Trans. geol. Soc. S. Afr.. SO, 1--8
THE GLACIAL BEDS OF THE GRIQUALAND WEST SUPERGROUP AS REVEALED BY FOUR
DEEP BOREHOLES BETWEEN POSTMASBURG AND SISHEN
by
P. R. DE VILLIERS and J. N. J. VISSER
ABSTRACT
The glacial beds of the Makganyene Formation which forms part of the Middle Precambrian Griqualand
West Supergroup were examined in detail in cores from four boreholes drilled along a north-south line.
Petrographic investigation shows that the sedimentary succession consists of a rhythmic alternation of
arenaceous and argillaceous diamictite, conglomerate, clayey sandstone, subgraywacke, cherty iron carbonate, dolomitic limestone, banded jaspilite and a cherty rock. The clastic rocks consist predominantly of
grains of quartz and chert with minor amounts of feldspar, carbonate and quartzite. Calcite/dolomite, siderite,
chamosite, stilpnomelane and chlorite occur disseminated in the ground-mass of the clastic rocks. The clasts
in the diamictite consist largely of chert and quartzite.
The diamictite was probably deposited from floating ice, while the arenaceous beds resulted from subaqueous gravity mass flows. The non-clastic beds were deposited during interglacial periods. The underlying
Gamagara Formation and dolomite of the Campbell Group acted as sources for the sediments. The Ongeluk
volcanism commenced before sedimentation had ceased and volcanic material was incorporated in the upper
sedimentary beds.
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
I. INTRODUCTION
During the course of a drilling program by the South
African Iron and Steel Corporation, Ltd (ISCOR) in the
Sishen-Postmasburg area several boreholes penetrated rocks
of the Makganyene Formation. Examination of the cores furnished evidence which is in support of the glacial origin of the
sediments.
Before the seventies the glacial beds received but scant
attention, as research was directed mainly to the underlying
strata of the Gamagara Formation. Previous workers correlated the Gamagara Formation with the Matsap sequence
of the Langeberg and explained this unique deduction by
thrust faulting. Wessels (1967) showed, however, that the
Gamagara Formation is part of the stratigraphic sequence of
the Griqualand West Supergroup. Messrs P. Smit and D.
Wilke of the Geological Survey of the Department of Mines
and De Villiers (1967) have subsequently proved this conclusion.
The peculiar rocks immediately overlying the Gamagara
Formation were described by Visser (1971, pp. 187-199). He
measured five stratigraphic sections and showed that there
can be very little doubt about the glacial origin of these
sediments. He proposed the term HGriquatown Glacial
Member" for this sedimentary unit.consisting of glaciomarine
and fluvioglacial sediments in the Griqualand West sequence.
The South African Committee for Stratigraphy has suggested
lhe name HMakganyene Formation" for this unit.
FIG. 1.
SKETCH-PLAN INDICATING LOCATION OF BOREHOLES
N
t
GAB
GAMAGARA
BK 1
BREDENKAMP
ML 2
' - -_ _ _2.1.-,5_ _--'5~ km.
I
OLiFANTSHOEK GROUP
VVV
II. GENERAL GEOLOGY
In the Kuruman area the Makganyene Formation occurs
directly below the Ongeluk Andesite Formation. The former
rests unconformably on sediments of the Gamagara Formation and in places transgresses on to the Campbell Group (old
Dolomite Series).
Borecores from the farms Gamagara (GA 13),
Bredenkamp (BK I), Magoloring (ML 2) and Langverwacht
(LV I) were examined (Fig. I). The thickness of the
Makganyene Formation increases from 68 m in the north to
552 m at Magoloring. Borehole LV 1 does not intersect the
MAGOLORING
LANGVERWACHT
ONGELUK ANDESITE FORMATION
~=~ }
GAMAGARA FORMATION
CAMPBELL GROUP
complete succession (Fig. 2) so that the stratigraphic
relationships of the sedimentary units in the sequence are uncertain in the south, The considerable increase in thickness
southwards is due to the presence of a thick sandstone in the
bottom portion of the section at Magoloring, as the six
horizons of diamictite have a total thickness of only 128 m (all
six are not shown on Fig. 2).
en en
~
N
~(1)::r'
FI G. 2.
STRATI GRAPHIC SECTIONS AS RECORDED IN THE FOUR BOREHOLES.
ALL DEPTHS ARE IN METRES.
CT ~ (=). ~
~
-.:;o;-~
p:l O::s ~
'<; ;. ~
~ ::r' en ~
~ (1) 0 ~
:;0;- en ....., ~
§.
