Diagenesis and burial conclusion of the GdA Fm.*

*in the areas of Lauzanier, Annot and Peira Cava - see map

The stability of heavy-minerals in deep-burial sandstones depends on environment in which they are situated. Besides the burial, which dominates the control of temperature and pressure, the pH - values of pore fluids are also assigned as conducting the order of heavy-mineral stabilities (Morton, 1984). When focusing on studies with orders of stability of detrital heavy-minerals in sandstones subjected to acidic pore fluids (Morton, 1982a, Friis et al., 1980, Grimm, 1973, Nickel, 1973), a modified heavy-mineral order is displayed compared to the mildly alkaline pore fluids. Particularly, the apatite, but also garnet and chloritoid is behaving sensible between acidic- and alkaline environment, relatively stable in alkaline, but unstable in acidic pore fluids (Morton, 1984). The heavy-minerals derived from the southern outcrops of Annot and Peira Cava indicate a saline and mildly alkaline environment, because the garnets and especially the apatites are part of the major heavy-mineral appearances, without any considerable traces of etching facets. This also implies that weathering did not influence the heavy-minerals during the geological past or recent. The same pH-environment interpretation and non-weathered arkoses theory are also valid for the northern located Lauzanier area, since apatites have preserved their habit. However, the garnets do show severe etching facets and their population is significantly lower than in the southern areas (Table 8.1.2d). The garnets emphasise an exceptional position when approaching an interpretation of diagenesis grade and burial in the Lauzanier area. Their presence in samples of the upper super-cycle (including bed 6 and 7 in table 8.1.2d or appendix 2: Log Correlation) is still significant, whereby the garnets either are strongly- or moderately etched. In contrast, samples of the lower super-cycle (including bed 1, 2, 4 and 5 in table 8.1.2d or appendix 2: Log Correlation) display rare occurrences of moderately - strongly etched garnets (Table 8.1.2d).
Apparently, this feature indicates an existence of at least two types of garnets, whereas one of them is more stable than the other (Picture 9.2.1a and b). An application of EPMA (work method, chapter 5.2.5.) assigned the more stable garnet as an almandine (Fe+++3Al2(SiO4)3) (Picture 9.2.1a and b) and the unstable as a pyrope (Mg3Al2(SiO4)3) (Picture 9.2.1a). Furthermore this device detected and determined rare examples of spessartine (Mn++3Al2(SiO4)3) in the upper super-cycle, which also showed strongly etched facets. Between this range of 350m in the succession, analysing of vertical applied samples might imply a continuously decreasing stability of the garnets (sample overview, figure 5.1.4 and table 8.1.2d). Most likely, the samples of the lowest part of the Lauzanier succession are adjacent the base of the garnet appearance. According to Morton (1984), the garnet is still present at depths of 2800m and more. His study concentrated on localities in the North Sea; an extensional basin, which enables depth-studies of heavy-minerals in situ. This is not featured in the remnants of the Grès d'Annot basin; a compressional foreland basin. Nevertheless, when neglecting this, the Lauzanier succession might be interpreted as of having experienced a burial exceeding 2800m. A burial interpretation of the southern outcrops of the Annot- and Peira Cava area is somewhat more complicated. This heavy-mineral study revealed rare presence of epidotes and titanites, but no kyanites were identified. According to Morton (1984), the kyanites, though, should still appear when the epidote and titanites are present (Table 9.2). When applying Stanley's (1975) description, kyanites are present, which cannot be disproved since this study only analysed four samples from the southern area. Thus, the epidotes are used as a measure of previous burial. Morton (1984) assigns the initiating depth-absence of epidotes as ranging widely, based on North Sea studies, from normally 1100m but may even occur at depths below 2000m. However, these depths are only observed in low-porosity units, which hinder percolation of fluids. The arkoses from the southern areas comprise high-porosity units (Table 8.1.1b), suggesting a rather ancient moderately burial of the southern areas that probably did not exceed depths of 1100m.