Etude stratigraphique, pétrographique et diagénétique des grès d ...

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tel-00534181, version 1 - 9 Nov 2010 Chapter I Sample LPR126/04 LPR136/04 LPR68/04 LPR74/04 LPR35/04 LOK05/04 Na2O 0,41 0,65 11,23 0,01 MgO 0,36 Al2O3 45,93 40,51 18,17 18,18 19,43 25,6 SiO2 54,07 46,99 63,54 63,64 69,43 65,91 K2O 12,09 18,29 17,52 0,53 2,62 FeO 1,7 Total 100,00 100,00 100,00 99,99 100,62 96,20 Na 0,05 0,06 0,94 0,00 Mg 0,03 Al 2,00 3,02 1,00 1,00 0,99 1,84 Si 2,00 2,98 2,97 2,97 3,01 4,01 K 0,98 1,09 1,04 0,03 0,20 Fe 0,09 Oxygen 7 11 8 8 8 11 Total Cations 4,00 7,03 5,07 5,08 4,98 6,17 XNa 0 0 0 0 0 0 XMg 0 0 0 0 0 0 XAl 1 1 1 1 1 1 XSi 2 2 2 2 2 2 XK 0 0 0 0 0 0 XFe2+ 0 0 0 0 0 0 S Oxygen 3 3 3 3 3 3 Fact K 2 4 3 3 3 4 Table 9. SEM-EDX chemical analysis and the computed structural formulae on selected detrital minerals and cements of the LSF. The elemental ratios used for the calculation of structural formulae are based on a total of 7, 11, 8, and 8 oxygen atom(s) per analysis formula unit of kaolin, muscovite, microcline and albite respectively. Figure 22. Petrographic micrographs of the studied sandstones (LP = polarised light; LPA = cross polarized light). (a) Detrital grains in the LSF include polycrystalline quartz, monocrystalline quartz and feldspars. Well preserved feldspar grains (such as the microcline showing characteristic crosshatched twinning), are not common in the LSF. The cement in this sample is poikilotopic calcite (sample LPR 19/04, LPA). (b) Most K-feldspars have undergone various stages of dissolution, resulting in the creation of secondary intracrystalline porosity, here visible in blue (sample LOK 05/04, LPNA). (c) Some microcline grains have undergone a certain amount of albitization. This is however only evident under SEM-EDX element mapping (sample LPR 74/04, SEM-EDX mapping, MAG: magnification, HV: high voltage, WD: working distance). (d) Metamorphic rock fragments (MRF) contribute a significant amount of framework grains in the LSF (sample LOK 05/04, LPA). (e) Cathodoluminescence (CL) shows the presence of at least two zones of luminescence in calcite cements of the LSF; a thin bright orange outer zone and a thicker, dull orange zone towards the pore centre. Quartz grains (Qtz) are very dark (very low luminescence) while feldspar (Kfs) luminesces deep blue. CL photograph. Sample LPR 25/04. (f) Late fracture-filling calcite (Cal) cement occurring towards the top of the LSF. Note the fracture cuts across kaolin (Kln) cements as well as hematite (Hem) cements, indicating it is a late cement (sample LPR 132/04, LPNA photograph). (g) Relationships between hematite and kaolin precipitation and K-feldspar dissolution. A first type of hematite cement (Hem- 1) is a rather continuous rim of euhedral grains which grew around the detrital minerals toward the center of the primary pores. The detrital grains of feldspar show an extensive dissolution resulting in secondary intragranular porosity which is posterior to Hem precipitation. Vermicular kaolin cement filled the remaining primary porosity after Hem precipitation and could be contemporaneous to the K-feldspar dissolution. A later type of opaque oxide cement (MnOx) filled partially the secondary porosity in the feldspar grain. This later cement is manganiferous (LPA, Sample LPR 94/04). (h) Vermicular pore-filling kaolin and the abundant microporosity associated with it. Polarised non-analysed light LPNA. Sample LPR 119/04. LPA = Polarised analysed light; LPNA = Polarised non-analysed light; Mc = microcline, Ms = muscovite, Kln = kaolin, Hem = hematite, Ab = albite, Qtzp = polycrystalline quartz, Qtzm = monocrystalline quartz, Cal = calcite, Ês = secondary porosity (mineral abbreviations adapted from Kretz, 1983). 60 �
tel-00534181, version 1 - 9 Nov 2010 Cal Chapter I (a) (b) 200 μm (c) (d) Cal Mc Qtz m Qtz Ab Qtz Qtz p Ab Qtz p Mc Qtz (e) (f) MnOX Qtz Hem 1 Mc 200 μm 600 μm 200 μm Kfs Ab 200 μm 200 μm (g) 200 μm (h) 50 μm 61 Kfs Mrf Kfs Qtz Qtz Kln
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