-
~E ~~
~
BK 1
GA 13
ML2
~g3Q
;.(")~~
g~
(JCl .....
::r'
~
en en
~ ~
(1)
ONGELUK
:i" 61
~
~ ~ ~
343
~ar6e:
~ .... (")
p:l
o
p:l (;
3
o
:=..
~ 'g ~~.
::s
ONGELUK LAVA
CLAYEY
BROWN-GREEN, RED-BROWN,YELLOWGREEN TO BLACKISH CL AYEY
SANDSTONE WITH INTERBEDDED
DIAMICTITE 1347-34Bml. FRAGMENTS
OF QUARTZITE AND CHERT IN
SANDSTONE.
SANDSTONE
-l
o~Q::r'
en (1) 0
(") 0 ::r':J.
(");4.0N
~ (1) - 0
.., 0.. (1)
::r' en t::c
ciQ. p:l ~
::r'
(1) en·
::s
~
......
'-'
p:l
.....
..,
451
~-
::s
::s (1)
(1) p:l
-·0 (")
QUARTZITE OF GAMAGARA FORMATION.
(")
en
567
::r'
C.'"Tj
(1) CT::S -.
@(1)(JClqt/
O~N
(") CT p:l - -
Oen~ .....
(1) ~ ::r'
3
~~ @~
~0..8.,p:l
~..,
_.::r' (1)
::s (1)
CT en
p:l 0 (1) ~
~
3
;4. ..,..0 (")
en (1) ~ ::r'
(") (1)
~.
Q~
~ (1) (1)
::r'
-.
(;j.5
p;. ~ g- ::
::r' en
0..t::c~(1)
3_. _(")
"-<:
::r'
p:l
a.p:l !2,.::S
,......::s _a,......
(1)
~
5'01
0.. (") ::r'
0..(1)
3::(1) CT
0..
~
-·N~.p:l
r.n.
1_
r'g
600
DIAMICTITE WITH NUMEROUS
CHERT FRAGMENTS.
CLAYEY SANDSTONE. WHITE AND
PURPLE QUARTZITE FRAGMENTS
BETWEEN 599 AND 600 m.
315....f--1--1-..1
-l
tTl
Cl
tTl
or
o
Cl
DIABASE
129
SUBGRAYWACKE ARGILLACEOUS AT
TOP BUT CALCAREOUS LCMR DOWN.
GRAY-BROWN TO BLACK CHERT
IN PLACES
QUARTZITE OF GAMAGARA FORMATION.
o'Tl
:r:
GREENISH -GR~Y, BROWNISH -GRAY AND
REDDISH CAL CAREOUS SUBGRAYWACKE .
DOLOMITIC LIMESTONE AT 47-49m.
IN PLACES INTERBEDDED SILTSTONE.
SLUMP STRUCTURES.
DOLOMITIC LIMESTONE WITH INTE RBEDDED ARENACEOUS DIAMICTITE.
GR ITTY SANDSTONE
en
~ £ ~ ~
~::s~-­
DIAMICTITE
A LTE RNATING ARENACEOUS DIAMK:TITE
AND CHERTY CARBONATE. UPPER
DIAMICTITE GRADES UPWARDS INTO
CONGLOMERATE .
ARENACEOUS DIAMICTITE
~
::r'::s~
(=)._.(1)
G"::r'g::s
(5
Z
LV 1
44
0
'(3
;a.
CLAYEY SANDSTONE
DIAMICTITE WITH SANDSTONE AT
TOP
~
~ ~E
(1)
CHERTY I RON CARBONATE
C/l
g.
§ ;.
::s :=.. (1)
•
ARGILLACEOUS DIAMICTI TE
511
uced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
(")
::r'
-l
20
390
::S..,::r'e.
~
»
z
V'l
»
(")
CLAYEY SANDSTONE
8. _:=..
::r'
0
;;0
ARGILLACEOUS DIAMICTITE WITH
THIN SUBGRAYWACKE AT TOP AND
INTERBEDDED CHERTY IRON
CARBONATE AT 16'Om.
ARGILLACEOUS DIAMICTITE
0...., CT (1)
o
ONGELUK LAVA
396
01 AMI (TIH
~
..,
LAVA
322
QUARTZITE AND SHALE OF GAMAGARA
FORMATION.
n
»
r
C/l
o
Q
tTl
-l
-<
DOLOMITIC
LIMESTONE AT 466m.
o'Tl
C/l
o
C
-l
:r:
CALCAREOUS SUBGRAYWACKE. GRAYBROWN SUBORDINATE CHERT IN
PLACES.
