Mesozoic Sedimentary Cover Sequences of the Congo Basin in the Kasai Region, Democratic Republic of Congo - Geology and Resource Potential of the Congo Basin - page 171

brown color to dark reddish brown color is diagnostic of
this facies. Beds have steep dip angles and abundant micro-
faults. Interlaminated siltstone and claystone beds are regu-
lar in places are less regular in other areas.
LF10 is a dark brown diamictite composed of randomly
distributed granule, pebble and cobble size clasts floating in
a weakly laminated to massive sandy mudstone matrix
(Fig. 9.8B ). Clasts range from angular to moderately well
rounded and include red sandstone, limestone, basalt, doler-
ite, quartzite and granite clasts. There appear to be striations
on some pebbles, but this is inconclusive. Most beds are
massive and ungraded, but with zones of weak parallel
bedding.

poorly sorted, poorly rounded and randomly oriented clasts
(Fig. 9.5 ). Matrix support is common in many units,
suggesting deposition by debris flow processes was com-
mon, most likely in mid-to-outer fan settings. LF3A, 3B,
4A and 4B represents deposition associated with high
energy, traction dominated depocenters. The high mud
matrix contents in LF3B and 4B suggest deposition during
flooding events associated with rapid sedimentation in mud-
choked rivers. In some units, pedogenic calcrete covers
many of the clasts indicating prolonged surface exposure
under arid conditions, as would be expected in arid alluvial
fan settings (Fig. 9.5 ). Thickened rinds on the top surfaces
suggest prolonged surficial exposure and paleosol develop-
ment during extended periods of surface stabilization.
The abundance of alluvial fan depocenters is associated
with the tectonic complexity of the Kasai area, specifically
the diverse, uneven and faulted basement topography. The
majority of FA2 deposits are observed from core located
along inferred fault zones. The basal portions of Cores 172-
X286, -X111, and -X146, 173-X009, 157-X015, -X016, and
-X017 all have substantial FA2 deposits (Fig. 9.9 ), which
appear to be associated with synorogenic deposition around
basement uplifts in the eastern part of the study area. FA2
deposits commonly transition to FA3 deposits and the
contacts between the two facies associations ranges from
erosional (unconformable) to gradational.

9.4.2 Facies Associations

Five different facies associations (FA) were identified and
described below based on the repeated associations of differ-
ent lithofacies, sedimentary structures and architectural ele-
ments. These five FAs were interpreted in terms of major
depositional environments and form the basis for our under-
standing and interpretation of the paleoenvironments and
depositional systems of the Mesozoic cover sequences
across the Kasai study area.

9.4.2.1 FA1: Glaciolacustrine Environments
The glaciolacustrine facies association is composed of an
important succession of lithofacies including basal
laminated shales and siltstones (LF8A) grading upwards
into LF9 and LF10 (Figs. 9.8B ), which are unconformably
overlain by poorly sorted conglomerates (LF2A and 2B)
with thin sandstone (LF5A) partings. LF9 is interpreted as
lacustrine shale with dropstones, deposited under glacio-
genic conditions and LF10 as a glacial diamictite deposited
as till in a shallow subaqueous to subaerial setting. This
lithofacies association is only found at a single locality in
the basal 60 m of core 172-X050; with a clearly unconform-
able fault contact with overlying FA2 strata. The overall
upward shallowing and coarsening succession appears to
represent a deepwater lake setting with glacial rafting and
progressive shallowing and infilling. The transition from
LF9 to LF10 marks a transition from lacustrine to subaerial
glacial tillites, followed by reworking and re-deposition by
glacial outwash processes (LF2A, 2B, 5A).

9.4.2.3 FA3: Fluvial Environments
The most common depositional environments identified
across the basin are relatively high-gradient, low-accom-
modation (probably braided) fluvial environments of FA3.
Lithofacies are typically dominated by sandstones of LF5B
and 6B (with less common examples of LF5A and 6A) and
interpreted to represent classic bedload-style fluvial channel
deposits deposited under alternating lower-upper flow
regime conditions.
Trough and tabular cross-bedding is a common feature of
this facies association, commonly showing evidence of soft
sediment deformation and slump features in outcrop expo-
sures. Deformed and overturned crossbedding is common,
and are typically simple, unfaulted, flat-lying structures with
weakly to moderately curved hinges that face up-current. The
sediment was likely water-saturated at the time of defor-
mation, and possibly also liquidized as suggested by the
smooth and unfaulted fold shape and extended upper limb
of the fold. Deformation may be attributed to fluid drag on the
upper surface of the cross-bedded sand body as indicated by
the upstream facing and recumbent nature of the folds, the
confinement to a single cross-bed set and by truncation
upwards by the erosion surface beneath the set above. The
origin of these structures is unclear, but likely a result of rapid
sedimentation or seismic activity. No paleocurrent information

9.4.2.2 FA2: Alluvial Fan Environments
Alluvial fan environments are common within the Kasai
cover sequences and commonly include any combination
of the following lithofacies: LF1A, 1B, 2A, 2B, 3A, 3B,
4A and 4B (with less common examples of LF6A, 6B, and
7). LF1A, 1B, 2A and 2B are dominant and characterized by