61

СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, год. 80, кн. 2, 2019, с. 61–69REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 80, part 2, 2019, p. 61–69

A new view to the spatial distribution of the Paleogene lithostratigraphic units in the offshore part of the Dolna Kamchiya basin based on seismic profile’s interpretation

Boris Valchev, Hristo DimitrovUniversity of Mining and Geology “St. Ivan Rilski”, 1700 Sofia, Bulgaria; E-mails: b_valchev@mgu.bg, hristo_dimitrov@mgu.bg

Един нов поглед върху пространственото разпространение на палеогенските литостратиграфски единици от акваториалната част на Долнокамчийския басейн на базата на интерпретация на сеизмични профили

Борис Вълчев, Христо ДимитровМинно-геоложки университет „Св. Иван Рилски“, София 1700

Резюме. Настоящата статия има за цел да хвърли светлина върху пространственото разпространение и връзките на палео­генските литостратиграфски единици от акваториалната част на Долнокамчийския басейн. На базата на литостратиграфска ин­терпретация на шест сеизмични профила и реинтерпретация на пет сондажни разреза са разпознати пет официални единици – Беленска свита (Палеоцен), Двойнишка свита с Армерски и Гебешки член (Долен–Среден Еоцен), Долночифлишки член на Авренската свита (Среден–Горен Еоцен) и Русларска свита (Олигоцен). Получени са нови данни за литологията, стратиграфски­те и латералните взаимоотношения, дебелината и вътрешната структура на единиците.

Ключови думи: литостратиграфия, Палеоген, сеизмични профили, Долнокамчийски басейн.

Abstract. The present article aims to elucidate the spatial distribution and relationships of the Paleogene lithostratigraphic units in the offshore part of the Dolna Kamchiya basin. Five formal units were identified on the base of lithostratigraphic interpretation of six seismic profiles and reinterpretation of five boreholes – the Byala Formation (Paleocene), the Dvoynitsa Formation with the Armera and Gebesh Members (Lower–Middle Eocene), the Dolni Chiflik Member of the Avren Formation (Middle–Upper Eocene), and the Ruslar Formation (Oligocene). New data on the lithology, stratigraphic and lateral relationships, as well as the thicknes and internal structure of the units were obtained.

Keywords: lithostratigraphy, Paleogene, seismic profiles, Dolna Kamchiya basin.

Introduction

The offshore part of the Dolna Kamchiya basin has been investigated for oil and gas since the beginning of the 60s of the 20th century and thus it is the lon­gest studied petroleum area in Bulgaria. On the base of geophysical and borehole data (the offshore drill­ing was done in the interval 1984–1993), outlining the deep geologic structure of the basin, the geolog­ical investigations have been focused on litho­ and biostratigraphy (Juranov, 1991), seismostratigraphy (Georgiev et al., 2004; Dimitrov, Georgiev, 2011), sequence stratigraphy (Dimitrov, 2007), lithofacies

analyses and palaeogeography (Dimitrov, Geor­giev, 2005; Dimitrov, 2008), and sedimenthology (Stefanov, 2018). Thus, a large amount of data and geologic interpretations has been collected.

The Dolna Kamchiya basin (DKB), also known as Dolna Kamchiya depression (DKD – Bokov et al., 1987) or Dolna Kamchiya sub­basin (Geor­giev, 2012), comprises a small onshore area in the central Bulgarian Black Sea coast and extends off­shore to the southeast into the Western Black Sea basin (Figs 1, 2). A brief review on the geological setting was published by Dimitrov and Georgiev (2005), and Stefanov (2018).

62

The present article aims to elucidate the spa­tial distribution and relationships of the Paleogene lithostratigraphic units in the offshore part of the Dolna Kamchiya basin on the base of lithostrati­graphic interpretation of seismic profiles and bore­hole sections. The interpretation of seismic profiles for lithosttatigraphic purposes was successfully ap­plied (Valchev, Dimitrov, 2018) to the Paleogene in the onshore part of the basin.

