Geology Reference
In-Depth Information
Lacustrine stromatolites are already known from
Archean and Proterozoic sediments. Common features
of Phanerozoic stromatolites are a significant morpho-
logical variety (undulations, columns, domes, arbores-
cent growth forms), light and dark macrolaminae com-
posed of many dark and light micritic and microsparitic
laminae, and radial fibrous textures reflecting growth
patterns of cyanobacterial and algal threads. A survey
of ancient lacustrine stromatolites was given by Stapf
(1989).
large oncoids (up to 20 cm) occur at the margins of
fluvial channels (Anadon and Zamarreno 1981). Many
oncoids are irregularly shaped.
The cores may be very small and formed by sedi-
mentary grains or rock fragments, or smaller oncoids,
or they may consist of gastropods and bivalve shells.
The cores are surrounded by often densely spaced, ir-
regularly concentric, up to 1 mm thick macrolaminae.
These macrolaminae are composed of dark micritic and
bright microsparitic microlaminae. The laminae are
arranged in parallel, often crinkled, dome-shaped or
columnar layers. Spongy and thrombolitic fabrics are
common. Radial, fan-shaped microstructures are caused
by the growth patterns of cyanobacteria (see Monty
and Hardy 1976 for illustrations). Microstructures of
lacustrine oncoids are often better preserved than ma-
rine oncoids.
Oncoids
Freshwater oncoids have been discussed in Sect.
4.2.4.1. The subspherical or concave-convex oncoids
are scattered or densely packed within a micrite matrix
or in marls. Sizes vary between less than 1 mm and
several centimeters (see Fig. 15.8C; Pl. 131/5). Very
Box 15.1.
Common lacustrine microfacies types (LMF).
LMF 1
:
Lime mudstone
consisting of very fine-grained
micrite crystals. Can originate from cyanobacterial and al-
gal blooms or by abrasion of limestones. Sometimes
strongly bioturbated (Fig. 15.9A). Occurs in deeper and
protected lake parts, but the sediment may be reworked by
storms and redistributed also in shallow areas.
LMF 2
:
Inhomogeneous laminated lime mudstone
with
very rare or no fossils (Fig. 15.8A). The laminated fabric
is caused by the alternation of light and dark layers, the
latter often rich in organic matter, plant debris and/or cy-
anobacteria forming very thin laminae that can be recog-
nized by means of fluorescence microscopy. Clay and fine,
silt-sized terrigenous quartz may be common and contrib-
ute to the lamination. LMF 2 characterizes a relatively deep
quiet-water basin with stratified water column and seasonal
variations. Poor oxygenation is indicated by the lack of
benthic organisms and burrowing. Fossils may be repre-
sented by rare fish remains, ostracods and conchostrakes.
LMF 3
:
Lime mudstone and wackestone with leaves of
plants
accumulated on bedding planes. The leaves may be
encrusted by carbonate. This microfacies is common in
Late Paleozoic and Cenozoic lacustrine sediments (e.g.
Potomogeton
layers, Miocene). LMF 3 characterizes a
quiet-water far-shore or near-shore environment.
LMF 4
:
Inhomogeneous fine-bedded intraclast wacke-
stone
(Fig. 15.8D) characterized by reworked laminated
mudstone; LMF 2) and nektonic fossils. Frequent in near-
shore and shore lake-floor areas affected by waves.
LMF 5
:
Densely packed nodular, peloidal wackestone
or floatstone
with reworked lacustrine grains and with black
pebbles of lacustrine limestone, sometimes also with ex-
traclasts. Common near the shoreline of large lakes. Formed
by storm events.
LMF 6
:
Ostracod mudstone, wackestone, packstone or
grainstone
characterized by double-valved or disarticulated
thin or thick ostracod shells, often monospecific (Pl. 130/
2; Fig. 15.9B). This type occurs in different near-shore,
far-shore and basinal parts of lakes.
LMF 7
:
Charophycean mudstone or wackestone
(Pl.
130/4; Pl. 65/1, 3) with abundant oogonia and stem remains.
Often associated with ostracods or ostracod debris. This
microfacies characterizes the shore region of many humid
and semiarid lakes and brackish environments of coastal
regions.
LMF 8
:
Gastropod packstone and grainstone
(Pl. 130/
3; Fig. 15.9C). Often composed of monospecific com-
munities. The gastropods may be associated with bivalves
and ostracods.
LMF 9
:
Bivalve floatstone
(Fig. 15.8B) composed of
centimeter-sized, often parallel arranged shells. LMF 9
occurs on near-shore lake floors and is rarely affected by
waves. Coquinas may represent storm deposits.
LMF 10
:
Oncoid floatstone, packstone and wacke-
stone.
Two subtypes:
Large oncoids (commonly porostro-
mate or agglutinated oncoids; Pl. 131/5; LMF 10A) occur
either associated with stromatolites or separately. Micro-
oncoids with submillimeter sizes (Fig. 15.8C; LMF 10B)
accumulate in packstones. These oncoids have nuclei of
quartz or silt grains, surrounded by only a few micritic
laminae. The large oncoids exhibit various microfabrics
reflecting different growth forms of different cyanobacte-
ria and algae. The micro-oncoid packstone facies occurs
in near-shore and shore areas. Floatstones with centime-
ter-sized oncoids are common in higher-energy fluvial-
lacustrine and brackish-water settings.
LMF 11
:
Stromatolite bindstone
(Pl. 131/1, 3). Very
variable in geometry, shape and internal fabrics (see above
and Sect. 9.1 for more information). Contribute to the for-
mation of 'algal reefs'.
LMF 12
:
Green algal bafflestone and wackestone
(Pl.
130/1). Tubular green algae (e.g.
Cladophorites
) living in
near-shore and shore-settings contribute to the accumula-
tion of sediment and the formation of lacustrine 'algal
reefs'.
LMF 13
:
Lithobioclast rudstone
(Fig. 15.9D) composed
of reworked, poorly sorted and sometimes strongly rounded
carbonate lithoclasts and worn fossil fragments. Storm
deposits.
