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

¹ Department of Geology, Trinity College, Dublin 2, Ireland
² Department of Earth Sciences, University of Cardiff, Main Building, P.O. Box 914, Park Place, Cardiff CF10 3YE, UK
³ Department of Geology, Trinity College, Dublin 2, Ireland
⁴ Department of Geology and Geophysics, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK

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 carbonate ramp at the distal featheredge of the underfilled north Alpine and southwest Alpine foreland basin. We present the results of a numerical model that links carbonate sedimentation at the distal featheredge of a peripheral foreland 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 constrained by geological and geophysical observations of the Alpine orogen and its peripheral foreland basin and from literature on the ecology of benthic foraminifers.

The generic Alpine model shows that the carbonate ramp accumulates as a number of sedimentary cycles before rapid drowning, giving rise to a retrogradational stratigraphic package terminated by a surface of accelerated transgression and backstepping. Running the model with sets of parameter values appropriate to the Paleocene to middle Eocene Nummulitic Limestone in central-eastern Switzerland, and the late Eocene Nummulitic Limestone of France, results in a successful replication of the first-order characteristics of the carbonate successions in each case. The model highlights the sensitivity of the stratigraphy to variations in environmental parameters (light-extinction coefficient) and controlling tectonic parameters such as convergence rate and flexural rigidity. The results therefore also provide an interesting 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 rigidity for the European plate during Tertiary time.

Key Words: Alps • carbonate ramps • foreland basins • numerical models • Nummulites

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