Geology Reference
In-Depth Information
dolomite cements have been reported from various di-
agenetic environments. Early diagenetic dolomite ce-
ments forming bladed rims around grains occur in in-
ter- and supratidal (e.g. Müller and Tietz 1966; Land
1973; Mazzullo et al. 1987; Lasemi et al. 1989) as well
as shallow subtidal settings, including reefs (e.g.
Mitchell et al. 1987). Dolomite cements from burial
settings were described by Dorobek et al. (1993) and
Reinhold (1998).
potential for dolomitization. Burial compactiondoes not
seem to be an efficient mechanism for regional dolo-
mitization of platforms.
7.8.2.1 Dolomites Associated with Evaporites
Various dolomitization models can be seen in Fig. 7.23:
A:
The
evaporative model
is based on the comparison
with modern evaporative and sabkha dolomite and ex-
plains ancient dolomites as supratidal in origin. Ho-
locene dolomite is well documented from tidal settings
in the Persian Gulf, the Bahamas and Florida: Supra-
tidal dolomites are typically microcrystalline (1-5
m)
and non-stoichiometric, with weakly ordered crystal
structure. These dolomites are found within muddy car-
bonate sediments or as surface crusts on supratidal flats.
Dolomitization is explained as being formed by hy-
persaline brines derived from intense evaporation in
sabkhas. Dolomite replaces preexisting metastable car-
bonate sediment. Dolomitizing solution is a brine with
a high Mg/Ca ratio resulting from Ca removal through
precipitation of aragonite, gypsum or anhydrite. Re-
evaluation of mass balance calculations sheds some
doubt on the replacement origin of all sabkha dolomites
and discusses the possibility of direct precipitation
(Hardie 1987; Lasemi et al. 1989).
7.8.1.3 Value of Dolomite Textures
Textures seen in thin section of dolomites offer some
hints to the origin of dolomites, but the assignment to
the main models depends on geological and geochemi-
cal data. However, many ancient dolomites have un-
dergone multistep episodes of textural and geochemi-
cal changes via dissolution-reprecipitation and mineral
stabilization (e.g. Ghazban et al. 1992; Nielsen et al.
1994; Reinhold 1998) and many dolomites have been
subject to repeated recrystallization. Petrographic evi-
dence of recrystallized dolomite includes an increase
in crystal size and increase in the number of non-pla-
nar over planar crystal interfaces. Further evidence is
given by CL patterns and SEM observations. Geochemi-
cal characteristics include dolomite stoichiometry and
ordering, depletion of stable isotopes and of Sr and Na,
and enrichment in Fe and Mn.
References: Ambers and Petzold 1996; Butler 1969; Carballo
et al. 1987; Chavetz and Rush 1994; Friedman 1980;
Gebelein et al. 1980; Gunatilaka 1991; Illing et al. 1965;
Lasemi et al. 1989; Mazzullo et al. 1987; McKenzie 1981;
Patterson and Kinsman 1982; Shinn et al. 1965.
7.8.2 Some Dolomitization Models
B:
The
seepage-reflux model
is a popular model for
dolomites associated with evaporites. It was developed
from studies of a supratidal gypsum-precipitating lake
on Bonaire (Netherlands Antilles), and favors a high
Mg/Ca ratio and Mg
2+
-rich hypersaline fluids, perme-
ating underlying carbonate sediments. The model in-
volves the generation of dolomitizing fluids through
evaporation of water or tidal flat pore water. The seep-
age-reflux model has been applied to shelf and lagoonal
carbonates of the Permian Reef Complex of western
Texas, Late Permian Zechstein dolomites of England,
and the Early Cretaceous Edwards Formation in Texas.
Dolomites form under different conditions and in depo-
sitional environments. The simple distinction between
primary dolomites believed to be directly precipitated
and secondary dolomites interpreted as products of
the replacement processes has become somewhat ob-
solete. Primary dolomites seem to be very rare and re-
stricted to saline lakes and lagoons (see Fig. 7.23F).
Most of the dolomite in the geological record appears
to be of replacement origin.
Many models have been developed in order to ex-
plain the origin of ancient dolomite rocks. Fig. 7.23
illustrates some of the currently discussed models. Nu-
merical simulation indicates that some of these models
may be more and others less reliable with regard to
fluid flow patterns in carbonate platforms (Kaufman
1994): The seepage-reflux (Fig. 7.23B) model appears
to be a viable dolomitization mechanism, but requires
millions of years. Thermal convection of seawater
through carbonate platforms (Fig. 7.23G) offers a high
References: Adams and Rhodes 1960; Clark 1980; Deffeyes
et al. 1965; Fisher and Rodda 1960; Kendall 1988; Lu and
Meyers 1998; Smith 1981.
C:
The
evaporation-drawdown model
tries to explain
the dolomitized intertidal and subtidal facies, often
formed beneath evaporites. Dolomitization is regarded
as a response to sea-level changes (Magaritz and Peryt
