Geology Reference
In-Depth Information
data-point error ellipses are 2 σ
8.7
Regional Synthesis
0.0335
210
The J-K succession of the CB (the Congo Supergroup)
attains a maximum thickness of 1000 m in the center of the
basin (Lepersonne 1974 ; Linol 2013 ). It comprises shallow
marine, aeolian and fluvial-lacustrine sequences (the
Stanleyville, Loia, Bokungu and Kwango Groups), depo-
sited following the period of initial break-up between East
and West Gondwana (ca. 160-180 Ma) and during the
subsequent opening of
0.0325
200
0.0315
0.0305
the South Atlantic Ocean, ca.
19 90
80-130 Ma (Fig. 8.18 ).
Lower intercept at
190 ±10 Ma
MSWD = 0.0031
0.0295
8.7.1 Upper Jurassic to Lower Cretaceous
Sequences
0.0285
0.195
0.205
0.215
0.225
0.235
0.245
207
Pb/ 235 U
The lowermost, Upper Jurassic Stanleyville Group (370 m
thick) comprises in the lower part black shales with one
limestone intercalation (
Fig. 8.17 U-Pb Concordia diagram for the two youngest detrital
zircons of sample WP19. The lower intercept age is 190
10 Ma,
which provides a maximum age for this lower Kwango sequence
), which contains a dis-
tinct juvenile fish fauna from marine spawning sites during a
short Kimmeridgian transgression of the proto-Indian Ocean
in the northern CB (Taverne 1975 ). This MFS is correlated
to a marker bed of black shales overlying tidal and flood
deposits in the lower part of the Samba section (Unit S5),
and is in turn overlain by upward-coarsening fluvial-deltaic
red sandstones and mudstones with ferricretes that record a
major regression and an emersion of the CB (Fig. 8.14 ).
Geochemical analyses of the upper part of the Stanleyville
Group in this section (Myers et al. 2011 ) have further
suggested that hot and arid conditions, between 25 and
40 C, prevailed during this period.
The Stanleyville Group is overlain to the south by aeolian
dunes in the Gilson (Unit G4) and Dekese sections (Unit
D4), and here named the Dekese Formation (Fig. 8.14 ). This
new aeolian unit also correlates with widespread aeolian red
sandstones of the Lower Kwango Group mapped in the
southwestern CB. U-Pb dates of detrital zircons from this
sequence indicate a maximum age of aeolian deposition
younger than Early Jurassic, with two dates at 190 Ma
(Fig. 8.17 ). With this new chronostratigraphy, the Lower
Kwango Group (and the Dekese Formation) can now be
correlated confidently with the J-K aeolian sequences in
Namibia and eastern Brazil (Miller 2008; Scherer and
Goldberg 2010 ), both of which are covered by Lower Creta-
ceous flood basalts of the Paran ´ -Etendeka LIP emplaced at
ca. 132 Ma (Renne et al. 1992 ). These correlations attest to
the presence of a giant,
'
Lime Fine
'
8.6.2.5 Mid-Phanerozoic (ca. 430 Ma, 240-280 Ma
and 190 Ma)
One zircon grain has a
206 Pb/ 238 Udateof426
8Ma
(mid-Silurian), although no sources of this age have been
reported in central Africa (Cahen et al. 1984 ). There are,
however, large potential source rocks within the Andes in
South America (e.g. Patagonia; Pankhurst et al. 1998 ;
Vaughan and Pankhurst, 2008 ), as well as detrital zircons
with this age-range within lower Paleozoic sequences in
southern Africa, such as the Cape Supergroup (Fourie et al.
2011 ).
Two zircons in sample D470 have concordant dates of
240
7 Ma, and two youngest zircons in
sample WP19 date at 204
6 Ma and 286
4Ma
( 206 Pb/ 238 U dates), which provide a maximum Early Jurassic
age for the Kwango Group (Fig. 8.17 ). These Mesozoic dates
may possibly correspond to kimberlite activity, as recently
discovered in northern Angola (Jelsma et al. 2011), and/or to
volcanism of the Central Atlantic (CAMP) and Karoo Large
Igneous Provinces (LIPs), ca. 200 Ma and 182 Ma, respec-
tively (Marzoli et al. 1999 ; Svensen et al. 2012 ; Blackburn
et al. 2013 ). However, these rock sequences (kimberlites and
basalts) are unlikely to yield abundant zircons. A more likely
source for these zircons, therefore, is the extensive late
Paleozoic to Jurassic silicic volcanic belt in southern South
America (the Choiyoi and Chon Aike Provinces; Kay et al.
1989 ), from which zircons in abundant air fall tuffs are also
recorded in the Permo-Triassic rocks of Paran´ Basin in
Brazil (e.g. Rocha-Campos et al. 2011 ) and the main
Karoo Basin in southern Africa (Stollhofen et al. 2000 ;
Fildani et al. 2007 , 2009 ; Rubidge et al. 2013 ).
4 Ma and 192
'
'
paleo-desert across
central West Gondwana, just before the separation of Africa
from South America (Fig. 8.18 ; see Chap. 13 , this Topic).
Additional zircons dates of ca. 430 Ma (mid-Silurian) and
240-290 Ma (Permian-Triassic) from these aeolian
sediments of the CB also further support a far-field influence
Sahara-like
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