Geology Reference
In-Depth Information
Plate 8 Microbial Contribution to the Formation of Micrites: Some Examples
1 Modern microbialite (accretionary organomicrite) forming cm-sized thrombolitic columns without distinct laminations.
The structure consists of microcrystalline and peloidal Mg-calcite. Note the numerous inclusions of detrital grains (ar-
rows). These automicrites exhibit specific soluble organic matrices which are regarded as responsible for the nucleation
of calcite crystals. Bacteria within biofilms contribute to the degradation of organic matter (see Fig. 4.2). Reef cave,
Lizard Island, Great Barrier Reef.
2 Bacterial contribution to the formation of mud mounds: Crudely laminated stromatolite interpreted as organic matrix
mediated automicrite (organomicrite, see Figs. 4.1 and 4.2). Note the morphological similarity with -> 1. Settlement by
sponges, foraminifera and polychaetes (not shown in the figure) and the criteria discussed on Pl. 6/5 indicate calcification
at the surface. Mud mound. Cretaceous (Albian): La Sia, Vasco-Cantabrian basin, Spain.
3 Bacterial contribution to the formation of non-marine oncoids: Cyanobacteria contribute significantly to the formation of
oncoids in non-marine, transitional and marine environments (see Pl. 12/1, 5). The taxonomic composition of the oncoids
reflects differences in environmental conditions (e.g. salinity). The figure displays details of the cortex of an oncoid
formed in a freshwater environment (Leinfelder 1985). The vertical sequence of the cyanobacterial morphotypes tells us
the environmental history of the oncoid: a -
Bacinella? sterni
Radoicic (alga?, intergrowth of alga and nubeculariid
foraminifera?) interwoven with cyanobacteria; b -
Scytonema/Calothrix
; c - crust consisting of
Phormidium
; d - bush-like
tufts
Schizothrix
; e - another
Phormidium
crust. The oncoid-forming organisms can be compared with morphotypes of
modern calcifying cyanobacteria. Changes in the morphotypes reflect response to changing microenvironments. The
Scytonema/Calothrix
morphotype points to an episodic toleration of subaerial exposure, the
Phormidium
morphotype
indicates increased hardwater conditions of moderate to high energy levels, and the
Schizothrix
type represents fairly
agitated freshwater conditions. The oncolite horizon occurs within terrestrial siliciclastica and is interpreted as deposits of
an ephemeral hardwater lake with varying salinities. Late Jurassic: Albuquer, Portugal.
4 Bacterial contribution to the formation of micritic deep-water limestones: Dendrolite microbialite. Erect or pendant mi-
critic mm-sized calcitic shrubs occurring in cryptic habitats of aphotic environments (
Frutexites
Maslov; arrow), and
interpreted as being bacterial or fungal in origin, are often present in ferromanganese crusts of ancient deep-water stroma-
tolites (Myrow and Coniglio 1991; Böhm and Brachert 1993).
Frutexites
is an important element of deep-water and
peritidal carbonates formed during intervals of low sedimentation. Shallow-water examples appear to represent coelobites,
deep-water examples grew within the sediment or on the sea floor of dysaerobic basins. Note the penetration of the
sediment and the downward growth (indicated by the internal infill) of the shrubs. Note the change in shape of the more
globular fibrous
Frutexites
growing into the cavity as compared with the black opaque shrubs within the sediment. The
arrow points to the erosional boundary of
Frutexites
on the cavity ceiling. Morphologically similar bacterial shrubs form
in a variety of inhospitable marine and non-marine environments (Chavetz and Guidry 1999), including travertines. An
inorganic origin of some of these structures, however, can not be ruled out. Early or Middle Jurassic (Obersee Breccia):
Lunz, Austria.
5 Bacterial contribution signalized by 'black/white' peloids: Tiny micritic peloids (arrows) surrounded by a clear rim of
acicular calcite cement crystals are interpreted as bacterially induced or as microbial fossils (e.g.
Baccanella
, see Pl. 99/
8). Similar peloids have been described from numerous shallow marine reefs, both modern and ancient (Macintyre 1985,
Chavetz 1986). In-situ 'benthic' peloids occur as cavity-fill precipitates or as laminated crusts, both between and within
skeletal material. Most peloids range from 10 to 60
m in diameter, have cloudy centers, and clear exterior rims. In
modern peloids, the rim consists of dentate High-Mg calcite crystals. The cores are interpreted as bacterially induced
precipitates. The sample represents small cavities within boundstones forming a sponge/algal/carbonate cement reef.
Middle Permian: Straza near Bled, northwestern Slovenia.
6 Bacterial/algal contribution indicated by thrombolitic fabric: 'Peloidal micrite', exhibiting a distinct clotted fabric, is a
common constituent of ancient deep-water mud mounds. Formation and accumulation of the sediment by binding effects
is explained as a result of the binding effects of bacteria and/or algae. Similar textures originate from the bacterial decom-
position of sponges (see Fig. 4.2). Note the fenestral clotted packstone fabric (F: fenestrae). A clotted microstructure is
taken as having been influenced by organically mediated calcification, either forming clots of bacteria or cementing
peloids together. Central part of an algal mound. Late Carboniferous (Westfalian): Cantabrian Mountains, Northern Spain.
7 Cryptic microbialite containing small encrusting foraminifera (arrow). The composition of micro-encruster associations
allows paleoecological conditions and sub-environments to be distinguished (Schmid 1996). Sessile foraminifera are
often attached on macerated skeletons of sponges. Note faint relicts of a siliceous sponge (S) at the bottom. The decompo-
sition of the tissue of siliceous sponges and subsequent microbial carbonate precipitation can result in the formation of
peloidal and minipeloidal micritic fabrics (Warnke 1996). The association of sponges and microbes is a common feature
of Phanerozoic mud mounds and reef mounds. Upper Jurassic reefs and massive buildups characterized by the abundance
of siliceous sponges (Pl. 78) and calcified microbes are common on the European northern Tethyan shelf (Portugal;
Spain; Southern Germany; Poland; Romania). Most mounds were formed within peri-/epicontinental seas on homoclinal
mid and outer ramp settings. The mounds were composed of early lithified automicrites and of allomicrites. Late Jurassic
(Oxfordian) sponge reef: Dobrogea, Romania.
-> 1, 2, 7: Keupp et al. 1993; 3: Leinfelder 1985; 4: Böhm and Brachert 1993; 5: Flügel et al. 1984; 6: Hensen et al. 1995
