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

tel-00534181, version 1 - 9 Nov 2010
Chapter I
4.4.2. Cements
The most important cements in the LSF are calcite, kaolin and hematite. Calcite cements are more
abundant at the base and towards the top of the formation, hematite cements are most abundant in the numerous
Fe-crusted zones which are concentrated within the middle part of the formation (Fig. 10a), while the kaolin
is most abundant in the upper parts of the formation. On the basis of the predominant cement type, the
Lapur Sandstone Formation has been subdivided into three diagenetic zones as presented above (Fig. 10).
4.4.2.1. Zone I. Calcite cement. Calcite is the most abundant cement in Zone I and it occurs most
abundantly in the lowermost 85 m of the formation (Fig. 10a). In this zone, the calcite occurs in abundances
ranging from 10 % up to 40 % (by visual estimation). The SEM-EDS gives the typical composition of a very
pure calcite grain with only trace amounts of iron and magnesium in some of the samples. Calcite occurs
mainly in poikilotopic textures where one large optically continuous crystal of calcite cements together several
individual detrital grains. The heterogeneous colours of theses crystals as observed under CL (sample LPR
25/04) (Fig. 22e) suggest that the calcite is actually cement precipitated into empty pore space and that the poikilotopic
textures results from recrystallization. The cement is made up of two luminescent zones: an outer
brightly luminescent zone that follows the outline of the pore space and measures 20-50 µm on average, and
an inner zone of dull orange luminescence formed by the pore-filling cement measuring up to 200 µm.
The cement is apparently pore-filling as there is no evidence of dissolution of previous minerals as
these would be expected to leave relics which are not observed in the samples. It also occurs in places as a partial
replacement of feldspar, this is observed in CL where the feldspar or more precisely, its remains, luminesces
deep blue while the calcite cement luminesces orange. The feldspar is microcline, based on SEM petrography.
The calcite cement is usually so pervasive that the primary intergranular porosity is often completely
filled. Presence of a rim of hematite cement around detrical grains followed by calcite cement suggests that
hematite is earlier than calcite and the presence of planar grain to grain contacts indicates that the sediment
had undergone some mechanical compaction prior to cementation. The high amount of calcite cement filling
primary porosity suggests a rather early diagenetic episode, only preceded by some early mechanical compaction
and the precipitation of a rim of hematite cement. Another phase of calcite which is non luminescent (i.e.
very dark) made of a succession of irregular rims of small crystals of calcite which grew around some detrital
grains is observed in a few samples. This cement is also interpreted as having been precipitated early after sediment
deposition.
A later phase of calcite cementation within this zone took place after the fracturing of quartz grains
as indicated by the presence of calcite within the fractured grains (sample LPR 19/04) (Fig. 22a). This phase
of calcite cementation is less significant volumetrically compared to the earlier poikilotopic cement. Other
cements in Zone I include early-generation hematite and kaolin resulting from the dissolution of feldspars as
well as from the kaolinization of detrital clays which are most developed in the other zones (see below).
Nevertheless, Zone I is also characterized by rare occurrences of silica-rich clay cements.
Silica-rich clay cement. This has been identified in a few of the samples studied under SEM by its
high Si/Al ratio (?2) on the EDS. It occurs as a more homogeneous clayey material adjacent to more crystalline
vermicular kaolin (samples LPR 19/04, LPR 35/04 and LOK 13/04; Fig. 10). Some EDS analyses are
close the theoretical structural formula of pyrophyllite Al2Si4O10(OH)2 (sample LPR 35/04; Table 9) with
minor amounts of potassium and iron. Nevertheless other analyses show higher Si/Al ratio (?4) and various
low contents of Fe, K, Mg, Na or Ti. It is characterised by lack of observable structure as opposed to the ver-
63