Studied material

Six selected seismic profiles (Figs 2, 4–6) were lithostratigraphically interpreted on the base of cor­relation to five borehole lithologic logs (Figs 2, 3), that were reinterpreted here. The primary lithologic data were collected from the geological reports kept in the National Geological Fund and they concern the following boreholes: LA­3 (Staples, Pierpoint,

Fig. 1. Tectonic subdivision of the territory of Bulgaria (af-ter Dabovski, Zagorchev, 2009) with location of the studied area

Фиг. 1. Тектонска подялба на територията на България (по Dabovski, Zagorchev, 2009) с разположението на из-следвания район

Fig. 2. Tectonic subdivision of the onshore part of Central East Bulgaria and adjacent offshore area (after Bokov et al., 1987; Georgiev, 2012 with modifications) with location of the studied seismic profiles and boreholes Tectonic units: 1, Moesian Platform, 2, Provadia syncline, 3, Dolna Kamchiya basin, 4, Western Black Sea basin, 5, Eastern Balkanides, 6, seismic profile, 7, borehole; Faults: 1, Venelin­Aksakovo, 2, Balchik, 3, Kaliakra, 4, Bliznatsi, 5, East Balkan frontal thrust

Фиг. 2. Тектонска подялба на сухоземната част на централна Източна България и прилежащата ѝ акватория (по Bokov et al., 1987; Georgiev, 2012 с изменения) с разположение на изучените сеизмични профили и сондажиТектонски единици: 1 – Мизийска платформа, 2 – Провадийска синклинала, 3 – Долнокамчийски басейн, 4 – Западночер­номорски басейн, 5 – Източни Балканиди, 6 – сеизмичен профил, 7 – сондаж; Разломи: 1 – Венелин­Аксаковски, 2 – Бал­чишки, 3 – Калиакренски (Западночерноморски), 4 – Близнашки, 5 – Източнобалкански челен навлак

63

1995f1), LA­1 (Pettit, Pierpoint, 1994f2), R­1 Sa­motino Sea (Jelev et al., 1989f3), R­1 Samotino East (Bogatskaya et al., 1986f4), and LA­2 (Staples,

1 Staples, G., N. Pierpoint. 1995f. Well LA IV/91-3. Final Well Report. Ministry of Energy, National Geological Fund, report LA IV­346, 14 p. (unpublished).

2 Pettit, W., N. Pierpoint. 1994f. Well LA IV/91-1. Final Well Report. Ministry of Energy, National Geological Fund, report LA IV­283, 28 p. (unpublished).

3 Jelev, S., A. Vasilev, R. Veneva, S. Juranov, G. Kovachev, Y. Kova cheva, R. Koleva, N. Kostova, T. Manchev, G. Marinova, L. Mo­nahova, K. Mutafova, G. Pavlova, G. Radev, R. Stanev, G. Stoyanova. 1989f. Report on the Results of the Deep Prospecting Drilling of R-1 Samotino Sea Borehole. Ministry of Energy, National Geological Fund, report ІІІ­366, 360 p. (in Bulgarian, unpublished).

4 Bogatskaya, G., M. Vakarelska, A. Vasilev, R. Veneva, D. Grib­neva, S. Juranov, S. Jelev, Y. Kovacheva, T. Manchev, L. Monahova,

1994f5). For additional correlation an onshore seis­mic profile, located along the sea coastline, was used (Fig. 7).

Lithostratigraphic units

The lithostratigraphic reinterpretation of the studied borehole lithological logs allowed identifying of five

G. Radev, S. Rangelov, V. Stanev. 1986f. Report on the Results of the Deep Prospecting Drilling of R-1 Samotino East Borehole. Ministry of Energy, National Geological Fund, report ІІІ­356, 280 p. (in Bulgarian, unpublished).

5 Staples, G. 1994f. Well LA IV/91-2. Final Well Report. Ministry of Energy, National Geological Fund, report LA IV­242, 18 p. (unpublished).

