The Saint-Triphon Formation is a new formally established Early Middle Triassic lithological unit of the Prealps and the Western Alps. The type locality occurs in the vicinity of the village of Saint-Triphon in the Rhône Valley of Western Switzerland. Subdivided in 3 Members and 19 levels, this For­mation, 220 m thick in the type area, consists of 5 main shal­lowing upward carbonate sequences, Appearing as the base of the internal part of the Prealpes Medianes Nappe ("Préalpes médianes rigides"), the Saint-Triphon Formation is also pre­sent all along the Briançonnais demain of the Western Alps, from the Barrhorn area (N of the Matterhorn) to the Ligurian Alps in the S.
The palinspastical reconstruction of the Middle Triassic marine area shows that the shallow marine carbonate sedimentation occurs in an intracratonical subsident half-graben of estima­ted 500 km length and 100 to 150 km width, Its orientation was to the NE and E in relation to the actual alpine trend. During the time of the Saint-Triphon Formation deposition, the more subsident area was emplaced in the original position of the Prealpes medianes and the calculated rate of sedimen­tation is 100 Bubnoffs. This rate decreases from a 2/3 ratio in the direction of the Ligurian Alps with an average there of about 30 Bubnoffs.
The original aragonitic sediment with high Sr content is now made of dolomite and low Mg calcite. Accessory minerals con­sist of detrital and authigenic quartz, chalcedony (quartzine), clays with illite, chlorite and smectite, authigenic silicate with K feldspath, albite and tourmaline, fluorite and sulfate (celestite). More than 40 samples are analysed in respect to the clay and silicate contents. The interpretation of the illite cristallinity shows that in the Prealps the internal part is affected by a lower greenschist metamorphic facies (about 300 ° ). The adjacent external part (Rigides externes) shows an anchimetamorphic facies.
Diagenetic fluorite, celestite and quartzine spherulite attests a fluctuating diagenetic from restricted marine to meteoric environment.
The evolution of the flora and the fauna communities is pre­sented. Attention is given to the underestimated role of the siliceous sponge with respect to biosedirnentary (sponge mats) and diagenetical (chert) processes. The expression crypto­spongial11 is introduced for the spicule rich micritic texture of vanished siliceous sponges. The Schizophytes (Cyanophytes and Bacterias) are the main lime rnud producer and act also as mound builders in the upper part of the Formation (Andonce member).
Also an important feature are the deformational structures that appear in the shallow ramp to lagoonal rythmicaly layered lime sediments. Vertical, "en chevron" and sigmoidal slab joints, pseudo-folding, crumpled beds and pseudobreccia or conglome­rate are illustratea and 2 processes of the synsedimentary deformations are analysed. These processes are influenced by reversed viscosity gradients and by the "soft" and "hard" layers thickness ratio.
The dynamic aspect of the sedimentation and the facies models are presented through the 3 main stages of shallow water car­bonate platform development:
1. the birth and initial developpment stage occurs during the end of the Early Triassic (?) and the beginning of the Anisian and is characterised by a multiphased transgression of the peritidal dolomites followed by the shallow ramp to lagoonal vermicular. limestone;
2. after an important eustatic regression and emersion of the platform, the juvenile stage (Early to Middle Anisian) is represented by a complete tidal flat succession:
3. during the mature stage of the carbonate platform {Middle to Early Late Anisian), the depositional model consist of 4 main paleoenvironnments: - the periti<lal flat, - the proper tidal flat, - the "barrier" consisting in lime sand shoals and patch algal-sponqe mounds (lithoherme), - the ramp and shallow basin.
The quick overlapping of the shallow basin facies by the peritidal dolomites indicate a lowering of the sea level and corresponds to the end of a 3rd, order eustatic cycle. A tentative eustatic curve for the Anisian is given. Paleo­geographical and geodynamical trends are under the in­fluence of the tardi-variscan structural directions and of a regional tentional activity. To the S (Southern Alps area) strike slip movments better explain the obser­ved features,