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

tel-00534181, version 1 - 9 Nov 2010
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Chapter II
5.2. Cements
The MSF is not generally a well-cemented formation. Total cement ranges in abundance between
trace amounts to approximately 30 %, on average it is about 10 %. The three main cements in the MSF are
kaolin, calcite and hematite. Kaolin occurs with abundances ranging between 5-10 %, calcite ranges between
5-10 % and hematite 5-10 %. Other cements which occur in unquantified amounts include pyrophyllite and
quartz overgrowths. The top unit contains beds of quartzitic sandstones which have undergone post-depositional
silicification. Faulting is extensive in the MSF and slickensides are common along fault planes.
Calcite
This cement is present in significant abundance at the base of the formation and towards the top. It
occurs as pore-filling cement associated with brown clays in sample MUR 01/04 and as in-fill between
concentric rings in the hematitic oolites (sample 50/04; Fig. 30a, b). In other samples, (MUR 71D/04; Fig. 30c)
it occurs as a product of the replacement feldspars. The calcite is relatively pure and it has a typical chemical
composition of Ca1.00CO3 (sample MUR 50A/04).
Kaolin
Kaolin occurs as vermicular pore-filling cement, well distributed throughout the vertical profile of the
formation. Its abundance ranges from 5 to 10 %. Samples MUR 56/04 (Fig. 30d) and MUR 64B/04 contains
a variety of kaolin containing stacked thin fibrous crystals forming coarse vermicular structures measuring
about 60µm in width. The typical structural formula of the kaolin computed from SEM-EDX analysis is
Al2.01Si2.00O5(OH)4 (sample MUR 50/04).
Hematite
This cement is well distributed throughout the formation but it always occurs in small quantities
except in the samples containing hematitic oolites (MUR 50/04; Fig. 30a, b; MUR 56/04, MUR 61/04 and
MUR 64B/04). Abundance in the samples studied ranges from 5 to 20 %. In certain instances it becomes difficult
to distinguish which hematite is cement and which is detrital in origin. Hematite occurs as pore-lining
cement (e.g. sample MUR 07/04 and MUR 20/04), pore-filling cement (sample MUR 26/04; Fig. 30e) and disseminated
tiny grains in fine-grained matrix (MUR 20/04 and MUR 21/04).
Figure 29. (a) Compaction of grains and fault-related deformation is a common occurrence in the Muruanachok Sandstone
Formation (MSF); resulting in pressure solution contacts (Psc) and extensive grains shattering (sample MUR 71A/04; LPA
photo). The fractures have been partially filled by calcite cement (Cal). (b) Severe compaction in certain cases has resulted in
grain interpenetration in some samples (MUR 64B/04; LPA photo). (c) Corrosion of quartz grains has resulted in the formation
of secondary intragranular porosity (sample MUR 56/04; LPNA photo). (d) Alkali feldspars (Kfs) have undergone extensive
dissolution resulting in secondary intragranular porosity (Êsi) (sample MUR 71F/04; LPNA photo). (e) Calcite (Cal) is
an important cement in the MSF; the calcite is later generation than hematite (Hem) note coarse feldspar grain (Kfs) and smaller
quartz grains (Qtz) in bimodal size distribution (sample MUR 01/04, LPA photo). (f) Some units especially in the lower parts
of the MSF have samples containing abundant detrital clays (Sample MUR 20/04; LPNA photo). Note partially dissolved feldspar
(Kfs) and secondary dissolution porosity Êsd. (g) Mica grains are normally good indicators of the intensity of compaction,
here a grain of muscovite (MS) is squashed between two grains of polycrystalline quartz (Qtz), note incipient kaolinization
(Kln) at edges of muscovite grain (sample MUR 35/04, LPA photo). (h) Pressure solution contacts are well developed between
muscovite and quartz grains (sample MUR 35/04; LPA photo). LPA= Polarized analysed light; LPNA= Polarized non-analysed
light.
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