Geology Reference
In-Depth Information
on interfluve plateaus in northern Angola. The Kalahari
Group has been divided into a lower formation (Gr
`
s Poly-
morphs or Gr`s Polymorfico; Kalahari B of Cahen et al.
1946
) and an upper formation (Kalahari Sands, Ochre
Sands or Sables Ochres, Kalahari C of Cahen et al.
1946
).
The
9.3
Methods
This investigation focuses on cores recovered from explora-
tion drill hole localities from three primary areas: (1) the
Tshikapa area in the western Kasai; (2) the Kananga to
Mbuji-Mayi area in the central Kasai; and (3) the Kabinda
area, in the eastern Kasai (Fig.
9.3
). In total, 196 holes were
drilled between 2006 and 2008, 22 of which were selected
and studied (Fig.
9.3
), representing 16 localities. The
selected cores have a combined intersect of 2,816 m (with
individual cores varying in length between 59 m and 282 m).
Fifteen holes terminated in basement and seven in sedi-
ments. All cores were photographed, and detailed core logs
were constructed following the methods outlined by Johnson
(
1992
). Typical sedimentary features, including grain size,
sorting, bed thickness, color, sedimentary structures, minera-
logy and fossil content, were observed and recorded while
particular attention was paid to the distribution, thickness,
thickness variation and clast lithologies of conglomerate
units within the sequences. Cores were sampled for micro-
fossils, and these were investigated and imaged using scan-
ning electron microscope. Conchostracan fossils were
identified and used to assist in correlations and help interpret
age and depositional environments. A suite of other samples
was collected for geochemical and provenance studies. In
addition, detrital zircons were analyzed from a core sample
collected near the base of core 173-X009 in the Kabinda area
via U-Pb laser ablation inductively coupled plasma mass
spectrometry (LA-ICPMS) at the Advanced Analytical
Facility at James Cook University.
Field-based investigations were conducted across various
portions of the Kasai area. Good outcrop exposures were
identified and studied, primarily along river drainages, but
also within large erosional cirques to supplement the core
facies data and constrain alluvial architecture (Fig.
9.3
).
Photo mosaics and measured sections were constructed for
certain outcrops, as well as facies and alluvial architecture
analysis. Paleocurrent measurements and sandstone prove-
nance samples were also collected where possible. Exami-
nation of cores and outcrop data permits the subdivision of
strata into 10 lithofacies (LF), based on distinctive sedimento-
logical characteristics (sensu Miall
1978
). Each lithofacies is
interpreted in terms of its basic depositional characteristics
(Fig.
9.4
). Repeated combinations of lithofacies have been
grouped together to form six lithofacies associations (FA),
which form the basis for interpreting the architecture and
depositional environments of the cover sequences in the
Kasai region. Core and outcrop data was synthesized, along
with surface geology and SRTM imagery to identify regional
structures and to construct a fence diagram for correlating
the Mesozoic cover sequences in the Kasai portion of the
Congo Basin.
20 m thick Gr`s Polymorphs reflect the end-
Cretaceous to mid-Tertiary planation event and associated
aeolian deposition (see Giresse
2005
and references therein)
and comprise calcretised and locally silcretised sands over-
lying deeply kaolinised weathering profiles. The overlying
Sables Ochres Sands reflect a return to fluvial conditions.
The last planation event was formed in the Pliocene and
characterized by gravels with clasts of reworked Gr
`
s Poly-
morphs as well as laterites and phosphatic sediments
(Giresse
2005
). These are overlain by Quaternary alluvial
deposits.
Renewed interest in the sedimentary cover sequences in
the DRC has recently led to the re-examination of original
cores. In this volume, Linol et al. (Chap. 8, this Topic) review
and update the Mesozoic and Cenozoic cover sequences of
the Cuvette Centrale, whilst this paper focuses on the exami-
nation and characterization of the southern Kasai portion of
the Congo Basin, and is discussed in detail below.
<
9.2.3.4 Diamonds and Kimberlites
Direct age constraints on the stratigraphy of the cover
sequences are scarce and limited to kimberlite volcanoes.
Alluvial and eluvial diamonds have been recovered and
mined within the southern margin of the Congo Basin
since the beginning of the twentieth century with total pro-
duction to date of
600 million carats. Recognition in the
1940s of diamondiferous kimberlite pipes near the town of
Mbuji-Mayi first demonstrated a local source for at least
some of the diamonds in the region. To date,
>
500 occur-
rences have been discovered within the southern margin of
the basin and two major episodes of kimberlite emplacement
have been recognized, first during the Early Cretaceous,
documented in northeastern Angola (Albian-Aptian,
145-113 Ma, Luxinga and Catoca clusters: Eley et al.
2008; Robles-Cruz et al.
2012
), and again during the Late
Cretaceous, documented in the DRC (Maastrichtian,
~70 Ma, Mbuji-Mayi and Tshibwe clusters: Davis
1977
;
Sch¨rer et al.
1997
; Kerschhofer et al.
2000
). In northeastern
Angola, the Early Cretaceous kimberlites are overlain by
Calonda Formation sediments, which are an important col-
lector of diamonds. The basal Calonda
>
“
”
is a
30 cm-2 m thick matrix-supported gravel with intra-
formational and extra-basinal clasts in a fine-grained and
poorly sorted muddy sandstone matrix and is characterized
by a clast assemblage that includes agate and chalcedony
pebbles. The kimberlite emplacement ages provide maxi-
mum constraints on the age of the Calonda Formation and
correlative Loia Group in the DRC.
conglomerate
Search WWH ::
Custom Search