Sedimentological features and palaeoenvironmental regime of bivalves-bearing (Lithiotis-type) deposits of the Albanian Alps (Lower Jurassic; Pliensbachian) – preliminary results

The large (up to 40 cm long), Lithiotis, Cochlearites, Lithioperna, Mytiloperna, Gervileioperna bivalves, which dominated within “Lithiotis” facies, are most significant representatives of buildup-maker of shallow... [ view full abstract ]

The large (up to 40 cm long), Lithiotis, Cochlearites, Lithioperna, Mytiloperna, Gervileioperna bivalves, which dominated within “Lithiotis” facies, are most significant representatives of buildup-maker of shallow marine/lagoonal bivalve mounds (reefs) in numerous places around the Pangaea during Pleinsbachian-Early Toarcian time. They are world-wide distributed fossils, which occupied shallow-water shelfs of the south-western Tethys and easternmost part of the Panthalassa Ocean. In Europe they are known from Alpine Spain, Italy, Slovenia, Croatia, Albania, Greece and constitute of the Early Jurassic Alpine-Adriatic-Dinaridic-Hellenidic carbonate platforms with different kind of shallow sea environments, including peritidal to subtidal sedimentation regimes which are typical for Lithiotis-type bivalves. One of the most spectacular area with continuous section (up to 200 m in thickness, approximately) with limestones rich in these bivalves is located in north-western part of the Albanian Alps (so-called external Albanides, N of Shkodra). In three outcrops were studied preliminary Pliensbachian in age Lithiotis-type bivalves-rich limestones: vicinity of Ducaj, near Bogë and close to Grabom village. Several horizons of bivalves-rich limestones are intercalated by grey-dark bluish marly limestones and bioclastic limestones. Some horizons are more softy with marly matrix where bivalve shells are isolated and relatively well preserved. Taphonomic and autecological analysis of bivalves-rich horizons based on semi-quantitative observation of orientation of shells and density of their occurrence indicate dominance of parauthochthonous associations (the ratio of horizontal to vertical shells) with a few places with record of shells in life position (dominance of vertical orientation of shells and so-called bouquets). Some beds full of bivalves have oblique, lens-shape character with sharp boundaries both with under- and overlying beds and maybe correspond to “biostrome” nature in origin. Bivalve-rich limetsones/marls are intercalated by oolitic/oncolitic leyers which indicate shallow-water environments (subtidal?) with high-energy regimes. In neighboring Greece such accumulations of bivalves are observed in Gavrovo-Tripolitza (= Kruja Zone in Albania), Pelagonian and Parnassos carbonate platforms and usually are also intercalated both by oncolitic-rich beds and megalodont-bearing limestones as well. In Albanian case the Lithiotis-bearing part of sections are practically without megalodontids. Such preliminary look and comparison between Pliensbachian bivalve-rich sequences of Dinaridic-Hellenic realms need more detail investigations both macro- and micropalaeontologicaly.