Geology Reference
In-Depth Information
contribute to the discussion of the origin of parasequences.
One expects non-random patterns for those produced by
orbital forcing (allocyclicity; Goldhammer et al.
1990
).
Were the cyclicity was produced by sedimentary processes,
for example of tidal-flat progradation (autocylicity), then the
Fischer plots should display more random patterns of cycle
thickness and one would not expect the plots to correlate over
any significant lateral distances. Limitations of the use of
these plots concern principally
five subgroups: BIIa, BIIb, BIIc, BIId and BIIe. Basic igne-
ous rocks have been identified as (i) basaltic lavas overlying
the BII Group at the confluence of the Mbuji-Mayi and
Sankuru Rivers (Cahen et al.
1984
); (ii) dolerite sills
emplaced within the succession close to the BI and BII
contact in the Lomami area; and (iii) within the BI group
along Kiankodi and Lovoy Rivers, with sills of dolerite
(Cahen and Mortelmans
1947
). Along the Kibaran Belt in
the southern part of the SMLL Basin (Fig.
4.1
), polymictic
conglomerates (Kabele and Kabenga conglomerates) with
more than 50 % of clasts material derived from the
Mbuji-Mayi carbonates, were correlated with the Grand
Conglom
´
rat Formation of the Katanga Supergroup (Cahen
and Mortelmans
1947
).
that occur
where oscillations in relative sea level have not been
preserved in the sedimentary strata through, for example
emergence, so that the complete cycle history is not recorded.
Fischer plots can also pick up noise where autocyclic pro-
cesses, such as tidal flat progradation, or periodic faulting,
overprint a eustatic signature.
The purpose of our work is to report advances in the
understanding of the tectono-sedimentary evolution of
the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin in terms
of paleoecology and sea level variations, recorded in the
late Mesoproterozoic—middle Neoproterozoic carbonate
successions of the Mbuji-Mayi Supergroup in the Demo-
cratic Republic of the Congo (DRC), and thereby contribute
to a better understanding of global Late Precambrian climate
fluctuations.
missed beats
'
'
4.3
Geochronology (Table
4.1
)
In the north, oldest siliciclastic BI Group rests unconform-
ably on 2648
22 Ma migmatitic gneisses with granitic to
tonalitic compositions of the Dibaya Complex (Delhal et al.
1976
). In the south, the BI Group overlies the 1155
15 Ma
Kibara Supergroup (Delhal et al.
1966
). An upper limit to the
age of deposition of the Mbuji-Mayi Supergroup is provided
a K-Ar date of 1152
15 Ma (Delhal et al.
1989
,
recalculated to 1118
15 Ma using the decay constants
from Steiger and J¨ger
1977
) on biotite, pyroxene and
amphibole from E-W trending syenodiorite dykes that out-
crop in the eastern part of the Lulua Complex. Concordant
U-Pb dates on detrital zircon grains from the BId Subgroup
yield and age of 1174
4.2
Geological Setting
The Sankuru-Mbuji-Mayi-Lomami-Lovoy (SMLL; Fig.
4.1
)
Basin is located between 6
S and 8
S latitude and 23
E and
26
E longitude (Kasai-Oriental region, DRC), flanking the
Archean-Paleoproterozoic Kasai Craton to the southwest,
the Mesoproterozoic Kibara Supergroup to the east
(Fig.
4.1
). The Mbuji-Mayi sedimentary sequence is undis-
turbed and only weakly affected by regional metamorphism.
The strata have a maximum dip of 3
in the west and
between 20 and 45
in the southeastern parts of the SMLL
Basin (Cahen
1954
). The Mbuji-Mayi Supergroup (Fig.
4.1
,
Table
4.1
) is divided, from oldest to youngest, into
siliciclastic BI and carbonate BII groups (Raucq
1957
,
1970
). In the Sankuru-Mbuji-Mayi area, the lower
siliciclastic series of the Mbuji-Mayi Supergroup (BI
group) is ~500 m thick, and consists of five subgroups:
BIb, BIc, BId, BIE (the latter only visible in the Kafuku
region) and BIe. The BIa subgroup is missing in the
Sankuru-Mbuji-Mayi area, but has been observed near the
Makululu and Kiankodi villages in the southern part of the
SMLL Basin. Detailed descriptions of this subgroup have
been given by Cahen and Mortelmans (
1947
). It consists,
from base to top, of ~1,500 m-thick red quartzites and shales
with an interbedded pink chert horizon. The BII group
consists of a ~1,000 m-thick carbonate successions embed-
ded of thin levels of organic-rich shales, and subdivided into
22 Ma, which is consistent with the
maximum K-Ar age of 1152
15 Ma (Delpomdor et al.
2013a
). The transition between the BI and BII groups,
constrained by
207
Pb/
206
Pb dates on presumed syngenetic
galena, yielded 1040 Ma and 1065 Ma (Cahen
1954
; Holmes
and Cahen
1955
). Recent data from
13
C and
87
Sr/
86
Sr
chemostratigraphy on BII carbonate samples reveal primary
marine signals that are coeval with the Bitter Springs nega-
tive anomaly around c. 810 Ma (Delpomdor et al.
2013a
).
Amygdaloid basaltic pillow lavas that overlie the Mbuji-
Mayi Supergroup at the Sankuru-Mbuji-Mayi confluence,
yield an
40
Ar-
39
Ar date of 928
δ
20 Ma.
40
Ar-
39
Ar dating
on dolerite located in the eastern part of the SMLL basin
yield and age of 888.2
8.8 Ma (Delpomdor et al.
2013a
).
4.4
Sampling and Analytical Methods
4.4.1 Sampling
Drillcores were studied for this investigation. They are
situated within the Sankuru-Mbuji-Mayi area between the
Lubi and Luembe Rivers (Fig.
4.1
). The cores are stored in
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