9. Diagenesis

The XRF-data do not display significant difference between the southern- or northern derived samples (southern samples: number 45 or higher, in table 8.1.2a). When considering the southern areas dispose the same sediments as the northern areas, this might be somewhat unexpected, because the arkoses of northern areas are substantially more consolidated and consequently comprise considerable lower porosities than arkoses of southern areas (Table 8.1.1b).
These characteristics most likely indicate that the northern areas experienced a higher grade of diagenesis, which is normally accompanied with an advanced mobilising of unstable elements. If still applying an equal composed sediment that was deposited in the southern- as well as in the northern part of the basin, the mobilising of elements in the higher-grade diagenesis-influenced arkoses either remained in the arkose and only modified- or developed new minerals, or mobilising of elements had not advanced further than the southern located lower-grade diagenesis-influenced arkoses. Thus, a theory of dissolved elements escaping from the arkoses can be excluded, since samples from all areas nearly comprise the same amount of elements.

The heavy-mineral concentrate-slide microscopy contradicts the uniform XRF-data. There are significant variances in heavy-mineral suites between the northern- and southern located outcrops (details, chapter 8.1.2. and table 8.1.2d). Stanley (1975) explains these location-dependent differing heavy-mineral suites as sediments deriving from more than one source area. However, recent research has demonstrated that these varying heavy-mineral suites may not decisively apply differing source areas. Authors like Morton (1983), Füchtbauer (1988), Scavnicar (1979), Nickel (1973) and Wieseneder & Maurer (1959) have done comparison of orders of detrital heavy-mineral stabilities in sandstones under deep-burial conditions from a number of sedimentary basins. Nickel (1973) has simulated stability sequence for deep-burial conditions, determined by dissolution experiments on heavy-minerals using mildly alkaline solvents. Morton (1983) generalised an order of stability for heavy-minerals in sandstones undergoing burial-related intrastratal solution, based on various authors and predominantly studies from the North Sea by himself (Table 9.2). When applying Morton's generalised order of stability for heavy-minerals in sand-stones undergoing burial with the results of heavy-mineral analysing of the Grès d'Annot Formation, this study features a striking resemblance concerning the differing heavy-mineral stability order of the southern- and northern outcrops. The stability order further enables an convincing interpretation of; that the northern region was definitely undergoing a significantly higher grade of diagenesis and most likely experienced a more intensive burial. However, caution is necessary when interpreting the higher-grade diagenesis theory in the northern part of the basin that was caused by a more advanced burial than in the southern outcrops. This intensified diagenesis may also have been a result of the northern region to be more adjacent the Alpine orogenic belt and thus experienced severer tectonic activity.

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.