Sedimentary and biological response to sea-level and palaeoceanographic changes of a Lower–Middle Jurassic Tethyan platform margin (Southern Alps, Italy)

Abstract

The sediment succession formed at the platform to basin transition along the western margin of a Jurassic Tethyan carbonate platform records a cyclicity in the order of 3–10 My. These cycles are interpreted as due to sea-level fluctuations associated to palaeoceanographic changes of the surface waters, as inferred by the evolutionary dynamics of the calcareous nannofossils. Overimposed to these variations, the skeletal composition of the platform succession records a longer-term (order of 50 My) trend of progressive environmental deterioration, related to the break-up of Pangea. Our case study deals with the Venetian Platform margin and the Lombardian Basin of the Southern Alps (Italy): a Lower to Middle Jurassic platform — basin system of the Western Tethys. Four depositional sequences have been recognized in the Lower to Middle Jurassic. The local sea-level curve derived from sequence analysis records a short-lived (<1 My) regression each during the latest Domerian, Early Aalenian and earliest Bajocian. The origin of these sea-level drops is probably related to climato-eustatic events. On the Venetian Platform, the skeletal sediment composition of the carbonate platform changed from Sinemurian–Domerian photozoan to Toarcian heterozoan assemblages, dominated by suspension feeders. This change appears due to increased trophic resources. The older event of platform demise correlates to the sea-level drop of the latest Domerian. The final drowning of the Venetian Platform occurred after the Early Aalenian drop in the sea-level. Among the basin biota, calcareous nannofossils represent the major pelagic carbonate producer since the beginning of the Jurassic. Nevertheless, the Sinemurian–Bajocian deep-sea carbonates of the Lombardian Basin consist mainly of allochthonous mud exported off-platform with a low nannoplanktonic contribution. Moreover, the platform demise correlates with condensation, gaps and mass-flows in the basin. This relationship suggests that the growth history of the carbonate platform directly controlled the carbonate sedimentation in the adjacent basin. The nannofloras recorded in the Lower and Middle Jurassic successions of the Lombardian Basin consist of two different taxonomic groups: the genus Schizosphaerella, a group of uncertain systematic affinity, and several genera of coccolithophorids. An inverse correlation between the abundance of schizospheres and coccoliths has been recorded throughout the whole interval investigated. Moreover, during periods of platform and sea-level fall, Schizosphaerella spp. dropped in abundance, whereas rates of coccolith diversification, calculated according to Roth's equations, reached peak values. Our evidence shows that schizospheres could be oligotrophic forms, while coccolithophorids on the contrary probably diversified during periods of increased nutrient resources. The observed sea-level falls are in-phase with increased trophic resources and holds on up to the transgressive interval. The biological response includes a dramatic drop of the platform carbonate productivity, and increased C org accumulation on the sea-floor. The long term evolution of the sedimentary system records increasing eutrophication from Liassic to Dogger, as reflected by the compositional changes of the carbonate producers.