Geology Reference
In-Depth Information
E
and
F:
Meteoric-marine mixing zone models
- Dorag
model (Dorag =mixed blood) involve mixing of me-
teoric and marine waters and postulate precipitation
from coastal unconfined or deep confined aquifers. The
meteoric-marine mixing zone model has been used to
explain subtidal dolomites that were formed near-sur-
face relatively early and prior to compaction, and that
are not associated with evaporites. Mixing-zone dolo-
mites should be located in the more landward parts of
carbonate platforms.
7.8.2.3 Subsurface Burial Dolomites
The principal prerequisites of the
burial dolomitiza-
tion model
(I) are sufficient sources of Mg
2+
, appropri-
ate driving (transport) mechanisms, and favorable con-
ditions for the precipitation of dolomite. The principal
process invoked in this model is the compactional de-
watering of basinal mudrocks and expulsion of Mg
2+
-
rich fluids from porewater during the transformation
of clay minerals (conversion of smectite to illite) with
increasing depth and rising temperature. Other sources
for deep burial dolomites are pressure solution and pos-
sible metamorphic and hydrothermal fluids. Hydrother-
mal dolomitization (Cervato 1990; Boni et al. 2000) is
the latest dolomite bandwagon.
Many reef and platform carbonates have been ex-
tensively replaced by burial dolomites (e.g. Devonian
of western Canada: Machel et al. 1994). Criteria of
burial dolomitization are coarse crystals, saddle dolo-
mite, the iron content of dolomite crystals, syngenetic
or younger formation of dolomite together with stylo-
lites, and specific isotope values. Often only the ma-
trix is dolomitized (matrix dolomite).
Saddle dolomite (Pl. 39/5) or baroque dolomite are
coarse, milky-white, or brown dolomite crystals, gen-
erally a millimeter or larger in size, with curved saddle-
like crystal faces due to rotating c-axes. The large crys-
tals consist of subcrystals giving the crystal a steep-
ened surface. Fluid inclusions are abundant. In thin sec-
tions saddle dolomite appears cloudy, exhibits undu-
lose extinction, and displays growth layers. It occurs
in moldic and vuggy pores, less commonly as a mas-
sive replacement of carbonates, and often in sulfate-
bearing carbonate host rocks associated with hydrocar-
bons and epigenetic sulfides (e.g. Mississippi Valley
Type ore deposits). Saddle dolomite is commonly in-
terpreted as having formed under deep burial or hydro-
thermal conditions from high-saline brines and under
high temperatures, or as a by-product of thermochemi-
cal sulfate reduction.
References: Badiozamani 1973; Burns and Rossinsky 1989;
Cander 1994; Choquette and Steinen 1980; Dunham and
Olson 1980; Gill et al. 1995; Humphrey 1988, 2000;
Humphrey and Quinn 1989; Land 1973; Meyers et al. 1997;
Narkiewicz 1979; Ruppel and Cander 1988; Ward and Halley
1985.
G:
Reports of dolomites forming from marine or only
slightly modified seawater in marine settings, includ-
ing platforms, reefs and pelagic (chalks) environments,
led to the development of
seawater dolomitization mod-
els
(G).
The models emphasize that seawater itself may be
able to dolomitize if there is an efficient mechanism
for pumping the water through carbonate sediments. A
mechanism for moving seawater through the sediment
is tidal pumping, resulting in the formation of tidal do-
lomites (Carballo et al. 1987).
Driving mechanisms for seawater circulation through
atolls (Minoura 1992), reefs, and platform margins can
be oceanic tides and oceanic currents (Saller 1984), the
downward reflux of higher saline waters over platforms
(Simms 1984) as well as thermal convection caused by
higher heat flow through a volcanic basement (Aharon
et al. 1987). Thermally driven convective flow pro-
cesses (endo-upwelling) may operate within the upper
part of volcanic foundations and overlying carbonates
(atolls, platforms) and can eventually be responsible
for carbonate dissolution and formation of massive re-
placement dolomites.
References: Aharon et al. 1987; Flood et al. 1996; Kastner
1984; Kaufman 1994; Land 1985; Machel and Burtin 1994;
Mattes and Mountjoy 1980; Mullins et al. 1985; Saller 1984;
Tucker and Wright 1990.
References
:
Barnaby and Read 1992; Dix 1993; Eren 1993;
Gawthorpe 1987; Gillhaus 2000; Lee and Friedman 1987;
Machel and Anderson 1989; Machel et al. 1994; Mattes and
Mountjoy 1980; McHargue and Price 1982; Sternbach and
Friedman 1984; Mountjoy 1991; Mountjoy and Halim-
Dihardja 1991; Mountjoy et al. 1999; Reinhold 1998; Rosen
and Holdren 1986; Sachan 1993; Schofield and Adams 1986;
Sternbach and Friedman 1984, 1986.
H:
Large-scale and prolonged circulation of seawater
into carbonate platform margins is inferred in the
Kohout convection model
(H). Kohout convection oc-
curs in response to a horizontal density gradient be-
tween cold marine waters adjacent to carbonate plat-
forms and geothermally heated groundwater within the
platforms.
7.8.3 Dedolomitization
Dedolomitization is the diagenetic replacement of
dolomite by calcite, particularly under the influence of
References: Kohout 1967; Mullins et al. 1985.
