Geology Reference
In-Depth Information
Tsodilo Section
a
b
1821C3
Approx.
ages
This study
T T T T T T T T
T od
o o o o o o o o o o od
od
o od
o i i i i i i i i
i i i i
T T T T
T T T T
T
o o o o o d
T T T T
T T
T T T T T T T
T
T
T T T T T T T T
o od
o d
i i
i i o
C SfSmS cS Gr P
H l
H H H H H H H H H H H H H H H H H H H H H
H
l l l l l l l l l l l l l
l
l l l l
l l
l l
l l
l
l
l s
H
H H
H H H
0
1821C10 B4 A15
A37
C15 KPH1
Sands
TSODILO1
> 50 ka
O O O O O O O k
O k
1821C16 B6
Ok
O k
O k a a a a a
Ok
a a a a
a a a a a a av a a a a a a a a a a a a a a a an
a n g g g g g g g g g g g g g g g g g g g g g
g g g
g
Carbonates
and calcretes
D D D D D De
D D D De
D D D D D D D D D D D De
D D D De
D e l l l l l
l l l l l l
l l l l l l l l
De
De
De
De
De
De
D De
De
l l l l l l l
l l t a
l l
l l
l l l
l
21641A1
BM1
ca. 80 Ma
BM3
Nxau Nxau
kimberlite field
Red beds
D Dr
D D D D D D D D D D Dr
o o o t s s s s s s s s s s s s s s s
y y y y y y y y y y y y'
D r
D D D D r
D D o
o o o o o o
y s
y'
100 m
C C C C C C C C C C C C C
C C C
C C
C
v v v v v v v v v v v
G3
v e
G1
Turbidites
JB7
11471A
Maun
11741A
JB1
Black shales
JEB/1
L
L L
L L L L a k
k
k
k k
m
m m m m m m m m m m m m m m m
L L L L
k k k k
k
m m
m m m m m
m i
k
k
k k e N g g g g g g g g g g
g g g g g g g g g g g g g g g g
g
g
g
g
Diamictites
NGAMI2
NGAMI3
350 Ma
Damara Quartzites
(Neoproterozoic)
200 m
Fig. 10.3 ( a ) Location map of the Kalahari drilling project in the Ngamiland region of northwest Botswana, showing the studied boreholes ( blue
and red dots ), and ( b ) simplified stratigraphic section (for more details see Linol 2013 )
Figs. 10.1a and 10.2 ) that have great potential to preserve
long Cenozoic sedimentary records. Below, we describe new
sequence analysis of the Kalahari Group from drilling up to
ca. 100-125 m depth at the edge of the Tsodilo Hills and into
Lake Ngami (Fig. 10.6 ). Details of the drilling technology
and core recovery are provided online: http://www.aeon.org.
za/content/pdf/kalahar_drilling_project_v7_10308.pdf .
10.3.2.2 Lake Ngami
Near the center of Lake Ngami (Fig. 10.6b ), the Ngami-
2 borehole (20 29 0 39 00 S; 22 43 0 44 00 E) is 125 m deep, without
reaching the base of the lake sediments. The recovered
Kalahari succession comprises calcareous fine sands with
at depth, below
60 m, seven intervals of darker brown,
organic-rich muds that record successive episodes of lake
expansion (i.e. flooding). These mud-rich horizons are fos-
siliferous with ostracods, charophytes and charophyte
gyrogonites (Fig. 10.7 ) that suggest an age younger than
Miocene and confirm the existence of a deep paleo-lake.
The ostracod shell fragments, from depth—106-107 m,
were dated using AMS 14 C technique (e.g. Rethemeyer
et al. 2012 ) and resulted in an age greater than 50 ka, beyond
the precise radiocarbon detection (Fig. 10.6c ). Near the
surface, at a depth of
10.3.2.1 Tsodilo Hills
At the foot of the Tsodilo Hills, the Tsodilo-1 borehole
(18 46
S; 21 43
E) intercepted a ca. 42 m succession
of sands and calcretes of the Kalahari Group. This section
overlies pre-Kalahari (e.g. Karoo-age) heterolithic
sandstones and green mudstones that are increasingly
silicified upward to form a 4 m thick silcrete. This distinct
hardpan, likely formed by deep weathering and groundwater
activity at the discontinuity that defines the base of the
Kalahari Group, possibly correlates with the Nxau-Nxau
(and Etosha) Calcrete Formation. The overlying Kalahari
succession at Tsodilo Hills comprises a lower, 32 m thick
sand-dominated fluvial sequence, overlain by 8 m thick
sandy carbonates and a 2 m thick fossiliferous calcrete that
cap the flat present day topography (Fig. 10.6a ). Previous
radiocarbon dates (Thomas et al. 2003 ), between 32 ka and
36 ka, obtained from molluscs and shells collected at sur-
face, date this uppermost sequence of pan-lacustrine and
calcrete deposits to the late Pleistocene (Fig. 10.6c ).
'
58
'
11
3.2 m, a horizon of organic-rich
sediments was previously 14 C dated at 18 ka (Huntsman-
Mapila et al. 2006 ).
In summary, the Kalahari Group in the Ngamiland region
of northwest Botswana comprises basal clastic-carbonates
and calcretes (the Nxau-Nxau Calcrete Formation) that pass
laterally and upward into a fluvial sequence of Kalahari
sands, which in turn intercalates near the top with Pleistocene
pan-lacustrine and younger calcrete deposits (e.g. Thomas
et al. 2003 ; Huntsman-Mapila et al. 2006 ). Correlations of
these sequences (at the Tsodilo Hills and Lake Ngami;
Fig. 10.6c ) with the Kalahari stratigraphy established at the
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