Geology Reference
In-Depth Information
between 20-40 Ma (e.g. Hofman et al. 1997 ; Pik et al. 2008 ),
Cenozoic rifting continued between Arabia and Africa with
the opening of the Red Sea (Fig. 13.5g ). Further extension and
uplift resulted in the East African Rift System (EARS) that
extends from Afar and Kenya, along the eastern and western
flanks of the Tanzanian Craton farther to the south, and with a
possible embryonic branch towards the Okavango Delta in the
central Kalahari (Reeves 1972 ; Modisi et al., 2000; Chorowicz
2005 ).
Neoproterozoic (Pan African) aged sources in north-central
Africa (e.g. the Central Saharan Belt). The youngest zircons
dated at 540 Ma provide a maximum early Cambrian age to
these sequences (the Inkisi Group; Jelsma et al. 2011 ).
These three earliest Paleozoic sequences surrounding the
CB are herewith correlated with the Upper Nama Group of
southwestern Namibia, deposited with paleocurrents to the
south and to the southeast from the Damara Belt (Germs
1995 ), and which unconformably overlies the Precambrian-
Cambrian boundary (Grotzinger and Miller 2008 ). U-Pb
dating of detrital zircons from this section indicate two
major peaks at 1050 Ma and 540-600 Ma (Blanco et al.
2011 ). These dates are similar to those from the CB, and all
indicate dominant Kibaran and Pan African sources for the
red-beds across a vast area of south-central Africa. Equi-
valent (post-Pan African) sequences are apparently absent in
the PB (Milani et al. 2007 ), except possibly within local rifts,
such as the Camarinha Basin along the western Brasiliano
fold belt (Fig. 13.7 ). Thus, at this stage, southwest
Gondwana red-beds were preferentially preserved across
proto-central Africa.
13.4
Chronostratigraphic Correlations and
Paleogeography
Based on modern basin analysis of the CB (Linol 2013 ) and
the PB (Milani 1997 ), we derive new stratigraphic corre-
lations between their Phanerozoic sequences (Fig. 13.6 ).
Together with some references to the better-known CKB of
southern Africa, as well as detrital zircon provenance studies
from these basins, this regional synthesis aims to improve
the paleogeography and paleo-topographic models of the
interior of southwest Gondwana.
13.4.1.2 Ordovician-Silurian and Devonian
In the PB, the oldest preserved sediments are part of the
Ordovician-Silurian Rio Iva´ Group (Fig. 13.6 ). This lower-
most succession comprises fluvial-marine sandstones and
conglomerates (the Alto Garcas Formation), covered by
diamictites of the Iapo Formation and followed by deglacia-
tion black shales of the Vila Maria Formation that record a
first, Early Silurian (Llandovery) maximum marine trans-
gression in the southern part of the basin (Mizusaki et al.
2002 ). The overlying Devonian Paran´ Group is also a
transgressive marine platform, and includes conglomerates
and sandstones with trilobite tracks (the Furnas Formation)
and black shales of the Ponta Grossa Formation that record a
second maximum flooding of the PB during the Late Devo-
nian (Milani et al. 2007 ). Both, the Rio Iva´ and the Paran´
Groups (in total 1,400 m thick) have been correlated with the
Upper Cape Supergroup of South Africa, including the
Upper Ordovician Pakhuis glacial deposits (Milani and de
Wit 2008 ). These correlations suggest the development of a
giant shallow marine shelf along the southwestern margin of
Gondwana during the Silurian-Devonian, more than
1,000 km wide, stretching from southern Brazil to South
Africa (Fig. 13.8 ).
13.4.1 Late Neoproterozoic to Early Paleozoic
13.4.1.1 End Precambrian-Cambrian
Following the Pan African orogens (ca. 500-800 Ma), wide-
spread upper Neoproterozoic to lower Paleozoic red-beds
were deposited across the North and Central African Shields
(e.g. Poidevin 1985 ; Morag et al. 2011 ). This succession
outcrops abundantly south of the Oubanguides Belt along
the northern margin of the CB, and was originally referred
to as the Aruwimi Group (upper part of the Lindian Super-
group; Lepersonne 1974 ). It comprises massive quartzites (the
Galamboge Formation), red schists with limestone
intercalations (the Alolo Formation) and relatively thick
(~1,000 m) arkoses of the Banalia Formation, totaling
1,500 m in thickness (Fig. 13.6 ). Only the uppermost
(Banalia) sequence is now considered to post-date the Pan
African deformation (Tait et al. 2011 ; see also Chap. 6 ,this
Topic). Based on this new lithostratigraphy,
the revised
'
is correlated with southward prograding con-
glomeratic red sandstones of the Inkisi Group (~1,000 m
thick) along the West Congo Belt (Alvarez et al. 1995 ), and
rippled red siltstones of the Biano Group (~1,000 m thick),
also deposited southward, at the southeastern margin of the
CB (Master et al. 2005 ). Detrital zircon provenance analyses
from these red-beds (Master et al. 2005 ; Frimmel et al. 2006 ;
Jelsma et al. 2011 ) reveal two prominent age-peaks: at
1000 Ma and 650 Ma that characterize predominant late
Mesoproterozoic to early Neoproterozoic (Kibaran), and late
Banalia Group
'
13.4.2 Late Paleozoic to Early Mesozoic
13.4.2.1 Carboniferous-Early Permian
In southeastern Brazil, the lower Paleozoic sequences of the
PB (e.g. the Furnas Formation) are truncated by a regional
unconformity marked with glacial pavements, such as the
Search WWH ::




Custom Search