Geology Reference
In-Depth Information
9.1.4 Stromatolites
are Laminated Microbialites
I recommend the
definition
proposed by Riding
(1999): Stromatolites are
laminated benthic microbial
deposits.
This definition emphases the two main ele-
ments included in the original definition by Kalkowsky:
lamination and biogenicity.
In describing the siliciclastic Rogenstein deposit of the
Early Triassic Buntsandstein of the German Harz Moun-
tains, Kalkowsky (1908) introduced the term stromato-
lith for 'limestone masses showing a fine, more or less
planar, layered structure' composed of a set of laminae
('stromatoid') and believed to be of vegetal origin (see
Paul and Peryt 2000). The use and definition of the
term stromatolite have undergone gradual revisions
(Riding 1977; Krumbein 1983) and oscillate strongly
between descriptive and genetic approaches (Monty
1977; Semikhatov et al. 1979; Awramik 1984, 1990).
Recent stromatolites occur predominantly in fresh-
water (Freytet and Verrecchia 1999), marginal marine
(Fig. 9.2) and shallow subtidal environments. Ancient
stromatolites are known from the same settings, but
from deep subtidal and basinal environments. Stroma-
tolite fabrics can also form in geothermal systems as
well (Jones et al. 2002) and result from anaerobic meth-
ane oxidation at cold seeps (Greinert et al. 2002).
Plate 50 Microbialite and Stromatolite Fabrics
Biogenic laminated and non-laminated microbialite crusts forming various growth forms and consisting of mi-
crite, grains and fenestral pores occur in different settings of shallow- and deep-marine as well as nonmarine
carbonates. The fabric corresponds to that of stromatolites (skeletal stromatolite, -> 1; agglutinated stromatolite,
-> 2) or non-laminated microbialites (e.g. thrombolite, -> 5). The microbialites are built by the combined effect
of biota, sedimentation and diagenetic processes. Microbial contributions, rapid cementation as well as the
assistance of encrusting organisms (foraminifera, serpulids) are essential in the formation of these microbialites.
1
Skeletal stromatolite
crust growing on a colonial reef coral. The crust consists of slightly undulated spongiostromate
micritic layers (ML) separated by layers of porostromate cyanobacteria filled with sparry calcite (SC). Late Triassic
(Dachsteinriffkalk, Norian): Gosaukamm, Austria.
2
Laminated fine-grained agglutinated stromatolite
produced by trapping/binding of sediment. Detail of microbial crusts
consisting of thicker peloid layers (PL) and thinner micrite layers (ML). Peloids are very small and surrounded by calcite
rims. They are isolated or occur in amalgamated masses. Micrite layers consist of micritic laminae with peloids and
(white) bodies composed of radially arranged calcite crystals (
Baccanella
, see Pl. 99/8). Both, calcite-rimmed small
peloids and
Baccanella
point to a microbial origin. These crusts are abundant in mm- to cm-sized framework voids of
Triassic reefs. The peloidal texture is characteristic of 'container organomicrites' (see Sect. 4.1.1) formed in semi-re-
stricted cavities. Late Triassic (Norian): Gosaukamm, Austria.
3
Microbial crusts
around and between rugose corals
(Peneckiella).
Microbial circumcrustations around reef-building or-
ganisms contribute significantly to the stabilization and preservation of reef structures. The crusts exhibit peloidal and
clotted structures. Note that the crust around adjacent corallites is interconnected, forming a framework. This sequence
can be explained by the leeward position of the
Peneckiella
thickets on the forereef slope (Gischler 1995) that hampered
a rapid early cementation. Interskeletal pores are filled with burial calcite cements. Black spots between the septa of the
corals and between coral calices are ipsonite, an asphaltic probitumen derived from the metamorphism of hydrocarbons
due to thermic effects. Atoll reef, formed by stromatoporoids and corals on top of a volcanic seamount. Late Devonian
(Frasnian): Iberg, Harz, Germany.
4
'Spongiostromate' stromatolite crust
covering the wall of a cryptic reef cavity.
Micritic stromatolite
according to the
classification by Schmid (1996). The term spongiostromate refers to the variable, undifferentiated microfabric. Note the
variable thickness of the micritic laminae. These crusts which are several centimeters thick, are interpreted as microbially
induced carbonate precipitation within biofilms coating the interior of semi-closed reef voids and forming autochthonous
micrites (container organomicrite). The interior of the cavity is occupied by
Baccanella
(arrow), supposed to be of bacte-
rial origin or as diagenetic products caused by the recrystallization of micritic High-Mg calcite and aragonite (Pratt 1997).
Late Triassic (Norian): Gosaukamm, Austria.
5
Agglutinated microbialite
consisting of amalgamated peloids (AP) leaving space for spar-filled cavities (C) forming a
'laminoid fenestral fabric' (see Sect. 5.1.5.2). The absence of a distinct lamination prohibits an assignment as stromato-
lite.
Poorly structured thrombolite
according to the classification by Schmid (1996). SMF 21. Late Triassic (Norian):
Zelenica, Begunjscica Mountains, Slovenia.
6
Tufa stromatolite.
Cement/algal bindstone, characterized by alternating thick layers of bladed elongate calcite cement
crystals (CC) and layers consisting of radiating bundles of algal threads (arrow). The algal character of these threads is
supported by distinct bifurcations. Schizohaline near-coastal environment. Tertiary (Miocene): Gulf of Suez, Egypt.
