Geology Reference
In-Depth Information
Alternating currents
A
x10 m
sub-section
Depth Facies
Grain-size
C
S
fS
mS
cS
Gr
P
sub-section
Fluvial
-1120 m
Depth Facies
Grain-size
Stf-b
C
S
fS
mS
cS
Gr
P
Upper tidal
flat deposits
Flood
deposits
Sm
Sm
-1005 m
Bottom beds
Flaser-bedded
sands and muds,
conic burrows
Monocraterion
Black shales,
planar burrows
Planolites
Fm
Fl
-1125 m
Fl
-1010 m
Fb-b
Stf-b
-1130 m
Flood
deposits
St
Stf-b
Heterolithic
sands
-1015 m
Fig. 8.9 Shallow marine facies successions from the lower part of the
Samba core-section (Unit S5; Fig. 8.8b ). Sub-section A: bioturbated
and flaser-bedded fine sandstones (Stf-b facies), characteristic of tidal
flat deposits (e.g. Dalrymple 1992 ). Sub-section B: heterolithic
sandstones with a thin bed of bioturbated black shales (Fb-b facies)
and soft-deformed rhythmites (e.g. slump 4 m thick)
Gilson and Mbandaka sections, re-interpreted from the well-
logs. Along this correlation profile (Fig. 8.14 ), 880 km long
across the central CB, the J-K sequences correspond to the
Stanleyville, Loia, Bokungu and Kwango Groups, as
described below.
Lueki Group; Chap. 7 ,
1,167 m
(Fig. 8.8b ), and includes two main depositional sequences:
• A lower sequence (sub-Unit S5a) is 157 m thick and
mainly transgressive. It comprises fine sandstones with
cross-laminations and mud drapes, and abundant shallow
conic burrows, interpreted as tidal (shallow marine) and
flood deposits (Fig. 8.9 ). In the upper part, a marker bed of
black shale records the first main maximum transgression,
or Maximum Flooding Surface (MFS). Overlying thinly
laminated shales and sandstones are truncated by an ero-
sion surface at depth
this Topic), at depth
8.5.1 The Stanleyville Group
The Unit S5 (322 m thick) of the Samba section is correlated
with the Upper Jurassic Stanleyville Group, based on litho-
and bio-stratigraphy (Cahen 1983b ). It overlies a marked
erosion surface across red quartzitic sandstones (the Haute
995 m and, in turn, overlain by
ferruginous and fractured red sandstones (Fig. 8.8b ).
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