Évolution stratigraphique et paléo-environnementale de la Formation à Microcodium et des Calcaires à Nummulites dans les Alpes Maritimes franco-italiennes

Abstract

The present study offers a detailed description and a palaeogeographic interpretation of the Eocene deposits of the Dauphinois-Provençal domain in the southern Maritime Alps. The studied area is 40 km 2 in extent and was mapped at 1:10.000. Five sections were measured and sampled. On the basis of lithological and paleontological features, two lithostratigraphic units have been distinguished: the Microcodium formation and the Nummulitic limestone. The Microcodium formation includes five facies: (1) the Microcodium breccias, (2) the clayey limestones, (3) the Microcodium conglomerates, (4) the dark micritic limestones and (5) the white quartz sandstones. The Nummulitic Limestones include three facies: (6) a basal lag deposit, (7) the Nummulitic facies and (4c) the Orbitoidacea facies. The paleoenvironmental analysis shows that the first Eocene sediments of the Dauphinois-Provençal Domain (facies 1 to 5) represent incised valley fills. The valleys were incised on the Mesozoic deposits during their subaerial exposure from Late Cretaceous to Early Eocene. Fluvial to estuarine deposits ( Microcodium breccias, 1; clayey limestones, 2; Microcodium conglomerates, 3) accumulated during the middle-upper Eocene in the valleys that were limited towards the sea by sand ridges (white quartz sandstones, 5). Theses ridges, located in the distal part of the estuary, isolated a brackish environment where the dark micritic limestones (facies 4) deposited. A transition to a fully marine environment occurred only during the Nummulitic limestones deposition. This unit represent a wave-dominated, middle to inner ramp, characterized by a low sedimentation rate. The storm waves action resulted in a mixing of bioclastic and terrigenous grains in the inner ramp (facies 7), while transported material prevailed in the middle ramp (facies 8). As a general trend, the facies indicate an ininterrupt deepening of the depositional environment. The passage from continental to paralic environments was followed by a marine transgression (fades 6) beginning with a inner-middle ramp mixed sedimentation and ending with a more pelagic sedimentation. In the present study a sequential analysis based on the recognition of major stratigraphic surfaces was attempted. The first stratigraphic surface is a sequence boundary located between the Upper Cretaceous facies and the Eocene continental deposits. This sequence boundary is linked to the geodynamical evolution of the Alps that caused a general uplift during the Late Cretaceous to Early Eocene. The second surface is a ravinement surface located at the base of the Nummulitic limestones. It is a regional marker of Lutetian-Bartonian age. A significant discrepancy between the depositional depth evolution of the studied facies and the second and third order eustatic cycles of Haq et al. (1987) is evidenced. The major eustatic signals referred to the Eocene, are not registered in the studied sections. In conclusion we can assert that, in the Maritime Alps, during the Lutetian-Priabonian, the eustatic sea level changes have been masked by the geodynamic evolution of the Alpine foreland basin.