Dolomite Perspectives on a Perplexing Mineral

Modes of Dolomite Formation
Many environments of dolomitization have been 
identified. Some result in unique reservoir geom-
etries that bear directly on exploration strategy. 
Rather than describe every type of dolomite 
formation, the following discussion focuses pri-
marily on modes that permit dolomites that are 
thick enough to be targeted for exploration. This 
also implies that the discussion mainly covers 
secondary, or replacive, dolomite. In some cases 
the distinction between modern and ancient con-
ditions must be drawn, because current settings 
do not necessarily reflect the conditions in which 
ancient dolomites were formed. Three well-
established hydrologic models and settings, along 
with some of their variants, are discussed first, 
and the section concludes with hydrothermal and 
bacterial cases. 
Brine-Reflux Model—Perhaps the most popu-
lar concept of dolomite formation is embodied in 
the brine-reflux model and similar variations. In 
this instance, seawater in a restricted lagoon evap-
orates to form a hypersaline brine that sinks to the 
lagoon floor and seeps through underlying lime 
sediments as it escapes, or refluxes back to the sea 
(above)
. As it filters through the pores of the 
underlying rock, magnesium from the brine 
replaces part of the calcium contained within the 
aragonite and calcite components of the lime-
stone, converting it to dolomite. 
This scenario was proposed in 1960 to explain 
extensive lagoonal and reefal dolomites associ-
ated with platform evaporites of the Permian 
basin in West Texas, USA.
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Reflux dolomitization 
has since been recognized in cores from other 
areas, where the intensity of dolomitization 
decreases with distance from the evaporite- 
carbonate contact. Today, hypersaline environ-
ments—where water salinity rises above that of 
normal seawater—are widespread in a belt 
between about 30° north and south latitude. In 
the Permian basin, lagoons developed behind 
barrier reefs on a broad shelf inundated by the 
shallow waters of Permian seas. The reefs 
impeded the surface exchange of water between 
lagoon and sea. Restricted circulation, combined 
with loss of water by evaporation, lowered water 
levels in the lagoon, raised the salinity of brines 
and promoted the precipitation of evaporites. As 
the specific gravity of the concentrated brine 
increased, it sank through the water column and 
migrated to the lowest depressions in the carbon-
ate floor of the lagoon.
Displacing the connate water in the underly-
ing rock, the dense hypersaline brine seeped 
downward along vertical migration pathways, fol-
lowing bedding planes only when vertical paths 
were exhausted. In rocks with varying permeabil-
ities, the seeping brines migrated mainly through 
porous zones while bypassing denser limestone 
lenses. Thus, coarse-grained and porous Permian 
dolomites are limited to beds previously com-
posed of coarse-grained and porous limestones. 
By contrast, fine-grained dense dolomites occupy 
open-shelf positions, where extremely fine-
grained, mud-based lithographic limestones 
would normally form. Dolomite textures were 
seen to be caused by primary permeability and 
crystallinity, rather than by dolomitization. 
In this model, the down-and-out migration of 
the hypersaline brine was responsible for dolomi-
tizing broad expanses of carbonate rock in the 
Permian basin. Within the carbonates, the brine-
reflux pathways shifted seaward as the shelves 
regressed. The lagoons, which sourced the brines, 
also followed progressively forestepping reef 
deposits. As established escape zones for the 
brine became sealed off by advancing evaporites, 
they would be replaced by similar outlets farther 
seaward. With each forestep, previously unin-
vaded reef limestones were exposed to the
dolomitizing brines. The pace of regression was 
geologic, and so slow that most of the limestones 
were dolomitized before the supply of brine was 
cut off.
Most modern dolomite is associated with 
hypersaline solutions.
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Modern brine-reflux con-
ditions have been documented on a smaller
scale in settings such as the San Andrés Islands
offshore Colombia, the Canary Islands, Spain
and the Caribbean island of Bonaire in 
Netherlands Antilles.
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>
Brine reflux in an evaporitic setting. A sill to seaward restricts circulation of waters. Some of the 
seawater evaporates, causing water density to increase. The dense brines sink below the sediment, 
reflux through the basin or lagoon floor and dolomitize any carbonate sediments that they pass 
through. (Adapted from Allan and Wiggins, reference 19.) 
MattV_ORAUT09_Fig_8
Evaporation
Increasing water density
Free flow
Open marine
Sill
Seepage reflu
x
18. Lucia, reference 12.
19. Allan JR and Wiggins WD: