New data on the stratigraphy of the Middle Jurassic siliciclastic rocks from the West Balkan Mountains (West Bulgaria)

Abstract

The present study is a contribution to the lithostratigraphy of Middle Jurassic strata in the area of the Zimevitsa Plateau (West Balkan Mts, West Bulgaria). Based on its lithological and regional properties, the Dobravitsa Member is distinguished from the surrounding rocks of the Etropole Formation. The new member consists of alternating shales, siltstones and fine-grained sandstones. It also contains abundant siderite and calcite concretions, which are distributed in discrete stratigraphic horizons in both the mudrocks and coarser lithologies. Fossils are extremely rare. Poorly preserved ammonites and belemnites are found only. Based on several ammonite finds, the Dobravitsa Member of the Etropole Formation is assigned to a narrow stratigraphic interval of the lower Bajocian (unlimited, probably lower part of the Witchellia laeviuscula Zone). Although it has restricted lateral distribution, which is exclusively confined to its type-area, the Dobravitsa Member has a proper place in the formal Bulgarian Jurassic lithostratigraphic scheme, as it represents a discernible atypical development of the Etropole Formation in the West Balkan Mts region. Another emphasis of this study is laid on concretions from the Etropole Formation. Besides siderite and calcite concretions, a few examples of iron oxyhydroxide concretions, from localities outside the Zimevitsa Plateau area, are also described. The latter are also an atypical component of the Etropole Formation, which deserves a further special study and systematization, as it implies unknown history of this lithofacies, in both depositional and diagenetic terms.