Geology Reference
In-Depth Information
turn to the environmental data (mainly sedimentology) and
thicknesses provided by elementary parasequences. The dis-
tribution of cycle thicknesses of Mbuji-Mayi carbonates
follow a bimodal pattern with
phototrophic microbial communities near the base of the
photic zone (Batten et al.
2004
). Absence of shallow-water
structures (cross-laminations, mudcracks) and other
indicators of shallow-water flows (ripples or trough-cross
bedding) are consistent with deposition in relatively deep
water (tens to hundreds of meters).
The transition between the deep- and shallow-water
microfacies is highlighted by crinkly laminated
dolomudstones associated with stromatolitic bioherms
(MF5) consisting of muddy-dominant non-branching and
branching columnar stromatolites, developed in upper
subtidal and lower intertidal middle-ramp zones (Schr¨der
et al.
2005
). This interpretation is supported by the nature of
stromatolites:
Conophyton
- and
Kasaia
-forms are
characteristics of the subtidal zone (Bertrand-Sarfati
1972a
),
while
Baicali
- and
Tungussia
-forms developed in shallow
subtidal and intertidal zones (Hoffman
1976
; Bertrand-
Sarfati
1972b
). The importance of carbonate muds
surrounding the stromatolitic carbonates of the Mbuji-Mayi
Supergroup suggests that these microbial forms were built in
quiet water environments without emersion (Bertrand-Sarfati
1972a
). The absence of storm wave and shallow-water
structures, point to quiet water deposition in relatively deep
water (tens to hundreds of meters) on the Mbuji-Mayi outer
ramp. The middle-ramp is bracketed offshore by the storm
wave base and inshore by the fairweather wave base
(Burchette and Wright
1992
).
A similar context with stromatolite accumulation on
slumps and deep-water environments has been reported
elsewhere in Mesoproterozoic reefs of the Victor Bay For-
mation near the Stathcona River on the northern Baffin
Island, Canada (Narbonne and James
1996
). However, the
abundance of evaporite casts in our inner-ramp microfacies
(MF6 to MF9) suggests that Mbuji-Mayi bioherms composed
a barrier, allowing development of a restricted environment,
probably lagoonal, with episodic evaporitic precipitation.
Desiccation cracks and fenestrae associated with bioherms,
point to episodic peritidal environment from shallow upper
subtidal to interdidal zones with high-energy sandy carbonate
'
thin
'
peritidal cycles and
thick
subtidal cycles. The former are from
1 m (or
'
'
less) to
6 m (or more) in thickness, with an average
cycle thickness of 3.4 m (S70 Tshinyama drillcore,
n
¼
35); 3.5 m (Kafuku 15 drillcore,
n
¼
31
), 4.3 m (Bena
Kalenda drillcore,
n
¼
13
); 3.1 m (Bena Tshovu drillcore,
n
¼
12
); 4.8 m (B13 Kanshi drillcore,
n
¼
37
). This leads to
an average thickness of
4 m for peritidal cycles. On the
other hand the subtidal cycles range from
20 m
in thickness, with an average cycle thickness of 17.7 (S70
Tshinyama drillcore,
n
10 m to
¼
7
); 16.3 m (Kafuku 15 drillcore,
n
¼
4
); 4.3 m (Bena Kalenda drillcore,
n
¼
18
); 16.8 m
(B13 Kanshi drillcore,
n
¼
13
).
4.6
Interpretation and Discussion
4.6.1 Paleoenvironmental Reconstruction
(Fig.
4.7
)
The shale microfacies (MF1) consists of organic-rich ben-
thic microbial mats (0.16-0.69 % in TOC; Delpomdor and
Pr´at
2012
) formed at the interface between a quiet basin
and a distal outer ramp, with sporadic sedimentary influx of
detrital minerals and planktonic matter deposited in deeper
parts or in small depressions under anaerobic and dysaerobic
conditions. The transition from the distal outer ramp to the
middle ramp is recorded in the MF2-MF5 evolution. MF2
and MF3 represent lower subtidal dolomitic muds deposited
by suspension, probably below storm wave base as no storm-
generated sedimentary structures were observed. MF4
represents therefore the shallowest water depth in the
subtidal facies zone as suggested by the preserved microbial
filaments or remnants of microbial mats. This suggests that
light may have been a limiting factor for the growth of
Fig. 4.4
(continued) by dolomite crystals. MF6, BIIb subgroup,
sample ULB359, depth:
45.50 m, Bena Kalenda drillcore; (
b
)
submillimetric-thick laminae of fine-grained organic-rich dolomicritic
matrix and light medium-grained organic-poor dolomicrosparitic
matrix. MF7, BIIb Subgroup, sample ULB41, depth:
435.80 m, B13
Kanshi drillcore; (
c
) submillimetric-scale wavy laminae of dark
micritic and
white
to
light grey
dolomitic laminae. MF8, BIIc Sub-
group, sample ULB4352, depth:
like internal foresets. MF9, BIIb Subgroup, sample ULB467, depth: -
388.80 m, B13 Kanshi drillcore.—
Lacustrine environment
:(
f
) dark
planar to undulate dolomicritic with thick laminations (
bottom
),
overlain by dark dolomicritic stromatolitic margin clots (at the
top
).
McVG
micrometric vuggy porosities. MF10a, BIIb Subgroup, sample
ULB353, depth:
47.00 m, Bena Kalenda drillcore; (
g
) rounded to
irregular anastomosing patches of stromatolite-margin clots, selvages
and
214.00 m, B13 Kanshi drillcore; (
d
)
small folded structures of deformations in wavy laminae during the
displacive growth of evaporitic minerals. MF8, BIIc subgroup, sample
ULB4469, depth:
filled by
drusy white
dolomite cements. Notice the
pendant beard-like microcrystalline cements. MF10b, BIIc subgroup,
sample ULB4457, depth:
grumeaux
'
'
128.20 m, B13 Kanshi drillcore; (
h
) dis-
continuous microbial clots of stromatolites filled by drusy dolomitic
cement and bitumen. MF10c, BIIc Subgroup, sample ULB4368, depth:
118.10 m, B13 Kanshi drillcore; (
e
) tightly packed,
fine-grained
dark grey/brown
dolomitic matrix, often silicified, with
tangential ooids (ooid dolopackstone). Notice the wedge-shaped cross-
lamination with planar to irregular layers inclined (average 20-30
)
207.80 m, B13 Kanshi drillcore
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