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was obtained on cross bedding in cores and too few
palaeocurrent indicators were measured in outcrop to present
a clear indication of paleoflow.
The bases of many fluvial sequences across the field area
are sharp and characterized by thin conglomeratic units.
Conglomerate beds are quite variable within this FA, and
may include beds of LF2A, 2B, 3A, and 3B, with 4A, and 4B
(e.g., Fig
9.5D
). These conglomeratic units are interpreted as
either basal channel thalweg deposits or gravel bars within
large channels. It represents deposition associated with high
energy, traction dominated depocenters, similar to LF3.
Fine-grained deposits, interpreted as overbank floodplain
facies, as well as in ponded depressions within channels, are
limited to LF7 and are quite rare within this FA, suggesting
the presence of high topographic gradients and minimal
subsidence of the basin during these depositional phases.
What rare fine-grained facies are observed, tend to preserve
features indicative of moderate to intense pedogenesis.
These features include carbonate nodules, deep oxidation,
slickensides and desiccation cracks (Fig.
9.7C, D
), all of
which suggest that semi-arid conditions prevailed, and that
ephemeral flow conditions existed in many of these channel
systems.
A distinctive characteristic associated with some FA3
deposits are heavy mineral laminations, particularly along
foresets in cross-bedded sandstones (LF5A, B) (Fig.
9.6A,
B
). Many FA3 deposits also have conglomeratic lenses com-
posed of intraformational mudstone rip-up clasts, suggesting
local derivation, due to channel incision.
fish bones are also an important component of this FA and
provide further support of a general lacustrine origin and
arid to semi-arid climate.
9.4.2.5 FA5: Aeolian Dune Environments
Aeolian Environments
FA5 is a fairly localized, but important depositional environ-
ment in the Kasai cover sequences. It is characterized by
LF5A, 4A and 6A beds, which are interpreted as aeolian
wind ripple and dune deposits, along with less common beds
of LF8B and 2A that are possibly associated with fringing
interdune environments. The key features of LF5A and 6A
that suggest an aeolian origin for these deposits are the fine-
grained, mature nature of these sandstones, most of which
represent quartz arenites (Fig.
9.6C
). Evidence of frosted
grains and distinctive inverse grading along cross-bedding
foresets (
laminations) are also present. Large-
scale tabular foresets, in the order of several meters, are
observed at several localities in outcrop and certain core
samples are suggestive of this and appear to have high-
angle foresets, relative to those observed in typical fluvial
facies.
Additionally, we commonly observe an interbedded rela-
tionship between FA4 and 5 with sharp to gradational
contacts. Overall, the thickness of aeolian sand dune
deposits are quite small and this is suggestive of relatively
arid conditions, but without a large or consistent upwind
sand source to produce larger dunes or dune fields. Although
difficult to reconstruct from core, these deposits give the
sense that they represent more isolated barchan style
dunes. They do not appear to be regionally extensive across
the Kasai at any one interval, but rather fairly isolated dunes
or dune fields.
“
pinstripe
”
9.4.2.4 FA4: Ephemeral Lake Environments
The most diagnostic facies associated with FA4 ephemeral
lakes are finely laminated siltstones and claystones (LF8A
and 8B) and less commonly, massive mudstones (LF7)
(Fig.
9.7
). Other lithofacies present may include LF6A, 6B,
4A, 5A, 5B, 2A, 2B, 1A, and 1B. Sedimentary structures are
relatively common, dominated by small-scale ripple cross-
laminations and small mudcracks. Fine-scale current and
wave ripples suggest shallow conditions with alternating
tidal and current processes operating. Although not varved,
many examples show a very cyclical alternation between
fine claystone and siltstone laminae suggesting alternating
energy conditions. Some lacustrine units such as in cores
173-X009 and 173-X030 also preserve thin (1-5 cm thick),
irregular zones of a white, partially laminated mineral
identified as chert in XRD that appears to be curled up or
deflected upwards (Fig.
9.7F
). We interpret these features as
likely expansion cracks, or teepee structures, in primary
bedded evaporitic chert deposits. The abundance of
mudcracks in FA4 supports this idea, as does the abundance
of conchostracan fossils in these beds, which are also typi-
cally characteristic of ephemeral lake settings (Eugster
1986
). Other fossils, such as ostracodes, rare bivalves and
9.4.3 Stratigraphic Correlation
We investigated drill core from along a ~500 km long (E-W)
by 100 km wide (N-S) transect along ~6
South latitude,
from west of Tshikapa, through Kananga, to east of Kabinda
(Fig.
9.3
). A fence diagram was constructed from 17 drafted
core logs and one outcrop section, along with basement
intercept and surface topography data to correlate facies
and depositional units across the study area (Fig.
9.9
). The
stratigraphy is complicated and more variable than expected,
making stratigraphic correlations with units described in
other parts of the basin extremely difficult and tenuous. We
identified and subdivided the stratigraphy into six different
lithostratigraphic units that are based on gross patterns of
lithofacies associations within the study area (Figs.
9.4
and
9.9
). Rather than trying to prematurely force stratigraphic
correlations with beds hundreds of kilometers away (e.g.,
in the Cuvette Centrale), we have assigned these units to
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