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Plate 39 Dolomite Fabrics, Dolomite Types (Saddle Dolomite) and Dedolomitization
The basic terminology of dolomite fabrics used in thin sections considers crystal shape, the mutual relationships
of crystals and uniform or non-uniform size distribution (-> 1-3). Saddle dolomite (-> 5) or baroque dolomite
is a specific dolomite type, commonly interpreted as having formed under deep burial or hydrothermal condi-
tions. Dedolomitization refers to the early or late diagenetic replacement of dolomite by calcite (-> 4, 6-8).
Criteria of dolomite fabrics
The fabric of dolomite rocks varies between two end-members: Limestones with scattered dolomite rhombs
(-> 2) and sucrosic mosaic dolomites (-> 3).
1
Criteria : Inequigranular; non-rhombic crystals; tightly packed anhedral crystals; xenotopic fabric; intercrystalline bound-
aries usually curved, lobate, irregular; preserved crystal junctions rare; grain size between 0.4 and 1.5 mm. Classification:
Friedman - Decimicron- to millimeter-sized inequigranular, xenotopic dolomite: Gregg and Sibley - Polymodal non-
planar matrix replacement dolomite; Randazzo and Zachos - Inaequigranular mosaic dolomite. Replacement dolomite.
Crystal size scale (Folk) - Coarsely to very coarsely crystalline. Black spots are artifacts. Interpretation: Late burial
replacement dolomitization is indicated by the amoeboid fabric and the serrate crystal boundaries.
2
The sample is characterized by clear euhedral and subhedral dolomite crystals (0.2-0.4 mm) exhibiting cloudy centers
and clear rims. Cloudy centers can be caused by calcite relicts, non-carbonate or fluid inclusions. The dolomite crystals
exhibit straight compromise boundaries. Descriptive criteria : Equigranular; rhombic crystals; crystal-supported; idiotopic
fabric. Intercrystalline areas are still microcrystalline calcite. Fenestral voids (FV) are filled with recrystallized fibrous
calcite cement. Classification : Friedman - Decimicron-sized idiotopic dolomite. Gregg and Sibley - Unimodal planar
partial matrix replacement dolomite. Randazzo and Zachos - Equigranular sieve mosaic dolomite. Size scale - Medium
to coarsely crystalline. Interpretation : Dolomite crystals postdate shallow-marine calcite cements of fenestral tidal lime-
stones. Missing evidence of compaction prior to dolomite crystal growth and the restriction of the crystals to fenestral
boundaries indicate replacement dolomitization of tidal sediment in a shallow burial setting.
3
Criteria: Inequigranular; non-rhombic crystals; tightly packed anhedral and subhedral crystals; hypidiotopic fabric; inter-
crystalline boundaries lobate and straight; some crystal-face junctions preserved; grain size between 0.1 and 0.4 mm.
Classification: Friedman - Centimicron-sized inequigranular, hypidiotopic dolomite; Gregg and Sibley - Polymodal
nonplanar matrix replacement dolomite; Randazzo and Zachos - Inequigranular mosaic dolomite. Size scale - medium to
coarsely crystalline. These fine- to medium-grained mosaic dolomites formed by replacement of fine-grained calcite
matrix. The hypidiotopic fabric indicates recrystallization of burial dolomites.
-> 1-3: Triassic: Roman mosaic stones, Hemmaberg near Globasnitz, Carinthia, Austria.
Dedolomitization (-> 4, 6-8) and Saddle Dolomite (-> 5)
4
Rhombohedral dolomite crystals have been dissolved. Molds were filled with equigranular microcrystalline Low-Mg
calcite. Preferred solution of the core zones of dolomite rhombohedrons is a common phenomenon, because these zones
often are richer in Ca than the periphery of the crystals (Randazzo and Cook 1987). SEM microphotograph. Late Jurassic:
Southern Franconian Alb, Germany.
5
Late diagenetic 'saddle dolomite' cement crystals within a vug characterized by curved crystal faces (black arrow). The
zonar dolomite has been internally corroded by low-saline formation-water migrating along rhombohedral cleavage planes
and causing burial dedolomitization and the formation of intracrystalline pores (white arrows). VP: Vuggy porosity. Fluid
inclusion analysis provides direct information about fluid movement and the timing of hydrocarbon migration. Saddle
dolomites are characterized by large, mm-sized crystals, curved crystal faces and cleavages, undulose extinction, com-
monly ferroan composition, often fluid inclusions that make the crystals appear cloudy in thin sections and pearly in hand
specimens. Saddle dolomites are commonly associated with sulphide mineralization, hydrothermal activity, and the pres-
ence of hydrocarbons at relatively deep burial depths.
6
Burial dedolomitization of a late diagenetic dolomite. Calcitization is restricted to the inner Ca-rich zones of former zonar
dolomite crystals (white arrows).
7
Near-surface early meteoric-vadose dedolomitization: Solution-enlarged molds of the former dolomite crystals are filled
with meteoric calcite cements (arrows).
8
Near-surface early meteoric-vadose dedolomitization. Crystals molds are geopetally infilled by gray vadose crystal silt
(arrows) and granular meteoric cement.
-> 5-8: Jurassic (Kimmeridgian): Subsurface, TB-3 Saulgau well, southwestern Germany.
-> 1-3: E. Flügel and Ch. Flügel 1997; 4-7: Meder 1987
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