Microbialites in the Middle–Upper Jurassic Ammonitico Rosso of the Southern Alps (Italy)

Abstract

The Trento Plateau (South-Alpine domain of the western passive continental margin of the Apulian plate), during the Middle-Late Jurassic was the site of extended microbialite development in Ammonitico Rosso fades. The setting was a current-swept, pelagic plateau located far from sources of terrigenous supply, characterized by very low average sedimentation rate. Both stromatolites and leiolites occur, commonly associated and passing into one another. For the carbonate-dominated Ammonitico Rosso depositional setting, it may be inferred that microbialites grew in well-oxygenated waters of normal salinity, arguably characterized by very low nutrient availability and probably located in the photic zone. A decimetre-scale basic rhythm occurring in some intervals of the local succession consists of a slightly clayey nodular unit stabilized by a carbonate, microbial unit. Transition from the lower to the upper unit of the rhythm may imply increase in hydrodynamics and oxygenation at the bottom and a progressive reduction in trophic level and bioturbation rate by burrowing metazoans. These rhythms are tentatively attributed to a precessional control.