The Lower Jurassic in the Troyan Region and the south-eastern part of the Teteven Region, Central Balkanids (brachiopod-, bivalve-, and ammonite stratigraphy and palaeotectonics)

Abstract

As a result of the biostratigraphic study of brachiopods, bivalves and ammonites made in eight instructive sections it was established that most of the Lower Jurassic rock sequences were more or less incomplete. The sections in the valleys of the rivers Kostina and Cherna Reka, Teteven Region, are most complete (from Hettangian to Toarcian). They are situated in the central, the most deeply sinking block one – called Kostina Step of the Izdremets Complex Graben of the Moesian Early-Middle Jurassic Platform. There, the most deep-sea Early Jurassic distal facies were existing. The section near the village of Gorno Shipkovo, Troyan Region, has more shallow-water character and is situated in the Shipkovo Step, the most deeply sinking step of the southern edge of the Izdremets Graben. There, the Lower Jurassic section begins with the Lower Sinemurian. The Lower Jurassic rock sequences in the Central Stara Planina Mts. between the hamlet of Neshkovtsi and the locality Brousovete, Troyan Region, are connected with the Neshkovtsi Step (the sections near the hamlet of Neshkovtsi) and the Ambaritsa Step (the section near the mountain hut Ambaritsa and in the locality Brousovete). There, the Lower Jurassic sequences are most incomplete and they begin with the Upper Sinemurian (near the hamlet Neshkovtsi) or with the Pliensbachian (in the rest sections). The lithofacies and the thanatocoenoses possess a more coastal character and are referred to the proximal part of the basin. During the Early Jurassic the studied south-eastern part of the Izdremets Graben subsided gradually, so that in the end of the Early Jurassic, when the maximum deepening took place, the depths corresponded to the deep sublittoral (150-200m). At that time the marine transgression extended considerably to the south.

Jurassic stratigraphy and geodynamics in the eastern part of the Central Balkan Mountains (Gabrovo and Kazanluk Districts), Bulgaria

“…It overflowed the Kostina Step of the Izdremets Early-Middle Jurassic Graben (figs 2, 4). The marine branch on the Kostina Step moved to the west, and afterwards turned to the north-east into the Selanovo Step of the Mihaylovgrad Early-Middle Jurassic Complex Graben (Sapunov et al, 1996;CanyuoB et al, 1988). In these parts of the grabens, the main marine ingression was realized.…”

“…Fig. 4 shows the correlation of some chosen instructive sections from the studied area and from the Troyan and Teteven Districts (by data of Sapunov et al, 1996). They are bounded with the palaeotectonic structures having predetermined the regime of sedimentation in time and space.…”

“…The examined area in the eastern part of the Central Balkan Mountains is situated in specific palaeogeodynamic conditions where four Early-Middle Jurassic graben structures have intersected one another. The Toundzha Fault-Angle Valley of the Thracian Massif finished there from the south -south-east (Sapunov, 1999); the lzdremets Complex Graben of the Moesian Early-Middle Jurassic Platform reached this place from the west (Sapunov et al, 1996); the Sevlievo Graben of the Moesian Early-Middle Jurassic Platform terminated there (Canyuoo et al, 1991) and the Veliko Turnovo Graben of the Moesian Early-Middle Jurassic Platform started from this area to the east (Canyuoo, qyMal.feHKO, 1987 ;Canyuoo et al, 1991). This complicated junction of four graben structures, having been formed in this way, played an important role during the Early Jurassic.…”

“…It contains late Graphoceratinae, which indicate the presence of the lowermost Lower Bajocian Hyper/ioceras discites standard ammonite Zone. Beside that, in the neighbour sections in the locality Brousovete (Sapunov, 1968;Sapunov et al, 1996) and at Neshkovtsi Hamlet, Troyan District (CanyHoB, 1964;Sapunov et at., 1996) there are abundant proves about the presence of the Aalenian in the upper part of the Ozirovo Formation. In addition, it is proved by the cited publications, that the level with chlorite ore and the overlying argillites and clayey siltstones of the Etropole Formation belongs to the lowermost part of the Lower Bajocian.…”

The pre-Callovian stratigraphic gap in the Central Balkanids: a key for the interpretation of other Early-Middle Jurassic gaps in Bulgaria

“…A good example in this respect is the section of the Toarcian near the h;1mlet of Neshkovtsi, Cherni Osum Village, Troyan District, Central Balkan Range. There the ammonite zones are very thin ( 4-60 em) and with sharp boundaries (CanyHoB, 1968, p. 133;Sapunov, Tchoumatchenco, Shopov, 1996, p. 1 0).…”