Geology Reference
In-Depth Information
a
b
TSODILO-1
NGAMI-2
C
S
fS mS cS Gr P
C SfSmS cS Gr P
0 m
0 m
36 Ka
Thomas
et al. (2003)
18 Ka
Huntsmann-
Mapilaet al. (2006)
PALUSTRINE CARBONATES
AND CALCRETES
10 m
10 m
20 m
20 m
30 m
30 m
FLUVIAL
SANDS
40 m
40 m
hardpan
50 m
50 m
60 m
60 m
1
2
3
70 m
70 m
DEBRIS-FLOW
80 m
80 m
4
5
90 m
90 m
6
100 m
c
>50 Ka
This study
7
110 m
120 m
Fig. 10.6
AEON drill sites (
a
) at the Tsodilo Hills and (
b
) at Lake
Ngami, and (
c
) sequence correlations of the Kalahari Group between
these two sites (Figs.
10.2
and
10.3a
for locations), with location of
14
C
dated samples (in
red
). In the Tsodilo-1 borehole, the Kalahari Group
comprises a lower, 30 m thick fluvial sequence overlain by an upper
sequence of palustrine carbonates and calcretes, in total 10 m thick. In
the Ngami-2 section, the equivalent lacustrine succession is more than
125 m thick, including at depth, below
60 m, seven horizons of
fossiliferous, dark organic-rich mud. These correlations indicate wetter
conditions during deposition of the lower Kalahari Group
age for this lower Kalahari sequence. Lithostratigra-
phically, this sequence of duricrusts correlates well with
the similar formations of calcrete and silcrete described at
the base of the Kalahari Group in northern Namibia and
Botswana (the Nxau Nxau and Etosha Calcrete
Formations). Differences in lithology between these
deposits (silcretes versus calcretes) likely reflect region-
ally variable weathering conditions related to more humid
and hotter climate in central Africa.
2. An upper sequence of unconsolidated sands, up to 120 m
thick, overlies another weathering surface, marked by
ferricrete deposits at
the top of
the
'
Polymorph
Sandstones
(Cahen and Lepersonne
1952
). This
sequence comprises fine ochreous sands, silts and kaolin-
itic muds devoid of stratification, possibly due to intense
bioturbation, and which are interpreted as sheet wash
'
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