614~
GREENISH -GRAY BANDED JASPILITE.
SUBGRAYWACKE.
655~
QUARTZITE OF GAMAGARA FORMATION.
»
'Tl
;;0
n
»
THE GLACIAL BEDS OF THE GRIQUALAND WEST SUPERGROUP
FI G. 3.
COMPOSITE STRATIGRAPHIC COLUMN IN BOREHOLE ML 2 ON MAGOLORING.
NAME OF ROCK
D I A GNOS TIC MIN ERAL SAN D ST RUCTURES .
ONGELUK LAVA
SUBGRAYWACKE
LLJ
PYROCLASTIC
DI A MI CT I TEA RGIL LA CEO US
MATERIAL ( PL.I,
FIG. 1.)
CHERTY IRON CARBONATE
DIAMICTITE ARGILLACEOUS
CHERTY IRON CARBONATE
STILP NOME LANE
AND
CHAMOSITE
PYROCLASTIC
MATERIAL
STI LPNOMELANE
SANDSTONE
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
DIAMICTITE
CHAMOSI TE
AND
CHLORITE (MINOR). FELDSPAR CLEAR
CHERTY CARBONATE
CONGLOMERATE
DIAMICTITE ARENACEOUS
CHERTY ROCK
DIAMI CTiT E ARENACEOUS
CHERTY CARBONATE
LIMESTONE, DOLOMITIC
DIAMICTITE ARENACEOUS
LI MESTON E , DOLOMITIC
DIABASE
CALCAREOUS
SUBGRAYWACKE (ARGILLACEOUS)
CHERT (GRAY-BROWN BLACK)
MANGANESE-BEARING (Mn 0= 13%).
BEDDING INDISTINCT.
I N PLACES.
NUME ROUS SLUMP STRUCTURES (377 TO 399 m I.
SUBGRAYWACKE
(CALCAREOUS)
CLASTIC QUARTZ, SIDERITE / CHAMOSI TE ( PL ATE
I, FIG. 5 ),
LIMESTONE, DOLOMITIC.
IN PLAC ES CHERTY ROCK
(GRAY-BROWN) SUBORDINATE.
SU BGRAYWACKE
(CALCAREOUS)
a.UARTZ FRAGMENTS AND LAMINAE
OF CHERT,
CARBONATE, SIDERITE / CHAMOSITE,
DO LOMITIC
STILPNOMELANE
(DISSEMINATED IN SUBGRAYWACKE ) AND
HEMATITE
I
MAGNETITE.
SLUMP STRUCTURES (476 TO 614m l.
BANDED JASPILITE
(GREENISH -GRAY)
SUBGRAYWACKE
(ARGILLA CEOUS )
a.UARTZITE OF GAMAGARA
FORMATION.
ALTERNATING BANDS OF CHERT AND SIDERITE WITH IDIOBLASTIC
MAGNETITE.
IN
PLACES BEDDI NG DISTURBED.
THE
VI
...::
LLJ
a::
:z:
u
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
4
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
PLATE I
Figure 1
Matrix of diamictite . Shard (left centre) and quartz (white or light grey) in argillaceous material. Magoloring . 30 x. Nicols half-crossed. Transmitted
light.
Figure 2
Striated clast consisting of chert imbedded in diamictite. Moos-fontein. 0,4 x. Oblique illumination.
Figure 3
Matrix of conglomerate . Fragments of quartz (white), quartzite with secondary growth (lower right) and silicified siderite/chamosite (with sandy
material). Magoloring. 30 x . Transmitted light.
Figure 4
Subgraywacke with lamina of siderite (left. dark-grey).- Magoloring. 30 x. Nicols half-crossed . Transmitted light.
Figure 5
Poorly sorted medium-grained ironstone consisting of large grains of iron rich siltstone (sideritelchamosite) surrounded by smaller ones of iron rich
siltstone, cemented by carbonate, occur as thin beds in subgraywacke. Magoloring. 30 x.
Figure 6
Bedded cherty iron carbonate with rafted pebbles of chert in borecore . Magoloring. 1,0 x . Oblique illumination.
THE GLACIAL BEDS OF THE GRIQUALAND WEST SUPERGROUP
followed in turn upwards by sandstone, carbonate and again
diamictite. In borehole ML 2 three complete cycles are present (diamictite ~ sandstone ~ carbonate ~ diamictite) as
well as a number of incomplete ones (diamictite~carbonate
----+diamictite). The cycles in borehole BK 1 are incomplete in
the sense that the carbonates are lacking (Fig. 2). It is also
noteworthy that non-clastic beds are absent in the north and
abundantly present towards the south. Most of these carbonate beds contain a fair amount of silica and iron.
