Fluvial depositional evolution of Buntsandstein of the Teteven Anticlinorium (Central Fore-Balcan, Northern Bulgaria)

The Buntsandstein of the Teteven Anticlinorium in the Central Fore-Balkan (Northern Bulgaria) is a submature coarse sandy to gravelly fluvial succession that formed mainly as channel bars and sheets in shallow braided river systems in a proximal to near-medial position in front of the source area. The Buntsandstein succession exhibits a more or less (neglecting some probable minor Internal recurrent incisions) monocyclic evolution of fluvial style. The depositional history starts in the Lower Conglomerates with highly-braided cobbly to bouldery river systems operating both in a river plain and on alluvial-fan lobes that are patchily lined up along the mountain front and that pass both laterally and longitudinally into the braidplain system which is in places also directly emanating from the foothill belt. The evolution proceeds in the Middle Conglomerates and Sandstones by equalization of the differences of palaeoslope of the low-gradient alluvial fans and the braidplain with amalgamation to only one river plain system of highly to moderately-braided cobbly to pebbly type (lower part) and later moderately-braided pebbly to sandy nature (upper part). In the Upper Sandstones and Mudstones, more mature stages are reached including moderately- to weakly-bralded sandy river systems in the lower part and weakly braided sandy stream complexes in the upper part which finally pass into a sandy and muddy inland floodplain. With the inversion of the sand/mud-relationship and the first marine influences, the continental inland braidplain and floodplain system is converted into a coastal muddy and sandy floodplain in the terminal Mudstones.