Fig. 3. Lithological logs and lithostratigraphic interpretation of the borehole sections in the offshore part of the Dolna Kamchiya basinAbbreviations: By, Byala Formation; Dv, Dvoynitsa Formation (A, Armera Member, Gb, Gebesh Member); DCh, Dolni Chiflik Member of the Avren Formation; Ru, Ruslar Formation

Фиг. 3. Литоложки колонки и литостратиграфска интерпретация на сондажните разрези от акваториалната част на Долнокамчийския басейнСъкращения: By – Беленска свита; Dv – Двойнишка свита (A – Армерски член, Gb – Гебешки член); DCh – Долночифлиш­ки член на Авренската свита; Ru – Русларска свита

64

formal units (Fig. 3): the Byala Formation (Pale­ocene), the Dvoynitsa Formation with the Armera and Gebesh Members (Lower–Middle Eocene), the Dolni Chiflik Member of the Avren Formation (Mid­dle–Upper Eocene), and the Ruslar Formation (Oli­gocene). A brief description of their lithology, age, spatial distribution, and relationships is given bellow.

The Byala Formation

The rocks of this unit were described as “whitish limy marls” (Zlatarski, 1907), “Byala clayey marls” (Bontchev, 1926), “limestone­marl formation” (Ju­ranov, 1984). It was formalized by Juranov (1991; see also Juranov, 1993a). In the offshore part of the Dolna Kamchiya basin the unit is represented by dark gray solid marls, established in two borehole sec­tions – LA­1 and R­1 Samotino Sea (Fig. 3b, c). In the first one, it overlies the Upper Cretaceous base as the boundary is an unconformity and the thickness is 65 m. In the second section the lower boundary was not penetrated and thus it could be assumed that the thickness is more than 106 m. The upper bound­ary is a sharp lithologic contact with the Dvoynitsa Formation (represented by the Gebesh Member in LA­1 and by the Armera Member in R­1 Samotino Sea). In the southernmost boreholes R­1 Samotino East and LA­2 (Fig. 3d, e) the pre­Paleogene base was not penetrated and therefore this boundary was not studied. The Byala Formation was identified in all seismic profiles (Figs 4–6) underlying the Gebesh or Armera Member of the Dvoynitsa Formation and showing almost constant thickness. In the northern part of B92­16 seismic profile (Fig. 4) there is prob­ably a lateral transition between the Byala Forma­tion and the Komarevo Formation (Thanetian). The planktonic foraminiferal data obtained from R­1 Samotino Sea borehole (Juranov, 1991) determined Paleocene age for the formation.

The Dvoynitsa Formation

The unit was introduced as formal one by Juranov and Pimpirev (1989) in the coastal region of the East­ern Balkanides. Later on, Vangelov and Sinnyovsky (2011) formalized three members of the Dvoynitsa Formation (the Armera, Gebesh and Goritsa Mem­bers), and Valchev et al. (2018) elucidated the inter­nal structure of this unit in the onshore part of the Dolna Kamchiya basin by 3D lithological modelling recognizing all the three members. The Gebesh and Armera Members were established in all five off­shore borehole sections, while the Goritsa Member was not recorded. The Armera Member comprises thick­bedded sandstones with interbeds of thick­bed­

ded to massive conglomerates. The Gebesh Mem­ber is composed of thin­bedded alternation of sand­stones, marls, siltstones and shales. In LA­3 borehole only the Armera Member is represented and it over­lies the Lower Cretaceous base, as the boundary is an unconformity (Fig. 3a). The lower boundary with the Byala Formation was discussed above. The two members show great varieties of their thickness, as they do not demonstrate an exact stratigraphic posi­tion in the Dvoynitsa Formation volume. The Gebesh Member was recorded at two or three levels in four borehole logs (Fig. 3b–e) and its presence was identi­fied in all seismic profiles. The thinnest (100 m) it is in R­1 Samotino Sea borehole (Fig. 3c) and the thick­est (over 2000 m) – in LA­2 borehole (Fig. 3e). The Armera Member occurs usually at one (Fig. 3a, d, e) or two levels (Fig. 3b, c). Its thickness varies between 54 m in LA­2 borehole (Fig. 3e) and 902 m in R­1 Samotino Sea borehole (Fig. 3c). The total thickness of the formation is from 395 (Fig. 3a) to over 2000 m (Fig. 3e). The lithostratigraphic interpretation of the seismic profiles (Figs 4–6) shows clearly the com­plex and variable stratigraphic and lateral relation­ships of the two units and the variety of their thick­ness. The chronostratigraphic range of the Dvoynitsa Formation was determined as Lower–Middle Eocene on the base of rare planktonic foraminiferal remains (Juranov, 1991).

The Dolni Chiflik Member of the Avren Formation

It was known as “Dolen Ciflik Seria” (Pollak, 1933), “marl­sandy complex” (Efremochkin et al., 1974f6; Vavilova et al., 1978f7), “Dolni Chiflik For­mation” (Cheshitev et al., 1994; Kânčev, 1995), and formalized as the Dolni Chiflik Member by Jura­nov (1993b). Generally, the unit comprises alterna­tion of sandy marls and clayey sandstones, as it is possible two distinct levels to be recognized. The lower one is composed predominantly of clayey sandstones and siltstones alternating with thin­bedded shales and marlstones. The upper level is represented mainly by sandy marls alternating with siltstones, sandstones and shales. The thickness of the lower level varies from 653 (Fig. 3e) to 789 m

6 Efremochkin, N. V., R. Y. Yovchev, V. P. Strepetov, R. I. Plot­nikova, L. Bryukner, S. Dimovski, A. Agopyan, V. Parashkevova, M. Vavilova, M. Nikolova, Y. Aleksiev. 1974f. Report on the Results from the Subsurface Iodine-bearing Water Prospecting in Dolna Kamchiya Source with Available Reserve Calculation (up to October, 1, 1974). Ministry of Energy, National Geological Fund, report V­223, 1202 p. (in Russian, unpublished).

7 Vavilova, M., S. Dimovski, L. Bryukner, V. Parashkevova, M. Nikolova. 1978f. Report on the Results of the Deep Drilling in Dolna Kamchiya Area Conducted in 1955–75. Ministry of Energy, National Geological Fund, report ІІІ­294, 3171 p. (in Bulgarian, unpublished).

65

Fig.

4. L

ithos

trat

igra

phic

inte

rpre

tatio

n of

seis

mic

pro

files

SP7

866

and

B92

-16

Figs

4–7

: Geo

logi

c­ge

ophy

sica

l sec

tions

to d

oubl

e­tim

e de

pth

(TW

T, s)

. Abb

revi

atio

ns: B

y, B

yala

Fm

.; K

om, K

omar

evo

Fm.;

Gb,

Geb

esh

Mb.

; A, A

rmer

a M

b.; D

Ch,

Dol

ni C

hifli

k M

b.; R

u,

Rus

lar F

m.;

B+D

+Al,

Bel

osla

v, D

ikili

tash

and

Ala

dan

Fms;

N, N

eoge

ne c

over

Фиг

. 4. Л

итот

рати

граф

ска

инте

рпре

таци

я на

сеи

змич

ни п

роф

или

SP78

66 и

B92

-16

Фиг

. 4–7

: Гео

лого

­гео

физи

чни

разр

ези

по д

войн

о вр

еме

дълб

очин

а (T

WT,

s). С

ъкра

щен

ия: B

y –

Беле

нска

сви

та; K

om –

Ком

арев

ска

свит

а; G

b –

Гебе

шки

чле

н; A

– А

рмер

ски

член

; D

Ch

– Д

олно

чифл

ишки

чле

н; R

u –

Русл

арск

а св

ита;

