The Halimeda dominated prograding wedge of the Pietra di Finale (Middle Miocene, Liguria)

Rend. Online Soc. Geol. It., Vol. 21 (2012), p. 918. The 'Pietra di Finale' allows to analyse the evolution of Mediterranean during the Chattian to Serravallian interval. The 'Pietra di Finale' deposits unconformably overlies the Alpine substrate. Three main stratigraphic units may be recognized in the sedimentary succession of the Finale area. The first unit is represented by metamorphic and dolostone conglomerates and sandstone that may be interpreted as a fan delta deposits and are Chattian to Aquitanian in age. This deposits pass upward to marls and marly limestone with some sandstone intercalation. This unit, Aquitanian to Burdigalian in age, is known as Torre di Bastia unit and it is overlain with a sharp contact by mixed terrigenouscarbonate deposits. Clasts of terrigenous fraction were mostly derived from the Alpine orogenic wedge, whereas sandstone lithic fragments have a more varied provenance. To reconstruct the paleolatitude position of the Finale succession we made plate kinematic reconstructions of the main blocks in the western and central Mediterranean area, during a time interval t=25 Ma. We explored tectonic evolution in the Eurasia-fixed reference frame, and the hotspot framework, testing both the present-day Atlantic/Indian hotspots and the present--day Pacific hotspots, for the definitions of the reference system. All the cases studied do not provide any significant variation of block positions, resulting in a range of displacements of few degrees of latitude. The carbonate fraction mixed siliciclastic-carbonate unit is dominated by Halimeda. Halimeda is restricted to tropical and subtropical marine environments where they grow either on unconsolidated sediments or on the reef itself. Halimeda green algae tolerate a range of hydrodynamic regimes, flourishing from the back-reef lagoon to the forereef slope, at depths ranging from <1 to 150 m. These algae grow rapidly, attaining heights up to 15 cm over their 1- 3 month lifespan. Halimeda tolerates high nutrient conditions. It is demonstrated that there is an increase of the algal rate of photosynthesis directly via nutrient enrichment. Palaeolatitudinal reconstruction of the investigated area during the Chattian to Serravallian interval evidence that this region was already near the Modern latitudinal position. This evidence an expansion of tropical zone (waters) during this interval up to the Northern Mediterranean. Typically Mediterranean Lower to Middle Miocene carbonate platforms are dominated by seagrass environments in the euphotic zone and coralline algae in the oligophotic zone promoting the development of carbonate ramp depositional profile. In this example regional condition had promoted a Halimeda dominated carbonate production that have substituted in the euphotic zone the seagrass carbonate factory, less tolerant to the eutrophic conditions. Halimeda substituted also the red algal production in the oligophotic zone. Halimeda dominance has also controlled the depositional profile. Wedgeshaped profile is the results of important nearshore siliciclastic sedimentation produced by erosional processes on the rocky shoreline and limited carbonate sedimentation in the deepest euphotic zone and in the oligophotic zone. The Halimeda dominated prograding wedge of the Pietra di Finale (Middle Miocene, Liguria)