Bivalve pavements from shallow-water black-shales in the Early Jurassic of northern Italy: A record of salinity- and oxygen-depleted environmental dynamics

Thin shelled-bivalve pavements which constitute the Eomiodon horizon occur in a distinctive black-shale unit in the lowermost part of a Lower Jurassic carbonate platform sedimentary system, of the Southern Alps, Italy. The Eomiodon horizon is characterised by laminated mudstone and fissile, dark grey and black organic-rich marlstone and claystone, yielding abundant bivalve pavements with monotypic to paucispecific assemblages dominated by Eomiodon, an opportunistic, brackish, shallow-infaunal veneroid bivalve. Litho- and biofacies indicate environmental conditions ranging from nearshore euhaline waters to oxygen-depleted brackish lagoon, with a high input of terrestrial organic matter, culminating in marsh and terrestrial conditions. These pavements were analysed, using a microstratigraphic approach, while considering their taxonomic composition, the geometry of the beds, their biofabric criteria (size-sorting, abundance, fragmentation, orientation, and disarticulation), origin and time-averaging. These characteristics led to reconstruction of the following palaeoenvironmental dynamics: (1) the event concentrations, represented by mass mortality of stunted and juvenile individuals caused by anoxia, are indicative of deeper and predominantly stagnant parts of the lagoon; (2) an increase in shell sizes, higher growth rate and a decrease in size sorting and winnowing illustrate higher and more continuous bottom oxygenation and enhanced life in oxygen-fluctuated environmental conditions; (3) poorly sorted, highly fragmented and bioturbated bivalve accumulations indicate the Eomiodon habitat. Here, large-sized Eomiodon specimens record a decrease in water depth and sedimentation rate and almost constantly oxygenated bottom conditions; (4) the appearance of byssally attached bivalves (Lithioperna) marks the transition from the muddy–soupy substrate of the Eomiodon habitat to firm/hard bottom conditions. This new substrate allowed the settlement of Lithioperna, a byssate isognomonid, which records a shallower and landward marsh setting. The Eomiodon horizon testifies to a short-term warm–humid climatic phase and yielded Lithioperna, the first genus of the Lower Jurassic Lithiotis fauna to appear in Southern Alps. This fauna is composed of large and aberrant bivalves which are spread throughout the Lower Jurassic Tethyan and tropical Panthalassian carbonate platforms. As a result, the Eomiodon horizon and its salinity- and oxygen-depleted environmental setting represent a possible progenitor incubator for the Lithiotis fauna