During the Mid-Cretaceous thin siliciclastic and carbonate sediments rich in glauconite and phosphorites were deposited on the Southern part of the Helvetic Shelf . These deposits overlie the shallow-water Urgonian Limestones, and they are covered by pelagic biomicrites of the Seewen Limestones.
New stratigraphie units are proposed for these Mid-Cretaceous deposits :
- the Aravis Formation composed of two members, the Bossetan Member (0-40 m - Upper Aptian) and the Platé Member (0,1 - 20 m - Late Aptian to Mid-Upper Cenomanian). These are separated by the important late-Aptian unconformity,
- the Freu Beds (0 - 6 m, late Albian to early Coniacian), with facies of the Aravis Fm. reworked in an Albian to early Coniacian micritic matrice.
The palaeontology of the ichnofauna and the microbial encrustations are given close attention. The vertical sequence of taphocenoses indicates a deepening trend from Aptian to Turonian.
The study of the Albian-Cenomanian deposits gives evidence of sedimentary areas partially characterized by erosional unconformities. As the fauna indicates a marine environment and no signs of emersion were detected, the erosional surfaces are probably a consequence of a submarine erosion. These discontinuities lie closer when going from N-E to S-W, i.e. from distal to proximal part of the shelf. This shrinkage is strongest in the Eastern part of the Bauges massif where the top of the Urgonian limestones contains solution cavities measuring several meters.
A synthetis of the data provides an account of the sedimentary evolution of the Southern Helvetic shelf. This evolution began in the early Upper Aptian by the drowning of the Urgonian platform. A mineralised hard-ground bearing phosphatized lag-deposits and ammonites veneers the rudistidmiliolid limestones.
During the Upper Aptian, sandy biocalcarenites containing small phosphoclasts (Aujon beds) were deposited in the restricted area of the "Platé-Dents du Midi". Then, after a period of fine-grained siliciclastic deposition (Borderan Beds), mixed-sedimentation with bio- and siliciclastic components took place (Colombière Beds). Faunal composition and depositional features indicate a storm-influenced environment below fair-weather wave base.
This sedimentation was interrupted by the late-Aptian unconformity ( = L.A. unconformity), an irregular surface showing plurikilometer scours cut into the Upper Aptian deposits.
From late Upper Aptian to late Upper Albian, phosphate-rich greensands, containing fossiliferous condensed horizons (Lindars Sandstones) were deposited. The main internal unconformities identify two depositional units within the Lindars Sandstones. In the first, late Aptian to Middle Albian, the sediments show numerous lateral facies and thickness changes. Some of these changes are partly controlled by the irregular shape of the L.A. unconformity : phosphate-rich condensed deposits with microbial encrustations were deposited on the high parts, while siliciclastic-rich deposits, five to ten times thicker than the phosphate-rich deposits, were accumulated within the erosional scours.
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