The Carboniferous of Svoge Coalfield (Bulgaria)

GEOLOGICA BALCANICA, 36, 1—2, Sofia, Iun. 2007, p. 5—15.

Yanaki Tenchov

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia; Å-mail: [email protected] (Submitted: 10.10.2006; accepted for publication: 24.11.2006)

ß. Òåí÷îâ – Êàìåííîóãîëüíàÿ (êàðáîíñêàÿ) ñèñòåìà Ñâîãåíñêîãî óãëåíîñíîãî áàññåéíà (Áîëãàðèÿ). Â êàìåííîóãîëüíûõ ïîðîäàõ Ñâîãåíñêîãî óãëåíîñíîãî áàññåéíà âûäåëåíû: Öà- ðè÷èíñêàÿ ñâèòà ñ Äûëáî÷èöñêèì ÷ëåíîì (250 m – íàìþð À), Ñâèäíåíñêàÿ ñâèòà (220 m – íàìþð C – âåñòôàëü À), Äðûìøàíñêàÿ ñâèòà (180 m), Ñâîãåíñêàÿ ñâèòà (500 m – âåñòôàëü À), Áåðîâäîëñêàÿ ñâèòà (280 m – âåñòôàëü A-B) è ×èáàîâñêàÿ ñâèòà (420 m – âåñòôàëü B-C). Çà èñêëþ÷åíèåì Äðûìøàíñêîé ñâèòû, êîòîðàÿ ïðåäñòàâëåíà ãëèíèñòûìè ïîðîäàìè, âñå îñòàëüíûå åäèíèöû ñëîæåíû ïðåèìóùåñòâåííî ïåñ÷àíèêàìè ñ íåçíà÷èòåëüíûì ó÷àñòèåì ãëèí, ñ êîòîðûìè ÷àñòî àññîöèèðóþò óãîëüíûå ïëàñòû. Êîíãëîìåðàòû ïðèñóòñòâóþò â îñ- íîâàíèè Öàðè÷èíñêîé, Ñâèäíåíñêîé è ×èáàîâñêîé ñâèò è ïåðåñëàèâàþò ïîðîäû Ñâèäíåí- ñêîé, Ñâîãåíñêîé è ×èáàîâñêîé ñâèò. Óãëè ïðåäñòàâëåíû ñóïåðàíòðàöèòîì (95—98% ëåòó- ÷èõ). Èìè ñëîæåíû ãëàâíûì îáðàçîì òîíêèå (0.6—2 m) ïëàñòû, êîòîðûå òîëüêî ìåñòàìè äîõîäÿò äî 12 m, ãëàâíûì îáðàçîì â ðåçóëüòàòå òåêòîíè÷åñêîé àêêóìóëÿöèè. Ïåðåðûâû â îñàäêîíàêîïëåíèè óñòàíîâëåíû ìåæäó Öàðè÷èíñêîé è Ñâèäíåíñêîé ñâèòàìè è â îñíîâà- íèè Áåðîâäîëñêîé ñâèòû. Ôóíäàìåíò ïðåäñòàâëåí ïîðîäàìè îðäîâèêñêîãî, ñèëóðèéñêîãî è äåâîíñêîãî âîçðàñòà. Êàìåííîóãîëüíûå îòëîæåíèÿ ïåðåêðûâàþò èõ ïî ýðîçèîííîìó íåñîã- ëàñíîìó (äî 30°) êîíòàêòó. Ñëîæåííûé â íîðìàëüíûå è îïðîêèíóòûå ñêëàäêè ïàëåîçîéñêèé ðàçðåç â çàïàäíîé ÷àñòè óãîëüíîãî áàññåéíà ïåðåêðûò íèæíèì òðèàññîì. Ñ òåêòîíè÷åñêîé òî÷êè çðåíèÿ óãîëüíûé áàññåéí èìååò ñòðîåíèå êîìïëåêñíîãî ãðàáåíà, ðàçìåùåííîãî â ÿäðå àëüïèéñêîé Ñâîãåíñêîé àíòèêëèíàëè. Â åãî þæíîé ÷àñòè ðàçâèòû íîðìàëüíûå ñèììåòðè÷åñêèå, à â ñåâåðíîé ÷àñòè – ëåæà÷èå ñêëàäêè: ñåâåðîâåðãåíòíûå – â åå âîñòî÷íîé