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Jurassic age by comparison with equivalent biostratigraphic
data from the USA (the Morrison Formation; Sames et al.
2010
) and Brazil (the Alianca Formation; Kr¨mmelbein and
Weber
1971
). Along the northeastern margin of the CB, the
Stanleyville Group intercalates near its base with a 40 cm
thin marker horizon of limestones, known as
Both the Loia and Bokungu Groups are fossiliferous,
containing numerous non-marine phyllopods
(
Bairdestheria kasaiensis
,
Pseudoasmussia banduensis
,
P. dekeseensis
,
Asmussia dekeseensis
,
A. ubangiensis
and
Euestheria sambaensis
; Marli`re
1950
; Defretin-
Lefranc
1967
) and ostracods (
Cypridea elisabethaensis
,
C. maringaensis
,
C. yakokoensis
,
Ilyocypris lomamiensis
,
I. minor
,
Metacypris polita
,
M. pustulosa
and
Pattersoncypris rotunda
; Grekoff
1957
; Colin
1994
), but
the complex biozonation of these (endemic) species does
not allow differentiating the two groups chronostratigra-
phically. However, based on this fossil record, the two
groups were also clearly identified in all the other three
boreholes (Cahen et al.
1960
; Boulouard and Calandra
1963
; Colin
1981
; Colin and Jan du Ch
ˆ
ne
1981
; Chap.
7
, this Topic). New palynostratigraphy in the Samba sec-
tion (Unit S4), between
'
(Cahen
1983b
), which includes a well-preserved
Kimmeridgian fish fauna (e.g.
Ophiopsis lepersonnei
; Saint
Seine
1953
,
1955
; Saint Seine and Casier
1962
; Taverne
1975
). Furthermore, the large quantity of small and juvenile
fish fossils within this horizon suggests proximity to marine
spawning grounds (i.e. laguna; Taverne
1975
).
Biostratigraphically, the Stanleyville Group is also
identified in the center of the basin in the Samba-1 and
Gilson-1 boreholes (Cahen et al.
1959
; Colin
1981
), as
well as from borehole cores at Kinshasa (Defretin-Lefranc
1967
). It is best preserved in the northern part of the central
CB (ca. 322 m at Samba) and thins westward across Gabon
with more continental deposits (e.g. the M
'
Lime Fine
676 m depth,
indicates a late-Lower to Middle Albian age by compari-
son withWest Africanmarine sections (Palynozone CXIIa
of Doyle et al.
1982
; Moron, personal communication,
2011).
3. The uppermost Kwango Group is about 300-400 m thick,
and defined from outcrops in the Kwango region of
southwest DRC (Lepersonne
1945
,
1949
,
1951
). It
overlies a major unconformity across Precambrian base-
ment (e.g. the Kasai Craton) and, in some places, relics of
the Carboniferous to Triassic successions (the Lukuga
and Haute Lueki Groups) and the middle Cretaceous
Loia Group (Cahen
1951
). It is commonly divided into
two
622 m and
'
vone Series;
Mounguengui et al.
2008
).
8.2.2 The Loia, Bokungu and Kwango Groups
The overlying sequences are assigned to the Loia, Bokungu
and Kwango Groups, which outcrop extensively within the
CB (Cahen
1954
; Fig.
8.1
). Their biostratigraphy is less
detailed than that for the Upper Jurassic Stanleyville
Group, and is based on a number of isolated fossiliferous
localities (Cahen
1983a
). Paleontology and palynology indi-
cate a middle Cretaceous age (Albian-Cenomanian) for the
Loia and Bokungu Groups in the center of the basin, and a
Late Cretaceous age for the Kwango Group in the south-
western part of the central CB (Maheshwari et al.
1977
;
Colin
1994
). These three groups correlate biostratigra-
phically with post-rift sequences outcropping along the
Atlantic margin,
(c.f. Lepersonne
1951
):
- A lower, Inzia Formation comprises 50-110 m thick
red sandstones intercalated with red-brown
mudstones. At Tshikapa, in the Kasai region of south-
ern DRC (Fig.
8.1
for location), its basal conglomerates
are diamondiferous (Cahen
1951
).
- An upper, Nsele Formation comprises conglomerates
and coarse red sandstones, between about 100-200 m
thick.
The age of the Kwango Group is essentially based on
paleontology of another distinctive fish fauna (e.g.
Rhipis
moorseli
, Decertids and
Diplomystus
; Saint Seine
1953
;
Casier
1965
), and freshwater phyllopods (
Bairdestheria
kitariensis
,
Pseudoestheria lepersonnei
and
Estheria
lerichei
; Marli
`
re
1950
; Defretin-Lefranc
1967
) and
ostracods (
Afrocythere?
536,
Cypridea kitariensis
,
Darwinula kwangoensis
,
Ilyocypris compressa
,
I.
luzubiensis
,
Metacypris
K3099,
Dolerocypris kinkoensis
and
Paracypria makawaensis
; Grekoff
1960
; Colin
1994
), collected at Kipala and Kitari-Kimbau-Schwetz
in the southwestern part of the central CB (Fig.
8.1
for
location). Paleo-ichthyological studies have suggested
a Cenomanian-Turonian age for the lower part of the
group (Casier
1965
; Taverne
1976
). In the upper part
'
formations
'
in Gabon (e.g.
the Madiela Series;
Mounguengui et al.
2008
).
The Loia and Bokungu Groups are best described in
subsurface from the Units S4 and S3 of the Samba section,
respectively (equivalents of
'
Couches
'
[Beds] 4 and 3 of
Cahen et al.
1959
), overlying a distinct erosion surface etched
across ferruginous sandstones of the Stanleyville Group.
1. The lower, Loia Group (Unit S4) is 280 m thick, and
comprises green and analcime-rich, poorly sorted,
sandstones and mudstones with black shale intercalations
(Cahen
1983a
). The top of this sequence is a distinct
silicified layer (see Sect.
5.2
).
2. The overlying Bokungu Group (Unit S3) is 363 m thick,
and comprises alternating red sandstones with purplish
carbonated mudstones and marlstones that are commonly
highly fractured (
'
clotted texture
'
described by Cahen
et al.
1959
).
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