Probable Fungal Colonization and Carbonate Diagenesis of Neoproterozoic Stromatolites from South Gabon, Western Congo Basin - Geology and Resource Potential of the Congo Basin - page 79

(Fig. 5.2 ) (Pr´at et al. 2010 ). One of the primary facies
consists of flat-laminated (Fig. 5.3b ) to low domal stromato-
lite columns (Fig. 5.3c,d ), the latter having laminae that form
overlapping domes, with younger laminae truncating against
older ones and leaving no intercolumn space. Laminae are
produced by alternation of organic-rich and organic-poor
horizons, with some individual laminae traceable over a
few centimetres. The organic-rich horizons are generally
richer in pyrite framboids (

facies associated with the stromatolitic layers. Silica is the
last diagenetic phase observed.

5.2.2 Paleoenvironmental Interpretation

Micritic, millimeter-scale laminae interstratified with
organic-rich thin horizons (benthic microbial mats,
Fig. 5.4a ) indicate initial deposition in a tidal-flat environ-
ment (Purser 1973 ; Hardie 1977 ; Sellwood 1986 ). The soft
peloidal mud contains wavy and discontinuous lenticular
laminae. Crinkled fenestral laminae (Fig. 5.3f ), being either
flat or domal, even (Fig. 5.3e ) or pinching (Fig. 5.3f ), are
probably related to cyanobacterial mats. Despite strong dia-
genetic overprinting (microsparitization, dolomitization),
slightly altered bacterial filaments are still observable by
SEM imaging of the stromatolite laminae (sample MOU26,
Fig. 5.4a ).
The sediment laminae of the Mouila facies are typically
disrupted by mudcracks and sheet-cracks a few millimeters
to a few centimeters long associated with irregular small-
sized fenestrae. Similar characteristics are observed today in
the low

m). The brighter
layers contain micropeloidal micrite or clotted mudstone
draping over dense mat layer. Mat constructors in thin sec-
tion are poorly preserved morphologically (filamentous mat
ghosts are present), but are still visible on the SEM despite
the dolomitization process (Fig. 5.4b ). In the outcrop, this
'

1 μ mupto10 μ

<

'

is composed of submillimeter-scale
white and gray micritic laminae couplets (Fig. 5.3e ). Smooth
flat laminated dolostone (Fig. 5.3f ) associated with disrupted
fenestral and crinkled fabrics are common. The latter typi-
cally exhibit near horizontal sheet-cracks associated with
vertical and step-like thin mudcracks isolating micritic
lumpy patches (Fig. 5.3f ).
The major diagenetic alteration of the facies consists
of a thin pervasive hypidiotopic dolomitization, probably
related to episodes of anhydritization since sulphate
microenterolithes developed inside the mats. Consequently,
the former greyish microlaminar micritic sediment is pro-
gressively replaced by a relatively fine-grained homoge-
neous whitish dolo-microsparite, which may still contain
thin discontinuous microbial mat relicts (Fig. 5.3g,h ). At
the beginning of the replacement process the dolomite is a
mimetic fabric-preserving dolomite with crystal size varying
between micrite and microsparite (

cryptomicrobialite

marshes fringing the ponds of channeled belts
at Andros Island (Hardie and Ginsburg 1977 ), particularly
along the backslope of the levees and the beach-ridge
washovers where very shallow (millimeter range), closely
spaced (around 1 cm) mudcracks are present. The cracking
process may be quickly stopped by rapid growth of
cyanobacterial colonies (microstromatolites) giving incom-
plete mudcracks as those present in Fig. 5.3f .
Sedimentological evidence also reveals that the Mouila
series consists of a succession of plurimetric-thick
shallowing-upward sequences which correspond to early
diagenetic salinity cycles (Fig. 5.2 , cycles A-F,) with well-
developed upper parts related to subaerially exposed
mudflats in a marginal marine sabkha. The cycles start
with open marine subtidal-intertidal sedimentation in asso-
ciation with stromatolites (MF1 and MF2) and grade into

' algal '

m). As mentioned
above, some evaporite minerals remain present in the matrix
(i.e., not dolomitized) and consist of laths, rosette-like
aggregates, enterolithic small nodules and castellated
crystals ( sensu Clark 1980 ) which often grow inside the
mat levels.

50 μ

<

with micropinnacles and
net-like structures (Gerdes et al. 2000 ) are common in the

' Elephant skin texture '

Fig. 5.3 (continued) discontinuous thinner silicified zones are present
in the stromatolites (see Fig. 5.3d ) and in the laminar dolostones.
Massive stromatolitic layer (1 m thick), Mouila old quarry, Gabon
(photo 1458/2006/ap). ( d ) Domal hemispherical stromatolites (LLH
Logan ' s type, 1964) with very thin blackish chert layering. Stromato-
litic greyish and whitish laminae are gently convex, without pro-
nounced asymmetry in the domes. They are only interrupted when a
slope is encountered. Massive stromatolitic layer, wide of the stromat-
olitic dome is 16 cm, Mouila old quarry, Gabon (photo 1467/2006/ap).
( e ) Smooth flat laminated dolostone composed of the alternation of
millimetric fine-grained well-sorted peloidal laminar dolopackstone
and thinner homogeneous dolomustone. The laminae are rather parallel
at this scale of the microphotograph but pinch out laterally at at
pluricentimetric scale. Small-sized vertical mudcracks cut several
laminae. Sample MOU26, Mouila old quarry, Gabon (photo 0400/
2006/ap). The sample has been taken in the domal stromatolite of

Fig. 5.3c .( f ) Crinkled fenestral laminar dolostone of the same type as
Fig. 5.1e . The laminae are wavy, some contain sediment ' clots ' or
irregular peloids, particularly near the fenestral fabric. Sample
MOU26, Mouila old quarry, Gabon (photo 0401/2006/ap). The sample
has been taken in the domal stromatolite of Fig. 5.3c .( g ) Irregularly
dolomicrosparitized mudstone with remnants of thin layers of homoge-
neous blackish dolomudstone of the same type of those illustrated in
Fig. 5.3e, f . The dolomicrospar is greyish and displays a patchy distri-
bution, it contains slightly recrystallized dolomicrite matrix. Sample
MOU22, Mouila old quarry, Gabon (photo 0417/2006/ap). ( h ) Strongly
dolomicrosparitized mudstone with very thin uneven remnants of
blackish dolomudstone. The dolomicrospar is coarser than in
Fig. 5.3g and more whitish. Sample MOU23, Mouila old quarry,
Gabon (photo 0409/2006/ap). Scale bars ¼ 1mm( h ) and 400 m

m

(Fig. 5.3f, g )