Geology Reference
In-Depth Information
(5) various microfossils encrusting micrite and peloids.
Many thrombolites contain micro-encrusters.
Riding (2000) distinguished
calcified microbial
thrombolites
typically displaying well-defined clots,
coarse agglutinated thrombolites
that incorporate sand-
and even gravel-sized sediment and commonly exhibit
complex internal variations (e.g. Shark Bay), some of
which are associated with sponge-tissue degradation
(Reitner 1993; Reitner et al. 1995);
arborescent throm-
bolites
associated with decimetric dendritic fabrics
(Schmitt and Monninger 1977, Armella 1994);
tufa
thrombolites
occurring in freshwater lakes and streams
(Moore and Burne 1994).
Thrombolites are essentially subtidal, common in
deeper-water settings, and form columns, domes, lay-
ers and thick crusts, typically dome-shaped meter-scale
doughnut-like masses (kalyptrae: Luchinina 1975;
Rowland and Gangloff 1988). They reflect an irregular
and uneven supply of sediment on surfaces that were
patchily colonized by microbes.
Thrombolites range from the Neoproterozoic to the
modern and were important as constituents of small-
scaled reefs throughout much of the Cambrian and Early
Ordovician (Shapiro and Rowland 2002; Webby 2002).
Reports of post-Ordovician thrombolites are rare (Sil-
urian: Kahle 2001; Late Jurassic: Leinfelder and Schmid
2000; Cretaceous: Neuweiler 1993; Miocene: Riding
et al. 1991).
Box 9.2.
Classification of organosedimentary deposits
controlled by benthic microbial communities
(after
Riding 2000). The classification is based on macro- and
microfabric criteria. The presence or absence of lamina-
tion is used in distinguishing two major groups. Note
that some authors restrict the term microbialite to non-
laminated microbial carbonates, whereas others use the
term as a synonym for all microbial carbonates.
Microbialites
(Burne and Moore 1987): Non-laminated
benthic microbial deposits:
Thrombolites
(Aitken 1967): Clotted mesofabric.
Dendrolites
(Riding 1988): Dendritic mesofabric, that
may be distinct, crude or diffuse.
Leiolites
(Braga et al. 1995): Structureless, aphanitic
mesofabric.
Cryptic microbialites
(Riding 1991): Lack distinct meso-
fabrics, but typically possess micritic, clotted or peloidal
microfabrics.
Stromatolites
(Kalkowsky 1908): Laminated benthic
microbial deposits:
Agglutinated stromatolites
(Riding 1991): Produced by
trapping/binding of particulate sediment. Well-laminated
and fine-grained, or crudely laminated and coarse-
grained.
Skeletal stromatolite
(Riding 1977):
Produced by in-place
organisms that are commonly preserved as calcified fos-
sils.
Freshwater tufa stromatolites
(Riding 1991): Produced
by encrustations of external sheets of cyanobacteria and
green algae.
Terrestrial stromatolites
(Riding 2000): The term refers
to fabrics seen in laminar calcretes.
of shapes (subrounded, amoeboid, grape-like, arbores-
cent, digitate, pendant) and occur isolated, intercon-
nected, or amalgamated. The clots can make up in ex-
cess of about 40% of the volume of a thrombolite rock.
Mesoclots may be lobate, cellular, microspherulitic,
grumous, peloidal, vermiform, or mottled. Calcified
microfossils such as
Renalcis
,
Girvanella
and
Nuia
are
common in Paleozoic thrombolites. The clots are in-
terpreted as a complex of irregular agglutination, in-
situ calcification of coccoid or coccoid-dominated mi-
crobial communities, skeletal encrustation, and ero-
sional processes, but may result also from calcified fila-
ments (Burne and Moore 1987). Walter and Heys (1985)
consider some thrombolites as being disturbed stroma-
tolites in which the original laminated structure was
disrupted and modified by bioturbation or subsequent
diagenesis.
In
thin sections
thrombolites show a complicated
microstructure. The individual micrite masses may con-
sist of (1) dense micrite, (2) clotted micrite, (3) peloids
of irregular shape and size, sometimes retaining poro-
stromate structures, (4) calcite spherulites consisting
of radially-oriented, non-ferroan calcite, typically 12-
20
m in diameter with a cloudy micrite core, and
Dendrolites
(Pl. 8/4). These microbialites form large
domes and columns that may be roughly layered, and
exhibiting bush-like dendritic fabrics, either vertically
erect or pendant. The fabric differs in color from adja-
cent areas and is distinct, crude or diffuse. Dendrolites
formed by porostromate microfossils (e.g.
Epiphyton;
Pl. 82/4) are common features of Early Paleozoic reefs,
growing at the surface, within dark cavities (
endo-
stromatolites:
Monty 1982) or in fissures (Pl. 8/4).
Rapid calcification and early cementation between the
bushy fossils resulted in the formation of an organic
framework that provided hard substrates for attached
metazoans (e.g. archaeocyaths, stromatoporoids).
Dendrolites were most conspicuous during the Early
Cambrian and Early Ordovician, Late Devonian and
Early Carboniferous. Bushy fabrics of dendrolites con-
tribute significantly to the formation of nonmarine car-
bonates, e.g. travertines (Pl. 2/1).
Leiolites
form large domes in association with stro-
matolites and dendrolites.
These microbial deposits with
structureless micritic fabrics often appear 'homoge-
neous' or 'massive' and lack clear lamination, clots or
dendritic fabrics. Leiolite formation is favored by a
