Environmental control and dynamics of Lower Jurassic bivalve build-ups in the Trento Platform (Southern Alps, Italy)

Abstract

Bivalve molluscs show a short and peculiar Early Jurassic evolutionary phase characterized by the Lithiotis sensu lato fauna, mainly represented by the three monospecific genera Lithiotis, Cochlearites and Lithioperna. These bivalves with stick-like or flattened large shells are mostly restricted to the Pliensbachian. They have a wide geographical distribution, occurring both in Tethyan and Panthalassian carbonate platforms, where considerable biogenic accumulations formed. They were common on the Trento Platform (Southern Alps), where they occur in the Rotzo Formation (Calcari Grigi Group). This formation was deposited in a tropical lagoon which was closed seawards by oolitic shoals and bars and bordered landwards by marshes. The Lithiotis s.l. fauna of the Trento Platform seems to be restricted to the upper Pliensbachian. It appeared after a eutrophic phase with recurrent low oxygen and brackish conditions and flourished in meso- to oligotrophic waters.The stratigraphic and geographic distributions of each genus are compared to the environmental development of the Rotzo Formation in order to highlight limiting environmental factors and ecological requirements. Lithioperna occurs throughout the whole formation; it is the most eurytopic genus, well adapted to dysaerobic habitats and with higher ecomorphic plasticity than the other two genera. It frequently occurs in the life position, giving rise to densely packed fan-like or book-like aggregates. Lithiotis required aerobic and euhaline marine conditions and is common in the middle part of the formation where mesotrophic conditions prevailed. This genus is rarely preserved in the theoretical vertical life position probably due to lower shell stability and a less effective attachment modality. Cochlearites is the most common genus of the Rotzo Formation. This genus inhabited euhaline to slightly hypersaline marine environments occurring in the upper part of the formation, prevailing in oligotrophic conditions.Lithiotis s.l. fauna gave rise to biogenic accumulations with an elevation of a few decimeters to several meters above the sea-bottom and with a lateral extension ranging from ten to several hundred meters. Therefore, these accumulations represent biological build-ups, which document four developmental phases: colonization, aggradation, progradation and demise. The colonization phase is represented by pioneering individuals which were cemented or byssally attached to a bioclastic, coarse-grained storm layer, which constituted the hard substrate for larval attachment. The aggradation phase produced the core of the build-up and its elevation above the sea-bottom. The core is mostly represented by individuals which acted as bafflers, commonly with an apparently chaotic fabric. The Lithiotis s.s. and Cochlearites cores are generally mud-supported and contain loosely packed to dispersed specimens, occasionally preserved in life position. The Lithiotis s.s. and Cochlearites build-ups can be defined as bivalve mounds. The cores of the Lithioperna build-ups are skeletal-supported with shells mostly preserved in life position, which characterize the bivalve reefs. The prograding phase produced lateral additions to the build-up flanks of oblique and imbricated articulated individuals. The flanks grew centrifugally from the core and represent the most conspicuous part of the build-ups, ranging from 10 to 20 times the size of the core and occasionally extending over several hundred meters.