Looking
south onto about 800m of cliff section of folded turbidites. Note the inclined fold packet verging to
the R (west).Parts of the old Alpine "foreland" basin offer great opportunities for examining the styles of deformation developed under compressional tectonics in turbidite sequences. Studies here focus on work carried out in the Tertiary turbidites on the southern edge of the Ecrins national park, in Champsaur and in the Dourmillouse sector to the east. The two settings have contrasting depositional origins, different tectonic conditions and radically different structural signatures. They are separated by the Selle Falt Zone – a zone of oblique thrusting that was active during and after turbidite deposition.
This sector is rather undeformed – forming a classic site for the study of large-scale depositional systems. But there are a few spaced thrust zones that apparently developed under low confining pressures (not cleavage etc), presumably before the emplacement of the far-traveled "internal" thrust sheets.
Although on a large scale the thrust zones appear to be simple, in detail their internal architecture is complex, apparently in consequence of precursor buckling. The effect is to increase sand-on-sand contacts in the thrust zones.
For further information why not download the pdf on this:
Butler, R.W.H. & McCaffrey, W.D. 2004. Nature of thrust zones in deep water sand-shale sequences: outcrop examples from the Champsaur sandstones of SE France. Marine and Petroleum Geology 21, 911-921.
The structures here largely developed under an overthrust sheet of "internal" Alpine rocks that provided significant burial conditions. The turbidite sequence here has deformed by poly-harmonic folding. The folds face in the presumed direction of shear (westwards).
Fold style varies depending on the layering characteristics of the turbidites.
The following photographs show the style of folding in the upper Fournel valley.
Looking
south onto about 800m of cliff section of folded turbidites. Note the inclined fold packet verging to
the R (west).
Foreshortened photograph of folding in turbidites on the north side of the Fournel valley (looking
N, about 700m of hillside visible).
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