Geology Reference
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seal rocks and assist in the understanding of climati-
cally controlled cyclic sedimentation patterns. Abun-
dant evaporite minerals are chlorides (halite) and sul-
fates (gypsum and anhydrite). Gypsum only occurs near
the earth's surface, whereas anhydrite is formed at the
surface and in the subsurface where it replaces gyp-
sum. Gypsum may replace anhydrite on the uplift of
evaporite sequences or after removal of overburden
sediments.
Fig. 13.3. Framboidal pyrite consisting of tiny euhedral crys-
tals, formed within residual clays of stylolites. The clay lay-
ers are distinctly displaced by the growing pyrite. Compare
Pl. 109/1. SEM photograph. Cretaceous: Subsurface, Ras al
Khaimah, United Arabian Emirates.
Depositional settings of evaporites
Evaporites originate in a number of environments:
Continental (subaerial and subaqueous) settings are
characterized by rapid variations in basinal flooding
and mineralogically and geochemically enormously
varied deposits (e.g. playa deposits) originating from
the waters of inland desert basins.
Supratidal (marine sabkha). These evaporite depos-
its form by evaporation in marginal-marine arid envi-
ronments (e.g. Persian Gulf and Red Sea). They are
characterized by either/or both siliciclastic and marine
carbonate matrices and affected by meteoric and sub-
aerial conditions. Distinct facies belts from seaward to
landward or from the base of the sedimentary sequence
upward are common (Alsharhan and Kendall 1994). A
normal sabkha deposit consists of marine microbially
controlled laminated carbonate or siliciclastic muds,
nodular sulfates, desiccation crusts and windblown
sands. The sulfates are usually gypsum or anhydrite
and occur in microbial beds as well as in beds charac-
terized by nodular or enterolithic masses. The fine-
grained carbonates are commonly dolomitized. Well
preserved gypsum and anhydrite can be distinguished
in thin sections by their crystal morphology (Schreiber
et al. 1982).
Intertidal to shallow subaqueous settings (hypersa-
line sea). Evaporites occur as isolated crystals within
microbial mats, or build gypsum beds. The morphol-
ogy of the gypsum crystals and the mats is commonly
well preserved as described from ancient examples.
Deep subaqueous settings below the wave base and
photic zone. These evaporites are characterized by a
long-distance continuity of laminar beds consisting of
gypsum, anhydrite or halite, sometimes by co-occur-
rence with turbidites.
Marine evaporites originate from ocean waters in
barred basins, shallow epeiric seas, or in deep-marine
settings. For case studies of ancient evaporites see
Handford et al. (1982) and Rouchy et al. (2001).
because of the rarely dissolved sulfate, which is a hun-
dred times lower than in sea water.
Microscopic spheroidal clusters of interlocking crys-
tals, termed framboidal pyrite because of the resem-
blance to raspberries, are known from shelf carbonates
and deep-marine limestones (Fabricius 1961; Honjo et
al. 1965). They have been explained as petrified sulfur
bacteria; these are crystal aggregates whose formation
was triggered by bacterially controlled processes, or as
inorganic crystallization out of iron sulfide gels.
Framboidal pyrite occurs scattered or as rounded clus-
ters within fine-grained micritic limestones (Pl. 109/1)
and marls and residual clays (Fig. 13.3) as well as in
the form of pyrite fillings within fossils (e.g. radiolar-
ians) and synsedimentary cavities. Early diagenetic geo-
petal fillings are potential tools for recognizing top and
bottom structures and tectonic overturning.
Reworked pyrite (nodules, tubes and fossil stein-
kerns) is believed to occur widely through the Phan-
erozoic, where currents eroded muddy shelves or slopes
in anaerobic to minimally dysaerobic settings and ex-
humed pyrite grains from the underlying beds (Baird
and Brett 1986).
13.1.2.3 Sulfates: Evaporite Minerals
Authigenic evaporite minerals are common in lime-
stones and dolomites. Evaporites are often interbedded
with dolomite, limestone and fine-grained siliciclastic
shales, e.g. thin red beds. Gypsum crystals can be iden-
tified by characteristic lath-like crystal shapes, weak
birefringence and low reliefs. Anhydrite is distinguished
from gypsum by a higher relief and stronger birefrin-
gence. For thin sections of evaporites see Garrison et
al. (1978), Scholle (1979) and Adams et al. (1984).
Evaporites are sedimentary deposits composed of
minerals precipitated from brines concentrated by ex-
tensive or total evaporation of water. Evaporites form
Criteria
Primary sedimentary structures of evaporites include
lamination, cross bedding, graded bedding, ripple marks
and mud cracks. A laminated anhydrite or gypsum se-
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