ïîëîâèíå è þæíîâåðãåíòíûå – â çàïàäíîé. Ìåæäó íèìè ïðîõîäèò âûðàçè- òåëüíûé ñáðîñî-ñäâèã. Þæíàÿ ÷àñò áàññåéíà ïîäâåðãëàñü ñêëàä÷àòîñòè â äîïåðìñêîå âðå- ìÿ, ïî âñåé âåðîÿòíîñòè â õîäå ëåîíèéñêîé ôàçû. Ìîæåò áûòü âî âðåìÿ àñòóðèéñêîé ôàçû ãðàáåí òðàíñôîðìèðîâàëñÿ â ãîðñòîâóþ ñòðóêòóðó è îñàäêîíàêîïëåíèå ïðåêðàòèëîñü. Íî- âîå ñêëàäêîîáðàçîâàíèå (ñåâåðîâåðãåíòíàÿ ñêëàä÷àòîñòü) ïðîèçîøëî ïî âñåé âåðîÿòíîñ- òè â ïîçäíåìåëîâîå âðåìÿ. Ðåãèîíàëüíûé ñðûâ (decollement) îòäåëÿåò òåððèòîðèþ ñ þæ- íîâåðãåíòíîé ñêëàä÷àòîñòüþ â çàïàäíîé ÷àñòè ãðàáåíà îò ïåðåêðûâàþùèõ òðèàññîâûõ ïîðîä. Ïî âñåé âåðîÿòíîñòè Ñâèäíåíñêèé ïëóòîí íå ïîääàëñÿ íàïðàâëåííîìó ê ñåâåðó ñæàòèþ. Abstract. The Svoge Coalfield Carboniferous is subdivided into Tsarichina Formation with Dalb- ochitsa Member (250 m – Namurian A), Svidnya Formation (220 m – Namurian C-Westphalian A), Dramsha Formation (180 m), Svoge Formation (500 m – Westphalian A), Berovdol Forma- tion (280 m – Westphalian A-B) and Chibaovtsi Formation (420 m – Westphalian B-C). Except Dramsha Formation, that is built of claystones, the other formations consist mainly of sandstones, with few levels of claystones with or without coal seams. Conglomerates occur at the base of Tsarichina, Svidnya and Chibaovtsi Formation and are intercalated in the sequences of Svidnya, Svoge and Chibaovtsi Formation. The coal seams are superantracites (95—98% volatile) mainly thin, some are 0.6 to 2.0 m, but in place reach even 12 m most frequently in result of tectonic accumulation. Breaks in sedimentation are between Tsarichina and Svidnya Formation and at the base of Berovdol Formation. The basement is built of a continuous Ordovician, Silurian and Devonian sequence covered with erosive contact and angular (up to 30°) unconformity by the Carboniferous. The folded (in normal and reversed folds) Palaeozoic section is unconformably covered in the western part of the coalfield by Lower Triassic.

