Geology Reference
In-Depth Information
Fig. 5.6. Stromatactis limestone. Slope of a mudmound. Note the difference in color and microfacies between the red lime
mudstones with spar-filled stromatactis structures and the overlying red and pink echinoderm wackestones, interpreted as
storm-transported sediment. The stromatactis cavities are filled with several generations of fibrous calcite cement. The
matrix contains sponge spicula. Early Devonian (Emsian): Notre Dame de Cros quarry, Montage Noire, southern France.
Scale is 5 cm. After Vigener (1996).
subsequent internal erosion) with that of the host lime-
stone and also considers the origin of the red color of
the limestone (a feature common in many Cambrian to
Devonian stromatactis carbonates). The model postu-
lates that the formation of the stromatactis mounds was
controlled by sponges, whereas stromatactis and the
common red color of many stromatactis mounds are
early-diagenetic features formed a few meters below
the substrate-water interface. The model was developed
for Cambrian to Devonian red stromatactis mounds
formed in deeper shelf settings. Criteria used as indi-
cations of former sponges are the occurrence of strom-
atactis together with peloidal relicts of sponges and the
abundance of sponge spicula in limestones with stro-
matactis.
(b) Microbial model (Pratt 1982; Flajs and Hüssner
1993; Kaufmann 1996): This model (Nos. 6-11 in Box
5.2) underlines the role of microbial mats in contrast
to that of metazoans.
Criteria taken as indications for microbial origin are
the occurrence of stromatactis in extended layers at-
taining sizes of several hundred square meters, the joint
occurrence of stromatactis and cryptalgal structures,
the presence of a reticulated former cavity system, the
digitation of the structures towards the top, the asso-
ciation with peloidal fabrics as well as scarce relics of
cyanobacterial structures.
(Flajs et al. 1996). The spar network is often preferen-
tially aligned along the bedding planes and can be con-
tinuous for tens of centimeters to a few meters.
Origin of stromatactis
The origin of stromatactis is still controversial (see
reviews by Bathurst 1982; Flajs and Hüssner 1993; Lees
1988; Boulvain 1990; Monty 1995). Central problems
are the reasons for the shape, how the cavities were
formed, and the filling mechanism of the cavities. Up
to now more than twenty-five, partly related possibili-
ties have been discussed for processes leading to the
formation of stromatactis (Box 5.2). The fundamental
question is to ascertain whether stromatactis reflects a
taphonomic process (e.g. the decay of a former soft-
bodied organisms) or is a product of physical and
chemical processes (e.g. winnowing, internal erosion,
dissolution).
Two models, both assuming dominant organic con-
trol, are currently under discussion. A key question is
the relative role of sponges versus microbes in creat-
ing cavity systems:
(a) Sponge model (Bourque and Boulvain 1993):
The model (Nos. 12 and 13 in Box 5.2) integrates the
origin of stromatactis (decay of sponge soft tissue and
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