Understanding the geological record of carbonate platform drowning: examples from the Lower Jurassic of the Apennines (Italy)

ABSTRACT - In the geological record a drowning process is documented by various types of shallow water-to-pelagic or shallow water to mixed benthic/pelagic carbonate transitions. Drowning unconformities are paraconformities, to disconformities, to angular unconformities, and their drowning surfaces range from planar to highly irregular morphologies. Drowning successions display a mix of products of both the benthic and pelagic carbonate factories. These successions can be also bounded by unconformities. Drowning unconformities and drowning successions are contrasted through a description and discussion of examples of escarpment-bounded platforms from the Jurassic of the Northern Apennines and, subordinately, from Sicily and other Tethyan sectors. The areal distribution of drowning unconformities and successions is discussed with reference to a complex depositional system whose architecture was the product of the riftinduced fragmentation of a regional Hettangian carbonate megabank. The examples in this study show drowning unconformities to be exclusive to intrabasinal highs, while drowning successions are found both on highs and in hangingwall basins. Drowning unconformities, with their long associated hiatuses, and drowning successions are often seen to merge laterally into one another over very short distances on the same intrabasinal high. The deposits of the drowning succession are sometimes missing on the top of the platform, while they are found forming clinoforms along its flanks, evidence that sediment could be permanently swept from the highs in these depositional systems at this stage.