Geology Reference
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In light of this, attempting to correlate isolated strata and
cores across the Congo Basin using a regional layer cake
style stratigraphy is challenging and possibly misleading,
particularly in the Kasai region where a complex network
of faulting appears to have played a role in topographic
development across the region, minimally from the late
Mesozoic onwards. We constructed a simple 2-D fence
from 17 core logs and one measured section, from Tshikapa
to Kabinda, which clearly demonstrates this point and helps
explain the complex stratigraphy (Fig.
9.9
). Two prominent
sets of faults are inferred from the basement topography and
correspond with lineaments observed in the DEM images
(Fig.
9.3
). A combination of NW-SE and ENE-WSW ori-
ented sets of faults appears to control stratigraphy on the
eastern side of the basin, whereas there appears to be a less
dominant structural control on the stratigraphy in the central
and western portion of the basin. Most faults are interpreted
as extensional normal faults, creating a series of basement
highs and associated grabens. These structures explain the
distribution of Proterozoic and rare Permian strata in the
region, as well as the highly uneven intercepts with crystal-
line basement rocks that prominently control topography
across the region. It is apparent from this study that
prolonged and sporadic movement appears to have taken
place on multiple faults, leading to heavily bisected and
uneven thickness and depth distributions of J1-C4 strata in
the region.
One of the most challenging aspects of this study revolves
around dating and correlation of the cover sequences. Dat-
able volcanic beds are rare and restricted to kimberlites that
are often isolated and difficult to access. However, a fair
abundance of both conchostracan and ostracode specimens
were identified from multiple cores and various stratigraphic
levels across the field area that provide relative dating
opportunities. Ostracode and particularly, conchostracan
biostratigraphy forms the framework for much of the Meso-
zoic continental stratigraphy of the Congo Basin, and hence
we studied the conchostracan specimens from cores,
although not yet the ostracodes, in an attempt to correlate
the Kasai strata with other parts of the basin. Surprisingly,
one of the most significant findings from this study is the
contrasting results that we observed between several bio-
stratigraphic age assessments, which are consistent with
records from other parts of the basin, and the much
younger-aged detrital zircon grains recorded from one of
two samples we analyzed from the C4 beds in the Mbuji-
Mayi/Kabinda area. Whereas the biostratigraphic data
suggests a Late Jurassic to Early Cretaceous (Lualaba
Group) age assignment for unit C4, consistent with the
geologic map assignment of these strata, our detrital zircons
from the lower part of C3 in the same core yielded a cluster
of three ~79 Ma (mean
fourth mid-Cretaceous (113 Ma) grain. These zircon ages
closely resemble the ages of local kimberlites (~70 Ma;
Mbuji-Mayi) and others further afield (~120 Ma; N.
Angola). Although it is possible that the grains are ano-
malous or erroneous ages, but they do correspond quite
well to known kimberlites in the region.
This is confounding on a number of levels. First, it calls
into question the usefulness of conchostracans in the basin as
index taxa for age assessment and correlation. Second, it
suggests that if units C3 and C4 do indeed correlate with the
Kwango Group in other parts of the basin, then the
Cenomanian-Turonian age typically assigned to the Kwango
Group (based on conchostracan and ostracode biostrati-
graphy) may be inaccurate, or that deposition of these strata
is much longer-lived. However, as we have observed even
within the Kasai region, a simpler explanation may be that
there is significant local stratigraphic variation across the
basin and that usage of terms like Stanleyville, Loia and
Kwango is at the root of the problem. These stratigraphic
terms seem to have limited utility outside of their type areas
and better local stratigraphies and nomenclature would prob-
ably help clarify this issue. Although this would add to the
confusion of having more nomenclature to deal with, it
would ultimately lead to less confusion and more precise
correlations.
This study provides important new data on the strati-
graphy of the Kasai cover sequences; however it also
highlights the previously underestimated complexity of the
basin and importance of further work. In particular,
expanded biostratigraphic and detrital zircon studies are
necessary before a formalized local stratigraphy can be
developed for the Kasai Region. The informal subdivisions,
P1-T1, used in this study to subdivide the local stratigraphy
are only placeholders until more data are available to con-
strain the age of these units and their stratigraphic relation-
ship with units outside of the Kasai region.
Conclusions
Detailed investigation of 22 cores and various outcrop
exposures from across the Kasai region provides a basis
for reinterpreting the regional stratigraphy and deposi-
tional environments of these extensive cover sequences.
Analysis of the sedimentary facies of the Kasai cover
sequences reveals numerous discrete lithofacies that com-
bine in various combinations to form five distinct facies
associations, FA1-5, which are interpreted as glacio-
lacustrine environments, alluvial fans, fluvial channels,
ephemeral lakes, and aeolian dune environments, respec-
tively. These environments dominate the stratigraphy and
suggest prolonged arid to semi-arid conditions through-
out the Late Jurassic to Late Cretaceous in the region.
These data, coupled with detrital zircon provenance,
ΒΌ
79.0
6.0 Ma) zircons and a
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