Geology Reference
In-Depth Information
Box 7.2.
Factors and conditions controlling the burial diagenesis of carbonates.
Factors
depending on the composition of the sediment
1
Mineralogy:
The diagenetic potential of shallow-marine carbonates consisting of aragonite and Mg-Calcite is lower
than that of deeper water carbonates consisting of Low-Mg calcite. Pelagic mud of chalks consisting of Low-calcitic
coccoliths generally exhibits few pressure solution criteria and little interparticle porosity, and therefore good preser-
vation of primary porosity. Pressure solution is accelerated by Mg-poor fluids. Carbonates with relatively low amounts
of clay minerals are more susceptible for compaction and pressure solution than carbonates with higher amounts of
clays (10% and higher).
2
Grain size and texture:
Fine-grained poorly sorted sediments are more rapidly dissolved and compacted because of
their larger grain surfaces and higher amounts of pore water.
3
Porosity and permeability:
Control diagenetic reactions, dependent on pore-water amount and flux rates.
4
Presence of early cement or early dolomite
: May increase rigidity and reduce or hinder stylolitization.
Conditions
1
Pressure:
Lithostatic, hydrostatic and tectonic pressure. Overpressed (geopressed) sediments may be formed in areas
of rapid sedimentation, in sequences where porous carbonates are bordered by permeability barriers (e.g. hard-
grounds, lateral stylolites, argillaceous sediments, evaporites). Pore fluid pressure may weaken hydrostatic pressure.
2
Temperature
: The solubility of calcite appears to decrease with increasing temperature. The increase in the overbur-
den favors temperature- and pressure-controlled mineral reactions (gypsum -> anhydrite, opal C -> opal A -> quartz,
smectite -> mixed-layer minerals -> illite). Increasing temperature controls the transformation of stable organic
matter into unstable organic matter and hydrocarbons. Free CO
2
resulting from these processes and organic acids
contribute to the dissolution of carbonate minerals and the creation of porosity.
3
Pore water chemistry
: Influences on transport and the speed of pressure solution, which is more rapid in Mg-rich
pore waters of marine environments than in Mg-poor burial pore waters.
more rapid. Typical rocks originating in this setting are
beachrocks (see Sect. 2.4.2.1).
Burial environments are conventionally subdivided
into shallow burial and deep burial, but the boundary is
by no means well defined.
The
shallow burial zone
includes the first few meters
to tens of meters of burial. Shallow burial near-surface
diagenesis are influenced by changing pore water chem-
istry in the mixing zone, temperature and pressure pro-
cesses.
Diagenetic processes in the
deep burial zone
are con-
trolled by conditions summarized below and in Box 7.2.
Pore water composition in deep burial environments
differs substantially from that of shallow diagenetic en-
vironments, where cementation usually takes place in
dilute meteoric waters that are oxic or only slightly re-
ducing. Deep-burial subsurface pore waters have el-
evated salinities and act in a reducing manner. As a
consequence redox-sensitive elements are mobilized
and Mn and Fe tend to be incorporated in the calcite or
dolomite cements. The cements are characterized by
distinctive morphologies (Sect. 7.4.2). Changing Mn
and Fe compositions of subsurface fluids result in min-
eral zoning recorded by CL patterns, specific fluid in-
clusion salinities (Dorobek 1989), and stable isotope
signals (Scoffin 1987).
7.2.1.3 Marine Diagenesis
Marine phreatic diagenesis takes place at the shal-
low or deep sea floor or just below, and on tidal flats
and beaches. High hydrostatic pressure, low water tem-
perature, and high partial pressure of CO
2
in the deep
sea lead to dissolution and to significant differences in
the calcium-carbonate preservation of deep-sea sedi-
ments. These differences are used to distinguish two
dissolution levels, the lysocline and the calcite com-
pensation depth (see Sect. 2.4.5.6).
7.2.1.4 Burial Diagenesis
Carbonates that have undergone burial diagenesis
are out of sight and somehow out of mind (Scholle
and Halley 1985). Microfacies criteria indicating burial
diagenesis are specific cements (Sect. 7.4.2.1) and di-
agenetic textures pointing to compaction and pressure
solution (Sect. 7.5). The study of burial diagenesis is
highly significant in the reconstruction of rock history
and the evaluation of rock properties and porosity. Re-
views on burial diagenesis have been published by
Wanless (1983) and Hutcheon (1989).
Processes in deeper burial environments include:
•
Physical compaction due to sediment overburden:
Reduces thickness of sediments, porosity and perme-
ability, leads to breakage and distortion of grains
