The Jurassic in the south-eastern part of Bulgaria (stratigraphy, geodynamics, facies and palaeogeographic evolution)

Abstract

In south-east Bulgaria the presence of Hettangian (by indirect data) and Sinemurian, Pliensbachian, Toarcian. Aalenian, Bajocian and Bathonian (on the basis of direct biostratigraphic proofs) has been established. The Kostina Formation is represented by quartzites, and almost everywhere is related to the Hettangian. However, in a restricted area, in the vicinity of Stara Zagora Town, the Kostina Formation probably belongs to the Upper Sinemurian. The Ozirovo Formation (represented by shallow water, sandy, bioclastic, partly ferrugineous limestones) and the Bliznak Formation (mainly consisting of fine-grained quartz sandstones) are related to the Sinemurian. The Ozirovo Formation and the Bliznak Formation comprise also the Pliensbachian, the Toarcian, the Aalenian and the lowermost part of the Lower Bajocian (the lower part of Hyperlioceras discites ammonite Zone). The Lower Bajocian (without the lower part of Hyperlioceras discites ammonite Zone) belongs to the Zvezdets Formation, represented by shales, rich in organic matter, with siderite and phosphorite concretions, in some places being interbedded with sandstones, or the Boyanovo Formation (new formation), represented by an alternation of terrigenous-clastic and pelitic rocks and bioclastic limestones. In the south-eastern part of the Strandzha Mountains, sandstones are more abundant in the Zvezdets Formation, and in addition some olistostromes, consisting mainly of limestone breccias, are found – Kazanski Member of the Zvezdets Formation. Only the Zvezdets Formation and its Kazanski Member belong to the Upper Bajocian. The Brushlyan Formation (represented by shallow water bioclastic limestones in alternation with phyllites) and the Hranova Formation (shallow-water calcareous phyllites and a lesser amount of limestones with bioclasts) belong to the Bathonian. All autochthonous Lower and Middle Jurassic rocks in southeast Bulgaria occurred within the limits of the Tundzha Early-Middle Jurassic Fault-Angle Valley which gradually subsided during Early Jurassic and the beginning of Middle Jurassic times. As a result of these phenomena, the sea rapidly ingraded from south-east, from the Vardar Ocean side. In the valley, 7 blocks (steps) were formed and separated bathymetrically from each other: Yambol, Indzhe Voyvoda, Botevo, Kraynovo, Stara Zagora, Elhovo and Kondolovo Steps. Along the valley, a sea sound was formed and carried out the connection between the Vardar Ocean and the epicontinental basin of the Moesian Early-Middle Jurassic Platform since the Hettangian up to the Bathonian time. The valley passed two stages in its development: (a) transgressive one (Hettangian-Bajocian) and (b) regressive one (Bathonian). During the transgressive stage the sea gradually overflowed the Yambol Step (during the Hettangian), the Indzhe Voyvoda, the Botevo and the Kraynovo Steps (during the Early Sinemurian), the Stara Zagora Step (during the Late Sinemurian), the Elhovo and the Kondolovo Steps (during the Early Bajocian). During Hettangian, Sinemurian and Pliensbachian, everywhere in the sea sound, a shallow sublittoral biotop with calcareous or sandy bottom and agitated waters existed. The depths did not exceed 50 m. During Toarcian, Aalenian and Early Bajocian, some bathymetric differentiation took place. The shallow sublittoral biotop remained near the sound coasts, while in the parts distant from the coasts, bottom subsidence took place. An environment of moderate deep sublittoral biotop mainly with calcareous and partly sandy bottom appeared there. The depths were of the order of 50-100 m. During the Early Bajocian, in the south-eastern part of the sea sound the most intensive deepening took place. There, environments of a deep sublittoral biotop with a silty and partly sandy bottom and relatively calm water have set in. The depths were 150-200 m. In the central parts of the sound, the sea overflowed new considerable parts of the Tundzha Valley. There, a shallow sublittoral biotop existed, and the sedimentation regime often changed (shallow-water limy silts, pelites and terrigenous-clastic sediments were deposited). In the north-western part of the sound, probably deeper-water pelitic sediments were deposited, but in the interval Callovian-Early Cretaceous, under conditions of emergence, they were eroded. During the Bathonian, a rapid general shallowing arose there, and under conditions of a shallow sublittoral biotop with calcareous and muddy bottom, limy silts with bioclasts and pelites were deposited. The depths did not cxceed 50 m. In considerably large parts of the Tundzha Fault-Angle Valley, a shallow sublittoral biotop with strong bottom currents existed and caused a sedimentation gap. At the end of the Bathonian, the Tundzha Fault-Angle Valley was uplifted over the sea level and joined the dry land of the Thracian Massif. As a result of this, the sea-strait between the Vardar Ocean and the Moesian Early-Middle Jurassic Platform stopped existing.