5 Tectonically the Coalfield is a complex graben superimposed on the core of the Alpine Svoge anticline. The south parts are built of symmetric folds. The north parts are of recumbent folds which in the east half are north vergent, whereas in the west part are south vergent, being separated by a prominent wrench fault. The south part is folded in pre-Permian time, possibly during the Leonian Phase, when the sedimentation stopped, and the graben has been trans- formed in a horst structure, maybe during the Asturian Phase. Both parts are refolded during an Alpine (Late-Cretaceous) north-vergent folding. The western south vergent folds are separated from the Triassic cover by decollement whereas most probably the Svidnya Pluton has resisted to the north directed pressure.

Tenchov, Y. 2007. The Carboniferous of Svoge Coalfield (Bulgaria). – Geologica Balc., 36, 1—2; 5—15. Key words: Svoge Coalfield, Carboniferous, Bulgaria

Introduction the field work. Obviously the prospecting geologist needs knowledge of the fossils. The present author Continental Carboniferous sediments are established studied the Carboniferous megaflora during two spe- in Bulgarian territory and aquatory in many outcrops cialisations: at the Charles University and the Muse- and in subsurface. The coal-bearing sediments from um in Prague (under Prof. F. Nemejc), and at Natur- outcrops near the town of Svoge (Fig. 1) had been historisches Museum in Berlin (Prof. R. Daber). The first indicated by Toula (1878) as Lower Carbonifer- lithostratigraphy, the age and tectonic concept had ous, based on determinations of megaflora by D. Stur. been proposed (Tenchov, 1966), as well as the estab- The mining of anthracite coals at the beginning of lished megaflora (Tenčov, 1977). the 20th Century provided new megaflora. Some spec- Many observed facts and some tectonic solutions imens seen by W. Gothan have provoked the opinion stay on my attention for many years. Now a new so- that Upper Carboniferous sediments are also present. lution is proposed together with larger information Krestew (1928) collected some new plants from the on facts that are not published or are published only mines and published them with conclusion about in Bulgarian language. Westphalian and Stephanian age. The Stephanian age had been rejected by Hartung, Gothan (1939). Haberfelner (1934) indicated a sequence: at the base Stratigraphy are black argillites with some sandstone lenses in the upper part, followed of whitish sandstones and con- The first lithostratigraphy of the basin has been pro- glomerates with three to six coal seams. Hartung posed by Tenchov (1966, 1971). The following for- (1935) gathered large collections of megaflora with mations have been introduced: Tsarichina Forma- conclusions for the presence of Namurian A and tion with Dalbochitsa Member, Svidnya, Dramsha, Westphalian A and B sediments. Nemejc (1943) es- Svoge, Berovdol and Chibaovtsi Formation (Fig. 2). tablished some more species, and confirmed the same The basement in the northern part of the coalfield time interval. consists of the Ordovician Grohoten Formation. It is The Bulgarian State Geological Survey starts eco- composed of irregular interbedding of sandstones, nomical prospecting with galleries and boreholes sandy claystones and claystones with numerous pure since 1953. In 1954, this prospection has been under white quartzite beds of 1 to 10 m thickness. In the the responsibility of the geologists P. Zaprianov and southern part the basement consists of Silurian grap- Y. Tenchov. Till this time the Carboniferous had not tolite shales and Devonian flysch-like alternation with been delimited well from the older sediments. Dur- 5 to 30 m intervals of dark shales. The Carbonifer- ing 1954—1956 the western part of the Svoge Coalfield ous sediments cover the basement with angular un- was mapped, and S. Stefanov was included in map- conformity. ping. A stratigraphic sequence (Tenchov, 1961) and The post-Carboniferous cover is preserved a few some perspectives of coal reserves were established kilometers from west to east and is represented by in internal reports. The prospecting had been stopped Lower Triassic red sandstones. The Permian red sed- for some years. The restarted economical prospec- iments are exposed as basement of the Lower Trias- tion (1968—1980) had been fulfilled by another team sic some 1 to 2 km from the Carboniferous outcrops, of geologists; it yielded some lithologic data and other but are never found in direct contact to the latter. stratigraphic ideas. In 1959 Tenchov entered the Geological Institute of Bulgarian Academy of Sci- ences and continued mapping the eastern half of Tsarichina Formation with Dalbochitsa the Coalfield. The western part has been also restud- Member ied mainly for collecting of fossil plants. The fossils collected during the first phase were determined of The formation starts with conglomerates and brec- S. Chernyavska although the results came late for cia-conglomerates. Their composition depends on the

6 Svoge C a rboni fe ro us – n or m a l d o v e rt u r ed

4 –

12 T ri a ss ic ; –

3 Svid n y a p lut o ; –

2 g e o l gi c a pr f i n s ( Fig. 5 ; –

11 pr e - Carbo ni f ro u s b ase m nt ; o b s e rv a l d c ll m nt; – –

1 10 a n ti c l i x s ; –

9 s yn c li n a l x i ; –

8 M og i l a t w r e n c h f u t; –

7 o v er t hr u s t; –

6 f a u l t; –

5 Tec t o nic ske tc h of he S v oge Carbo n i fe ro u s b as ( af er T e nc hov , 196 6 F ig . 10 w ith ch a g es ) F ig. 1. C o a l f i e d ; b e d i ng

