The Early Jurassic Tethyan carbonate platforms with Lithiotis-type bivalves-bearing levels – Albanian-Oman comparison

Giant bivalves experienced a significant evolutionary and ecological expansion during the Early Jurassic, particularly with the rise of Lithiotis-type taxa. This group includes genera such as Lithiotis, Cochlearites, Lithioperna, Mytiloperna, and Gervilleioperna (cf. Fraser et al., 2004). These gigantic bivalves were among the most important contributors to shallow marine/lagoonal bivalve mounds/biostromes („reefs”) in various locations around the Pangea during Pliensbachian-early Toarcian time. In Europe they are known from Alpine Mediterranean countries, including Albania, where they formed part of the Early Jurassic carbonate platform system along southern margin of the Tethys Ocean. These systems featured a range of shallow-water carbonate environments, from peritidal to subtidal settings.
Several years ago, during the XIIth Jurassica Conference (Smolenice, Slovakia), a unique and nearly 300-meter-thick continuous sequence of the Triassic/Jurassic transitional strata in the Albanian Alps was presented on this forum (Krobicki, 2016; see also –Barbacka et al., 2019; Gawlick & Schlagintweit, 2019). This section contains many leyers rich in Lithiotis-type bivalves. They occur in various modes: autochthonous assemblages, with vertically oriented shells in so-called “bouquet” forms preserved in life position within biostrome-like structures (Figure 1A); (ii) parauthochthonous assemblages, with non-disarticulated, horizontally aligned shells at the base of beds, likely deposited during weak storm events (Figure 1B); and (iii) allochthonous accumulations, consisting of large coquinas with heavily fragmented shells, possibly formed by strong storms (tsunamites?). These bivalves-rich levels are often interbedded with oolitic limestones (sometimes showing cross-bedding) and oncolitic limestones, indicative of extremally shallow carbonate sedimentation. At least four coal interbeds are also present in the sequence. They are associated with both clastic layers containing leaves and root-bearing beds, presumable of mangrove-type vegetation, that colonized the shoreline and was occasionally destroyed by storm events. The combination of faunal/floral remains, their taphonomic features, and the sedimentological characters of the carbonates indicate a subtropical, shallow-water regime with many emersion events, documented by palaeokarst features (e.g., omission surfaces, bioclasts dissolution, sparry calcite in micro-cavities).
Similar palaeontological and sedimentological features are observed in Early Jurassic successions in the Jabal Akhdar Mountains in Oman, though with notable differences in carbonate/clastic ratios (based on our own investigations in Wadi Mu’aidin and Wadi Sahtan sections; see also – Bendias & Aigner, 2015). Similarities include: (i) the presence of Lithiotis-type bivalves across several beds – ranging from elongated, lens-shaped biostromes to isolated valves in a full spectrum of preservation styles, including autochthonous bouquets (Figure 1C), parauthochthonous groupings (Fig. 1D), and allochthonous storm-deposited shell concentrations; (ii) co-occurrence of oolitic and oncolitic limestones with shell beds (Figure 1E, F); (iii) frequent omission surfaces, some with emersion-related features (indicative of palaeokarstification); and (iv) individual beds containing plant remains and preserved root systems. Notable differences between Albanian and Omani sequences include: (i’) a greater abundance of clastic intercalations in the Omani sections (e.g., sandy limestones, sandstones, and mudstones with ripple marks or cross-bedding); (ii’) the presence of dolomitic limestones; (iii’) multi-coloured marls and nodular limestones; and (iv’) rare occurrences of solitary corals and megalodonts in biodetrital limestones.
In conclusion, a preliminary comparison of the Pliensbachian carbonate and mixed carbonate-clastic successions from Albania and Oman containing Lithiotis-type bivalves reveals a high degree of similarity in warm, likely subtropical shallow-marine carbonate platform settings, with varying clastic input from adjacent landmasses along the southern margin of the Tethys Ocean. Despite the considerable palaeogeographic distance – ± 2500 km – between these regions, the observed sequences demonstrate relatively stable sedimentary and palaeoecological conditions along this sector of Pangea, which extended southeastward toward the Kioto Carbonate Platform, now preserved in the Himalayan region (Ladakh in India; Dolpo, Thakkhola, and Manang in Nepal) (Krobicki et al., 2023, 2025).
Acknowledgements: This paper was financially supported by AGH grant (16.16.140.315).