Sedimentology and Sequence Stratigraphy of the Late Precambrian Carbonates of the Mbuji-Mayi Supergroup in the Sankuru-Mbuji-Mayi-Lomami- Lovoy Basin (Democratic Republic of the Congo) - Geology and Resource Potential of the Congo Basin - page 62

reefal middle ramp (MF5; Fig. 4.3f-h ), a restricted back-reef
inner ramp (MF6-MF9; Fig. 4.4a-e ), closed lacustrine ponds
(MF10; Fig. 4.4f-h ) and carbonate-evaporite transitional
environments (MF11; Fig. 4.5a-h ). The major characteristics
of these microfacies are briefly described in Table 4.2 .

Tshinyama drillcore). Overlying parasequences (10-42) are
thinner, ranging from 1 m to

10 m (average thickness is
3.6 m) except for two of them (parasequence 23 is 25 m-
thick; and parasequence 30 is 16.4 m-thick) that are six times
thicker than the average cycle thickness. The Fischer curve
reveals three packages (parasequence sets I, II and

III), showing a slight thinning upward trend.
Parasequence thicknesses (43 to 57) in the BIIb Subgroup
(Bena Kalenda drillcore) fall in the same metric range of
thickness than the previous parasequences of the BIc-d/BIe
Subgroups. Their average thickness is 4.3 m (but for thicker
parasequences 48 and 49, 15.9 and 20.7 m-thick, respec-
tively) and the Fischer curve is stabilized without significant
thickness variations of parasequences, but for thicker
parasequences 48 and 49 (15.9 and 20.7 m-thick,
respectively).
The BIIc Subgroup in the B13 Kanshi drillcore contains
50 parasequences (55 to 104) recording shallowing-upward
sedimentation. As for previous parasequences, a bimodal
distribution of thicknesses is the rule with a 4.8 m-thick for
'

cyclic

' '

4.5.2 Sequence Stratigraphy

We present a synthesis of the sequence stratigraphic framework
of late Mesoproterozoic—middle Neoproterozoic series in the
Mbuji-Mayi Supergroup (Fig. 4.6 ) based on the recognition/
definition of elementary sequences or fifth order rhythms/cycles
resulting from the interpretation of our lithologic curve. The
recognized parasequences, identified as shallowing-upward
aggrading or prograding lobes from marine to near-shore
settings present a bimodal distribution of their thicknesses, the
most abundant being 5 m-thick on average, the others

> 10 m-thick on average (Delpomdor and Pr´at 2012 ).
Fischer plots are extremely useful for comparing and for
assessing variations in long-term sea level. The Fischer plots
were only used with cycles that show full regression to
intertidal-supratidal facies, since it is only then that one
can be confident that accommodation space has been
completely filled. In contrast, Fischer plots of subtidal cycles
may not necessarily reflect the full amount of available
accomodation space and in our case for

<

thinner parasequences (37 parasequences, not including

'

> 10 m-thick parasequences) and 16.8 m for ' thicker '

parasequences (13 parasequences, not including 10 m-

thick parasequences). The average parasequence thickness is
7.9 m. Parasequences 55 to 67 are similar to parasequences
described at the top of the Bena Kalenda drillcore and have a
similar average thickness (4 m-thick excluding

<>

10 m-thick

>

the first

'
MF1-MF5 cycle or standard parasequence that is dominated
by subtidal facies. The

'

parasequences 61 and 64).
Despite the discontinuous profile of the Bena Tshovu
drillcore in the BIId Subgroup, parasequence facies contents
and thicknesses (on average 3.1 m-thick for parasequences
105-117) are similar to those of the second part of the B13
Kanshi drillcore. Moreover, the upper part of the BIId
Fischer plot (parasequences 115-117) has a slight positive
slope. The following parasequences in the BIIe subgroup are
in the same range as the previous ones (i.e. 3.9 m-thick,
excluding

MF6-MF11 cycle or stan-
dard parasequence is associated with upper subtidal-
supratidal environments in very shallow water. The general
setting of both standard sequences can be considered as
subtidal-peritidal.

second

' '

4.5.2.1 Mbuji-Mayi Fischer Plots
Our Fischer curve (Fig. 4.6 ) starts with the clastic BIb
Subgroup and the clastic/carbonate lower part of the BIc-d
Subgroup (in the northern SMLL Basin with the S70
Tshinyama drillcore and in the central SMLL Basin with
the Kafuku 15 drillcore) and presents a strong positive slope
with succession of parasequences 1-9 (in the S70

10 m thick parasequences).

>

4.5.2.2 Parasequence Thicknesses
To determine the magnitude of relative sea level change
needed to produce observed metre-scale cyclicity, we now

Fig. 4.3 (continued) minerals in a shaly matrix. MF1, BIIc Subgroup,
sample ULB4244, depth: 278.00 m, B13 Kanshi drillcore. Marine
lower subtidal environment (outer ramp) :( c ) grey/brown , fine- to
medium-grained dolomitic matrix. McBC—Micrometric intercrystal-
line porosity. MF2, BIe Subgroup, sample ULB15, depth:

BIIe subgroup, sample ULB75, Handspecimen 628a. Marine upper
subtidal environment (middle ramp) :( f ) organic-rich dolomicritic
films in columnar stromatolite. McFR micrometric-scale fracture
porosity. MF5, BIIc Subgroup, sample ULB4238, depth: 281.40 m,

B13 Kanshi drillcore; ( g ) superimposed linear and coalescent
dolomicritic clots and grumeaux in a dolomicritic matrix. MF5, BIIc
Subgroup, sample ULB490, depth:

88.30 m,
S70 Tshinyama drillcore; ( d ) submillimetric-thick laminae of dark
grey/brown microcrystalline organic rich dolomite and light grey/
light brown medium-grained organic-poor dolomite MF3, BIIb sub-
group, sample ULB431, depth:

365.80 m, B13 Kanshi drillcore;
( h ) replacive drusy dolomites in pore-filling and space-filling
interparticules and vugs. Notice dark amorphous bitumen in the
cavities. McVG micrometric-scale vuggy porosities. MF5, BIIc Sub-
group, sample ULB4256, depth:

420.00 m, B13 Kanshi drillcore; ( e )
submillimetric-scale, continuous wavy laminites of dark grey/brown
organic-rich and light grey/light brown organic-poor dolomite. MF4, 262.40 m, B13 Kanshi drillcore