7 Fig. 2. Lithostratigraphic division and lithologic columns, based on the stratotypes. 1 – conglomerates with breccia-conglomerates; 2 – conglomerates; 3 – sandstone with pebbles; 4 – sandstones; 5 – siltstones; 6 – claystones; 7 – thin coal seams; 8 – coal seams; 9 – Localities with megaflora with Ns of locality after Tenchov (1977 – Abb. 1 and Anlage III); 10 – sampled Leaia; 11 – mined coal seam basement. In the next 100 m a coal-bearing alterna- an A, as determined by Hartung (1935). The lists of tion follows. It is built of sandstones, aleurolites, clay- localities and species compositions of this formation stones and 3 to 4 coal seams, two of them exploited (as well as of all others) are published by Tenchov near the village of Tompson. Above follow some 100 (1977). m of claystones and aleurolites with rare sandstones interbeds. The rare Stigmaria are with preserved spa- tial arranged appendices. The Formation crops out Svidnya Formation in the south part of the Coalfield. Megaflora is collected mostly in claystones from many levels and lo- This formation starts with conglomerates in alterna- calities. There are established 33 localities that yielded tion with breccia-conglomerates up to 100 m thick. 43 species, among them Sphenophyllum tenerrimum, They are composed mainly of Ordovician white Eleutherophyllum mirabile, Zeilleria moravica, Di- quartzites and black lydites. To the east they are thin- plotmema patentissimum, Pecopteris aspera, Neu- ner even missing in the places where the basement ropteris antecedens. They are typical for the Namuri- consists of Silurian and Devonian sediments. Up-

