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 56

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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)

Franck Delpomdor, Christian Blanpied, Aurelien Virgone, and Alain Pr ´ at

4.1 Introduction sedimentary evolution by the formation of carbonate

buildups, because they are controlled by biogenic production
variations as well as by sea-level fluctuations, accommoda-
tion, and sedimentation rates (Fl¨gel 2004 ). This is also
especially relevant in the Precambrian, because most of the
carbonate sediments were directly precipitated with micro-
bial assistance (Day et al. 2004 ). Precambrian sedimentary
successions have recently been discussed within the context
of sequence stratigraphy (Catuneanu et al. 2005 ; Martins-
Neto 2009 ). This approach interprets the sedimentary
changes as a response to the interaction occurring between
authigenic (i.e. the sedimentology and facies analyses), and
allogenic processes, (i.e. sedimentation including climate,
tectonics and sea level changes) regardless of scale and age
of the strata (Vail et al. 1977 ; Miall 1997 ; Posamentier and
Allen 1999 ; Catuneanu 2006 ). The largest sequences (Vail
et al. 1977 ; Krapez 1996 ), recognized as first-order, last
between 200 to 400 Myrs, and are subdivided into second-
order (10-100 My), third-order (1-10 My), and fourth-order
(0.01-1 My) sequences etc. (e.g. Vail et al. 1977 ; Miall
2000 ). The durations of these sequences are related to global
tectonic processes (first to third orders) and orbital controls
(fourth to fifth orders) with possible superimposed eustatic
fluctuations. These sea level fluctuations may be revealed
by Fisher plots, a popular tool in cyclostratigraphy to
graphically illustrate deviations from the average thickness
of a sedimentary cycle (Fischer 1964 ). Fischer plots provide
graphically and illustrate such thickness variations, and they
help identify also longer-term trends and patterns. The thick-
ness variations in the stacking patterns of meter-scale (or
more) may reflect third-order changes in accommodation
space. Because Fischer plots have often been criticized as
being too subjective, and speculative, statistical analyses
should be carried out, and ideally based upon more than
30-50 cycles to install confidence in the models (Saddler
et al. 1993 ). Fischer plots, combined with the analysis of
cycles, are useful and upward changes in facies can be exam-
ined in the context of upward changes in cycle thickness
through accommodation cycles. Fischer plots can also

The late Mesoproterozoic to middle Neoproterozoic times
heralded extraordinary climatic and biological changes deeply
linked to global tectonics (e.g. Kaufman et al. 1993 , 1997 ;
Halverson et al. 2007 ). These dramatic climatic changes,
global carbon cycling and atmospheric oxygen budget
(Knoll et al. 1986 ; Derry et al. 1992 ;Knoll 1992 ; Des Marais
1994 ; Strauss 1997 ; Hoffman et al. 1998 ;Canfield 1999 )are
recorded globally by chemostratigraphic fluctuations of C, Sr
and S isotopic compositions in the worldwide late
Mesoproterozoic (Stenian, i.e. 1200-1000 Ma; Fairchild
et al. 1990 ; Des Marais et al. 1992 ; Knoll et al. 1995 ;
Podkovyrov et al. 1998 ; Canfield 1999 ; Kah et al. 1999 ;
Bartley et al. 2001 ), and mid- (Cryogenian, i.e. 850-635 Ma)
to late (Ediacaran, i.e. 635-541 Ma) Neoproterozoic series
(Derry et al. 1989 ;KaufmanandKnoll 1995 ; Asmeron et al.
1991 ; Halverson et al. 2005 , 2007 ; Kaufman et al. 2006 ;
Tewari and Sial 2007 ). The origin of these large global isotope
fluctuations is still being debated. In spite of the abundance of
late Mesoproterozoic to middle Neoproterozoic sedimentary
rocks chemostratigraphically studied with stable isotope geo-
chemistry to establish regional or global inter-basinal
correlations (Melezhik et al. 2001 ; Shields and Veizer 2002 ;
Halverson et al. 2005 , 2010 ), their depositional environment
remains poorly constrained.
Phanerozoic carbonate rocks may reflect short-term
marine environmental changes with high frequency sea-
level fluctuations as well as long-term patterns of