Geology Reference
In-Depth Information
7.8.3.1 Textural Criteria for Recognizing
Dedolomitization
Box 7.13.
Selected case studies of dedolomitization.
Meteoric environment:
Al-Hashimi and Hemingway
1973; Back et al. 1983; Bischoff et al. 1993; Black et
al. 1983; Braun and Friedman 1970; Burger 1989;
Cussey et al. 1977; Deike 1990; Evamy 1967; Folk-
man 1969; Frank 1981, Friedman 1970; Fritz 1966;
Goldberg 1967; Jorgensen 1988; Kargel et al. 1996;
Lang 1964; Meder 1987; Qingan et al. 1979;
Shearman et al. 1961; Török 1997; Ulmer 1984;
Warrak 1974; Wirsing 1986.
Lacustrine environment:
Arenas et al. 1999; Canaveras
et al. 1996.
Shallow burial environment:
Al-Hashimi and Heming-
way 1973; Bausch et al. 1986; Black et al. 1983; Dun-
ham and Olson 1980; James et al. 1993; Kenny 1992;
Kyung Sij Woo and Moore 1996; Lucia 1961; Meder
1987; Purser 1985; Sellwood et al. 1984.
Deep burial environmemt:
Budai et al. 1984; Land and
Prezbindowski 1981; Mattes and Mountjoy 1980;
Stoessel et al. 1987.
Brown and reddish rock colors: Liberation of Fe
2+
from Fe-rich dolomite may form hematite occurring
as thin coatings on the crystals or ferric iron precipi-
tates. Many dedolomites are heavily stained with iron
oxides and hydroxides and exhibit brown or reddish
rock colors.
•
•
Weathering along crystal faces of a calcitized dolo-
mite may produce a loose sandy sediment.
•
Calcite pseudomorphs after sucrosic xenomorphic,
nonplanar dolomite.
•
Syntaxial calcite rims bordering rhombohedral crys-
tals.
•
Relicts of dissolved dolomites preserved inside the
pseudomorphs or at their boundaries. Dedolomiti-
zation may be centripetal or centrifugal (center to
periphery: Shearman et al. 1961).
•
Association of dedolomite with evaporite minerals:
Dedolomite is often associated with pseudomorphs
after evaporitic minerals, because the solution of
evaporite minerals favors calcitization.
meteoric water and porewaters of different composi-
tion, and often resulting in the formation of secondary
porosity. This process affects marine, lacustrine, and
terrestrial carbonates and occurs in meteoric and burial
diagenetic environments (Box 7.13).
Early diagenetic
dedolomitization
may result from (a) the instability of
Ca-dolomite within crystal cores facilitating the replace-
ment of the core by calcite, (b) near-surface recrystal-
lization under meteoric conditions (characterized by
rhombohedrons displaying microcrystalline calcite with
zonar hematite zones), and (c) meteoric dissolution of
dolomite rhombohedrons within the micrite and infill-
ing of the crystal molds by granular meteoric cements
(Pl. 40/ 4) and geopetal internal sediment.
Late diagenetic dedolomitization
controlled by sa-
linity variations of burial pore waters can result from
(a) the diagenetic instability of Ca-dolomite (similar to
the above mentioned early diagenetic dedolomite), and
(b) corrosion of zonar rhombohedron along cleavage
planes, formation of intracrystalline pores and later
growth of syntaxial calcite within these pores (Pl. 40/5).
The alteration of dolomite to calcite was first reported
by the Swiss geologist A. von Morlot (1847) based on
solution experiments and field studies in Styria. He
coined the term dedolomite and suggested that ground-
water leaching of preexisting evaporite deposits con-
taining gypsum or anhydrite would be enriched with
respect to calcium and sulfate, thus thereby resulting
in the calcitization of former dolomite. The term
dedolo-
mitization
is now used along with
calcitization
to de-
scribe the replacement of dolomite by calcite. Staining
with Alizarine Red S shows the distribution of calcite
and dolomite within the dolomite rhombs very clearly.
7.8.3.2 Origin of Dedolomite
Two general mechanisms have been proposed - a
reaction of dolomite with calcium sulfate solutions
(Morlot 1847; Shearman et al. 1961; Evamy 1963;
Goldberg 1967) or the alteration of ferroan dolomite
by oxygenated meteoric water. The sulfate ions needed
for the reaction are believed to be derived from the oxi-
dation of pyrite or from gypsiferous solutions. Near-
surface dedolomitization is often related to dissolution
of gypsum and dolomite in vadose or phreatic mete-
oric waters.
Meteoric waters falling on outcropping gypsum gen-
erate a solution with a very high Ca/Mg ratio. In some
cases, dedolomite may be indicative of subaerial expo-
sure. Gypsum dissolution drives the precipitation of
calcite, thus consuming carbonate ions released from
dolomite. This may lead to karstification.
Prerequisites for the formation of near-surface dedo-
lomite is a solution high in dissolved calcium and low
in magnesium, low CO
2
partial pressure, and tempera-
tures < 50 °C. Dedolomitization by ground water in car-
bonate aquifers may occur on a regional scale (e.g. Back
et al. 1986; Deike 1990). CO
2
-bearing waters contrib-
ute to the dissolution of dolomite, which sometimes
leads to the formation of dolomite sands under recent
subaerial conditions (Fritz 1966). Continuous replace-
ment of dolomite by calcite via solution/precipitation
