Sequence stratigraphy and condensed pelagic sediments. An example from the Rosso Ammonitico Veronese, northeastern Italy
Abstract Correlation between eustatic and sedimentary cycles in the Jurassic Rosso Ammonitico Veronese suggests that the existence of a genetic link between them should be explored. The red nodular limestones of this formation were deposited on a current-swept submarine plateau pertaining to the South Alpine passive continental margin. In this setting of fault-bounded blocks, shelf-derived sediments were trapped in the intervening troughs and only the very slow, particle by particle rain of planktonic organisms took place. The corresponding rock bodies typically show frequent omission surfaces of variable range. Current activity seems to be the prime controlling factor which affects the degree of winnowing, early cementation and reworking and is decisive in bringing about all lithologic changes. This hydrodynamic effect greatly depends on ocean circulation patterns which in turn are controlled by climatic and sea-level changes. Highstands lead to equable climates and hence to a weak regime of currents impinging on sea bottoms; the opposite happens during the following lowstands which reactivate ocean circulation through deteriorated climatic conditions. It is envisaged that, even though most of the geometric and sedimentologic criteria commonly used on shelf-slope sediments are lacking, depositional sequences, i.e., physically the stratigraphic record of complete eustatic cycles, may be recognized also in condensed, pelagic successions: they consist of thin rock units deposited during periods of sea-level highstand, when a slowed down current activity allows the preservation of the pelagic input, and are bounded by lowstand-induced hard-grounds when accelerated winnowing prevent deposition. These unconformities consequently don't correlate with condensed sections on shelves.