8 wards follows a up to 200 m coal-bearing alterna- Paripteris gigantea, Mariopteris acuta, Mariopteris tion of sandstones, siltstones, claystones, rarely 2—5 muricata, Eusphenopteris cf. obtusiloba. m interbeds of conglomerates, composed mainly of Ordovician quartzite pebbles, and 3 to 4 levels with coals, the middle level being exploited in western part Berovdol Formation of the field. To the East of Iskar River in the northern limits of the field the Formation is thinner, even The formation is about 260 m thick. It covers pre- missing. The conglomerates of the basal part are ap- Carboniferous rocks, Tsarichina, Dramsha and Svoge parently better represented in the midst of the field. Formation, but never Svidnya Formation (Tenchov, Maybe they indicate the main river bed in the valley. 1966, Fig. 8, 9). In the western part of the coalfield it In many places around the coal seams the claystones consists of 3 coal-bearing cycles in which conglom- are fine and contain numerous rests of megaflora. erates are lacking. The coal seams are the main ex- There are established 24 localities that yielded 39 ploitation target. To the east of Iskar River the se- species. Between them are Sphenophyllum amplum, quence is not stable, but conglomerates are more fre- S. cuneifolium, Asterophyllites gothani, Annularia quent. Some 10 km east of the river coals are practi- subradiata, Zeilleria frenzli, Crossotheca schatzla- cally missing, but conglomerates are well present. In rensis, Neuropteris schlehanoides, Neuropteris schle- the central part of the basin white quartzite-com- hani, Neuropteris rectinervis, Paripteris gigantea, posed conglomerates form the base and contain vol- Mariopteris acuta, and several specimens of Aletho- caniclastic sediments. The conglomerate level seems pteris decurrence, Alethopteris lonchitica, Eusphe- to indicate the central river valley about 3—4 km large. nopteris hollandica, Hexagonocarpus mosanus. The About the east end of the coal field 2 or 3 andesite species known as Namurian only, are found in lower layers and one body of diorites are observed. Due to part of the Formation, whereas these known as West- prevailing coarse sediments megaflora fossils are rare. phalian are from the upper part. This fact allows to There are established 28 localities mainly at worked accept that the sedimentation starts during Namuri- coals seams that yielded 35 species, among them an C and continues in the Westphalian A. Sphenophyllum cuneifolium, Sphenophyllum trichomatosum, several Lepidodendrons, Pinakodendron musivum, Renaultia schatzlarensis, R. rotundifolia, Dramsha Formation Eusphenopteris cf. obtusiloba. That composition is not indicating clearly the Westphalian B age, which It is easily recognised by its dark colour and mainly is accepted by stratigraphic position. claystone composition. The thickness is accepted as 150 m. It is difficult to say for measured sequence as far as in the outcrops are observed frequent internal Chibaovtsi Formation folds. From the tectonic structure of the basin it can be supposed accumulation and compression. In that This formation is about 400 m thick, and is built of 3 formation fossils are missing even as fragments. The coal-bearing cycles. Each cycle starts with quartz only established fossil locality is in the upper parts, conglomerates that are 40 respectively 30 and 10 where some sandstone interlayers appear. The Ca- meters thick. The lower is composed of pebbles up to lamites undulatus and Cordaites cf. principalis found 15 cm, the middle – up to 5 cm, the upper one – up does not allow to determine the age. According to to 2 cm pebbles. The sandstones are with prevailing stratigraphic position in the sequence Dramsha For- quartz and quartzite grains and white in colour. The mation is referred to Westphalian A. 3 coal-seams in the lower cycle have 1—2 m claystone covers. In the middle cycle there is one coal seam in about 40 m claystones, that contain frequent Svoge Formation plant remains. The upper coal seam over the third conglomerate also is exploited but has a restricted It is about 310 m thick, composed of 5 or 6 coal- extension. Over the coal seam in locality 46 are es- bearing cycles in which the white sandstones form tablished red sediments and the first specimens of packets of 30 to 70 m. Conglomerates are present in Neuropteris scheuchzeri. The claystones over the coal the upper two cycles. At least three of the coal seams seam contain frequent Leaia remains. In the forma- were mined. The upper two cycles are known from tion are established 13 fossil-bearing localities with the central outcrops, where are preserved from the about 42 species. There are found Sphenophyllum erosion beneath the next formation. The plant fossils majus, S. myriophyllum, S. trichomatosum, several from that formation are typical for Westphalian A, Lepidodendrons, Sigillaria elongata, Sigillaria brar- as determined by Hartung (1935) and Nemejc (1943). di, Corynepteris coralloides, C. pecopteroides, Neu- The 47 species are collected from 19 localities and ropteris heterophylla, Neuropteris scheuchzeri, Parip- include Sphenophyllum cf. majus, Sigillaria elegans, teris gigantea, Sphenopteris rotundifolia. The Sig- S. cf. decorata, S. polyploca, S. mamillaris, S. schlo- illaria brardi seams to be the oldest known level of theimiana, S. cf. rugosa, Corynepteris angustissima, this taxon. The flora from the upper parts seems to Crossotheca schatzlarensis, Sphenopteris laurenti, Re- be of Westphalian C age, whereas in the lower level naultia schatzlarensis, Neuralethopteris schlehani, still is near to Westphalian B flora.

