Geology Reference
In-Depth Information
15.4 Lacustrine
and Palustrine Carbonates
Lacustrine carbonates (Sect. 2.4.1.7) originate in fresh-
water lakes and salt lakes. Palustrine limestones (Sect.
2.4.1.2) are formed through pedogenic modifications
of lacustrine freshwater carbonates.
The history of lakes is studied by means of sedi-
mentological, geochemical and biological methods.
Mineralogical data of carbonate and non-carbonate lake
sediments, and trace elements and stable isotopes of
lacustrine limestones allow seasonal productivity, tem-
perature changes and lake paleohydrology to be recon-
structed (see reviews by Kelts and Talbot 1990; Talbot
and Kelts 1990). The isotopic composition is used to
distinguish between open and closed lakes (Talbot
1990; Anadón and Utrilla 1993) and between lacus-
trine and palustrine carbonates (Platt 1992). The inten-
sity and type of bacterial degradation of organic matter
is reflected by the δ
13
C signal of early diagenetic ce-
ments. The δ
18
O value of lacustrine carbonates reflects
the composition of the bottom waters from which they
were formed (Oberhänsli and Allen 1987; Bellanca et
al. 1992; Platt 1992).
Biological data relevant for facies analysis of an-
cient lacustrine carbonates are the composition and
zonation of the biota and the biotic controls on carbon-
ate production. Epilithic and endolithic organisms con-
Fig. 15.8.
Lacustrine microfacies types.
The pictures show
microfacies types of Early Permian lacustrine limestones of
the Saar-Nahe region in southwestern Germany. The lime-
stones comprise only a minor part of the predominantly si-
liciclastic sedimentary sequence and occur as thin units called
'Flöz' because of their association with coal beds. Carbonate
beds may have a thickness of only a few tens of centimeters.
Nevertheless, the microfacies types reflect distinct differences
in depositional settings (see Fig. 15.10).
A:
Laminated mudstone (LMF 2)
characterized by dark and
light layers that can be better recognized at lower magnifica-
tion. The laminae consist of carbonate and quartz grains.
Shallow eutrophic lake. Odenbach Flöz, Lauterecken beds.
B:
Bivalve floatstone (LMF 9).
Leached and recrystallized
shells of anthaconaiid bivalves were accumulated by bottom
currents. These shells bear some similarity to trilobite frag-
ments and phylloid algae. The former are differentiated by
calcium phosphatic shells (see Pl. 94), the latter by thicker
plates with relicts of cortical pores (Pl. 58). Odenbach Flöz.
C
:
Micro-oncoid packstone
(LMF 10B)
. Note the distinct dif-
ferences in size, shape and core/coating ratios. Wiesweil bed.
D
:
Intraclast wackestone (LMF 4)
consisting of reworked
mudstones (irregularly shaped white grains). Arrows point
to corroded fish scales. Odenbach Flöz. After Clausing (1990).
