Юрское развитие Северо-Западной Болгарии

I.G. Sapunov, P.V. Tchoumatchenco, P.A. Mitov. Jurassic development of Northwest Bulgaria. The geological development of this area proceeded through an Early-Middle Jurassic transgressive stage and a Late Jurassic stage of bathymetric differentiation. The marine transgression began during the Hettangian and ended during the Early Callovian. In individual isolated areas and during individual episodes of the Sinemurian, Pliensbachian, Toarcian, Aalenian and Bajocian it was preceded by continental (lacustrine, palustrine) accumulation. The bathymetric relationship of the gradually subsiding horst and grabens of the Moesian Early-Middle Jurassic Platform governed the progress of the marine transgression in time and pace. In the marine basin were deposited shallow-water terrigenous and terrigenous-carbonate sediments with crinoids, brachiopods, bivalves, belemnites and ammonites. The maximum depth was not over 200 m. The Late Jurassic stage of bathymetric differentiation was related to the formation of the Central Moesian Basin where deep-water, pelagic carbonate sediments were laid down: nodular limestone of ammonitico rosso facies and micrites with Globochaete, Saccocoma, Cadosina, pelagic foraminifers of g. Globulogerina, calpionellids, numerous ammonites of Lytoceratina and Phylloceratina and fewer Ammonitina, belemnites, aptychus and restricted brachiopods. The depths of the basin varied in a wide range, from 100 to 600 m. On both sides of the Central Moesian Basin formed the West Moesian and the East Moesian Platform. Small parts of these platforms were located in Northwest Bulgaria. They were areas of quite shallow-water carbonate sedimentation. Most common deposits are biodetrital, bioconstructed and restricted micritic limestone with corals, rudists, blue-green and green algae of sublittoral environments close to the lower boundary of the littoral zone. The maximum value of bathymetric difference between the platforms and the Central Moesian Basin were about 600 m. During the Kimmeridgian the tectonic setting was complicated by the formation of the Niš-Trojan Flisch Trough at the expense of the southern part of the Central Moesian Basin. A small part of the trough extended in Northwest Bulgaria. There, in epibathial conditions, under the influence of turbidity currents formed fine-grained ands, silts and clays.