Asymmetric Sedimentary Cycles in the Organic-Rich Italian Rhaetic

Daniele Masetti, Marco Stefani, Mark Burchell

In the Lombardy basin (northern Italy), the main part of the organic-rich Rhaetic depositional sequence consists of metric-scale asymmetric cycles, each divided into three parts: lower clay portion, a central rhythmic portion consisting of repeated marl-limestone couplets, the limestone part of which gradually thickens upward, and a wholly carbonate upper unit. The organic content is often high in the lower part of each cycle but decreases upward. The analysis of the diagenetic history demonstrates that these lithological alternations are fundamentally sedimentary. This cyclicity is derived from the interference of a lower-frequency (magnitude of 100,000 yr) asymmetric carbonate mud signal with a high-frequency argillaceous mud signal, as indicated by spectral power analy is. The calcareous basinal mud was predominantly allochthonous in origin, coming from the adjacent platforms. The associated asymmetric carbonate signal was a response to eustatic fluctuations; the exportation of platformal calcareous muds, which was negligible in the deepening phase, greatly increased during the shallowing evolution, and was eventually stopped by widespread subaerial exposition. In contrast, the argillaceous signal is likely to be climatically modulated. The study of these cycles improves our basin analysis, giving us a suitable method to correlate different sections both at outcrops and in the subsurface.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.