engleski

Lithiotis-type bivalves in the Lower Jurassic carbonates of the Central and Southern Velebit Mt., Croatia

Lithiotis-type bivalves are characteristic faunal element of many Lower Jurassic successions in southern Europe, western Arabia, western and central Asia, as well as western margin of the North and South America. These large bivalves were most significant buildup-makers in Early Jurassic shallow-marine environments of many Tethyan carbonate platforms. The study of carbonates with Lithiotis-type bivalves was carried out in the Central Velebit Mt. (Kubus section) and Southern Velebit Mt. area (Libinje and Mali Alan sections), which are 50 km apart. Benthic foraminifera assemblage (Lituosepta recoarensis, Paleomayncina termieri, Lituosepta compressa, Orbitopsella primaeva, O. praecursor, Pseudocyclammina liassica, Socotraina serpentina) indicates Late Sinemurian to earliest Toarcian age. The thickness of sections with Lithiotis-type bivalves varies from 210 m on Kubus, to 171 m on Libinje and 145 m on Mali Alan. The oldest Lithiotis-type bivalves were found in the lowermost Upper Sinemurian beds at Kubus and youngest in the Lower Toarcian beds in all studied sections. Lithiotis-type bivalve shells mostly occur in micritic limestones indicating growth on muddy substrates, as typical mud-stickers fauna, within low- to moderate-energy shallow-marine lagoonal environments of the inner carbonate platform. Shells are typically reworked, only sporadically preserved in their primary life position. All studied sections were characterized by similar shallow-marine environments during the Late Sinemurian and beginning of the Early Pliensbachian. Relative deepening started in late Early Pliensbachian in the Kubus section (as indicated by occurrence of coarser-grained limestones with slumps), while in Southern Velebit sections shallow-marine environments persisted until the latest Pliensbachian. The Pliensbachian–Toarcian transition and earliest Toarcian were characterized by deposition of dark grey micritic limestones with Lithiotis-type bivalves and clayey limestones with thin marly beds. Intensely bioturbated micritic limestones, known as “spotted limestones” sharply overlay the last beds with Lithiotis-type bivalves and S. serpentina indicating middle Early Toarcian age. Lithiotis-type bivalves are most abundant in the Upper Pliensbachian and lowermost Toarcian deposits in all sections and are generally more frequent in Southern Velebit Mt. Preliminary results show that Lithioperna scutata occurred as the first among Lithiotis-type bivalves, and is the most common species in the Upper Sinemurian and Lower Pliensbachian beds. First occurrence of Cochlearites loppianus can be noticed in the middle Lower Pliensbachian beds, while L. scutata and C. loppianus were equally frequent during the Late Pliensbachian. Lithiotis problematica was certainly determined only in the Lower Toarcian beds of the Kubus section. However, most of the Early Toarcian Lithiotis-type bivalves cannot be determined on generic level. The trend of increasing shell size is clearly seen in all Lithiotis-type bivalves from the Late Sinemurian to the Early Toarcian. Rapid development and wide diversity of these unique bivalves during the Early Jurassic indicate recovery of optimal conditions within shallow-marine environments before their final demise caused by the worldwide recorded early Toarcian Oceanic Anoxic Event (T-OAE).

Lithitiotis-type bivalves, Early Jurassic, Velebit Mt.

IGCP-710 Project of UNESCO