B+D

+Al –

Бел

осла

вска

, Дик

илит

ашка

и А

ладъ

нска

сви

та; N

– н

еоге

нска

пок

ривк

а

66

Fig. 5. Lithostratigraphic interpretation of seismic profiles BGK92-22 and 84581

Фиг. 5. Литотратиграфска интерпретация на сеизмични профили BGK92-22 и 84581

(Fig. 3b), while the upper one is from 180 (Fig. 3e) to 500 m (Fig. 3b) thick. It should be noted that the core intervals of R­1 Samotino Sea borehole are very scarce and this fact did not allowed us to rec­ognize the two levels. The total thickness of the unit is from 836 (Fig. 3e) to 1289 m (Fig. 3b). The chro­nostratigraphic range of the Dolni Chiflik Member was determined on the base of planktonic foramini-feral data (Juranov, 1991) as Middle–Upper Eocene.

The Ruslar Formation

It was introduced as “Ruslar sandstones” by Zlatar­ski (1927) and formalized by Aladjova­Chrisčeva (1991). In the offshore part of the Dolna Kamchiya basin the unit comprises predominantly claystones

with thin­bedded interbeds of sandstones, silt­stones and rare limestones. The last, as well as the sandstones, occur mainly at the lower levels of the formation. Its thickness varies from 126 (Fig. 3e) to 500 m (Fig. 3d). The boundary with the Dolni Chiflik member is a sharp lithologic contact. In the entire area of its distribution the Ruslar Formation is covered with Neogene sediments. Fossil remains have not been established, therefore the age of the unit was determined as Oligocene on the base of its stratigraphic position.

Discussion

The data, obtained from the studied borehole sec­tions and seismic profiles, allowed us to outline the

67

Fig.

6. L

ithos

trat

igra

phic

inte

rpre

tatio

n of

seis

mic

pro

files

BG

K92

-71

and

SP79

124

Фиг

. 6. Л

итот

рати

граф

ска

инте

рпре

таци

я на

сеи

змич

ни п

роф

или

BG

K92

-71

и SP

7912

4

68

Fig. 7. Lithostratigraphic interpretation of the onshore seismic profile 5506-84

Фиг. 7. Литотратиграфска интерпретация на сухоземния сеизмичен профил 5506-84

spatial distribution of the five established Paleo­gene lithostratigraphic units. Thus, we could claim that the Byala Formation is distributed in the entire southern board of the basin, showing almost con­stant thickness. It is covered with the Armera or Gebesh Member of the Dvoynitsa Formation in the entire area of its distribution. The last one was also recorded in the southern board of the basin, as it is difficult to outline exactly the northern boundary of its distribution. The unit demonstrates complex internal structure with varied thickness. The com­plicated tectonic structure of the southernmost area (Fig. 4 – profile SP7866, Fig. 5 – profile BGK92­22, Fig. 6 – profile SP79124) does not allow calcu­lation of the exact thickness, but we could assume that it is over 2000 m. Variation of the stratigraphic position and the thickness of the Armera and Gebe­sh Members confirms the data, obtained from the onshore boreholes and seismic profiles (see Fig. 7), and reveals the Dvoynitsa Formation as the most complex Paleogene lithostratigraphic unit in the Dolna Kamchiya basin.

The data, concerning the Dolni Chiflik Mem­ber of the Avren Formation, proved its broad spatial distribution in the basin, but we could not mark the lateral transition to the typical Avren Formation in the northern board of the Dolna Kamchiya basin. The member covers the Armera or Gebesh Members of the Dvoynitsa Formation as the boundary is unconformity. The thickness shows considerable variations, as it is maximal

in the central area of the southern board of the basin.

Compared to the onshore sections, the Ruslar Formation shows similar lithologic features and variations of the thickness. It is maximal in the cen­tral offshore areas.

The uneven distribution of the borehole sec­tions and seismic profiles, especially in the northern board of the offshore Dolna Kamchiya basin, is an obstacle for a more precise outlining of the spatial relationships with the Paleogene lithostratigraphic units recorded from the northernmost areas of the basin, and the probable lateral transitions to the Komarevo, Beloslav, Dikilitash, Aladan and Avren Formations, established in the onshore sections.

