Geology Reference
In-Depth Information
13 Integrated Facies Analysis
To understand the formation and diagenesis of carbon-
ate rocks mineralogical and geochemical data derived
from the study of non-carbonate constituents, trace el-
ements, and stable isotopes must be integrated. Other
indicators for depositional and diagenetic conditions
are organic matter and organic carbon.
The following shows how these data can be success-
fully combined with data based on microfacies analy-
ses. The first section deals with acid-insoluble residues
in carbonate rocks and with authigenic minerals (ox-
ides, silicates, sulfides, sulfates and phosphorites)
formed after the deposition of the sediment. The sec-
ond section discusses the value of trace elements in
tracing the depositional and diagenetic history of lime-
stones, and the importance of stable isotopes studied in
the context of microfacies analyses. The third section
gives a brief overview of organic matter in carbonate
rocks.
Common clay minerals in mudstones and also in
limestones are those of the smectite group, kaolinite,
illite and mixed-layer illite/smectites and chlorite. Ka-
olinite and other clay minerals are often used as paleo-
climatic proxies, but the post-depositional diagenetic
alteration of clays related to the depth of burial and the
pore-water geochemistry must be thoroughly consid-
ered (see review by Ruffell et al. 2002). As a conse-
quence of different transport behavior clay minerals are
deposited in the ocean at different distances from the
coast. The relative abundance of kaolinite as a detrital
mineral may reflect proximity to the sediment source
and deposition in relatively nearshore settings, whereas
smectite may be deposited in deeper more offshore set-
tings. The relative proportions of illite/kaolinite to
smectite in marine deposits have been used to infer shal-
lowing/regressive and deepening/transgressive epi-
sodes.
Techniques:
The use of IR as paleoenvironmental
indicators requires data on the amount of non-carbon-
ate constituents and the mineralogical composition. The
non-carbonate fraction of carbonate rocks is commonly
extracted by acid digestion with a variety of acids and
chelating agents (Müller 1967), or by the use of acidic
ion exchange resins (French et al. 1984). All these treat-
ments may attack or even dissolve clay minerals. Weak
organic acids may be more useful than diluted mineral
acids. Clay and silt fractions are separated by sedimen-
tation methods (see McManus 1988).
13.1 Non-Carbonate Constituents
Non-carbonate constituents of limestones and dolomites
include clastic terrigenous minerals as well as auth-
igenic minerals. The study of clay minerals in acid-
insoluble residues has a high potential for recognizing
changes in erosion, climate changes and sea level and
estimating the influx of siliciclastic material into shal-
low and deep-marine carbonate basins.
Authigenic minerals
grow after sediment deposition
during diagenesis. They describe the course of diagen-
esis and are proxies for varying chemical and physical
conditions.
Insoluble residues in carbonate rocks:
All the clay
minerals occurring in clays or sandstones are also
known from limestones. A diagenetic origin of kaolin-
ite found in limestones can be neglected because of the
protective effect of the carbonate matrix against the for-
mation of an acid milieu. The most common marine
clay mineral is illite, which may be subjected to mul-
tiple redeposition. A diagenetic origin of illites found
in limestones can not be excluded. Mixed-layer miner-
als are often the result of the transformation of illite to
smectites in soils and subsequent transport to marine
13.1.1 Insoluble Residues (IR): Clay Minerals
and Detrital Quartz
The input of terrigenous material into carbonate envi-
ronments takes place by eolian transport, fluvial trans-
port (e.g. during flash-floods from wadis), or by ero-
sion of underlying rocks (e.g. in tidal zones).
