Berriasian–Aptian black shales of the Maiolica formation (Lombardian Basin, Southern Alps, Northern Italy): local to global events

Abstract

The Lower Cretaceous Maiolica Formation represents the classic pelagic succession of the Apulian margin of the Tethys Ocean, in the Southern Alps. It consists of white calcilutites with minor marly layers and black shales. A multidisciplinary study including lithostratigraphy, nannofossil assemblage composition and organic geochemistry was performed on two composite sections in order to characterise the marl–shale layers that record production and storage of organic matter (OM) and terrigenous input in the Berriasian–Aptian interval. Such layers are associated with turbidite deposition and bottom current activity in the Late Berriasian–earliest Valanginian, Late Valanginian and Early Aptian. The uppermost Hauterivian–Barremian black shales alternate with pelagic calcilutites, forming an interval barren of redeposited layers. The quantity of organic carbon increases from the Berriasian to the Barremian; the background composition of the kerogen is continental-dominated, with varying proportions of marine-derived OM. Preservation of OM did not necessarily occur under anoxic conditions; on the contrary, storage of OM probably resulted from enhanced terrestrial input of nutrients, siliciclastics, woody and herbaceous fragments, inducing mesotrophic conditions and increased primary productivity. Higher fertility of surface waters during the Late Valanginian and Early Aptian is reflected by distinctive decreases in abundance of oligotrophic nannoconids and increase in abundance of mesotrophic Diazomatholithus in the Valanginian. During the Barremian, the rhythmic establishment of eutrophic conditions, documented by the highest amounts of marine OM, determined high-frequency fluctuations in nannoconid abundance. The Early Aptian nannoconid crisis is interpreted as the result of a major climate change and enhanced primary productivity. Variations of calcareous nannofloras, as well as changes of lithology and OM, which we describe in the Lower Cretaceous Maiolica Formation of the Lombardian Basin, can also be recognised in coeval units from other sites of the Apulian side of the Tethys. Similar episodes of siliciclastic and terrestrial OM accumulation and major changes in calcareous nannofossils are also recorded in the Upper Valanginian and Lower Aptian pelagic sediments of the Tethys, Atlantic and, partly, Pacific oceans. Such nannofossil variations are coeval with positive excursions of the δ 13C curve and suggest perturbations of the bio-geochemical cycles at a global scale. Additional minor changes in siliciclastic and terrestrial OM accumulation as well as in nannofloras are probably the result of local events, strictly related to the physiography and evolution of the Tethyan margin.