus, P. gollevillensis armenicus, P. gollevillensis gollevillensis, P. jacquoli, Anapachy discus lerniinus. In the lowermost part of that section Anapachydiscus freisvillensis, Pachydiscus neubergicus and Pachydiscus gollevillensis armenicus occur together. The species P. neubergicus has been considered to be characteristic for the upper part of Lower Maastrichtian (В 1 a s z k i e v i c z, 1980; Kennedy, 1986; Kennedy, Summesberger, 1986). Recently, the species has been found in the lower part of the Upper Maastrichtian (W a r d, 1990; Ward, Kennedy, 1993; Kenne- d y, 1993). Within that interval A. freisvillensis and P. gollefvllensis armenicus often occur. The latter realizes the transition in the phylogenetic lineage — P. neubergicus — P. gollevillensis armenicus — P- gollevillensis gollevillensis. The divided here Anapachydiscus freisvillensis Zone is defined as an interval of conçurent occurrence of A. freisvillensis, P. neubergicus and P. gollevillensis armenicus. The base of the zone could not be drawn. Its top is marked by the last occurrence Of 17 2 Geologica Balcunica, 24, 6 /1. freisvillensis and the first occurence of Pachydiscus gollevillensis gollcvillensis. The ammonite association comprises: Phylloceras (N.) ramosum, Anagaudryceras politissi mum, Veriebrites kayei, Pseudophyllitcs indra, Phylloplychoceras sp. indet., Hoploscaph- ites constriclus, Diplomoceras cylindraceum, Baculites anceps, Glyptoxoceras cf. subcompressum, Desmophylliles larieti. Anapachydiscus freisvillensis, Pachydiscus neubergicus, P. gollevillensis arméniens. Thickness — 12 m. Pachydiscus gollevillensis gollevillensis Zone is distinguished as an interval from the first occurrence of P. gollevillensis gollevillensis or the last occurrence of A. freis villensis to the first occurrence of Anapachydiscus terminus. The index species has a wider range and occurs in the next zone as well. According to Ward, Kennedy (1993) and Kennedy (1993) the first occurrence of this species in the Biscay sec tions is earlier, and its range is partly overlapping the distribution of A. freisvillensis. The ammonite assemblage comprises: Phylloceras (N.) ramosum, A. politissimum, P. indra, Saghaliniles wrighti, H. constriclus, D. cylindraceum, B. anceps, G. cf. subcompressum, Pachydiscus gollevillensis gollevillensis, P. jacquoti. Thickness — 15 m. The last Anapachydiscus terminus Zone is recognized by the first occurrence of the index species to the level of the complete disappearance of the ammonites immediately below the K7T boundary. Single specimens of the following species have been found there: Phylloceras (N.) ramosum, V. kayei, B. anceps, Pseudophyllites indra, Anugau- dryceras politissimum, Pachydiscus gollevillensis, P. jacquoti. Thickness — 18 m. Ammonite records were registered in the alochthonous part of Bjala 2c section, as well as in Bjala 3. They are not studied in detail yet. The preliminary data indica tes the presence of Pachydiscus gollevillensis gollevillensis Zone in Bjala 3 section, at 20—25 m below the K/T boundary. It should be mentioned, that in Bjala 2b section the ammonites occur commonly in the interval — 45 m to —10 m below the K T. Between — 10th and — 7th m they occur rare, and within — 7th to — lm no ammonites were found. In the top Im of the Maastrichtian, four unidentifiable fragments (at —1 m) and five specimens (at —40 cm) were documented. The Upper Maastrichtian ammonite assemblages recovered around Bjala are similar and correspond to these of the Biscay region — Zumaya, Sopelana, Hendaye, Bidart (W a r d et al., 1986; W i e d m a n n, 1986, 1988; W a r d, 1990; \Y a r d, Kennedy, 1993; Kennedy, 1993). Bulgarian ammonite fauna shows a great similarity with respect to the ammo nites of the other European Upper Maastrichtian sections — near Maastricht, Lim burg (Belgium) (K e n n e d y, 1986 a); Stevns Klint and Kjolby Gard (Denmark) (B i r k е I u n d, 1979, 1993; Kennedy, 1993); Vistula Valley (Poland) (В 1 a s- z k e w i c z, 1980). There are, however, a significant problems associated with the absence of the ammonite standard zonal scheme and the nonprecised local and total range of the im portant Upper Maastrichtian ammonite taxa. The recognized in the present paper three ammonite zones could be correlate with the zones of the Biscay sections. Anapachydiscus freisvillensis Zone, covering the lower part of the Upper Maas trichtian section around Bjala, is not complete. It is correlated with A. freisvillensis Zone sensu W i e d m a n n, 1986, 1988 and with the upper part of A. freisvillensis Zone sensu Ward, Kennedy, 1993. Pachydiscus gollevillensis gollevillensis Zone spans the middle part of the Upper Maastrichtian. It is characterized by the conçurent local-range of P. gollevillensis go llevillensis and P. jacquoti. This zone corresponds to W i e d m a n n’s (1986, 1988) zones P. gollevillensis, P. llarenai and P. jacquoti and probably to the uppermost part of W a r d, K e n n e d y ’s (1993) A. freisvillensis Zone. Anapachydiscus terminus Zone is based on the total range of the index species. It is well-correlated with the same-named zone, recognized in the Uppermost Maas-

