Geology Reference
In-Depth Information
biostratigraphy, and facies stacking patterns, provide
context for subdividing the Kasai cover sequences into a
temporary, informal stratigraphy until additional work on
the age of these units is available. The informal stratigra-
phy includes a basal glacio-lacustrine dominated
sequence of Permian age, termed P1. This unit has limited
spatial extent and probably represents an erosional rem-
nant of a potentially much more widely distributed
sequence of Permian-age rocks in the Kasai region.
Overlying this, but only in the western part of the Kasai
near Tshikapa, is a package of mature fluvial-aeolian
sandstones and thin lacustrine shales of probable Late
Jurassic age, termed J1. Also exposed exclusively in the
western part of the basin, is a distinctive succession of
maroon-purple, heavy mineral-rich, cross-bedded quartz-
ose sandstones of predominantly fluvial origin, termed
C2. Rare basal conglomerates, termed unit C1, sit con-
formably beneath C2 sandstones in isolated cores. In the
central and eastern side of the basin from Kananga to
Kabinda, immature, coarse-grained conglomerates of
alluvial fan origin rest on a highly uneven basement
topography. These synorogenic conglomerates are
conformably overlain by a heterogeneous sequence of
sandstones and laminated shales and siltstones of fluvial,
aeolian and ephemeral lake origins. They are collectively
termed C4 and represent the thickest and most wide-
spread unit in the Kasai region. A pilot detrital zircon
sample collected from the base of C3 yields a ~79 Ma
population of detrital grains, which sharply contrasts the
Late Jurassic to Early Cretaceous age suggested by
conchostracan biostratigraphy. These results suggest a
Late Cretaceous age (or younger) for units C3 and C4.
The unexpected local variability and stratigraphic com-
plexity observed in the cover sequences from the Kasai
region demonstrate a more tectonically active Mesozoic-
Cenozoic basin history then has previously been
assumed, with synorogenic movements on major fault
systems apparently playing a large role in the local stra-
tigraphy. These findings necessitate even greater care and
study in the Kasai and other portions of the basin before
basin-scale correlations can be fully assessed.
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Acknowledgements We would like to acknowledge the exploration
manager of De Beers Exploration for support in the research of
the Congo Basin geology and for permission to present this
work. Samantha Perritt is thanked for her contributions to our under-
standing of the geology of the region and Mike de Wit, Anthony Revitt,
Mike Roberts, Mike Lynn, John Ward and Renato Spaggiari for
valuable discussions over the years. Mike de Wit and Maarten J. de
Wit are thanked for their valuable review contributions. Former joint
venture partners MIBA, BUGECO and African Diamonds are grate-
fully acknowledged for permission to carry out research on the
drill cores.
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