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strongly reduced, causing an isotope shift towards higher
marine values; and (ii) during highstand systems tracts
(HST) circulation of ramp-top marine water masses is
reduced, and effects of meteoric diagenesis on ramp-top
carbonates much increased, leading to a negative
also cyclic with
thick
subtidal cycles characterized by an
'
'
average thickness of
17 m. Accurate relative sea level
fluctuations are difficult to assess in this
environ-
ment since the facies could have been deposited in a wider
range of shallow water and did not fill completely the
accomodation space. The likely magnitude for such sea
level fluctuations was around 10 to 20 m. Despite no
reliable data on the deepening direction have been recently
reported, we have to mention that Raucq ( 1970 )suggested
a general southward deepening of the basin.
In conclusion, the carbonate succession of the Mbuji-
Mayi Supergroup is a good example of sedimentary and
geochemical records in the late Mesoproterozoic to mid-
dle Neoproterozoic (1150-800 Ma), and in particular
preserves well the local variations of carbonate
deeper
'
'
13 C shift
(e.g. Immenhauser et al. 2003 ). Furthermore, the effects of
meteoric or freshwater diagenesis, observed within REE +
Y distributions (Delpomdor et al. 2013b ) coincides with
negative slopes of the Fischer curve and negative
δ
13 C
δ
excursions.
Similar carbon isotopic ratios that co-vary with third- and
fourth-orders have already been documented in numerous
Phanerozoic successions. For example, Wynn and Read
( 2007 ) show that carbon (and oxygen) ratios of Mississip-
pian ramp-slope muds tend to co-vary at the scale of fourth-
order sequences. There is a 1 to 2
13 C
that reflect the relative importance of circulation of
ramp-top marine water masses and effects of meteoric
or freshwater diagenesis on ramp-top carbonates.
δ
13 C carb in
the maximum flooding zone relative to the low-stand,
followed by a positive shift upward in the high-stand. Posi-
tive carbon isotope ratios reflect increased productivity due
to increased global circulation associated with transgressive
carbonates in the Mississippian. Our 100 m stromatolitic
biohermal succession in the BIIb/BIIc transition is also
transgressive (TST) and shows a positive shift of 8
decrease in
δ
Acknowledgements Financial support from the TOTAL PhD schol-
arship (grant ULB/TOTAL-FR00003322) is gratefully acknowledged.
The Royal Museum for Central Africa (Tervuren, Belgium) is thanked
for providing samples of the Mbuji-Mayi Supergroup. The manuscript
has been greatly improved following reviews and suggestions by M.J.
de Wit and H. E. Frimmel. We would like to thank M. Rottesman for
her critical encouragement.
in
13 C carb occurs at the end
of the HST. Late HST is also characterized in the Mississip-
pian carbonates by more negative values related to the influx
of light carbon by riverine waters. Our late HST at the top of
the BIIc and BIId Subgroups are characterized by lacustrine
environments influenced by riverine input as shown by
REE + Y analyses (Delpomdor et al. 2013b ; Delpomdor
and Pr ´ at 2013 ).
13 C carb while a decrease of 4
δ
in
δ
References
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Conclusion
The Mbuji-Mayi sequence (Sankuru-Mbuji-Mayi-
Lomami-Lovoy Basin), now dated between ca. 1155 Ma
- 800 Ma, consists of approximately 550 m of clastic (BI
group) and 1,000 m of stromatolitic carbonates (BII group).
Our study, based on microfacies analysis, sequence stratig-
raphy, and Fischer plots combined with carbon and oxygen
isotopes, indicates that the succession was deposited in
'
deep
shaly and stromatolitic subtidal environments and
'
subtidal/supratidal marine and non-marine
carbonate/evaporitic environments. Microfacies analysis
shows that carbonate succession, i.e. BIe to BIIe
Subgroups, consists of strata accumulated on a carbonate
ramp, and sequence stratigraphy indicates that the sedimen-
tation was cyclic in the inner ramp, represented by
plurimetric
very shallow
'
'
4 m on average,)
recording relative sea level of a maximum of 4 m, with
fluctuations in the range around 1 m to 4 m. These shallow
water depths, and the abundance of (photosynthesizing)
cyanobacteria, indicate that the water column was
oxygenated. The subtidal outer/middle ramp facies are
'
thin
'
peritidal cycles (
Etat, Universit´ des Sciences et Technologies de
Montpellier et du Centre national de la recherche scientifique.
Centre de Recherche des Zones arides, France No. 14, unpublished
Burchette TP, Wright VP (1992) Carbonate ramp depositional systems.
Geology 79:3-57
'
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