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steady uniform supply of well-sorted sediment on sur-
faces colonized by an even layer of microbes.
Records of leiolites are rather rare (e.g. Late Juras-
sic: Dupraz and Strasser 1999, Leinfelder and Schmid
2000; Miocene: Braga et al. 1995).
defined growth forms but without (or very rare) with
preserved organic microstructures (Pl. 50/4). Poro-
stromata comprise forms exhibiting tubular microstruc-
tures (Pl. 50/1).
Spongiostromate microstructures are very variable,
including micritic, spongious, vermicular, fenestral, and
peloidal textures (Gürich 1906). Comparable textures
occur in recent and ancient stromatolites (Monty 1976).
Porostromate microstructures are typified by loose
or tangled, variously oriented, straight or sinuous tubes.
Because these tubes record organisms of different sys-
tematic position Riding (1977) proposed the term 'skel-
etal stromatolites'. The terms spongiostromate and po-
rostromate are used in microfacies studies describing
textural variations of biogenic carbonate crusts and on-
coids (see Sect. 4.2.4.1).
Spongiostromate refers to a laminated, poorly dif-
ferentiated micritic and peloidal microfabric (Pl. 28/1)
that may include various encrusting fossils.
Porostromate microfabrics characterized by tiny cal-
cified tubes occur in skeletal stromatolites (Pl. 50/1),
but also in non-laminated microbialites (Pl. 124/1), and
as constituents of biogenic nodules (Pl. 12/3, 4), in reef
limestones as well as in carbonates formed in nonma-
rine and marine-nonmarine transitional environments
(Pl. 50/6).
Cryptic microbial carbonates (Pl. 8/7). Many fine-
grained limestones, especially from reefal environ-
ments, may be microbial in origin but lack distinctive
macrofabrics. They are characterized by inhomoge-
neous micritic, clotted or peloidal fabrics, showing lo-
cally some traces of filaments. Many of these fabrics
are also present in stromatolites, dendrolites and throm-
bolites.
Further classifications
Microbialite features can be differentiated at vari-
ous scales (Shapiro 2000) and according to the amount
of compositional constituents (Schmid 1996; Fig. 9.1).
Comments on Spongiostromata and Porostromata
These groups were proposed by Pia (1927) for
schizophycean blue-green algae of uncertain affinities.
Spongiostromata refer to nodules or heads with well-
Fig. 9.1. Differentiation of microbialites . A : The four scales of microbialite investigation. Microbialites should be differenti-
ated according to (1) the megastructure (referring to the large-scale features of microbial limestones, e.g. biostromal buildup),
(2) macrostructure (designating the shape of the microbialite, with typical diameters of centimeters to meters, e.g. domal or
columnal), (3) mesostructure (describing internal textures of macrostructural elements visible with the naked eye, e.g. lami-
nated, clotted, or dendritic), and (4) the microstructure (referring to microscopic feature observed under the microscope or in
SEM, e.g. peloids, filamentous microbes). The scale bars indicate the magnitude of the criteria described at each of the
stages. Slightly modified from Shapiro (2000).
B : Classification of Mesozoic microbialites (modified from Schmid 1996). This valuable classification relies on microstruc-
tures observed in thin sections. The system is apparently not applicable to the morphologically highly variable Precambrian
and Early Paleozoic microbialites. See Pl. 50 for examples.
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