Probable Fungal Colonization and Carbonate Diagenesis of Neoproterozoic Stromatolites from South Gabon, Western Congo Basin - Geology and Resource Potential of the Congo Basin - page 84

Fig. 5.7 SEM photomicrographs of relicts of fungal structures from
Neoproterozoic section of Mouila quarry, Gabon. Sp

probable spore mass attached to it. The traces of the ancient fungi are
clearly outlined by the later dissolution diagenesis. ( c ) The visible
shapes, compared to modern and ancient fungal analogues (e.g. in :
Alexopoulos and Mims 1979 ; Hermann and Podkovyrov 2006 ), depict
zygosporangium and suspensors. ( d ) For further demonstration, here is
an artificially enhanced negative image from Fig. 5.7c . The
permineralized structures shown in Fig. 5.7c are visibly outlined.
Note how well the structures are immersed in the matrix, indicating a
syn-sedimentary process

sporangi(a)um,
sph ¼ sporangiophore. Bar scale as indicated. Images are from
samples MOU22 (Fig. 5.7a-d 22-20, 22-16, 22-18, 22-17/2007/ap)
( a ) Fungal remains showing sporangia ( black rounded bodies ),
sporangiophores (sph), and a probable sporiferous region. In the larger
sporangium are seen two concentric perimeters, the internal one
(delineated by white arrows ) depicts the columella. ( b ) Richly
colonized substrate (not all traces shown) where sporangia are showing
an external black perimeter depicting a sporiferous region and a

¼

honeycomb pattern. The pore space boundaries contouring
the shape of dissolved crystals have a filamentous appear-
ance with fine crystal aggregations (Fig. 5.8a ). This
arrangement strongly resembles the pattern of interaction
of modern fungi with Carboniferous dolomites (Fig. 5.8b ).
Experimentally, fungal invasion of carbonate substrata
have been shown to selectively occur by hyphal penetration
along grain boundaries (Sterflinger 2000 ; Kolo et al. 2007 ).
This stage is followed by active microbial dissolution of
the crystals through organic acid generation, creating hol-
low dolomite crystals or whole rhombic-quadratic and
roundish pore spaces with fungal hyphae as boundaries
and authigenic mineral deposition (biominerals such
as Ca- and Mg-oxalates: weddelite, whewellite and
glushinskite). These hollow dolomite crystals, where only
boundaries are preserved, are diagenetically different from
dolomite crystals with hollow centres and preserved rims
(Vahrenkamp and Swart 1994 ; Feldmann and McKenzie

1997 ; Jones 2005 ) that may have precipitated on minute
particles or metastable material andwas subsequently
dissolved or from bacterially-formed dumbell-shaped
hollow-core dolomite crystals (Cavagna et al. 1999 ).

5.4.2 Intracavity Biomineralization in Natural
and Experimentally-Weathered
Dolomites

Figure 5.9a shows a circular pit surrounded by a visible
elevated mineral

that was also observed in some
thin sections of Neoproterozoic strata. Tiny crystals and also
per-mineralized filaments litter the interior and exterior
of the pit. The mineral

“ collar ”

consists of a mixture of
per-mineralized filamentous material (probably with EPS
material) and attached crystals. Similar diagenetic features
(Fig. 5.9b ) comprising pits formed by dissolution of

“ collar ”