18 trichtian by Ward, Kennedy (1993) and probably with P. epipleclus Zone of Wiedmann (1896), 1988). The correlation with the Seymour Island section (Antarctica) (M a ce 1 lari, 1986) is impossible due to the endemic character of the ammonite fauna.

Comparison of the biostrati graphic data This parallel study of nannofossil and ammonite successions across the K/T boundary allow us to draw the following conclusions: 1) The presence of Micula murus within the whole interval studied below the K T boundary proves a Late Maastrichtian age according to nannofossils. The same age is determined by the occurrence of the ammonites P achy discus gollevillensis gollevillensis, P. jacquoli, Anapachydisens terminus, Hoploscaphiies constriclus,Saghalinites wrighti, Vertebrites kayei. 2) In the lower part of Bjala 2b section, A. freisvillensis, P. neubergicus and P. gollevillensis armenicus were found together with Micula murus. 3) The ammonite species P. jacquoti occurs together with the CC26 marker species Nephrolithus frequens in the higher levels of the Bjala 2b section. 4) Anapachydiscus terminus has been found together with Nephrolithus frequens and rare specimens of Micula prinsii. 5) The “youngest” ammonites — Pseudophyllites indra and Anagaudryceras poli- tissimurn were found 40 cm below the K/T boundary. They are associated with M. prinsii. Thus, ammonites still existed during the Latest Maastrichtian. 6) The ammonite zones A. freisvillensis (represented by its upper part in Bjala 2b section) and P. gollevillensis gollevillensis correspond to the nannofossil zone M i cula murus. The topmost Cretaceous ammonite zone A. terminus almost coincides with the range of the nannofossil zone Micula prinsii. With respect to palaeobiogeography the Late Maastrichtian nannofossils and ammo nites found in the region of Bjala are characteristic of the low latitudes (“Tethyan”). This conclusion is indirectly corroborated by the almost complete lack of belemnites. An exception here is the occurrence of the genus Hoploscaphites, which is considered to be Boreal (W i e d m a n n, 1988).

Conclusion The K/T boundary is considered to be a surface, marked by a boundary clay bed and by considerable change in the calcareous nannoplankton assemblages. In the vicinity of Bjala, on the Black Sea coast, the K/T boundary has been located in an uninterrup ted sedimentary sequence. The biostratigraphic boundary is defined on the basis of reduction in abundance of the Cretaceous nannoplankton species and the regular appearance of Biantholithus sparsus. Seven nannofossil zones (CC25 — NP 5) covering the Upper Maastrichtian to Upper Palaeocene, were assigned to the succession. The Bjala 2b section is one of only a few complete Upper Maastrichtian sections in the world, where macro-invertebrates (ammonites, echinoids and inoceramids) occur commonly. Three ammonite zones have been erected here, based on members of the Pachydiscidae. The integrated zonation using nannoplankton and ammonites allows us to com pare and correlate biostratigraphically important events. Thus, in the uppermost Maastrichtian Nephrolithus frequens and Pachydiscus jacquoti have been reported to gether, while the first specimens of Micula prinsii occur together with Anapachydis cus terminus.

19 The ammonites commonly occur in the Upper Maastrichtian. The “youngest” ones are registered immediately below the K/T boundary, at — 40 cm. Acknowledgments

The authors express their grati tude to the Austrian Academy of Sciences and to tile Geological Institute of the Bulgarian Academy of Sciences for their financial support. We are particularly indebted to Prof. Dr. Anton Preisinger (TU — Vienna) for stirring up interest in the problems of the Cretaceous — Tertiary boundary and for giving us an opportunity to work on those problems at the Institute of Mineralogy of Vienna Technical University. Our discussions with him, in which he was always a rational opponent, on the problem of the “mass extinction” were very stimulating to us. K. Stovkova is grateful to Prof. Herbert Stradner (Geologische Bundensanstalt — Wien) for extremely helpful consultations and for literature used to determine nannofossil taxa. M. Ivanov is grateful to Dr W. J. Kennedy (Oxford) for literature kindly provided and for critical reading and notes on the ma nuscript. Both authors are indebted to their colleages — Prof. T. Nikolov (Sofia University) and Dr D. Tronkov (Geological Institute, BAS) for their critical examination of the manuscript.

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