Geology Reference
In-Depth Information
is indicated by xenotopic fabrics. Complete dolomite
solution and subsequent cementation is shown by dolo-
molds.
Reaction rates and the mode of dedolomitization in
experimental studies appear to depend on solution con-
centrations, and the temperature and crystal size of the
dolomite (De Groot 1967; Kastner 1984; Stoessel et al.
1987). Experiments point to a relatively slow process.
In contrast, dedolomitization of dolomitic marbles, re-
sulting in the formation of calcite crusts, seems to be a
rather rapid process as shown by the study of ancient
sculptures (Doehne et al. 1992).
tinuous dedolomitized horizons, therefore, may indi-
cate the presence of unconformities (Brown and Fried-
man 1970).
Porosity development:
Dedolomitization can in-
crease moldic and intercrystalline porosity, forming res-
ervoir rocks (Purser 1985). Open dolomolds may be
indicative of subaerial exposure and freshwater effects.
7.9 Metamorphic Carbonates and
Marbles
The deep burial diagenetic environment grades into the
zones of metamorphic control. Metamorphic processes
occur at various depths, under different pressure and
thermal conditions, and within a time range of millions
and more years. There is no clear, generally accepted
distinction between diagenesis and metamorphism,
7.8.3.3 Significance of Dedolomitization
Recognizing subaerial exposure and unconformities:
Most authors consider dedolomitization to be a near-
surface process related to weathering. Laterally con-
Plate 40 Selective Dolomitization and Dedolomitization of Platform Carbonates
Dolomitization, particularly multigenerational burial dolomitization, often causes obliteration of primary con-
stituents and textures. Many dolomites, however, exhibit selective dolomitization patterns (Murray 1964), indi-
cated by the dolomitization of only the fine-grained matrix (-> 2, 5) or only of individual grains (-> 6, 7).
1
Heterogeneous dolomitization
affecting only parts of the carbonate rock. Larger and smaller isolated euhedral dolomite
rhombohedrons (floating rhombs) occur within a microcrystalline calcite matrix. Using the Sibley and Gregg classifica-
tion, the sample would be described as partial dolomite exhibiting unimodal and euhedral planar crystals. Some crystals
are basically filled with calcite internal sediment (arrows), indicating meteoric dedolomitization (see Pl. 39/7). Lagoonal
facies. Late Jurassic: Yumakle, southern Turkey.
2
Fabric-selective dolomitization.
Distinctly zoned euhedral dolomite crystals occur only within the fine-grained matrix
and at the periphery of an auloporid coral. Zonation is accentuated by Fe-rich zones (appearing black) within the dolomite
crystals pointing to burial dolomitization. Matrix dolomitization by unimodal and euhedral planar crystals. Medium-
crystalline bioclastic dolomite. Early Devonian (Emsian): Erfoud, Morocco.
3
Partial burial dolomitization.
Fossils (echinoderms, E; styliolinids, S) are not dolomitized. Arrows point to a calcitic core
of dolomite rhombs. Note micrite relicts within the recrystallized matrix. Early Devonian (Emsian): Erfoud, Morocco.
4
Early diagenetic dedolomite.
Molds of former dolomite rhombs are filled with vadose meteoric calcite. Fenestral bind-
stone. Lagoonal facies. Early Jurassic: Korfu Island, Greece.
5
Selectively dolomitized ooid grainstone.
Replacement dolomitization is restricted to intergranular pores. Note the irregu-
lar boundary between tangentially-structured ooids and the dolomite crystals forming circumgranular rims (arrows) con-
sisting of dolomite crystals similar to the dolomite crystals occluding the remaining pore space. The circumgranular rims
represent isopachous dolomite cement, probably formed in a shallow burial setting. Late Triassic (Carnian): Central
Carinthia, Austria.
6
Selectively dolomitized ooid grainstone.
Note the distinct difference in comparison with -> 5. Only ooids are dolomitized
but not the echinoderms (E). The ooids are mimically replaced, e.g. they exhibit still traces of their concentric microstruc-
ture. Cambrian: Takkanawa Fall, Yoho Valley, British Columbia, Canada.
7
Selective dolomitization
restricted to micritic clasts (MC) and the micrite infilling of the gastropod (arrow). Restriction is
probably controlled by the High-Mg mineralogy of the microcrystalline calcite. Skeletal grains are echinoderms (E) and
gastropods. Back-reef facies. Shallow-burial replacement dolomitization. Late Devonian: Canning Basin, western Aus-
tralia.
8
Dolomitized ooids.
The different intensity in dolomitization of the ooid layers and nuclei possibly reflects differences in
their composition. Back-reef facies. Late Devonian (Frasnian): Canning Basin, western Australia.
