Flexural-eustatic numerical model for drowning of the Eocene perialpine carbonate ramp and implications for Alpine geodynamics

The Paleocene-Eocene Nummulitic Limestone Formation of the Alpine periphery occupies the lowermost part of a stratigraphic trinity above a major basal unconformity. It is thought to have accumulated as a foraminiferal carbona te ramp at the distal featheredge of the underfilled north Alpine and south west Alpine foreland basin. We present the results of a numerical model tha t links carbonate sedimentation at the distal featheredge of a peripheral f oreland basin to the flexural subsidence of an elastic plate subjected to a distributed load with a superimposed eustatic sea-level history. Carbonate accumulation is treated as depth dependent. Model parameters are constrain ed by geological and geophysical observations of the Alpine orogen and its peripheral foreland basin and from literature on the ecology of benthic for aminifers. The generic Alpine model shows that the carbonate ramp accumulates as a num ber of sedimentary cycles before rapid drowning, giving rise to a retrograd ational stratigraphic package terminated by a surface of accelerated transg ression and backstepping. Running the model with sets of parameter values a ppropriate to the Paleocene to middle Eocene Nummulitic Limestone in centra l-eastern Switzerland, and the late Eocene Nummulitic Limestone of France, results in a successful replication of the first-order characteristics of t he carbonate successions in each case. The model highlights the sensitivity of the stratigraphy to variations in environmental parameters (light-extin ction coefficient) and controlling tectonic parameters such as convergence rate and flexural rigidity. The results therefore also provide an interesti ng perspective on the likely range of geodynamical parameters, particularly equivalent elastic thickness, Te, at an early stage in Alpine orogenesis. Combined with previous estimates of Te at 25 Ma,17 Ma, and the present day, our results appear to rule out any possible secular increase in flexural r igidity for the European plate during Tertiary time.