Geology Reference
In-Depth Information
1996). The term
microbialite
(Burne and Moore 1987;
changed to microbolite by Riding 1991) characterizes
'organosedimentary deposits that have accreted as a
result of benthic microbial community trapping and
binding detrital sediment and/or forming the locus of
mineral precipitation'. Microbialite is often used as an
overall term, but also as a term restricted to non-lami-
nated microbial carbonates only in contrast to
stroma-
tolite,
which denotes laminated benthic microbial de-
posits (Riding 1999); see Box 9.1 using the classifica-
tion developed by Logan et al. (1964; Fig. 9.3). The
fabric describes lamination or non-lamination and the
spatial composition (e.g. clotted). Microstructure re-
fers to the type and fabric of the constituents (com-
monly micrite or microspar, various grains, and fenes-
tral spar-filled voids, sometimes encrusting fossils and
borings).
9.1.3.2 Classification of Benthic Microbial
Carbonates
Box 9.2 summarizes the current classification of mi-
crobial carbonates (examples in Pl. 8 and Pl. 50).
Comments on non-laminated microbialites
Thrombolites
(Pl. 8/1, 6, Pl. 50/5, Pl. 131/5). The
term was proposed as a field term for 'cryptalgal struc-
tures related to stromatolites but lacking lamination and
characterized by a macroscopic clotted fabric' (Aitken
1967). The name refers to micrite clots.
The clots differ in color, form and/or texture from
the intervening area and create a blotchy fabric. Kennard
and James (1986) called the components of thromb-
olites
mesoclots,
which are typically dark colored and
have a microcrystalline texture. They display a variety
Box 9.1.
Which thin section and SEPM criteria provide evidence of a microbial contribution to the formation of lime-
stones?
Very fine-grained dense micritic matrix:
Dead calcified bacterial cells can become calcified (Krumbein 1979) and form
discrete micron-sized bodies. Small varieties found in micritic carbonates have been attributed to nannobacteria (size
0.05 to 0.2
m) because the sphere- and bean-shaped objects seen in SEM exhibit similarities with regard to morphol-
ogy and cluster-like distribution patterns (Folk 1983). Caution is needed because similar objects can be also formed
abiotically (Kirkland et al. 1999, Abbott 1999).
Laminated and undulated micritic structures:
Comparing of laminated textures of fine-grained limestones with
microtextures produced by bacteria in laboratory experiments shows that micritic finely laminated and undulated micro-
structures (Pl. 6/5, Pl. 50/4) can be formed by microbes that contribute to the precipitation of seafloor automicrite (see
Sect. 4.1.1).
Constructive micrite envelopes:
Some of these envelopes (see Sect. 4.2.3) may represent biofilm calcification (Perry
1999). Bacteria adhere to surfaces for stability and create calcifying biofilm communities augmented by other microbes.
Clotted fabrics:
Clotted fabrics (Pl. 10/1) are widespread in stromatolites and thrombolites. The fabric appears to repre-
sent EPS calcification. It has been referred to as spongiostrome (Gürich 1906) and structure grumeleuse (Cayeux 1935),
and can grade into dense micrite. Diffusely clotted micrite often forms clusters of rounded aggregates within microsparite
and is associated with filamentous microfossils (Guo and Riding 1992).
Calcimicrobes:
Calcification of microbial, external polysaccharide-protected, sheets produces calcified fossils (calci-
microbes, e.g.
Girvanella
,
Cayeuxia
, Pl. 8/3, Pl. 53) which are represented by tiny tubes with micritic walls. Most of
these fossils are cyanobacteria, and occur in association with finely peloidal and clotted fabrics.
Peloids:
Silt to sand-sized micritic grains and aggregates (20-60
m) are common constituents of modern tropical
carbonates. In-situ precipitation has been observed in Holocene reefs (Lighty 1985). These Mg-calcite components have
been regarded as cement (Macintyre 1984, 1985), but also as calcified bacterial aggregates rimmed by euhedral calcite
crystals (Chafetz 1986; Pl. 8/5; see Sect. 4.2.2). Potential microbial peloids are widespread in ancient reefs (Pl. 8/6), in
stromatolites and thrombolites.
Microbial ooids:
The recognition of bacterially constructed carbonate crystals and grains (Fig. 4/24) in recent environ-
ments and the similarity of these grains with particles occurring in ancient carbonates (Gerdes et al. 1994; Reid 1987;
see Sect. 4.2.5) offer a clue to the microbial character of some ooids enclosed in laminated textures.
Microspar and spar:
Fibrous, equant and dendritic spar precipitates (Hofmann and Jackson 1987) occur as external
crusts on organic tissue and mineral surfaces of microbial carbonates, particularly those originating in fresh water (tufas
and travertines, karst) and schizohaline settings. These spars also form rosettes and spherulites around cyanobacterial
precipitates and on bacterial cells (Guo and Riding 1992; Defarge et al. 1996).
Pores and allochthonous grains:
Discrete voids ranging from tiny interstices to large growth cavities (Pratt 1995) are
often associated with microbial carbonates. They include irregular fenestrae and tidal flat birdseyes. Trapped grains are
important constituents of these carbonates (Pl. 20/4, Pl. 50/5, Pl. 124/1).
