Petroleum Potential of the Congo Basin - Geology and Resource Potential of the Congo Basin - page 362

discontinuities can be seen between the Aruwimi and the
Lukuga Groups, either in the Dekese core or in the seismic
profiles. However, a sedimentary hiatus may exist between
the two, corresponding to the Ordovician-Early Carbonifer-
ous period during which Gondwana drifted across the South
Pole (located, respectively, in Algeria in Early Ordovician,
in Guinea in Late Ordovician, in Angola in Late Devonian,
in northern DRC in Early Carboniferous, in South Africa in
Mid Carboniferous, and Antarctica in Late Carboniferous; e.
g. Torsvik and Cocks 2011 ).
The Lukuga Group contains a series of glacial to
periglacial massive diamictites and varval dark grey shales
in the Dekese core (couches F-G of Cahen et al. 1960 ),
interpreted to be deposited in a large basin adjacent to
mountain systems, or as moraine deposits in paleo-glacial
valleys outcropping in East DRC. They are overlain by post-
glacial brown sandstones and shales in the Dekese core
(couches D-E of Cahen et al. 1960 ), or sandstones with
coal seams in the Kalemie region along the western shore
of Lake Tanganyika (Fig. 18.1 ), in the Lukuga coal field
(Fourmarier 1914 ; Jamotte 1931 ; Cahen and Lepersonne
1978 ). In the latter locality, fossiliferous reddish sandstones
and mudstones of the Lower Triassic Haute Lueki Group
overly with a local unconformity Permian sediments
(Jamotte 1931 ; Cahen and Lepersonne 1978 ). The Triassic
has not been formally recognized in the Samba and Dekese
wells, but might be present in reduced thickness in the
Dekese well above the Lukuga Group (Linol 2013 ; see also
Linol et al., Chap. 7 , this Topic). Linol ( 2013 ) also proposed
that the red arkoses of the Samba core could be of Triassic
age. In absence of chronological evidence (to date, no spore
and pollens or other fossils have been found), we support
the views of Cahen et al. ( 1959 ) who correlated the red
sandstones of the Samba well to the Aruwimi, Inkisi and
Biano Groups (Fig. 18.2 ). Cahen et al. ( 1959 ) also men-
tioned that undetermined organic material was found in
one place. This would suggest that the red sandstones are
younger than Ordovician. But in their conclusion, Cahen
et al. ( 1959 ) did not take into account this observation as
an age constraint.
A subsequent regional unconformity separates the Paleo-
zoic (and possibly also the Triassic) from the overlying
Jurassic-Cretaceous and Cenozoic successions that comprise
a third sedimentary unit (Fig. 18.5 , seismic unit C of Kadima
et al. 2011a ; Linol et al., Chaps. 8 , 9 and 11 this Topic). Its
base is defined by the Stanleyville Group, which is exposed
along the upper course of the Congo River (locally named
Lualaba), south of Kisangani (Fig. 18.1 ), with up to 470 m of
lacustrine sandstones with organic-rich shales and some
limestone (Passau 1923 ; Sluys 1952 ; Grecoff 1957 ; Cox
1960 ; Lombard 1960 ; Lepersonne 1977 ; Cahen 1983a ;
Colin 1994 ). Towards the basin centre, in the Samba well,
323 m of fluvial-lacustrine red sandstones with thin layers of

bituminous shales are also attributed to the Stanleyville
Group; here they directly overlie the red arkoses attributed
to the Aruwimi Group (Couches 5; Cahen et al. 1959 ). In
contrast, the Stanleyville Group was not recognized in the
Dekese well (Cahen et al. 1960 ) but it occurs in a condensed
section in the Kinshasa area (Egoroff and Lombard 1962 ;
Defr ´ tin-Lefranc 1967 ). The depositional environment of
the Stanleyville Group is generally interpreted as lacustrine
(Grecoff 1957 ; Cox 1960 ; Cahen 1983a ; Colin 1994 ). A
limited marine influence was initially suggested by de Sainte
Seine and Casier ( 1962 ) on the basis of fossil fishes found
near the base of the Stanleyville Group. After a
paleoichthyological study, Taverne ( 1975a , b , and personal
communication, 2014), however, no clear evidence for a
marine influence during deposition of the Stanleyville
Group. A Kimmeridgian age was proposed by de Sainte
Seine ( 1955 ) on the basis of fossil fishes, but Colin ( 1994 ),
on the basis of palynological and micropaleontological
evidence, considered it as Middle Jurassic (Aalenian-
Bathonian).
After a long depositional hiatus, the succeeding Middle
Cretaceous Loia and Bokungu Groups were deposited across
the central CB (Lepersonne 1977 ; Maheshwari et al. 1977 ;
Cahen 1983b ). The Loia Group (Late Aptian-Early Albian;
after Colin 1981 ) was first recognized in the Dekese well
by254 m of aeolian sand dunes (Couches C of Cahen et al.
1960 ; Linol 2013 ) and in the Samba well, by 280 m of
shallow lacustrine sandstones and mudstones with a few
thin black shale levels (Cahen et al. 1959 , 1960 ). According
to the more recent chronostratigraphic correlations of Linol
( 2013 ; Linol et al., Chaps. 8 and 11 , this Topic) , the aeolian
sand unit (the Dekese Formation) is Late Jurassic-Early
Cretaceous and not part of the Middle Cretaceous, reducing
part of the hiatus between the Stanleyville and the Loia
Groups. The Bokungu Group (Late Albian, after Colin
1994 ), with fluvio-deltaic sandstones and siltstones
(372-439 m), is unconformably overlain by unconsolidated
siliceous sandstones of the Cenomanian Kwango Group
(250-280 m). The Paleogene is represented by the Gr`s
Polymorphe Formation (Eocene, after Colin 1994 ) compris-
ing silicified sands covering a prominent erosion surface,
and in turn by the Neogene

(70-90 m in total
for the Cenozoic; Cahen et al. 1959 , 1960 ; Cahen 1983b ;
Lepersonne 1977 ).
This above summary demonstrates that the sedimentary
sequences display distinct lateral thickness and facies vari-
ations at the basin scale. It highlights also some disagree-
ments about their age (e.g. Lower Paleozoic versus Triassic
for the red quartzites in Samba). Specifically, locally angular
discontinuities between the seismic packages B and C (not
shown here) range at least from the Early Paleozoic to the
Early Jurassic in the Samba well, and from the Late Triassic
to Early Cretaceous in the Dekese well; and is almost absent

' Ochre Sands '