III. MINERALOGY AND PETROLOGY
A. Magoloring KuQ 9-8 (Borehole ML 2)
The rock types of the Makganyene Formation intersected
by borehole M L 2 are shown in detail in Fig. 3 and are discussed below.
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
1. Diamictite and associated rocks
The upper three of the six diamictite horizons are dark in
colour and contain relatively few pebbles set in an
argillaceous carbonate-rich matrix, whereas the lower three
horizons are grey in colour, contain many pebbles, and have a
sandy matrix. The rock contains sand-sized grains of quartz
(PI. I, Fig. 1) and quartzite with some chert, feldspar and carbonate. The upper portions of the argillaceous diamictite contain numerous rounded fragments of fine-grained dolomitic
carbonate, which often enclose flakes of stilpnomelane. The
clasts in the diamictite are heterogeneous but chert is the
dominant rock type. They are poorly sorted, subrounded to
rounded, striated (PI. I, Fig. 2) and commonly vary from 6 to
12 mm but may reach up to 100 mm in diameter. The pebbles
decrease in number upwards in the succession; in fact pebbles
are rare in the upper portions of some individual diamictite
horizons. Occasionally the diamictite grades into conglomerate', sandstone and subgraywacke in which argillaceous
material and carbonate are subordinate (Fig. 3). The conglomerate consists of rounded to subrounded fragments of
chert, quartz, quartzite and subgraywacke. The conglomerate
contains siderite/chamosite which has been silicified (PI. I,
Fig. 3). The sandstone consists mainly of quartz and subordinate chert, partially cemented by chlorite. The subgraywacke, which is described below, is very similar to the
sandstone except for more clayey material and sporadic chert
fragments.
The more argillaceous diamictite contains angular
fragments typical of shards (PI. I, Fig. 1). These fragments occur from 182 m upwards, increasing in number to reach a
maximum at 115 m. From 106 to 115 m only a small number
of these was observed. The fragments consist of either
volcanic glass or near-isotropic chlorite or both. The
crystallization of the near-isotropic chlorite was from the outside inwards as the cores sometimes consist of isotropic
material. However, some of these fragments may also represent chloritized fragments but the distinction between the two
types is not always possible.
2. Subgraywacke
This rock type constitutes the basal unit of the
Makganyene Formation. In places it is calcareous and
between 650 and 655 m it resembles more a mudstone. The
subgraywacke consists of subrounded to angular grains (PI. I,
Fig. 4) of quartz, chert and minor feldspar and quartzite in a
fine ground mass of quartz, dolomitic carbonate, siderite,
chamosite, stilpnomelane, hematite/magnetite and chlorite.
The rock is fine-grained and in places grades into a sandy
siltstone.
Slump structures, the amplitude of which varies between a
few centimetres to approximately 100 cm, are fairly common
in the subgraywacke. Graded bedding occurs at 492,518 and
525 m.
The clastic material consists mainly of quartz and chert. The
quartz grains are rounded to angular and range in size from
0,01 to 0,2 m in diameter. Chert grains are less common but
more rounded than quartz grains. Fragments of quartzite are
present in the subgraywacke just above the quartzite of the
Gamagara Formation, between 650 and 655 m. Feldspar
(microcline and plagioclase) is rare.
Carbonate is present as cementing material in the groundmass and to a lesser extent as veins and as replacement
material. The carbonate is mainly dolomite, calcite and
siderite; the latter forms one of the main constituents of the
subgraywacke (PI. I, Fig. 4). In most samples siderite can only
be detected by means of X-ray diffraction. In a sample of
ironstone sand-sized grains consisting of aggregated finely
crystalline siderite with some chamosite (PI. I, Fig. 5). occur in
a conglomerate bed some 10 cm thick. The roundness of these
grains suggests transportation by water.
Chamosite is a major constituent of the subgraywacke, but
X-ray diffraction methods were required to detect it, except
in the conglomerate bed referred to before. Likewise, stilpnomelane is disseminated in the matrix of the subgraywacke.
Finely crystalline hematite and subordinate magnetite were
also detected in the matrix.
Chlorite occurs mainly as veins, but between 368 and 394 m
it is present as a dominant constituent of the matrix of the
subgraywacke, which here contains up to 13,9 per cent MnO.
