The sedimentology, diagenesis and palaeo-oceanogeaphy of diatomites from the miocene of Sicily

There were two bouts of diatomaceous sedimentation in Sicily during the Miocene: the first in the Aguitanian/Burdigalian and the second during the Messinian. The lower Miocene deposits are found in only a few widely distributed outcrops and their stratigraphic context is uncertain. They occur as thinly bedded cherts and porcelanites Which consist Of either opal CT or quartz. Although diagenesis has destroyed much of their original texture, a diatomaceous origin for the silica can be inferred from the presence of many corroded diatom frustules. Kineralogical diagenetic changes appear to be related to burial associated with early to middle Miocene tectoniam while textural variations probably reflect compositional differences in the original sediment. The Messioian diatomites ape moderately well exposed in central and southwestern Sicily and underlie the Mediterranean Evaporates. They belong to the Tripoli formation which consists of alternating diatcmite and claystone horizons with each of the latter comprising a grey dolomitic marl overlain by a brown terrigenous shale. Diatcnite deposition took place under normal marine conditions in response to the upwelling of deep nutrient, rich waters while isotopic evidence suggests that the claystones usually devoid of biogenic remains, were deposited in stagnant, ' highly evaporated waters, cyclic nature of Tripoli sedimentation is thought to be due to a combination of sea level fluctuations controlled by Antartic glacial activity and the existence of a shallow sill separating the Mediterranean from the Atlantic Restricted conditions developed during low sea-level, stands while the diatomites were deposited as sea levels were rising. The waters of the Mediterranean became increasingly saline and restricted during the Messinian until diatomaceous sedimentation ceased eventually communication between the Atlantic and the Mediterranean was severed and the Messinian Salinity Crisis followe