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.