2 Geologica Balcanica, 1—2/2007 9 The coals in the basin are superantracites – with with rare Alethopteris and no Lonchopteris. Svoge 2% volatile. In the southern part they are with 3—5% Formation flora is without Alethopteris, rare Mari- volatile. Thus, in the Svoge Coal-field palynological opteris and Eusphenopteris. Berovdol Formation flo- study is not realised. ra missed Mariopteris, Alethopteris, Lonchopteris and Eusphenopteris. Chibaovtsi Formation Flora missed Alethopteris, Lonchopteris, Mariopteris, Eusphenop- Permian teris and is poor in Neuropteris. Articulatae, Lepido- dendron and Pecopteris are well presented, whereas The Permian sediments are not subject of the present Sigillaria is rare in higher levels. Ferns are represent- article. They are widely distributed in the core of the ed but not of a large species diversity. Linopteris is Svoge anticline (unpublished data of the author) and represented by one specimen, recently found in Svoge formed two belts. The northern belt is in place 1 km Formation – it is not a whole pinnule but remem- from the Carboniferous and is built of red, yellow bers Linopteris subbrongniarti. On the base of simi- and rarely whitish sandstones with some breccia and larity with the more northerly-situated Moscow Ba- conglomerates at the base. Near to Svidnya pluton sin and Angara flora it seems that Svoge flora is sim- they are rich in pebbles from the pluton. The south- ilar to floras situated more to the north about 15—20° ern belt is in place at 2 km from the Carboniferous latitude. The temperature conditions at such latitude outcrops. It is with similar composition as the north- difference recently corresponds of about 2000 m al- ern one, but also has two or three levels of acid lava titude. The nearby Dobrudzha Coalfield that is of sheets near the base. Near to the Carboniferous, along about 200 meters altitude (Tenchov, 2004) is with a river Katinska it is interbedded with numerous con- floral composition in its Westphalian A-Westphalian glomerate-breccia, the pebbles of the latter being of C part which is richer even compared to western the Carboniferous sandstones. Several bodies of paralic basin. quartz porphyrites cut the Palaeozoic sediments. One of them cuts the Carboniferous in west part between the village of Chibaovtsi and village of Dramsha, with Tectonics (Figs. 3, 4, 5) contact change of the coals. The Svoge Coalfield is situated in the Palaeozoic core of the Alpine Svoge anticline. The core is intensively Comments faulted and folded. Another lithostratigraphic concept is accepted by the last prospecting team (Rousanov et al., 1997). They Consedimentary faults (Fig. 3) regard Dramsha Formation as Dalbochitsa Member; Svidnya, Svoge and Berovdol as lateral members of a The Tsarichina Formation is deposited during Na- new – Drenovo Formation and the topmost clay- murian A in a half graben delimited by a fault to the stones in Chibaovtsi Formation as a new member – north. The fault maybe inherited an older fault that Radova-Glava. In their description of the Drenovo separated Ordovician of the northern outcrops from Formation the places and nature of lateral connec- Silurian from the southern outcrops. The same fault tion between the members are not indicated. When later has been activated. The sequence of the Forma- dating the Drenovo Formation they used the mega- tion does not indicate clearly the presence of a river, flora lists of Tenchov (1977) and determined the lower but the Leaia testifies that the depression had con- part of Drenovo Formation as Namurian C indicat- nection with river system to basins with them. The ing the flora of Svidnya Formation; the middle part sedimentation finished with lacustrine sediments of as Westphalian A, indicating the flora of Svoge For- Dalbochitsa Member. mation; the upper part as Westphalian B, – indicat- The Svidnya, Dramsha and Svoge Formation were ing the flora of Berovdol Formation. In fact they ar- deposited in another graben north of the first. The range in time their lateral members in a vertical suc- conglomerates that are interbedded with breccia at cession, as it is in Tenchov’s concept. The type sec- the basal part of Svidnya Formation indicate a river tion of the Radova-Glava Member is indicated in of east-west direction. The outcrops of the eastern Dramsha Formation outcrops, where Leaia is missing. half of the Coalfield, where Dramsha Formation covers directly the Ordovician indicate some relief, covered with the expansion of the basin. The conglom- Peculiarity in the Svoge Carboniferous erates in Svoge Formation indicate a river of east- megaflora west direction the bed of which reaches a width of up to three kilometres. Numerous Leaia are found Tenchov (1976) analysed the species content of the in the Svoge Formation. Svoge Coalfield as a whole and in the vertical se- The Berovdol Formation indicates that after a sed- quence in comparison with the West-European paral- imentation break the graben enlarges and cover the ic basins and Donets Basin. The Tsarichina Forma- territory of the former two grabens. In the southeast- tion flora is with well presented all existing species ern part of the Coalfield it covers the Silurian and except Lyginopteris. The Svidnya Formation flora is Devonian sediments with basal conglomerates. The

