Taphonomy, sclerochronology and palaeoecology of lower jurassic lithiotid bivalves from the trento platform (italy)

Resumen

The Lower Jurassic lithiotid bivalves are mostly represented by the larger, gregarious and monospecific genera Lithiotis, Cochlearites and Lithioperna. These formed large shell accumulations in the shallow-water carbonate settings of the Tethyan and Panthalassa margins. This thesis analyses the lithiotid accumulations occurring in the Trento Platform (Southern Alps) through the study of a) the taphonomy of the accumulations; b) the skeletal and non-skeletal components along with pyrite forms of the bivalve accumulations to evaluate the microbial role in the shell accumulation geometry and to assess the redox conditions during the lithiotid accumulation growth; c) the shell growth pattern and stable isotope (δ13C and δ18O) variations as proxy of paleoecological limiting factors. The taphonomical analysis of lithiotid accumulations distinguished autochthonous, parautochthonous and allochthonous lithiotids in the accumulation core and flanks. Shell accumulation characteristics and taphonomic attributes, quantitatively analysed in the core and flanks (i.e., shell cover, shell density, percentage of articulated and complete shells), vary according to the dominating lithiotid bivalve. This quantitative approach is potentially useful as proxies for distinguishing core or flanks when the accumulation does not crop out as a whole. The occurrence of bacteria-like features along with calcareous green algae (Thaumatoporella parvovesiculifera), from thin section observations, could contribute to stabilise the soft muddy substrate on which the lithiotids grew and to produce carbonate mud. This is evident in Lithiotis- and Cochlearites-dominated accumulations, characterised by larger dimensions. On contrary, Lithioperna generated tabular bodies. The relative increase in occurrence of bacteria-like features and calcareous green algae (i.e., Palaedasycladus sp., Thaumatoporella parvovesiculifera) upwardly in the Rotzo Formation points to a general decrease in water runoff and an increase in marine water carbonate saturation state. Pyrite framboids, observed by SEM, locally registered lower dysoxic conditions in Cochlearites accumulation core and in both Lithiotis and Cochlearites accumulation flanks whilst Lithiotis core grew in fully oxic setting. The deposits overlying the lithiotid accumulations formed under oxic conditions. High resolution stable isotope (δ13C and δ18O) and shell growth pattern were investigated on well preserved lithiotid shells of Lithiotis, Cochlearites and Lithioperna. Shells of Opisoma excavatum and Pachyrisma (Durga) crassa, two bivalve species widespread in the Rotzo Formation, have been also analysed. Shell preservation was assessed by three different screening techniques (SEM, XRD and cathodoluminescence analyses). Finding of well-preserved and articulated specimens of Lithiotis clarifies the occurrence and structure of the outer calcitic layer in this species and the free valve morphology. In all the studied bivalves, daily growth increments were grouped in fortnightly bundles suggesting a tidal influence even in subtidal settings. This study confirms the long life of some individuals (e.g., up to ca. 40 years old for Cochlearites) which distinguished this aberrant-bivalve group. The δ18O and δ13C variations in the analysed bivalves respond to a complex of ecological conditions, mostly involving the effects of temperature, salinity and primary productivity. Lithioperna, Opisoma and Pachyrisma, which thrived in restricted environmental conditions with a reduced water circulation, could be influenced by water salinity. Their stable isotope profiles suggest higher riverine input and strong seasonal algal blooms. On contrary, Lithiotis and Cochlearites stable isotope profiles indicate a decrease of riverine input influence and more stable conditions. The inter-annual variability was probably related to inter-annual changes in rainy and dry seasons in a tropical climate.