Conclusions

The lithostratigraphic interpretation of the seismic profiles by correlation to the reinterpreted offshore borehole logs, gave us an opportunity to clarify the spatial distribution and relationships (stratigraphic and lateral) of the Paleogene lithostratigraphic units and to add new data of their lithology and thickness. This fact is a reliable base for further investiga­tion of the deep geological structure of the offshore part of the Dolna Kamchiya basin including 3D lithostratigraphic, chronistratigrapnic and lithologi­cal modeling, as well as more precise palaeogeo­graphic reconstructions.

References

Aladjova­Chrisčeva, K. 1991. Stratigraphic subdivision and correlation of Paleogenic deposits in Northeast Bulgaria. – Geologica Balc., 21, 2, 12–38 (in Russian with English abstract).

Bokov, P., G. Georgiev, I. Monahov, A. Atanasov, S. Jelev, Ch. Dachev, D. Yordanova, M. Vavilova, M. Nikolova, R. Ognyanov. 1987. Tectonic framework. – In: Bokov, P., Ch. Chemberski (Eds). Geological Premise for the Oil-gas

69

Bearing of the Northeast Bulgaria. Sofia, Tehnika, 109–119 (in Bulgarian).

Bontchev, G. 1926. Les roches dans les parties septentrionales du Balkan entre la mer Noire, le défilé de Kotel et les ri­vières Vrana et Goléma Kamtchia. – Rev. Bulg. Acad. Sci., 34, 16, 1–99 (in Bulgarian with German abstract).

Cheshitev, G., V. Milanova, N. Popov, E. Kojumdgieva. 1994. Explanatory Note to the Geological Map of Bulgaria on Scale 1:100 000. Varna and Zlatni Pyassatsi Map Sheets. Committee of Geology and Mineral Resourses, Geology and Geophysics Ltd., Sofia, Avers, 75 p. (in Bulgarian with English abstract).

Dabovski, H., I. Zagorchev. 2009. Alpine tectonic subdivision of Bulgaria. – In: Zagorchev, I., H. Dabovski, T. Nikolov (Eds). Geology of Bulgaria. Part II, Mesozoic Geology. Sofia, “Prof. Marin Drinov” Publishing House, 30–37 (in Bulgarian with English abstract).

Dimitrov, H. 2007. Analysis of relative changes of the sea level in the Kamchia sedimentary basin (offshore part) during the Middle–Late Eocene and Oligocene epoch. – Ann. Univ. Mining and Geol., 50, 1–Geol. and Geophys., 43–49 (in Bulgarian with English abstract).

Dimitrov, H. 2008. Paleogeographic reconstruction of the Kamchia sedimentary basin (offshore part) during the Mid­dle–Late Eocene and Oligocene epoch. – Ann. Univ. Min-ing and Geol., 51, 1–Geol. and Geophys., 28–34 (in Bul­garian with English abstract).

Dimitrov, H., G. Georgiev. 2005. Lithofacies analysis of the Kamchia basin (offshore zone) sedimentary sequences. – Ann. Univ. Mining and Geol., 48, 1–Geol. and Geophys., 47–52 (in Bulgarian with English abstract).

Dimitrov, H., G. Georgiev. 2011. Correlation between main seismic sequence boundaries in Kamchia basin (offshore Bulgaria) and Western Black Sea basin. – In: 73rd EAGE Conference & Exhibition incorporating SPE EUROPEC 2011, Vienna, Austria, 23–26 May 2011 (CD ROM).

Georgiev, G. 2012. Geology and hydrocarbon systems in the Western Black Sea. – Turkish J. Earth Sci., 21, 723–754.

