Paleogeography and Tectono-Stratigraphy of Carboniferous-Permian and Triassic ‘Karoo-Like’ Sequences of the Congo Basin - Geology and Resource Potential of the Congo Basin - page 118

TWT
(sec) SW NE Litho- and bio-stratigraphy Sonic log

AGE OF UNCONFORMITIES

0

Paleogene

M1

Cenozoic
Jurassic-Cretaceous
Triassic

Cenozoic

M2

Jurassic-Cretaceous

Albian

M3
M4

LATE PALEOZOIC MID-MESOZOIC

U3

M5

Permo-Carboniferous

-1

Triassic

M6

Early to mid-Paleozoic

M7

EARLY PALEOZOIC LATE PALEOZOIC

U2

M8

Late Neoproterozoic Permo-Carboniferous

Carboniferous

EARLY PAN AFRICAN LATE PAN AFRICAN

U1

M9

-2

Early Neoproterozoic Late Neoproterozoic

M10

5 km

Daly et al. (1992);
Kadima et al.(2011)

This study

Fig. 7.9 Re-calibration of the seismic reflection against the Mbandaka well logs (modified from Kadima et al. 2011 ), and ages of major
unconformities. Biostratigraphic data is from the original study of Colin and Jan du Ch ˆ ne ( 1981 )

1. A lowermost, thick and weakly banded supersequence
corresponds in the Mbandaka section to 583 m thick
red-brown siltstones coarsening upward into conglo-
meratic sandstones (Unit M9) and to 786 m thick
conglomerates fining upward into grey-brown conglom-
eratic sandstones with dark siltstone intercalations (Unit
M8). Along the seismic profile these two lithostra-
tigraphic units are conformable (Fig. 7.8 ), and in unison
they are truncated by a second regional erosion surface
(U2; interpreted as an early Paleozoic unconformity by
Daly et al. 1992 ). A palynological study of the Mbandaka
section (Colin and Jan du Chˆne 1981 ) indicates an age
younger than mid-Carboniferous within the underlying
grey-brown siltstones (Unit M8), on a core-sample
between

to the Albian-Cenomanian (Colin and Jan du Chˆne 1981 ).
This rather suggests a lateMesozoic age for the underlying
unconformity (U3; Figs. 7.8 and 7.9 ).
3. A third and uppermost supersequence corresponds in the
Mbandaka section to 683 m thick red arkoses and green
mudstones coarsening upward into carbonated sandstones
(Units M2 to M4), and to 161 m thick varicoloured
claystones (Unit M1), dated to the middle Cretaceous and
Paleogene, respectively (Colin and Jan du Ch ˆ ne 1981 ).
In summary, the seismic reflection data from the 1970s
reveals a Phanerozoic development of the CB punctuated by
two regional episodes of deformation that previously were
interpreted by Daly et al. ( 1992 ) to be related to: (1) the Pan
African collisions during the end Precambrian and early
Cambrian (~530-650 Ma), and (2) a possible phase of dis-
tant contraction induced by development of the Cape Fold
Belt in southern Africa during the Permian-Triassic
(~250 Ma). A recalibration of the seismic reflection against
the Mbandaka well logs (i.e. Integrated Travel Time of the
Sonic Log), tied to the litho- and bio-stratigraphy (Fig. 7.9 ),
shows that the two lower, Carboniferous-Permian and Trias-
sic supersequences (Units M5 to M9) unconformably overlie
deformed Pan African carbonate-siliclastic rocks (Unit
M10). These two supersequences are separated by a regional
late Paleozoic unconformity (U2) that possibly relates to far-
field deformations resulting of multiple collisional processes
flanking the margins of Gondwana (Daly et al. 1992 ; Trouw
and de Wit 1999 ; Newton et al. 2006 ; Dabo et al. 2008 ; see
Chap. 11 , this topic). The uppermost unconformity (U3) is
overlain by Jurassic-Cretaceous and Cenozoic sediments

2,825 m depth. This suggests
a late Paleozoic age for this second angular unconformity
(U2; Figs. 7.8 and 7.9 ).
2. A thick middle succession represented by a discontinu-
ously banded supersequence corresponds in the Mbandaka
section to 467 m thick red-orange conglomerates (Unit
M7), 619 m thick red-brown fine quartzitic sandstones
(Unit M6) and 692 m thick red-brown coarse sandstones
with siltstone intercalations (Unit M5). Along the seismic
profile (Fig. 7.8 ), these units are truncated by a very
conspicuous, third regional erosion surface (U3) that was
interpreted as a late Paleozoic unconformity by Daly et al.
( 1992 ). However,

2,805 m and

this unconformity is at a depth of

844 m in the Mbandaka section, and directly covered
by 40 m thick red arkoses (Unit M4) and fossiliferous
grey-green mudstones (Unit M3) dated by biostratigraphy