10 Fig. 3. Scheme for sedimentation grabens in the Svoge Coalfield. O – Ordovician; S – Silurian; D – Devonian; Ts – Tsarichina Formation; Sv – Svidnya Forma- tion; Dr – Dramsha Formation; Sg – Svoge Formation; Bd – Berovdol Forma- tion; Ch – Chibaovtsi Formation; P -Permian volcaniclastic interlayers and the andesitic layers near to Variscan tectonics, but in the eastern one they are the base of the Formation allow to adopt that the north-vergent, a fact that has not been discussed. faults become magma-conducting and later a dior- Many other observations have not been assimilated ite body has been intruded. and published by the author. Between them are: vis- The Chibaovtsi Formation indicates a relief-build- ible decollement between the Carboniferous and its ing process in three phases, that provides the con- Triassic cover in the western part; visible decollement glomerates. The quartz-rich sandstones testify a longer between Ordovician and Carboniferous north of vil- and more active weathering. lage Chibaovtsi at the southern slope of pick Brak- As a whole the graben system is formed at the back- iov kamak in the overturned limb of a syncline; the ground of strong uplift of a Variscan block in exten- continuation of some faults in the Mesozoic of the sion conditions. The graben subsidence is of about western part; difference in the metamorphism of the 1700 m, on the background of considerable uplift of coals in the normal and recumbent folds; the milled the Variscan block, so that the graben sedimentation coals in the oveturned limbs of the folds; the ore is realised may be of about 2000 m over the sea level mineralisation of faults in the eastern block, that are (Tenchov, 1976). After Westphalian C and prior to identical with the Late Cretaceous mineralisation in the Permian, most probably before the Stephanian east and south neighbourhood. C, inversion of block movements transforms the gra- From south to the north the Carboniferous sedi- ben in a horst which provides the Permian sediments ments cover Devonian, Silurian and Ordovician sed- of its two sides with pebbles from the Svoge Coalfield iments with angular unconformity reaching up to Carboniferous. The inversion is accompanied with 25°. An analysis of the pre-Carboniferous folds and compression and folding in the graben. From this faults is not performed completely for the core of the time may be starts the subsidence of the Variscan Svoge anticline but normal synclines of the Silurian block, but in the studied area there is not enough sediments are existing southeast of the eastern end of information. the Coalfield. The normal Carboniferous synclines in the southwest part of the Coalfield are covered by the Triassic with angular unconformity up to 15° and Folds (Fig. 4, 5) there is no trace of tectonically reworked contact. The Permian clastic sediments crop out to the south The Carboniferous folds are situated into two differ- and to the north of the Carboniferous but never cov- ent blocks, separated (Figs. 1, 5) by Mogilishte Fault er it. They exhibit a small angular unconformity with (Tenchov, 1966). In the western block the folds are the Triassic cover. The Permian from the southern south-vergent that was bound (Tenchov, Yanev, 1981) side of the Coalfield, along the river Katinska, con-

11 Fig. 4. Scheme of the folding of the Carboniferous from Svoge Coalfield tains sandstones with many subrounded cobbles of are of anthracite content with about 4 to 6% vola- Carboniferous sandstones. In the outcrops north of tile. In the overturned limbs they are more or less the Coalfield, at a distance of only 1 km, in the base accumulated or squeezed forming lenses up to 30— of the Permian the pebbles are mainly of the Svidnya 40 m long, followed by a 15—20 m zone where coals pluton and Ordovician quartzites (unpublished au- are squeezed. The accumulated coals are briquette- thor’s data). like, mixed with claystone and sandstone oval parti- The coals in the normal limbs of the Carbonifer- cles. The volatile in them is about 2—3%. In the sed- ous sediments possess a preserved layer structure and iments of the overturned limbs megaflora is present,

12 but that in the claystones is deformed – elongated ern part of the Coalfield (Fig. 5) are reduced by over- or shortened. In the sandstone it is not deformed, thursts, that to the west of Mogila Fault are south- but when some slab is cut it is decomposed in milli- vergent, but are north-vergent to the east of that fault. metre particles. The folds in the Carboniferous sediments in the south synclines are normal. They are slightly asym- Folding episodes (Fig. 4) metric and the south limbs are steeper. The synclines in the north part are recumbent parallel folds with Before the Carboniferous sedimentation in the Svoge dips 30 to 40°. To the west of the Mogila Fault they Coalfield the Ordovician, Silurian and Devonian are south-vergent, whereas to the east of that fault sediments were folded in flat folds up to 20°; possi- they are north-vergent. Some anticlines in the east- bly, a fault run east-west at the place of the northern

Fig. 5. Idealised geological profiles. 1—4 – Basement: 1 – Ordovician, 2 – Silurian, 3 – Devonian, 4 – Svidnya Pluton; 5 – Tsarichina Formation; 6 – Svidnya Formation; 7 – Dramsha Formation; 8 – Svoge Formation; 9 – Berovdol Formation; 10 – Chibaovtsi Formation; 11 – Triassic; 12 – overthrust; 13 – fault; 14 – observable decollement

