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 70

Omachtenia (?)

Stratifera

Pond

Shoal

Laminar
crusts

Collapse
breccia

Stromatolites

MF9 MF10c

MF10a MF10b

High Tide

Baicalia

Desiccation

Mud

Tungussia

Kasaia

Conophyton

MF11

Lacustrine
Environment

MF10

MF9

MF8

Evaporative
Environment

MF7

MF6

MF5

MF4

MF3

MF2

Marine
Environment

MF1

Lithology

Sedimentary structure

OM-rich dolomudstone

Planar laminae
Wavy laminae

Stromatolite

Dolomite Oolite/Oncolite

Desiccation crack

Evaporate

Dolomudstone

Fig. 4.7 Schematic reconstitution of depositional environments of the
carbonate Mbuji-Mayi ramp. Idealized 3D-model illustrating the dis-
tribution of microfacies (MF1-MF11) from the outer-, middle- and

inner-ramp system, with evaporative lacustrine ponds and sabkha. No
vertical and horizontal scales. Ecological distribution of stromatolites is
also shown in the middle ramp zone

Fischer plots: thin cycles characterize a stabilized curve with
slight falling limbs, while thick cycles dominate the rising
limbs of the plot. This suggests that fluctuations in eustacy
had a strong control on the cyclicity

masses in ramps affecting carbonates during early diagene-
sis. In contrast, Allan and Matthews ( 1982 ) argued that the
low isotopic values of carbonates in the ramp are mainly
related to (i) influence of 18 O-depleted early meteoric fluids
and a 13 C-depleted soil-zone CO 2 (overprinting heavier
marine values during a sea level fall and (ii) effects of
' 4.6.4 Sequence Stratigraphy Versus Carbon

Isotope Curve

with oxidation of organic matter in
CO 2 , which increase of bicarbonate concentrations in shal-
low poorly circulated sea water masses (e.g. Holmden et al.
1998 ). In our study, the relationship between cycle-thickness
variations reflecting third-order changes, and

sea-water ageing

'

13 C isotopic data
(Delpomdor et al. 2013a ) displays alternations of

Combining the Fischer plots with our δ

13 C
positive-negative shifts in tune with cycle-thickness
variations that reflect third-order and possible fourth-order
changes in accommodation space available. The BII
carbonates exhibit

13 C variations
in the Mbuji-Mayi carbonate Supergroup, are linked
(Fig. 4.6 ). We have shown that transgressive sequences
highlighted transgressive systems tracts (TST) while

δ δ

rela-

'

18 O isotopic trends towards
higher values, reflecting open-marine signatures episodi-
cally subjected to evaporitic conditions, and towards lower
values reflecting early meteoric and burial signatures
(Delpomdor and Pr´at 2013 ). Immenhauser et al. ( 2003 )
attempted to explain local variations of C and O isotope
shifts in the Late Carboniferous Atokan series in northern
Spain as a result of chemical changes of marine water

13 C and

δ δ tive stages of

highstand systems tracts (HST). Two trends are identified (i)
'

' regressive sequences developed during ' late '

long term ' negative slopes in the Fischer curve, with nega-

13 C excursions; and (ii)

tive δ long term positive slopes

' '

13 C excursions. These trends sug-
gest that (i) during transgressive systems tracts (TST) circu-
lation of ramp-top marine water-masses was enhanced, and
effects of meteoric diagenesis on ramp-top carbonates were

associated with positive δ