A dolomitized diatomite in an Oligocene–Miocene deep-sea fan succession, Gonfolite Lombarda Group, Northern Italy

An unique, 5–10 cm thick layer of a dolomitized diatomite, which contains traces of volcanic material, occurs intercalated between terrigenous mudstones, thin-bedded turbidites and pebbly mudstones of the Montana Member (Burdigalian) of the Oligocene–Miocene Gonfolite Lombarda Group (“South-Alpine Molasse”). Dolomite formation took place during early diagenesis under conditions of sulfate reduction as suggested by the strongly negative δ 13C values (∼−13‰ PDB). Stereoscan electron microscope observations show that the dolomite grew as a pore-filling cement enveloping deeply corroded diatom frustules. The dolomitized diatomite can be correlated with coeval occurrences of biosiliceous (“Tripoli”) and cineritic deposits in the Northern Apennines and the Southern Alps. Its preservation as a unique and distinct layer in the deep-sea fan facies of the Gonfolite Lombarda Group appears to reflect a particular event and suggests that it was deposited rapidly rather than during a longer time interval of slow pelagic sedimentation.