Geology Reference
In-Depth Information
able in discriminating carbonate cements formed in dif-
ferent diagenetic environments.
Carbonate cements are also common in clastic se-
quences. They profoundly influence the quality of hy-
drocarbon reservoirs. For reviews and case studies on
carbonate cementation in sandstones see Morad (1998).
Precipitation of cements and dissolution of carbon-
ate sediments is primarily controlled by
•
the composition of pore-fluids,
•
the flow rate of water through pores, triggered by
energy levels. Water energy (supplying seawater
from which carbonate cements can be derived) and
the rate of sedimentation are considered as impor-
tant factors in shallow-marine environments, particu-
larly in reefs,
7.4.1 Controls on Carbonate Cementation
•
the primary porosity (size and geometry of pores)
and permeability,
Common cements are aragonite, High-Mg calcite, Low-
Mg calcite and dolomite. Less common are ankerite,
siderite, kaolinite, quartz, anhydrite, gypsum and ha-
lite. The growth of carbonate cements is favored by
high pH and higher temperatures, quartz or chert ce-
ments by low pH and low temperatures.
The cementation of carbonates at or near the sedi-
ment-water interface requires an enormous input of
CaCO
3
and an efficient fluid flow mechanism. The
source of CaCO
3
in the marine realm is seawater, in
meteoric and burial environments it is usually a solu-
tion of the sediment itself.
•
the number of ions (oversaturation resulting in pre-
cipitation, undersaturation resulting in dissolution)
and carbonate supply rates,
•
the salinity of solutions and the Mg/Ca ratio. If Mg/
Ca ratios are low, calcite can crystallize freely, form-
ing large equant crystals. If Mg/Ca ratios are high,
calcite and aragonite crystals are prevented from
growing sideways, so that fibrous, acicular, micritic
or peloidal cements are common. The mineralogy
and Mg content of the cements are predominantly
controlled by the Mg
2+
/Ca
2+
ratio of the solution
Text continued on p. 295
Plate 32 Modern Marine and Meteoric Cementation: Cement Fabrics and Cement Types
Cement fabrics indicate whether the pores were totally or only partially filled by fluids. Cement rims consisting
of symmetrical cements around grains (-> 1-4) originate in pores completely filled with marine or meteoric
water. In pores containing both water and air, water collects at grain contacts, precipitating asymmetrical menis-
cus-style cements (-> 5, 6).
1
Marine aragonite fibrous cement
and
microcrystalline Mg-calcite cement.
A uniformly broad rim consisting of fibrous
aragonite crystals (AC) growing normal to pyroclastic grains (PG) is covered by a thin rim of microcrystalline Mg-calcite
(MC). Aragonite cement was formed in the upper subtidal or lower intertidal zone, High-Mg calcite cement within the
intertidal zone. Partially dissolved aragonite crystals indicate the influence of fresh water. SEM.
2 Thin section of -> 1. Isopachous (= crust of equal thickness) fringe of cements. Note the difference in the thickness of
aragonite (AC) and Mg-calcite cement (MC) rims. The cement grew within voids of pyroclastic sands. Open parts of the
void filled with resin appear white.
3
Marine microcrystalline Mg-calcite cement (MC)
and
acicular aragonite cement (AC).
Due to the evaporation of vadose
water in tidal zones, broad Mg-calcite rims were formed around coralline algal grains. A change from vadose to marine-
phreatic conditions favored the precipitation of aragonite cement. Note the square-ended habit of the aragonite crystals. If
this shape is preserved in limestones after neomorphism to calcite, it provides evidence of a former aragonite mineralogy
of the cement (Assereto and Folk 1976; Mazzullo and Cys 1977). SEM.
4 Thin section of -> 3. Dark Mg-calcite cement rims (white arrows) and isopachous aragonite cement (black arrows)
around coralline red algal clasts characterized by their fine network structure (CA; Pl. 54/6. 9). Coralline red algae consist
of High-Mg calcite. The overgrowth by Mg-calcite cement points to substrate control of cement growth. Open interpar-
ticle pores appear black.
5
Meteoric-vadose meniscus cement
characterized by concave calcite cement bridges (arrows) connecting adjacent grains.
The cement was precipitated from water films at grain contacts distributed between the grains in a meniscus-like fashion.
The overall effect of meniscus cements is to round off the pore spaces. Meniscus cements originate mostly in vadose
environments but can also be formed microbially in other environments (Hillgaertner et al. 2001). Open interparticle
pores black. SEM.
6 Thin section of -> 5.
Meniscus cement
(arrows) between coralline algal bioclasts (CA) and volcanic lithoclasts (VL).
Open interparticle pores black.
All samples from modern beach sands in Fuerteventura, Canary Islands, Spain.
