Petrograpic conclusion of the GdA Fm.

The XRD-application (work method, chapter 5.2.3.) identified the major mineral content qualitatively and some minerals quantitatively (Table 8.1.1a). The XRD-results additionally confirmed that the thin section estimations of quartz being the most abundant mineral with values commonly exceeding 50% (Table 8.1.1a). Feldspar content is pending around 40%. Further the feldspars could be divided into low-albite, oligoclase, microcline and orthoclase. No approach in quantifying these feldspar-types was started, since their peaks superimpose in the XRD-plots. However, microscopy displays a slight domination of K-feldspars. Carbonate is present in all samples, though at low rates, ranging from below 1- and up to 14.6% (Table 8.1.1a). Dolomites show more stable percentages, normally below 1- and not exceeding 3%. The XRD-application could not resolve any quantitative values of the micas, because of the equal superimposing problem like the feldspar-types. However, micas were estimated to constitute 4 - 6% provided by the implementation of thin section microscopy. Table 8.1.1a: Values obtained through the XRD-application. Minerals with "*"are not quantitatively measurable with an XRD-application. Thus, these mineral amounts were estimated with thin section microscopy.

The sampling was purposed to restrict fine- to coarse-grained sandstones. When applying thin section microscopy, this criteria is successfully accomplished (Table 8.1.1b). The sampled sandstones range from an average grain-size of 130 - 550µm. According to the Wentworth scale, this range is covering the entire fine- and medium sand spectrum (Wentworth, 1922). All analysed samples comprise angular - sub-angular grains, whereas the sub-angular grains dominate the thin section picture. Sub-rounded grains are rare or even totally absent. The porosities of the southern areas (Annot and Peira Cava) are much higher, varying from 11 - 17%, compared to the northern area of Lauzanier that only emphasise very poor porosities below 7% (Table 8.1.1b).

The XRF-data (Table 8.1.2a) (work method, chapter 5.2.4.) were used to classify the sandstones with sediment classification diagrams by Pettijohn (et al., 1987) (Figure 8.1.2b) and Herron (1988) (Figure 8.1.2c).
In both sandstone classification diagrams the 48 analysed samples of the Grès d'Annot Formation plot well assembled (Figure 8.1.2b and c) in the classification diagrams, implying an uniform SiO2/Al2O3-, and Na2O/K2O- and Fe2O3/K2O ratio content. However, the classification diagrams indicate differing lithology classifications, since samples plot in different lithology zones. According to Pettijohn's (et al., 1987) diagram, which applies the ratios between log(Si2O/Al2O3) and log(Na2O/K2O), the samples mostly plot in the lithology zone of litharenite and partly in arkose. In contrast, the diagram of Herron (1988) implements an y-axis of log(Fe2O3/K2O)- instead of a log(Na2O/K2O) ratio, whereby the samples strictly plot in the arkose lithology zone. After Rollinson (1993), the Pettijohn diagram is to be treated with caution, since Na and K are likely to be mobilised during diagenesis. Thus, Herron modified the diagram of Pettijohn (et al., 1987). He uses the ratio Fe2O3[total]/K2O that allows arkoses to be more successfully classified and is also a measure of mineral stability, for ferromagnesian minerals tend to be amongst the less stable minerals during weathering (Herron, 1988). Considering the SEM-analysing, which is emphasising an element mobilisation of Na-feldspar grains that probably initiated during diagenesis (details: Chapter 8.1.1.), it appears reasonable to apply the diagram of Herron. Thus, the sandstone of the Grès d'Annot Formation is classified as an arkose, whether it is derived from southern- or northern area. According to Macbride (1963, in Füchtbauer, 1988), if a sandstone contains more than 25% of feldspars and less than 10% of lithic grains, it may be assigned as an arkose. Estimations of thin section microscopy and XRD-calculated data regarding the mineral content are confirming the arkose theory. Both applications revealed low contents of lithic grains, definitely below 10%, and a feldspar constitution of significantly more than 25%. The petrography studies of Stanley (1961) and Jean (1985) suggest that 80% of the sandstones are arkoses; the remainder is a feldspathic sandstone.

Besides the application of thin section microscopy and XRD, which was suitable for determining the qualitative and quantitative composition of the major minerals, an effort in detecting some accessory minerals was accomplished by the implementation of heavy-mineral concentrate-slide microscopy (work method, chapter 5.2.2.). This technique showed that the heavy-mineral composition of the arkoses mostly is associated with zircon and apatite grains (Table 8.1.2d Picture 8.1.2e). Further significantly present are: tourmaline (Table 8.1.2d Picture 8.1.2e), rutile (Picture 8.1.2d Picture 8.1.2e), garnet (Table 8.1.2d Picture 8.1.2e), anatase, hematite, chlorite, chloritoid, mica and opaques. More uncommon appearance of heavy-mineral types is chrome-spinel and monazite (Table 8.1.2d Picture 8.1.2e). All these mentioned heavy-minerals are present at all studied areas and thus not locality dependent. However, other minerals do not show this ability of independence. Their appearance seems most likely to be controlled by the grade of diagenesis, since considerably more heavy-mineral types are only sited in the southern areas, like staurolite, epidote and titanite. Stanley (1975) has also described these differing heavy-mineral suites between northern- and southern areas. He reports presence of kyanites and staurolites, which only occur in southern localities, however he did not document any existence of epidote or titanite.

According to Stanley (1975), no garnets are sited north of the Argentera-Mercantour Masif (Figure 3.2: Locality map). This study does not share such an opinion. The heavy-mineral microscopy of the samples derived from the Lac du Lauzanier area, north of the Argentera-Mercantour Masif, emphasised garnets in all analysed samples. However, their population tends to decrease continuously towards the base of the Grès d'Annot succession. Commonly, analysed garnets in this area reveal strongly etched surfaces, which probably are resultants of severe etching processes.