Geology Reference
In-Depth Information
The shape of freshwater oncoids seems to be con-
trolled to a lesser degree by environmental factors (e.g.
water energy, terrigenous input) than texture and micro-
fabrics (Abell et al. 1982). The best way to gain a high
paleoenvironmental resolution is the taxonomic differ-
entiation of cyanobacterial and algal associations (see
Pl. 8/4). Cyanobacteria communities in lacustrine on-
coids and stromatolites have remained stable at least
since the Cretaceous (Monty and Mas 1981).
15.4.1.3 Distribution of Lacustrine Microfacies
Types
Fig. 15.10 is a rough sketch showing the occurrence of
more frequent microfacies types of freshwater carbon-
ates.
More facies types exist, of course, depending on
the climatic setting, the size and depth of the lake ba-
sin, and the topography of the surrounding regions.
A strict actuogeological approach to the environ-
mental interpretation of ancient lake sediments is dif-
ficult, in that most present-day lakes are shaped by gla-
ciers and therefore provide poor analogs for ancient
lacustrine sedimentation. This was predominantly con-
trolled by tectonic movements, and only few modern
lakes have an areal extent and a sediment thickness
comparable with those of ancient counterparts, particu-
larly in the Paleozoic.
15.4.1.2 Microfacies Types of Lacustrine
Limestones
Thirteen common and widely distributed microfacies
types can be recognized when the facies of lacustrine
limestones of different age are compared (see Box
15.2). These LMF Types are listed in Box 15.1 and
shown in Pl. 130, Pl. 131 and in Figs. 15.8 and 15.9.
Plate 130 Lacustrine Limestones: Common Microfacies Types
The plate displays Tertiary and Cretaceous lake carbonates. The Tertiary samples (-> 1-3) are taken from the
Miocene Ries Lake, which formed within a large impact crater. Bioherms were built by green algae and cyano-
bacteria, and multiple lacustrine-vadose sinter veneers at the margin of the humid, hydrologically closed soda
lake basin (Arp 1995). The lacustrine sediments are overprinted by pedogenic and early diagenetic processes
modifying the primary depositional fabric and implying an adaption of traditional limestone classifications (see
Sect. 8.4.2; Pl. 48).
Algal limestone, ostracod grainstones, wackestones and packstones, as well as gastropod grainstones and
wackestones are common microfacies types in lacustrine sediments. Unlike marine limestones, gastropods and
ostracods are represented by very low-diversity associations, often by just one or two species. Other common
type of lacustrine limestones formed in oligotrophic or brackish lakes are wackestones and mudstones with
charophycean algae (-> 4). Lacustrine microfacies types (LMF) are defined in Box 15.1.
1
Green algal (Cladophorites) bafflestone:
LMF 12. The basic elements of the algae are erect and branched tubes, arranged
in cushion- or tuft-like thalli that sometimes coalescence forming meter-sized structures. Rhythmic growth patterns indi-
cate seasonal controls. The tube walls consist of micrite.
Cladophorites
(C) is regarded as a green alga identical with or
closely related to the modern genus
Cladophora
. Note associated gastropods (G) and charophycean oogonia (CH). Ter-
tiary (Miocene): Hainsfarth, Ries Basin, southern Germany.
2
Ostracod grainstone:
LMF 6. Some valves are stacked into each other (white arrows). The ostracods comprise 40-50% of
the very porous rock. The shell interior is filled with sinter cement and meteoric-phreatic sparite. The shells are covered
by a clear dolomite cement (black arrows). Calcite appears black due to staining with Alizarin-S. Paleoenvironment:
Wave-exposed eulittoral zone, sands near the algal bioherm. Note that comparable microfacies types occur in shallow-
marine hypersaline environments as well. Same locality as -> 1.
3
Gastropod grainstone
with some ooids: LMF 8.The gastropod shells (
Hydrobia)
are leached and preserved as micrite
envelopes only. The envelopes are covered by thin layers of microcrystalline cement.
Hydrobia
was the only aquatic
gastropod that flourished in the soda lake. Some calcitic ooids exhibit a radial structure (white arrow), other ooids are
dolomitized and appear black (black arrow). Paleoenvironment: Wave-exposed shallow eulittoral zone near the algal
bioherms. Same locality as -> 1.
4
Charophycean algal wackestone:
LMF 7. Most algal fragments are internode parts (arrows), see Pl. 65, Fig. 10.20, and
Sect. 10.2.1.8. Thin shells are ostracod fragments. Paleoenvironment: Brackish-water lagoon. Cretaceous: Spain.
-> 2 and 3: Arp (1995)