Georgiev, G., H. Dimitrov, F. Raid, J. Pringle, N. Botoucharov. 2004. Seismostratigraphy and 3D model of the Dolna Kam­chiya sedimentary basin (offshore part). – In: Proc. Intern. Scientific and Technical Conference “Problems of Oil and Gas”. Varna, 142–147 (in Bulgarian).

Juranov, S. 1984. Lithostratigraphy of the sedimentary rocks from the Senonian–Middle Eocene interval near the vil­lages of Byala and Goritsa, Varna District. – Ann. High. Inst. Mining and Geol. Sofia, 30, 2, 13–23 (in Bulgarian with English abstract).

Juranov, S. 1991. Stratigraphy of the Upper Cretaceous series

and the Paleogene system in the marine borehole sections at the village of Samotino. – Rev. Bulg. Geol. Soc., 52, 3, 19−29 (in Bulgarian with English abstract).

Juranov, S. 1993a. Byala Limestone­marl Formation. – In: Ten­chov, Y. (Ed.). Glossary of the Formal Lithostratigraphic Units in Bulgaria (1882–1992). Sofia, Bulgarian Academy of Sciences Publishing House, p. 32 (in Bulgarian).

Juranov, S. 1993b. Dolni Chiflik Member of the Avren Marl Formation. – In: Tenchov, Y. (Ed.). Glossary of the Formal Lithostratigraphic Units in Bulgaria (1882–1992). Sofia, Bulgarian Academy of Sciences Publishing House, p. 118 (in Bulgarian).

Juranov, S., H. Pimpirev. 1989. Lithostratigraphy of the Upper Cretaceous and Paleogene in the coastal part of East Stara Planina. – Rev. Bulg. Geol. Soc., 50, 2, 1–18 (in Bulgarian with English abstract).

Kânčev, I. 1995. Explanatory Note to the Geological Map of Bulgaria on Scale 1:100 000. Dolni Ciflik Map Sheet. Committee of Geology and Mineral Resourses, Geology and Geophysics Ltd., Sofia, Avers, 113 p. (in Bulgarian with English abstract).

Pollak, A. 1933. Geologische Untersuchungen über das End­stuck Ostbalkans. – Abh. Math.-Phys. Kl., Sächsischen Akad. Wiss., 41, 7, 1–60.

Stefanov, Y. 2018. Illite/smectite diagenesis and thermal evo­lution of Lower Cretaceous–Paleogene successions in the Dolna Kamchiya Depression, Eastern Bulgaria. – Geologi-ca Balc., 47, 1, 3–21.

Valchev, B., H. Dimitrov. 2018. Lithostratigraphic interpreta­tion of seismic profiles on the example of the Paleogene in the onshore part of the Dolna Kamchiya basin (Eastern Bulgaria). – Rev. Bulg. Geol. Soc., 79, 3, 105–106.

Valchev, B., H. Dimitrov, D. Sachkov, S. Juranov. 2018. New data about the Dvoynitsa Formation distribution in the on­shore part of the Dolna Kamchiya basin (Eastern Bulgaria) on the base of 3D lithological modeling. – C. R. Acad. Bulg. Sci., 71, 12, 1252–1256.

Vangelov, D., D. Sinnyovsky. 2011. New data about the Dvoynitsa Formation distribution, lithology and chron­ostratigraphic range, East Bulgaria. – Ann. Univ. de Sofia, Fac. géol. et géogr., 49, 1–géol., 43–70 (in Bulgarian with English abstract).

Zlatarski, G. 1907. Le Sénonien dans la Bulgarie orientale, au nord des Balkans et sa division en Emschérien et Aturien. – Ann. Univ. de Sofia, Fac. Physico-mathématique, 2, 31–51 (in Bulgarian with French abstract).

Zlatarski, G. 1927. La Géologie de la Bulgarie. Sofia, Publ. House “Hudozhnik”, Univ. Library, 65, 268 p. (in Bulgar­ian with French abstract).

Постъпила на 27.09.2019 г., приета за печат на 06.11.2019 г.Отговорен редактор Димитър Синьовски