The study was carried out in the southern and northern Central Zones of the intra-cratonic arm of the Pan-African Damara Orogen. The study area straddles the Omaruru and Okakoara Lineaments and stretches from Karibib and Usakos in the south to Omaruru and Omatjette in the north. Although the area lies on the edge of the Namib Desert where vegetation cover is sparse the area is extensively covered by sand and calcrete plains resulting in poor outcrop.

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22 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

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The study was carried out in the southern and northern
Central
Zones of the intra
-
cratonic arm of the Pan
-
African Damara
Orogen. The
study area straddles the Omaruru and Okakoara
Lineaments and
stretches from Karibib and Usakos in the south to
Omaruru and
Omatjette in the north. Although the area lies on
the edge of the
Namib Desert where vegetation cover is sparse the area is extensively
covered by sand and calcrete plains resulti
ng
in poor outcrop.

Two major and one minor phase of folding have deformed the
sediments, but the grade of metamorphism is medium and
sedimentary and volcanic structures are often well preserved.
The
grade of metamorphism increases slightly from north to south
in the
study area but does not reach upper amphibolite facies.

This study resulted in a major revision of the correlation
of
lithostratigraphic units in the Central Zone. The lower
schist,
lo
wer marble, upper schist and upper marble units north
of the
Omaruru Lineament were previously correlated with the Khan,
Rossing, Chuos and Karibib Formations respectively. The
present
investigation shows that the stratigraphic sequence in the
northern Cen
tral Zone, from the exposed base of the lower
schist
unit to the top of the upper marble unit, is a
stratigraphic
unit above the Chuos Formation and thus a facies equivalent of the
Karibib Formation. This correlation is based
on the assumption that
a simil
ar stratigraphic sequence is found
above the Chuos Formation
diamictites in the southern Central Zone. The suitability of the
Chuos Formation diamictites as a
chrono
-
stratigraphic marker
horizon is also discussed. The
origin for the Chuos Formation
diamict
ites is considered to be glacio
-
marine, where moraine till
was reworked in a sub
-
marine
environment during warmer intervals
and also transported deeper
into the basin by mass flow processes.

The sedimentary facies changes of particular
lithostrati
graphic u
nits above the Chuos Formation were studied
from south
to north in four stratigraphic sections. These
stratigraphic
units are from the base upwards the Spes Bona Formation
(schist),
the Okawayo Formation (marble), the Oberwasser
Formation
(schist), the Dah
eim Formation (volcanics), the Karibib
Formation
(marble) and the Onguati Formation (marble/schist).

The Spes Bona Formation consists of a sequence of graded
rhythmites of calc
-
silicate rock, metagreywacke and schist
exhibiting sedimentary structures such
as flat
-
topped ripples and
planar cross
-
bedding. This sequence is interpreted as represen
ting
a sub
-
tidal to intertidal environment on a
siliciclastic
-
dominated shelf. Reduced influx of siliciclastic
material
initiated the first carbonate deposition (Okawayo
Formation).
The Okawayo Formation consists of interbedded
calc
-
silicate rock
and marble and synsedimentary carbonate
breccias in the south.
The sequence changes northwards into
cross
-
bedded calc
-
silicate
ro
ck and marble rhythmites with rip
-
up
clasts and to the far
north into a marble unit with high proportions
of schist. This change from south to north in the Okawayo Formation
represents
carbonate deposition on an upper slope through mid
slope to a
deep basi
n away from a carbonate platform situated to
the south
of the study area.

A reverse back to siliciclastic deposition followed with the
deposition of the Oberwasser Formation. The base of this
sequence
"comprise relatively thick packages of graded metagrey
-
wackes and
schist exhibiting partial Bouma sequences. This
sequence is
interpreted as representing mid to outer fan
environment of a
turbiditic fan system (from south to north). Towards the top of
the Oberwasser Formation there is a calc
-
silicate rock unit

characterised, by stacked wave ripple,
oncolites and stromatolitic
structures. The combination of these
sedimentary structures
indicates sedimentation in a shallow water
wave
-
dominated sub
-
tidal
environment for the final stages of the
siliciclastic sedime
ntation
over the whole of the study area.

The
Daheim Formation volcanics which occur only south of the
Omaruru
Lineament are stratigraphically situated in the upper
parts
of the Oberwasser Formation. The volcanic sequence
consists of pil
low lavas and lava breccias (extruded underwater)
at
the base overlain by fine and coarse pyroclastic material
(scoria)
that extruded sub
-
aerially. A return to carbonate
deposition
(Karibib Formation) over the whole of the Central Zone
was still
controlled

by the same platform, slope and basin
configuration
from south to north. During the initial stages of carbonate
deposition the carbonate platform was situated to the
south of the
Karibib
-
Usakos area. The platform edge was in the
Karibib area
terminating against a northwesterly inclined slope
characterised
by synsedimentary carbonate breccias. Mid slope
carbonate facies
lay to the north of Karibib, and open shelf
facies
(calc
-
silicate/marble rhythmites) to the far north. With
the
p
rogradation of this carbonate platform and shelf to the
north the
upper slope carbonate breccias in the Karibib area were buried by
intertidal to supratidal carbonates (oolites, fenestrai
structures,
conglomerates). During this stage the mid slope
carbonat
e facies
to the north were buried by outer reef
carbonates (stromatolites?)
and the outer ramp sediments to the
far north were buried by well
oxygenated shallow water fore
-
rest:
carbonates (white graphitic
marbles).

During the final stages of evolution of
the basin the
carbonate platform and shelf were terminally drowned and overlain
by
deep water siliclastic sediments with some carbonate detritus
washed
in from the surviving platform (Onguati Formation). With
continual
regional subsidence the whole area wa
s overlain by deep
water
turbiditic sediments (Kuiseb Formation).