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Geological Society, London, Special Publications; 1998; v. 149; p. 205-227;
DOI: 10.1144/GSL.SP.1999.149.01.11
© 1998 Geological Society of London

Carbonate sedimentation during early foreland basin subsidence: the Eocene succession of the French Alps

H. D. Sinclair1, Z. R. Sayer2 & M. E. Tucker3

1 School of Earth Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
2 Robertson Research International, Llanrhos, Llandudno LL30 1SA, UK
3 Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK

The Eocene Infranummulitic Formation and Nummulitic Limestones of the external chains of the French Alps in Haute Savoie and Alpes Maritimes unconformably overlie the Mesozoic passive margin succession, and were deposited during the onset of subsidence of the underfilled Alpine foreland basin. The Infranummulitic Formation represents detrital deposition in localized structural lows in braided fluvial, fan delta, coastal plain and lagoonal environments; the resultant facies record late lowstand and early transgressive sedimentation. A transgressive surface separates the fluvial-dominated conglomerates from lagoonal facies, and a ravinement surface separates lagoonal from ramp facies of the overlying Nummulitic Limestones. The combination of infilling of topographic lows by the Infranummulitic Formation, and the transgressive ravinement process, reduced local slope gradients inherited from the topography of the European foreland, thus encouraging the subsequent development of a low-angle carbonate ramp during continued transgression.

The Nummulitic Limestones represent highstand sedimentation on a low-energy, wave-dominated, carbonate ramp characterized by large benthic Foraminifera. The inner ramp accumulated bioclastic banks and shoals composed of peloids, calcareous red algae and Nummulites. The middle ramp is recorded by wackestones with a diverse assemblage of flat Foraminifera; occasional storm events generated winnowed packstones. The outer ramp is represented by mudstones and marls with a sparse biota. The Nummulitic Limestones pass upwards into hemipelagic marls.

The drowning of the Nummulitid-rich ramp is unlikely to have been caused by flexural subsidence of the foreland plate. Possible other contributing factors to the demise of the carbonate ramp were nutrient excess from the siliciclastic-rich orogenic margin to the basin, and/or glacio-eustatic sea-level rise.





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