Okavango Delta Management Plan - Ramsar Convention on Wetlands

Recent geomorphologic work has shown that the <strong>Delta</strong> (possibly as slowly diffusing multiple
streams) was much more widespread 200 000 years ago and flowed extensively southwards
to around Deception Valley in the northern Kalahari (Ringrose et al., 2005). Re-activation
of the faults to the north (around Gumare) and south (the Thamalakane and Kunyere) led
to the confinement of the present fan, likely around 40 000 years ago. More movement
along the faults may be anticipated in this earthquake-prone rift zone.
2.2.3 SOILS
The soils of the <strong>Delta</strong> have been investigated by Staring (1978). They are mainly sand-based
(arenosols) as a result of the sediment infill into the east African Rift basin (see
geomorphology section). However, the local geomorphology and the effects of weathering,
change the nature of the surface condition of the sands such that the original arenosols are
modified. A major characteristic of the sands is the lack of diagnostic horizons other than a
surface layer rich in organic material, which may extend from 30-100 cm.
Typically the arenosols have very low water-holding capacities because the sand in the soil
is not graded so that sands of varying coarseness are constantly mixed throughout the soil.
As most sands are highly siliceous and are also extremely low in essential nutrients,
especially phosphorus, most are acidic and therefore difficult to farm. In addition the low
and erratic rainfall experienced within the ODRS tends to lower productivity of the soil.
Dryland farming takes place on haplic arenosols while the more productive flood-recession
agriculture takes place on gleyic arenosols. In areas where intermittent clay has been
deposited as a result of former water ponding or vegetation induced surface processes (e.g.
under tall mopane) some clay-like structures are found in the soil leading to the
development of luvic arenosols. Some ferralic arenosols occur in the dune areas, peripheral
to the <strong>Delta</strong>.
2.2.4 GEOLOGY AND STRUCTURES
The <strong>Okavango</strong> <strong>Delta</strong> overlies solid basement bedrock, which occurs under 20-300m of
Kalahari sediments. The older basement rocks are referred to as:
1. Palaeoproterozoic (approx 2.05 thousand million years old) comprising gneiss, granites
and amphibolites exposed in the Qangwa area and granulites exposed in the Gweta area
2. Mesoproterozoic (approx 1.2-1.1 thousand million years old) gabbros, granites and
meta-rhyolites and meta-basalts which are exposed in the Kgwebe and Ngwanalekau
Hills.
3. Karoo supergroup comprises (approx 180 million years old) silica rich sedimentary rocks
with mafic lava and dolerites which are less exposed close to Tsodilo Hills and around
areas east of Ngamiland.
These are covered by the Kalahari Group sediments comprising silica rich sands and
carbonates such as calcretes and occasionally silcretes.
The geological map of the ODRS is given in Figure 2-5. As the older rocks may be emplaced
by mineral rich ores (such as kimberlite pipes) the possibility of mining occurring in the
<strong>Ramsar</strong> site over the longer term should not be discounted. The older rocks are aligned on
two trends, an older NW-SE trend and a younger NE-SW trend. Re-newed movement along
this latter trend has caused the <strong>Okavango</strong> <strong>Delta</strong> to form in a down faulted depression
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