| Abstract: | During the early Cretaceous, the carbonate platform attached to the northern Tethyan margin experienced a series of platform drowning episodes, which can be widely traced. The five episodes recognized cover the early Valanginian to early Hauterivian (D1), middle Hauterivian (D2), late Hauterivian to early Barremian (D3), early to early late Aptian (D4), and late Aptian to early Albian (D5). The particular sensitivity towards drowning in this platform system was probably related to its marginal paleoposition (ca. 30 degree N) with regard to reef growth, to its attachment to the European continent (a periodic source of reef-unfriendly weathering products), as well as to prevailing paleoceanographic conditions (sea-level change, upwelling intensity, presence or absence of connections to the boreal realm).
Drowning episodes are usually preceded by changes in carbonate production from a chlorozoan to a foramol mode, with important increases in accumulation rates. The drowning episodes themselves are documented by erosional surfaces and/or by the formation of strongly condensed beds rich in coarse sand, glauconite, phosphate, and biosilica. The presence of ammonites is a key to their dating.
If resolvable, the onset of drowning appears to be diachronous, and its evolution is by onlap onto the platform (D1, D4). Termination of drowning appears in all cases synchronous. Drowning episodes D1, D4, and D5 are correlated to positive excursions in the delta C-13 record, albeit with a slight diachronity in their onsets (drowning episodes lead the delta C-13 record), whereas episode D2 correlates to a negative excursion and episode D3 to a long and steady increase in delta C-13 values. This suggests that no uniform mechanism can be assumed for all five episodes but that each episode needs to be examined in its own context. An important element in all episodes is the change in weathering style and intensity in the continental hinterland, which profoundly affected detrital and nutrient fluxes. |