Geology Reference
In-Depth Information
significant. The second scenario implies that fungal
vegetative and reproductive parts (e.g., sporangia,
sporangiophores) could spread across the colonized sub-
strate surface and metabolic exudates that were released in
the growth environment would have caused substrate disso-
lution, authigenic mineral precipitation and cementation of
fungal parts within the substrate.
The Mouila sediments were probably partly or totally
lithified during early diagenesis through pervasive dolomiti-
zation in hypersaline brines allowing pits to be formed. In
this very shallow environment (backslope of the levees and
beach-ridge washovers) exposure was probably high, with
very dry conditions proving favourable to fungal coloniza-
tion. Numerous mudcracks seen in thin sections or under the
SEM support this interpretation. Cyanobacteria are partly or
entirely destroyed by the dolomicropar. This contrasts with
the fungal hyphae which are reasonably well-preserved
and intimately associated with the dolomicrospar and the
dolomitized EPS that constituted an integral component of
the original microbial mats.
Evidence for ancient life typically exists within sedimen-
tary environments, where microbial mats and colonies of
filamentous, coccoid or rod-shaped microbes have been
found in Early Archean strata such as in cherts of the Pilbara
and Barberton greenstone belts (Westall 2005 ). As fungi are
increasingly pushed deeper into the Precambrian, their role in
early Earth processes is also increasingly linked to two major
events: the
5.6
Implications
It is our contention that we have described here one of the
earliest physical records of fungi, and that these organisms
having inhabited the upper supratidal part of a shallowing-
upward carbonate sequence. We also show how the fungi
impacted the main petrophysical characteristics of the rock.
Despite this importance and stratigraphic distribution, fungi
are rarely reported in ancient series in the literature. This is
particularly the case in the Precambrian of West Africa,
where numerous stromatolites have been described in great
details (Amard and Bertrand-Sarfati 1997 ). Clusters of
closely packed meter-scale ellipsoid to upward expanding
cone-shaped bioherms several meters (up to 5 m wide and
3 m thick, Fig. 5.12e-h ) developed relief of several meters
above the top of laminar microbial bindstone and small-
sized LLH stromatolites associated with collapse breccia
containing anhydrite relicts (Fig. 5.12 ). The biohermal
level is 15 m-thick and belongs to the post-Marinoan
Neoproterozoic SCIc unit (Schisto-Calcaire Group)
recognized in the Niari Basin (the Republic of Congo) by
Alvarez and Maurin ( 1991 ). The Niari Basin extends over
more than 75,000 km 2 and is mainly constituted by two
depressions, the Niari depression in the Republic of Congo
and Nyanga depression in Gabon where the Mouila quarry is
located. Study of the microbial contents (cyanobacteria and
fungi) of the SCIc stromatolites is in progress (Yannick
Callec and Alain Pr ´ at) and is focused on the lamina micro-
structure forming irregular bands and lenses of grey and light
carbonate mud. They are associated with early diagenesis
related to replacement by evaporitive brines (Fig. 5.12a-d ).
and the rise of oxygenation in
the Neoproterozoic (Heckman et al. 2001 ;Canfield 2005 ;
Kennedy et al. 2006 ). How fungi may have impacted terres-
trial weathering, and to what effect this may have played a
role in the broader evolution of the Earth system remains
unclear. We feel that this work takes a step further into the
deep past by describing how fungal relicts within the
Neoproterozoic Mouila series points towards their coloniza-
tion and diagenesis of shallow sediments at the time.
Snowball Earth
Acknowledgements Kamal Kolo would like to thank Prof. Philippe
Claeys, Department of Geology/Vrije Universiteit Brussels for
supporting the experimental work in this study. We also thank the
Department of Metallurgy/Vrije Universiteit Brussels for kindly giving
access to their SEM and FE-SEM laboratories. The fieldwork was done
under the terms of the SYSMIN program (Eighth Fonds Europ´en de
D´veloppement, BRGM-CGS-SANDER-MRAC). KOK would like to
thank the Natural Sciences and Engineering Research Council of
Canada for continued support. The authors thank Dr Yannick Callec,
BRGM (Bureau Recherches G ´ ologiques et Mini ` res, Orl ´ ans, France)
for guiding Alain Pr ´ at on the field in the Niari area during dry season
(September 2012). We thank Prof. David Gillan for a comprehensive
review which helped improve the MS.
Fig. 5.12 (continued) interstratified in strongly deformed, slumped
evaporitic laminated dolomudstones (pictures c and d ). Sample CB9,
outcrop MAD8122-Yannick Callec, Republic of Congo, photo cb9252
and 9253/ap/2013). ( c , d ). Salt migration (slump, microenterolithe,
folding -c, tepee -d) in a dolo-microsparitized mudstone with remnants
of organic-rich microbial laminae. Same ouctrop as previous pictures,
sample CB10 (50 cm above CB9), photo cb9258 and 9269/ap/2013).
( e ) Flat to slightly domal stromatolites switched between irregularly-
laminated microbial dolomudstones. Outcrop MAD0165-Yannick
Callec, Republic of Congo, photo P1170717/ap/2012). ( f ) Massive
stromatolitic
reef (height 2 m) bordered by recent tufa deposits,
( g ) Concentric sheet stromatolitic bioherm, ( h ) Stacked patch reef units
flanked by intraclastic (angular stromatolitic chips) dolopackstones on
both sides.(f-g-h) : same ouctcrop as ( e ), respectively photos
P1170726/P1170734/ P1170744ap/2012). The stromatolites constitute
a 15 m-thick level interstratified in well-bedded dolomudstones and
ooid-pisoid dolopackstones and dolograinstones
'
table
'
Search WWH ::




Custom Search