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

tel-00534181, version 1 - 9 Nov 2010
Chapter II
Calcite cementation
The precipitation of calcite cements occurred in at least two phases, one a relatively early event while
the second one is late-generation, taking place after fracturing of quartz grains and partially healing the fractures.
The reaction for the precipitation of calcite is:
Ca 2+ + HCO3- = CaCO3 + H +
Poikilotopic calcite is common in early diagenetic primary cements in sandstones. 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 floodplains
and also develop in fluvial channel sandstones and they dominate in the coarse grained proximal facies.
Sources of Ca 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 case of the MSF, sources for the Ca may include
younger carbonate-rich Oligocene Miocene lavas, though there was no direct contact between the two was
observed in the field. Late generations of replacive calcite occur where the calcite replaces all types of older
grains (including quartz and feldspars) and cements such as hematite and kaolin.
The proposed diagenetic evolution of the MSF can be summarized Figure 31 below. There are 5 main diagenetic
events identified in the MSF. There are:
1. Precipitation of first generation of calcite cements.
2. Dissolution of feldspars.
3. Calcitization of feldspars.
4. Precipitation of kaolin cements.
5. Precipitation of hematite cements.
Minor events include compaction and precipitation of quartz overgrowth cements, corrosion of quartz
grains, precipitation of pyrophyllite cements and fracturing of quartz grains associated with tectonic events.
Kaolinization of detrital grains
Several possible sources of kaolin suggested for the MSF include the alteration products of K-feldspars,
the alteration of muscovite and the kaolinization of detrital clays. Alteration of feldspars is inevitable since their
detrital grains are commonly in disequilibrium with diagenetic pressure, temperature and pore fluid chemistry and
are therefore prone to diagenetic processes that help them reach equilibrium. The main processes of feldspar alteration
are dissolution, replacement by clay minerals (e.g. kaolinization) and albitization (Worden and Morad,
2000). In the MSF, there was no evidence of albitization. Kaolinization reaction is in the form:
2KAlSi308 + 2H + + H2O = Al2Si2O5(OH)4 + 4SiO2 + 2K +
K-feldspar kaolin
Due to the high Si/Al ratios of feldspars compared to the clay minerals that replace them, reactions
that involve production of clays at the expense of feldspars lead to the release of Si and thus creates potential
for quartz cementation. This reaction in many instances is accompanied by co-precipitation of carbonate
cements such as calcite (Worden and Morad, 2000).
92