13 boundary of Tsarichina Formation sedimentation site. folds (Fig. 2 – profiles IV to VII). All that east part is The folding is pre-Namurian, but post Devonian, cut by numerous wrench faults, some of them with most probably bound to the Late Visean Sudetian the typical Srednogorie polymetalic and porphyry Phase. copper mineralisations. About 10 km out of the map The next folding was post-Westphalian. Accord- at the village Vrachesh the Carboniferous sediments ing to regional data most probably it is pre-Stepha- deposited on the Ordovician are overthrusted by the nian C. It formed in the graben of Svoge Coalfield Ordovician and still mark the northern part of the normal folds with bed dips no more then 30°. It was Palaeozoic core of the Svoge anticline (personal ob- followed by inversion in block movements and the servations). Variscan block started subsidence, but the former graben slow and gradually reached the level of denuda- tion about Stephanian C or Early Permian: around Conclusion the horst Permian sediments contain subrounded cobbles and pebles of Carboniferous sandstones. The The recent tectonic structure of the west part of the Permian sediments show no clear discordance with Svoge anticline is a result of Late Cretaceous - Early the Triassic. A some discordance can be supposed Tertiary folding and faulting. It is realised by strong because the Triassic covers both the Permian and pressure to the North. To the west of Mogila Fault the Coalfield sediments. From lithologic point the the Svidnya pluton stopped the movement of the Triassic sediments are quartz oligomictic, whereas the blocks to the north. The sediments of the Svoge Car- Permian sediments are polymictic. The regional data boniferous graben, mainly the plastic Dramsha For- in the Variscan Range in Bulgaria (Tenchov, 1982) mation and the fault between Tsarichina Formation show a clear sedimentation break between Late Per- and Svidnya Formation controlled the south vergent mian and Early Triassic, but no data for substantial folds in the Carboniferous represented by numerous folding. The subsidence of the Variscan block since overthrusts at the south limb of the anticline and the Triassic to Lower Cretaceous times is considerable. asymmetric Izdremets syncline to the north. To the The anthracite coals of the Svoge Coalfield indicate east of Mogilata fault thrusting of the Ordovician that they had been covered of about 5000 m sedi- (with the Carboniferous on it) over the Jurassic oc- ments to reduce the volatile only to 4—5%. curred. Many wrench faults and faults east of Mogi- The Alpine folding in the region about the end of lata fault contain polymetalic ore mineralisation in- Early Cretaceous time created the symmetric Svoge clusive in the Carboniferous sediments (author’s un- Anticline. The next intensive folding about the Late published data). The tectonic architecture is realised Cretaceous or Early Tertiary with clear strong pres- beneath the Mesozoic cover at a depth of about 5000 sure to the north created some nappes and numer- metres, indicated by the anthracite coals. The plane ous overthrusts. In that time, the Palaeozoic core of of decollement shown on figure 1 is observable in the Svoge anticline was situated some 5,000 m bellow the outcrops south-west of the village Dramsha. surface after the anthracite coals of the Carbonifer- ous. That created conditions for decollement (Mat- tauer, 1973) The strong pressure to the North acted Acknowledgements on the west of peak Mogilata, where the Svidnya Plu- ton is situated, created south-vergent recumbent par- This paper is a contribution to the IGCP Project allel folds under the decollement plane bellow the 469 Late Variscan terrestrial biotas and palaeoen- hard Triassic sandstones (Fig. 2, profile I). To the vironments. The paper was presented at the Bucha- north some part of the Carboniferous sediments were rest meeting of the project, and the author is grate- compressed in vertical parallel fold (Fig. 2 profile ful to the project for financial support for attend- II). To the East of peak Mogilata the sediments ad- ing the meeting. Dr. Cleal is thanked for correc- vanced to north and formed a nappe of the Palaeo- tions in the English text, Dr. V. Sachanski for the zoic sediments (Ordovician to Permian) over the Ju- help in preparation of figures, and Dr. I. Zagorchev rassic (at 2 km north of Svoge). The Carboniferous for discussions and recommendations on the pre- sediments formed north-vergent recumbent parallel liminary manuscript.

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