Geology Reference
In-Depth Information
interpenetration, because the pseudomorphs of sulfates or
the microenterolithic levels have not collapsed. These
replacements were probably formed nondisplacively
in an enclosed volume of lithifying muddy sediment.
Dolomite and also fine- to coarse-grained silica have
completely replaced these original voids, thus
maintaining their original shapes without deformation.
The process was probably rapid, as no mechanical
compaction and fracturing are observed. This early
replacement-cementation (dolomicrospar) prevented late
fracturation due to overpressuring in response to burial.
2. The mudcracks and sheet-cracks are well preserved,
undeformed and filled with a dolomicrospar (and some-
times silica) having the same size (generally very-fine to
fine-grained) as the dolomicrospar that replaced the pri-
mary carbonate mud. Boundaries separating peloid and
muddy laminae are quite sharp and of constant thickness
inside a particular laminar structure. No interpenetration
of the different laminae is observed. Detailed fabric pres-
ervation of primary cracks suggests that dolomitization
and silicification occurred early in the diagenetic history;
they do not cross the cracks.
3. Evaporite facies display micro-slumped or contortion
structures without any signs of compaction. The tiny
contorted levels are folded, keeping their uniform origi-
nal thickness (
upwards percolation of underlying pore waters, ruling
out an increase in temperature (during burial) and an
early or late influence of meteoric fluids. In this context,
late-stage diagenetic alteration resulting from large-scale
convection of marine, meteoric or hydrothermal waters
during burial can be dismissed. The dolomicrospar is
therefore neomorphic and replaced the original fine-
grained (dolo)micrite without a dissolution phase.
Collectively, these points lead one to infer that the pri-
mary carbonate muds were rapidly lithified by dolomitiza-
tion associated with evaporitive marine or coeval marine
waters. Under such conditions, fungi were able to inhabit
this stressful environment and subsequently played an
important role in the pit formation in specific or particular
interstratified levels (here a stromatolite layer).
5.5.2 Fungal Colonization and the Pit
Formation Hypothesis
The different depth levels of these the fungally-generated
solution pits suggest a progressive process of pit formation
through incipient, moderate and advanced pitting stages
(Fig.
5.10a-d
), possibly caused by different stages of micro-
bial colonization-diagenesis. The incipient pit stage
(Fig.
5.10a, b
) has a shallow quasi-circular/oval form,
visibly corresponding to a bioweathered, fragmented,
micropitted, and decolorized original mineral surface com-
pared to the surroundings where some fine-grained
authigenic minerals had started to precipitate. Interestingly,
some incipient pits visibly show fungal form morphology
(Fig.
5.10a
) that suggests a sporangium and sporangiophore
relicts. This morphological resemblance between fungal
parts and pits
μ
m for the thinnest).
4. The pits and fungi are confined to the same stratigraphic
levels in a stromatolite horizon. In the field, the laminar
structure consists of irregular bands and lenses of dark
and light carbonate mudstone. The bands constitute sets
with uniform thickness.
5. The spheres attributed to spores are dolomitized at a
nanoscale level and embedded in the dolomicrospartic
matrix.
6. The pits are invaded by thin hyphae associated at a very
small scale (
<
200
leads us to assume a cause and effect process.
In moderately developed pits, colonization by fungal
forms and mineral authigenesis can already be observed
(Fig.
5.10c
) relating the two processes and suggesting an
interaction of fungally induced biochemical and biomechan-
ical factors with the mineral surface. The circular/oval shape
of the pits suggests therefore an inherited form after the
fungal parts (e.g., sporangia, as depicted by Fig.
5.10a
)or
by selective fungal attack on certain sites of weakness on the
mineral surface (e.g., Fig.
5.10c
, grain boundaries giving rise
to irregular pit shapes). Selective fungal attack that produced
alveolar-type mineral structure and pitting has already been
shown to occur with dolomite and limestone (Kolo et al.
2007
).
At the advanced stage, the pits have clear 3-D forms,
display visible inner walls, depth and variable diameters
(Fig.
5.10d
). Inside the pits, a dense network of colonizing
fungal hyphae are interwoven with the pit
'
m) with former quadratic crystals
which were probably primary oxalates (identical to our
experiments). Those oxalates were then dolomitized
through either the cycling of dolo-microspar or simply
via interaction with near-coeval seawater or seawater-
derived fluids.
7. Excellent fabric retention of highly-soluble evaporate
phases (rosettes, swallow-tails, laths, microenterolithes,
and nodules) during dolomitization indicate that dolomi-
tization had occurred under hypersaline conditions. These
conditions are also suggested by
18
O enrichment of the
facies constituting the upper part of the shallowing-
upward salinity sequences where the fungi developed.
Oxygen isotopes (
100
μ
<
18
O) (Pr´at et al.
2010
), have values
ranging from
5.1 to
1.3
‰
, recording stabilization in
normal Neoproterozoic marine water (Veizer et al.
1992
).
Lighter values
δ
s inner walls as
well as the bottom which was coated with EPS (Fig.
5.10d
).
'
indicate continued evaporation and
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