MECO and Alpine orogenesis: Constraints for facies evolution of the Bartonian nummulitic and Solenomeris limestone in the Argentina Valley (Ligurian Alps)

The Eocene represents the last greenhouse interval before Present. The maximum warming during the early Eocene was followed by a long-term cooling trend culminating in the Antarctica glaciation at the base of the Oligocene. Superimposed on this long-term cooling trend there is a prominent transient warming event known as the middle Eocene climatic optimum (MECO) occurring during the early Bartonian. The carbonate ramp succession cropping out in the Argentina Valley (Maritime Alps) offers new insights on the evolution of shallow water realms during this time interval. This ramp displays two main facies belts, middle and outer ramp. The middle ramp is recorded by larger benthic foraminifer floatstone to rudstone, passing to Solenomeris branches and nodule floatstone to rudstone evolving to branching coralline algal floatstone. The outer ramp is dominated by bioturbated marly wackestone to packstone alternating with larger benthic foraminifer floatstone with a silty matrix. The investigated ramp was affected by continuous dispersion and reworking of the skeletal components as other Tethyan Eocene ramps. During the Eocene, the Alpine foreland was influenced by fine terrigenous input controlling the trophic conditions and promoting seawater stratification and the development of a strong pycnocline, for which many perturbations could propagate as internal waves. The reworking of skeletal components of the ramp has been ascribed to the action of internal waves. The switch of carbonate production from a carbonate factory dominated by larger benthic foraminifera to a factory in which the encrusting foraminifer Solenomeris was the main carbonate producer biota, is indicative of a radical change in palaeoenvironmental conditions affecting the early Bartonian. The acme of Solenomeris often coincided with the crisis of carbonate producers during intervals of an evident deterioration of environmental conditions. In this case the acme is probably related to the adverse conditions linked to the (MECO) warming event. Finally, the drowning of the nummulitic ramp has been caused by light reduction for the photo-dependent biota due to progressively increasing depth linked to flexural subsidence of the foreland plate, and minor efficiency of the aphotic carbonate factory.

Automated summary

The study reveals the influence of long-term cooling and transient warming events on the evolution of carbonate ramps during the Eocene. The reworking of skeletal components and the crisis of carbonate producers are found to be associated with environmental deterioration. The drowning of the ramp is attributed to reduced light and inefficiency of the carbonate factory.