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

tel-00534181, version 1 - 9 Nov 2010
�
Chapter I
Calcite cement in this zone is therefore considered early cement and the product of shallow burial diagenesis.
Poikilotopic calcite (Fig. 22a) is a common early diagenetic primary cement in sandstones calcretes,
especially in fluvial channels (Morad, 1998). In continental sediments with low organic matter, dissolved carbon
is derived from the decay of plant remains within the soil horizons and from atmospheric CO2. Calcretes
occur as concretions and laterally extensive cements in flood-plains and also develop in fluvial channel sandstones
and they dominate in the coarse grained proximal facies. Sources of calcium for calcretes may include
windblown dust, pyroclastic material, or groundwater may bring carbonate ions from carbonate terrains to siliclastic
sequences. Additional sources include Ca dissolved in rainwater and dissolution of Ca-plagioclase
(Morad, 1998). In the LSF case, the source of carbonate ions may be from groundwater enriched with Ca2+
ions picked up by fluid circulating through the Cretaceous sediments deposited during the marine transgression
whose limit is to the east of Lake Turkana (Bosworth and Morley, 1994; Morley et al., 1999).
Nevertheless a marine source is may be more realistic to explain the large amounts of carbonate cement observed
(approximately 5.5 cubic km based on average of 20 % calcite content in a LSF outcrop measuring 4.4
km wide, 63 km long and 0.1 km thick calcite cemented zone). Other sources may include carbonate-rich lavas
overlying the LSF, which can be used to account for the calcite-cemented zone at the top of the formation
where the cement occurs as fracture filling calcite. Indeed numerous calcite geodes are present in the overlying
Turkana volcanics (Fig. 13h), attesting to the fact that the lavas are calcium-rich and that calcite is mobile.
Geochemical analysis of samples of lavas indicate that the calcium values, measured as % CaO, range between
9.1 and 11.1 (Table 6).
The numerous dykes that cut across the LSF could be a potential calcium source for calcite cementation
(See § 4.1.). Their alteration by late magmatic fluids or most probably by diagenetic fluid circulations
activated by the thermal effect of this magmatic phase could mobilize calcium by intensive alteration of magmatic
minerals (Daoudi and Potdevin, 2002). Field evidence has noticed the presence of well indurated, tightly
cemented sandstone units in the proximity of dyke and sill intrusions in the lower parts of the LSF. The overlying
Turkana Volcanics are likely sources of some of the calcium that forms calcite cement as evidenced by
presence of horizons with abundant calcite geodes within the lower parts of the volcanics (Fig. 13h).
Late generations of replacive calcite occur in some cases especially in the middle and top section of
the formation where the calcite replaces all types of older grains (including quartz and feldspars) and cements
such as hematite and kaolin. This late generation cement was most likely precipitated as a result of chemical
stability differences between mineral phases with respect to the prevailing circulating pore waters.
4.4.4.2. Hematite cementation. Hematite cementation is associated with ferricrust development and
is most pronounced in the middle section of the LSF. When deposition areas, especially alluvial flood-plains,
are starved of sediment supply for extended periods, in situ alteration of sediment may take place. This alteration
commonly takes the form of pedogenesis and the development of red hematite pigments through the
weathering of ferromagnesian minerals and the infiltration of weathering products into the sediment
(Collinson, 1996). Hematite is the main cement in the middle section of the LSF. At least 2 generations of dark
coloured oxide cements were observed, where the early first generation of hematite cement is succeeded by
precipitation of kaolin and later by the dissolution of K-feldspar. The dissolution of feldspar is in turn followed
by the precipitation of a second generation of a manganiferous variety and finally in some cases, the depo-
Figure 24. Lithologic log of the LSF with a porosity plot, showing measured porosities ranging from 3 to 25 %. The trend
of the porosities follows what is expected in view of the cementation profile of the LSF. Low porosities are generally associated
with cementation by calcite while kaolin cemented zones retain higher porosity values.
69