Dolomite Perspectives on a Perplexing Mineral

Marine-Meteoric Mixing Model—The strati-
graphic position, related fossil assemblages and 
lack of associated evaporite indicate that some 
dolomites do not form within a restricted-marine, 
supratidal setting. Instead, they are found in 
areas where Mg-rich saline waters mix with fresh 
meteoric water. Modern and ancient dolomite 
formations around the world support variations 
on this theme.
One such variation is grounded in widespread 
dolomitic facies associated with shallow epicon-
tinental shelves or structural highs, where—
unlike the previous models—evidence of saline 
brine evaporation is not seen.
28
The origin of 
these dolomites is explained by the dorag model, 
in which dolomitization occurs in the brackish 
zone that forms when fresh groundwater is mixed 
with seawater.
29
In this zone, seawater supplies 
Mg
2+
ions, and dissolution of CaCO
3
occurs as the 
two waters mix. Calculations show that mixing 
meteoric groundwaters with 5% to 30% seawater 
can cause undersaturation with respect to cal-
cite, while dolomite becomes supersaturated. 
Within this range, calcite can be replaced by 
dolomite. In general, the dolomitization process 
can be expressed by the chemical equation:
2CaCO
3
+ Mg
2+
=> CaMg(CO
3
)
2
+ Ca
2+
.
This model is based on Mifflin carbonate
outcrops of the Platteville Formation, in the 
Middle Ordovician Champlain Series of Wisconsin, 
USA. Here, the carbonates appear homogeneous 
over a broad area, and the thickness of the unit 
and general structure suggest a broad, shallow 
open-marine environment. The open-marine
fossils, along with a lack of mud cracks, algal 
mats and evaporites, preclude supratidal deposi-
tion and dolomitization in a physically restricted 
lagoonal environment.
Dolomitization of the Mifflin Member was the 
result of a relatively early diagenetic process fol-
lowing subaerial exposure of uplifted limestone 
and subsequent establishment of freshwater 
lenses. Dolomitization occurred in the brackish 
zone where seawater and fresh waters mixed, 
with a dolostone-limestone boundary established 
along the lower margin of the groundwater lens.
30
In a somewhat different mode, dolomite may 
be created through the circulation of saline 
groundwaters deep within a carbonate platform. 
In southern Florida, USA, cold, dense seawater is 
drawn through the platform margin from the deep 
Straits of Florida. Geothermally driven circulation 
causes the Mg-rich seawater to rise into the inte-
rior of the Florida carbonate platform, where it 
mixes with fresh meteoric water before discharg-
ing through an extensive aquifer system.
The interplay of fresh and saline waters
with geothermal heat flow is known as Kohout 
convection.
31
In this scenario, the resulting pore 
waters become undersaturated with respect to 
calcite and aragonite but still saturated with 
respect to dolomite, which is precipitated in the 
permeable aquifers.
Another environment for mixing of fresh and 
saline waters is found along the coastal plains of 
southeastern Australia. From the present, and 
extending throughout the Quaternary Period, 
microcrystalline dolomite and other carbonate 
minerals have been forming in shallow ephemeral 
lakes of the Coorong region. These lakes develop 
along a 100-km [62-mi] belt, in an interdune cor-
ridor located immediately inland from the present 
coastline, behind a calcareous sand barrier. The 
lakes are considered to be outcrops of the water 
table, and free water, resulting from rainfall and 
regional or local aquifer recharge, is found at their 
surface only during winter and spring.
32
Modern dolomite is found only in lakes sub-
jected to an annual desiccation phase. Those lakes 
occur mainly in areas receiving less than 500 mm 
>
Sabkha reflux environment. This schematic of peritidal sediments on a Qatar peninsula sabkha 
shows another variation on the reflux theme. Seawater is pushed onshore during storm surges, 
becomes concentrated through evaporation, then seeps into the underlying sediment to reflux to its 
source. (Adapted from Warren, reference 2.)
MattV_ORAUT09_Fig_9
Subtidal
Water level
Intertidal
Supratidal
High water
Low water
Evaporation
Seepage
reflux
Storm flo
od
26678schD5R1.indd 7
11/5/09 3:53 PM

---

Autumn 2009
39
[19.7 in.] of rainfall per year, and typically fill to a 
water depth of 0.5 to 1 m [1.6 to 3.3 ft]. When 
filled, these lakes have a carbonate-mud bottom 
that contains algae and other organic matter. As 
lake levels fall, the waters become increasingly 
saline before eventually exposing the mud bottom 
to sunlight and consequent desiccation. Ensuing 
brines form during the drying phase and are 
refluxed out of the system into seaward-flowing 
groundwaters. Fine-grained dolomites and other 
carbonates remain behind, while saline and sul-
fate evaporite minerals are flushed out of the sys-
tem. This dolomite is thought to precipitate from a 
carbonate gel suspension, not through replace-
ment of a preexisting carbonate.
The dolomites in this system accumulate above 
shallow continental groundwaters that flow toward 
the sea. During their coastward migration, the 
groundwaters traverse large volumes of predomi-
nantly carbonate aquifer sediments. The source of 
the Mg is poorly understood but is believed either 
to be supplied by a local Quaternary volcanic prov-
ince or to be scavenged by groundwater flow from 
other sources.