Field trip B26

Bulgarian part

Sofia Plovdiv Starosel Hisar Karnare Troyan (about 310 km)

Leaders: Ivan Zagorchev,Marin Ivanov

The itinerary of the first day is traced along the Maritsa fault belt in the southern edge of the Srednogorie zone. From Plovdiv it crosses towards North the Upper Thracian plain (Neogene Quaternary graben), the central parts of the Sredna-gora crystalline fragment, the Sub-Balkan neotectonic grabens, the Sub-Balkan normal fault and the Central-Balkan thrust, the Balkan (Stara-planina) zone, and enters the Fore-Balkan zone.

Stop 1.3. Temple of Sitalkes, village of Starosel
Excavations during the last 3 years exposed a Thracian temple complex referred to the times of King Sitalkes (5nd Century B.C.). The principal temple is built over Precambrian biotite gneisses (often garnet-bearing)and migmatites intruded by the biotite porphyric granites to leucogranodiorites of a Palaeozoic (c. 300320 Ma) granite pluton. Rarely, dykes of pegmatites and aplites are observed. The temple of Sitalkes is built with local granite blocks but the well-polished elements (columns, capitels, etc.) have been made from imported zeolitized tuffs.

Stop 1.4. Hisar From Starosel we continue to Hisar first a Neolithic site and a Thracian town transformed into a fortified spa (Diocletianopolis) during the Roman rule. The well-preserved walls are a typical Roman construction. The stone blocks come mostly from the local biotite porphyric granite of the Hisar pluton (c. 337 Ma, Sri 0.706).  Huge blocks of Late Cretaceous equigranular syenites have been transported from Plovdiv (stop 1.2) for to fortify points with major importance for the defense. Presently, 22 mineral springs have a total debit of 4000 l/min, the temperatures ranging between 27 and 52oC.The waters are alkaline (pH 7.6 9.02), sodium hydrocarbonic, with sulphate,fluorine, and more than 20 microelements. Sveti-Georgi granite (c. 270 Ma) cross-cutting the Hisar granite (c.337 Ma).The Hisar polje represents a peculiar irregular post-sedimentation (in respect to the Neogene) graben formed by
ENE-and NNE-striking steep (60-90o) normal faults. To the South, the Starosel and Hisar grabens are separated from the Upper Thracian depression by the Krasnovo normal fault that strikes ESE. Thus, a second-order step-like graben is formed between the depression and the Sredna-gora horst.

 Sveti-Georgi granite (c. 270 Ma) cross-cutting the Hisar granite(c.337 Ma)

Stop 1.6. Road junction after the town of Banya
The stop exhibits a neotectonic panorama of the Karlovo graben, the Sub-Balkan fault and the Stara-planina linear morphostructure. The neotectonic features of these parts of the Srednogorie zone are related to the formation and subsidence of a Sredna-gora swell trending approximately West-East. The subsidence is linked to the collapse of the back parts of the Stara-planina Palaeogene orogen. As a result, two rows of Neogene Quaternary grabens have been formed: the grabens of the Sub-Balkan poljes to the North (at the boundary between the Srednogorie and Stara-planina zones, along the Sub-Balkan fault), and the grabens along the Krasnovo fault, to the South. The internal structure of the Karlovo graben is complicated by second-order normal faults. It is filled with Neogene (Maeotian? Romanian) and Quaternary sediments with a total thickness of about 500 m. The sediments have been deposited in a fluvial system, and a lake (with diatomites and coal) has existed in the northernmost parts in latest Miocene earliest Pliocene times. During the Neogene Quaternary uplift a tectonic relief inversion favoured the formation of the numerous normal faults of the Sub-Balkan fault belt, and some of the old thrust surfaces rotated and were transformed into steep normal faults. Further on, some of the steep normal faults rotated in the extension process to low-angle (listric) normal faults. The itinerary continues through the Karlovo graben, and at the village of Karnare reaches the normal faults of the Sub-Balkan fault belt. After the village of Karnare the itinerary leaves a high mountain step and enters the Stara-planina zone.

Stop 1.11. Beklemeto Pass - quarry near the mountain ridge; altitude 1399 m
The quarry exposes strongly tectonized Jurassic limestones. The klippe exhibits in an overturned position (from bottom to top: Anisian dolomite(Bosnek Formation); limestones and intraclastic limestones (Anisian Mogila Formation); sandy limestones, marls, sandstones and fine dolomite beds(Spathian-Anisian Svidol Formation); flakey and variegated sandstones with separate quartz, quartzite and granite pebbles (Lower Triassic Petrohan Terrigenous Group); strongly tectonized Hercynian? granite. The neotectonic panoramic view displays the Neogene (Maeotian?) planation surface at 1500 - 1600 m over the Stara-planina morphostructure. To the South: the Karlovo graben, and (at the background) the Sredna-gora crystalline block (neotectonic horst). Several planation surfaces are visible, and they are clearly superimposed one upon the other due to the low-velocity uplift of the Sredna-gora horst, in contrast with the clearly-defined planation surfaces within the Stara-planina range.

Stop 1.12. Bridge at the entrance of the town of Troyan
Upper Jurassic - Lower Cretaceous (in their full range - from Middle Kimmeridgian to Lower Vallanginian) flysch sediments (Cherni-Osam Formation)are exposed in the road cuttings and the picturesque cliffs. They belong to the Central Balkan Flysch Group deposited in the so-called Nish-Troyan flysch trough. The sedimentary sequence consists of flysch rhythms built up of fine- to medium-bedded sandstones, siltstones and marls. Rarely,the flysch sequence is disturbed by thick turbidite beds. The flysch complex has been formed in the axial part of the basin, and shows transitions to the hemipelagic slope sediments (clayey limestones and marls) of the Salash Formation (to the West) and the coarse terrigenous sediments (conglomerates,sandstones and marls) of the Kostel Formation (to the East). The travel from the ridge of the Stara-planina (Balkan) mountain range towards the Danube plain (Moesian platform) allows for almost continuous observations of the Neogene surfaces. In contrast to the steep southern slope of the Balkan (along the Sub-Balkan fault belt), the northern slope exhibits a gradual lowering of the planation surfaces that corresponds to the flexural transition from the range towards the plain, and to a steady uplift. In Neogene times, the evolution of the relief was greatly controlled by this steady uplift, and by the draining of the fluvial systems towards the Paratethyan marine basin in the Moesian platform.

Geological Institute of the Bulgarian Academy of Sciences
Author: Ivan Zagorchev
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