Late Alpine tectonics and Neotectonics
International Conference - May 2001
Late Jurassic geology of Southwest Bulgaria
The Nish-Troyan flysch trough has been first recognized
and described by Nachev (1970, 1974). According to him, the basin covered
31000 km2, and was symmetrical and 20 - 40 km wide in its western
part. The coast lines can be traced along the coarser flysch deposits.
Neritic zones 200 - 400 m deep were situated near the coast but their coarse
and non-fossilized deposits have been redeposited in the central, bathyal
zone that extended at depth to several thousand meter. The bathyal zone
embraced up to 98% of the total area of the basin.
The Nish-Troyan flysch trough has been discordantly superimposed over the pre-existing different Jurassic environments. Three principal environments existed in Late Jurassic rimes, and namely:
1) A dryland that comprised most of the autochthonous to parautochthonous units of the future Late Cretaceous Morava-Rhodope Zone, and namely, the Osogovo-Vlahina Unit of the Strouma Superunit, and the Rhodope Superunit, together with the Ograzhden Unit; the Osogovo-Vlahina Unit has been exhumed after the Late Triassic folding, and has been apparently a dry land during the whole Jurassic and Cretaceous.
2) A shallow sea with carbonate deposition; the latter followed the formation of deeper-facies carbonate sediments ("ammonitico rosso") during the Oxfordian time.
3) A fast subsiding trough with flysch sedimentation at the margin between the dry land and the carbonate platform; the trough developed in most cases over Oxfordian "ammonitico rosso" or carbonate breccia filling in submarine furrows, or over Oxfordian - Kimmeridgian radiolarites and slates (Treklyano sedimentation zone).
The subsidence of the trough proceeded diachronously at least from the Kimmeridgian to Middle Tithonian times. However, new data (Zagorchev et al., 1998) point at a much earlier (end of Middle Jurassic time) onset of a pre-flysch sedimentation in the Konyava Mountain and in the border area with Yugoslavia, at the southern margin of the flysch basin in close proximity to the uplifting Morava (Serbo-Macedonian) and Rhodope areas. The sedimentation ended with a carbonate flysch (or post-flysch - Nachev, 1970) in Middle Berriasian time, and shallow-water carbonate sediments in the Vallanginian, possibly ending with Hauterivian - Aptian carbonate and terrigeneous rocks. Intense folding and denudation were followed by northeast-vergent thrusting of the Morava Superunit over the folded Struma Superunit (the formations of the Nish-Troyan flysch trough included), and thrusting of the Ograzhden Unit over the Pirin Unit of the Rhodope Superunit. Most typically, the Mid Cretaceous, Late Cretaceous, Palaeogene and Early Neogene thrusts cut obliquely and displace the Nish-Troyan flysch trough that was also displaced by Palaeogene and Neogene right-lateral strike-slip faults of the Struma Lineament.
The dry lands South of the basin had been built up mostly of the metamorphic and igneous rocks of the Rhodope Massif (s.l.), and possessed mostly a low relief. Provenance analysis on the pebble composition of the coarse Tithonian and Berrisian conglomerates and olistostroms points at three principal source rocks, and namely:
(1) metamorphic rocks of Serbo-Macedonian and Rhodope signature;
(2) Palaeozoic rocks (mostly Devonian) from the Morava Superunit;
(3) Triassic and Jurassic rocks that correspond to the principal formations from the close basement of the flysch, and came most probably from the cordilleras or the Mesozoic cover of the dry lands or from islands within the trough. Flow directions deduced from numerous measurements on hieroglyphs were either transversal and oblique or longitudinal to the trough direction.
Field work during the first day of the field trip will enable observations on different Triassic and Jurassic environments, the Upper Jurassic - Lower Cretaceous flysch of the western part of the Nish-Troyan trough included.