Geology Reference
In-Depth Information
et al. (
2003
). The fungi are systematically associated
with calcrete levels at the top of thin shallowing-upward
evaporitic tidal sequences, and their roles in bio-
mineralization were dominant as indicated by particular
coated grains, bridging grains, rhizoconcretions, sparmicri-
tization, micritization, perforations and secondary calcite
formation from excreted fungal Ca-oxalates.
Despite their importance, it is surprising that only a few
detailed studies have been undertaken with the view of
assessing carbonate alteration features induced by fungal
growth. For instance, Burford et al. (
2003a
) investigated
the role of fungi in the transformation of Paleozoic
limestones and dolostones, showing that fungal dissolution
of the carbonate substrates led to the formation of new
microfabrics, such as polymorphic growth patterns with
mineralized hyphae and crystalline material (Na plate-like
and Ca-blocky crystals) adhering to their external walls.
Although the precise mechanisms involved in the formation
of these new
finding was from the Devonian Rhynie chert (Stubblefield
and Taylor
1988
). Other recent studies describe
zygomycetes fungal forms from macerations of Late
Riphean (Neoproterozoic) raising thus again the issue of
when Fungi first appear on Earth (Hermann and Podkovyrov
2006
). Given this lack of a reliable Fungi fossil record, and
identification problems, the answer is perhaps better
resolved through molecular genetics., Based on protein fun-
gal sequence analysis, Heckman et al. (
2001
) extended all
major fungal group lineages back to ca. 1 Ga, and inferred a
fungal colonization of Earth deep into the Precambrian.
Nevertheless, compelling fossil evidence to corroborate
this hypothesis has not been produced to date (Krings et al.
2013
).
Finally, little attention has been given to fossilized fungal
communities or to their role in carbonate diagenesis
(Krumbein
1972
); and studies of fungal activity in carbonate
sediments older than the Quaternary are relatively limited in
number. Our study of the Neoproterozoic of South Gabon
revealed numerous per-mineralized fungal relicts: sporangia,
sporangiophores, columellae, zygosporangia, suspensors,
dichotomous hyphae and spores in the upper part of
shallowing-upward evaporitic peritidal sequences. It provides
a detailed evaluation of the fungal role in an attempt to better
understand the mechanisms involved in the paleo-weathering
of a Neoproterozoic carbonate substrate. As documented
below, comparison of our observations with in vitro
experiments allow us to define an eogenetic sequence driven
by fungal invasion and colonization of a Neoproterozoic
substrate. This finding, corroborated by comparison with
present day fungal-mineral surface interactions, also bears
upon the role of microbial activity in the shaping of the late
Neoproterozoic Earth surface.
fabrics were not described, it
seems that both metabolism-dependent and metabolism-
independent processes played integral roles. Kolo et al.
(
2007
) described the formation of small pits and alveolar
structure related to the dissolution of dolomite crystals by
fungi of Carboniferous aged dolostones, and showed that the
pits were occupied by a dense network of fungal hyphae
penetrating the dolomitic substrate and circumventing the
grain boundaries through dissolution paths.
There has similarly been a paucity of study directed to
observing fossilized fungal communities in the ancient rock
record, in part due to a bias directed at the more common and
easily recognizable land plants (Taylor et al.
2005
) and the
difficulties at resolving whether fungal forms in ancient
sediments can be reasonably considered
'
mycogenic
'
from
one or more of the four classical phyla: Ascomycota,
Basidiomycota, Chytridiomycota, and Zygomycota which
comprise the so-called true
“
fungi
”
5.2
Geological Setting
(Cavalier-Smith
1987
).
At present, the oldest definitive fungal fossils are associated
with lichen-like symbiosis is reported from the Doushantuo
Formation dated at 600 Ma (Yuan et al.
2006
). Butterfield
(
2005
) re-interpreted the presence of
Tappania
in the
Mesoproterozoic Roper Group of Australia and extended
the record of putative fungi to 1430 Ma. Along with other
Proterozoic acritarchs exhibiting fungal-like characteristics
(e.g.
Trachyhystrichosphaera, Shuiyousphaeridium,
Dictyosphaera, Foliomorpha
), this author considered that
there is a case to be made for an extended and relatively
diverse record of Proterozoic fungi. However, these reports
were later discounted (Cavalier-Smith
2006
), leaving us
with a reasonably confident fungal phylogeny (Glomalean
fungi: Redecker et al.
2000
) dating back to ca. 600 Ma.
Berbee and Taylor (
2001
) also suggested such a
Neoproterozoic age for this colonization. Compared to this,
nearly two decades ago, the oldest reliable fossil fungal
“
Fungi
”
The Nyanga synclinorium is an important geotectonic unit of
southern Gabon, consisting of a 250 km long deformed
basinal structure. It formed during the West Congolian orog-
eny, which forms a vast Neoproterozoic (Pan-African) meta-
morphic belt stretching from Angola, in the south, some
2,000 km northwards to Gabon (Thi´blemont et al.
2009
;
The Gabonese part of the belt, averaging 100 km in width,
runs parallel to the Atlantic coast and terminates in northern
Gabon. Among the five major lithostratigraphical units of
the Gabonese part of the West Congolian orogen, the West
Congo Supergroup outcrops in the Nyanga synclinorium
(G´rard
1958
) and consists of the succession of several
informal units. The
'
Schisto-Calcaire Group
'
directly
overlies the
), which,
based on chronostratigraphic and sedimentological studies
'
Niari Formation
'
(or
'
Niari Tillite
'
Search WWH ::
Custom Search