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eastward across south-central Africa likely reflects the influ-
ence of Cenozoic uplift related to the formation of the
EARS, between 20-40 Ma (e.g. Chorowicz
2005
; Pik et al.
2008
; Roberts et al.
2012
).
The classical
10.3 Drill-Cores from the Central Kalahari
Plateau, Northwest Botswana
, as first described in
Botswana by Passarge (
1904
), consists of basal gravels (the
Botletle Beds), calcretized or silcretized sandstones and
marls (
'
Kalahari type-section
'
In the Ngamiland region of northwest Botswana (Fig.
10.3
),
new borehole stratigraphy has also characterized widespread
calcretes in subsurface, between 10 m and 60 m thick, and
named the Nxau-Nxau Calcrete Formation (Linol
2013
).
These carbonate rocks overly a regional unconformity across
Precambrian basement (the Damara Supergroup; Haddon
and Roos
2001
) and diamictites with black shales and red-
beds of the Karoo Supergroup that are deformed and locally
intruded by kimberlites in the Nxau-Nxau area, dated at
83 Ma (Batumike et al.
2007
; de Wit
2013a
). Calcretization
also frequently penetrates into the Karoo Supergroup and
Precambrian basement, and xenoliths of calcrete occur in the
kimberlites (de Wit
2013b
).
'
Kalahari Limestones
'
) and aeolian sands (
'
Kalahari
Sands
). Lithostratigraphic equivalents were later also
recognized in South Africa (the Kalahari Beds; du Toit
1954
), Zimbabwe (
'
; Maufe
1936
), Zambia
(the Barotse Formation; Money
1972
) and DRC (
'
Pipe Sandstones
'
'
Poly-
morph Sandstones
; Cahen and Lepersonne
1952
). Much of
this historic mapping and research has been summarized
since in regional reviews (e.g. Thomas and Shaw
1990
,
1993
; Haddon
2000
; Giresse
2005
), but very little new
field data have been generated to provide detailed strati-
graphic sections and test these regional correlations.
Most recent studies across the Kalahari have focused on
near-surface pan sediments, silcretes/calcretes and aeolian
sand dunes to provide information about Quaternary paleo-
climate and landscape evolution (e.g. Summerfield
1983
;
Holmgren and Shaw
1996
; Ringrose et al.
1999
; Thomas
et al.
2003
; Huntsman-Mapila et al.
2006
; Kampunzu et al.
2007
; Burrough et al.
2009
;H¨rkamp et al.
2011
; Moore
et al.
2012
; Eckardt et al.
2013
). However, significant
reworking, bioturbation and post-depositional modifications
of these sediments often hamper reliable age-dating and
stratigraphic correlations. It has also been argued that a
significant part of the Kalahari Group results from in-situ
weathering of its basement rocks, further hampering con-
struction of a robust stratigraphy (McFarlane et al.
2010
).
Only in the Etosha Pan of northern Namibia (Figs.
10.1a
and
10.2
for location) recent borehole and paleontological
investigations have described in detail a more complete
Kalahari succession (Miller
2008
; Miller et al.
2010
). Here,
the Kalahari Group includes basal gravels of the Beiseb
Formation (probably equivalent to the Botletle Beds
described by Passarge
1904
), and two thick sand-dominated
fan sequences, named the Olukonda and Andoni Formations,
maximum 150 m and 550 m in thicknesses, respectively. It
intercalates at the top with 50 m of saline clays that consti-
tute the present-day pan bed, and which contain fossil verte-
brate suites dated from the late Miocene to Pliocene, ca.
4-6 Ma (Miller et al.
2010
). This relatively thick Kalahari
succession in turn passes laterally into an 80 to 120 m thick,
massive
'
10.3.1 The Nxau-Nxau Calcrete Formation
In cores (Figs.
10.4
and
10.5
), the Nxau-Nxau Calcrete
Formation comprises sandy and muddy carbonate rocks
that commonly have abundant micro-karst structures, root
moulds and/or burrows, and cracks filled with coarser sands
and gravels.
In thin sections (e.g. Fig.
10.5
), the matrix is generally a
dark brown micritic groundmass supporting detrital quartz.
It is partially or totally replaced by sparite due to subsequent
diagenesis. Fenestral fabric and circum-granular cracks
within the more muddy facies also indicate episodes of
immersion and desiccation (e.g. Wright and Tucker
1991
;
Tanner
2010
). Although biogenic features, such as alveolar
structures, rhyzoliths and burrows are abundant, indicating
intense biological activity (e.g. Goudie
1983
), no fossils
were observed. Thus, these facies have characteristics of
both lacustrine and pedogenic deposits (c.f. Alonso-Zarza
2003
), and are interpreted to be deposited in a shallow lake
wetland environment (Fig.
10.5
). This suggests a humid and
relatively hot climate during early deposition of the Kalahari
Group.
10.3.2 Kalahari Lake Sediments
Previous sedimentological studies of Kalahari sediments
have demonstrated wet conditions during the Pleistocene
and suggested the occurrence of extensive paleo-lakes (e.g.
Du Plessis and Le Roux
1995
; Holmgren and Shaw
1996
;
Thomas et al.
2003
; Huntsman-Mapila et al.
2006
; Eckardt
et al.
2013
). However, these studies often lack deep borehole
data, particularly into the Kalahari lakes and wetlands (e.g.
Okavango Delta, Makgadikgadi Pan and Lake Ngami;
named the Etosha Calcrete Formation,
considered to represent a gigantic groundwater deposit that
developed
'
calcrete
'
4 Ma under arid conditions (Miller
2008
).
These terrestrial carbonates, however, are complex because
of multiple phases of deposition and dissolution, and are thus
difficult to interpret chronologically (e.g. Nash and McLaren
2003
; Wanke and Wanke
2007
; Linol et al.
2009
).
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