Facies Analyses, Chronostratigraphy and Paleo- Environmental Reconstructions of Jurassic to Cretaceous Sequences of the Congo Basin - Geology and Resource Potential of the Congo Basin - page 146

H

Bed-load

G

x10 m

sub-section H

sub-section

Depth Facies Grain-size

Depth Facies Grain-size

C S f S m S c S G r P

C SfSmS cS Gr P

-540 m

Gmc

-400 m

Planar-bedded
carbonate
conglomerates

Gmc

-545 m

St

-405 m

Fmc

Massive
marlstones

Planar-bedded
coarse sands

Sl

-550 m

Shallow lake
deposits

Fluvial

-410 m

Gtc

Sm

Calcretized sands

Sr

Fluvial

Fig. 8.12 Semi-perennial lake facies succession from the upper part of
the Samba core-section (Unit S3; Fig. 8.8b ). Sub-section G: purplish
carbonated mudstones and marlstones (Fmc facies) with fossil remains.

Sub-section H: planar-bedded sandstones (Sl facies) interbedded with
intraformational (fluvial) carbonate-conglomerates (Gmc facies),
indicating alternating shallow lacustrine and fluvial conditions

silicified mudstone (hardpan) at depth -565 m that marks a
hiatus of sedimentation (Fig. 8.8b ).
The Loia Group (Unit S4) is 280 m thick and includes two
main depositional sequences:
• A lower sequence starts with relatively thick (8 m), thinly
laminated green shales and upward-coarsening flood
deposits (sub-Unit S4a) that record the first main maxi-
mum transgression (MFS). Overlying alternations of
sandstones with mudstones intercalate with several beds
of black shales (sub-Unit S4b) that indicate alternating
episodes of lacustrine stagnation and fluvial activity (e.g.
Fig. 8.11 ). At the top of this sequence, the most pro-
nounced relative lake-level

successive thin beds of horizontally laminated very
coarse sandstones, at depths 719 m and 690 m

(Fig. 8.8b ).
• An upper sequence also comprises mixed fluvial and
lacustrine deposits with beds of black shales that
record another main maximum transgression (MFS).
This sequence shallows upward with rooted sandstones
overlain by alternating carbonated mudstones and
sandstones (sub-Unit S4d).
These two sequences are also detected within the Gilson
and Mbandaka sections (Fig. 8.14 ). In the Mbandaka sec-
tion, the Unit M3 starts with 44 m thick green shales (sub-
Unit M3a) that signal the first main maximum transgression
(MFS). These are sharply overlain by 81 m thick, near

fall corresponds to two