Conclusion of Grès d'Annot Basin evolution in the Lauzanier area

regional basin evolution - see chapter 4

As the first Grès d'Annot sediments were encountering the basin, the paleo-surface of the Marnes Bleues was irregular. Thus, a common sight in the outcrop remnants of the basin is the onlap between the boundary of the two formations (Picture 6.7b). This irregularity of the paleo-basin floor was, after Apps (1987), caused by tilting of Early-Eocene Alpine loading, either sub-crustal blading or distant crustal stacking (details, Chapter 4.2). During the deposition of the Grès d'Annot Formation, the basin experienced a period of tectonic quiescence. The syn-sedimentary structures exposed in the lower super-cycle of the formation, probably only are resultants of gravity and slope balance features.

After the cessation of the Grès d'Annot deposition, another significant tectonic event was initiated as the Alpine loading reiterated its movement. While the Alps were compressing and moving, they produced nappes that were thrusted and therefore closing to the Grès d'Annot basin. The most outer part of the Alpine thrust system, the Brianconnaise Nappes, even overthrusted the Grès d'Annot basin in the northern region. Today, this event is observed as an angular unconformity between the boundary of the allochthonous Brianconnaise Nappe and the autochthonous Grès d'Annot Formation (Picture 7.1b and 7.1c). Apparently, the Grès d'Annot Formation was already consolidated enough to enable composite sandstone units to penetrate into the less consolidated allochthonous olisthostromes. Yet, the olisthostromes have eroded significant proportions of the upper part of the Lauzanier succession, since only 625m thick (Appendix 2: Log Correlation). Other northern successions exposures thicknesses of more than 1000m (Hilton et al., 1995), when their top is not eroded. Obviously, the olisthostromes caused very variable erosion amounts when thrusting into the northern part of the basin, whereas the Lauzanier area apparently was severely effected. This is evidently revealed, since the lower part of the Lauzanier succession is displaying similar composite logs when comparing other, more complete, composite logs. Logic is probably, if moving further on eastwards, into Italian Grès d'Annot exposes, an even stronger erosion has been acting that is caused by stronger influenced Alpine thrusting activity. However, this hypothesis is not studied more intensive.

When excluding the angular unconformity at the top of the Grès d'Annot succession, there are no internally structural features sited across the whole Lauzanier area that are resultants of compressional tectonic activity. Except the master set of joints that emphasises feather joint characteristics (Figure 7.1a), which indicate the former stress- or even strain regime of the body. Hereby the feather joints show that the northwest - southeast directed s1 is the highest compressional component of force. The s1 compressional force produced shear forces and probably caused the forming of the north - south trending master set of joint. Further, these shear-forces were able to rotate the feather joints from north - south- to a nearly east - west trend. Related perpendicular to the s1 is the s3 component, which reveals the northeast - southwest trending extensional force. Most likely, this joint system was produced during the Miocene and post-Miocene folding process of the Alps, since the direction of major compressional component of force is equally directed as s1 of the Lauzanier joint system.

The faulting characteristics in the Lauzanier area, comprising downtrown blocks (Appendix 3: Panorama Pictures, 3.4) and maybe a graben-like structure that even suggest a step fault system, implies resultants of extensional tectonic activity. Likely is to assign a cessation of compressional tension, providing an increasing space of the body, to advance an collapse. Thus, if representing this theory, the collapsing of the Lauzanier body is post-orogenic and therefore of post-Miocene age.