The manganese is apparently bound in the chlorite and/or
carbonate because X-ray diffractograms indicate no pure
manganiferous species.
3. Jaspilite
Banded jaspilite occurs interbedded with subgraywacke
(Fig. 3) as alternating bands of chert/jasper, siderite/chamosite
(with idioblastic magnetite) and stilpnomelane veined and
replaced by dolomitic carbonates. No clastic material was
detected in the banded jaspilite. The banding is generally disturbed, particularly at 614 m. Prominent slump structures
such as those in the subgraywacke, however, do not occur.
4. Cherty rock (ferruginous)
This rock type, which is very limited in the sequence, consists of chert/quartz subordinate siderite/chamosite, clastic
quartz and carbonate. In places siderite/chamosite is a major
constituent.
5. Diabase
Owing to the intrusion of the diabase, talc, magnetite and
andradite, but no siderite, has developed in the adjacent
carbonate-rich rocks (Fig. 3).
6. Cherty iron carbonate
Cherty carbonate is interbedded with diamictite from
292 m upwards. The rock is banded, light brownish grey in
colour and in some portions the banding is accentuated by
alternating light brown to grey bands. It consists mainly of
alternating layers of chert and chert/siderite. Siderite occurs
as small crystals in a chert matrix. Flakes of stilpnomelane
and rhombs of calcite and dolomitic carbonate are present in
varying quantities. In places the calcitic and dolomitic carbonates predominate. Small scale slump structures often occur. Scattered pebbles of chert which are nearly always wellrounded are present in places (PI. I, Fig. 6).
B. Gamagara VrQ 18-37 (Borehole GA 13)
On the farm Gamagara clayey sandstone is associated with
the diamictite. In places the rock is very sandy and the quartz
grains are well-rounded (PI. II, Fig. 7). Near the underlying
quartzite of the Gamagara Formation numerous fragments of
quartzite are included in the sandstone (PI. II, Figs. 8 and 9).
These quartzite fragments, some of which are ferruginised (PI.
II, Fig. 10), exhibit relict textures (PI. II, Fig. 8) as well as
rounded grains of zircon and tourmaline similar to the quartzite of the Gamagara Formation. Considering the strati-
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
6
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
PLATE II
Figure 7
Moderately sorted clayey sandstone. Rounded quartz grains. Gamagara. 30 x. Nicols half-crossed. 'transmitted light.
Figure 8
Poorly sorted clayey sandstone . Rounded fragment of quartzite. Note relict textures in the inclusion . Gamagara. 30 x. Transmitted light.
Figure 9
Pebbly sandstone . Fragments of quartzite in groundmass of-fine-grained quartz, carbonate and argillaceous material. Borecore from Gamagara.
0,7 x. Oblique illumination .
Figure 10
Poorly sorted clayey sandstone. Ferruginised fragments of quartzite (black and white). Gamagara. 30 x . Transmitted light.
Figure II
Well sorted coarse-grained sandstone. Grains of chert cemented by quartz. Bredenkamp. 30 x. Transmitted light.
Figure 12
Well bedded sandy siltstone . Note sharp contacts and graded bedding. Langverwacht. Oblique illumination. 2 x.
THE GLACIAL BEDS OF THE GRIQUALAND WEST SUPERGROUP
graphic position in which these fragments or grains occur, it
seems most likely that the underlying Gamagara quartzite
acted as source for the sediments. In places the clayey
sandstone shows banding and graded bedding.
C. Bredenkamp VrQ 18-1 (Borehole BK 1)
On the farm Bredenkamp clayey and gritty sandstone is
associated with the diamictite (Fig. 2). The lower portion of
the diamictite is arenaceous and the upper portion
argillaceous. The gritty sandstone contains well-rounded
grains of chert (PI. II, Fig. 11) and has sharp contacts with the
clayey sandstone.
D. Langverwacht M 89 (Borehole LV 1)
Subgraywacke is associated with diamictite on the farm
Langverwacht. The mineralogy of the subgraywacke is very
similar to that of the subgraywacke in borehole ML 2. The
interbedded sediments are calcareous and show banding,
graded bedding (PI. II, Fig. 12) and slump structures. The
amplitude of the slump structures varies from a few centimetres up to 1 m.
Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2010)
IV. MODE OF DEPOSITION
There can be no doubt about the glacial origin of the
diamictite as striated clasts were collected on surface to the
east of the line of boreholes. Visser (1971, p. 187), investigating the outcrops to the south-east and south, came to a
similar conclusion. The association of the diamictite with immature sandstones and non-clastic beds in a cyclic fashion
and the presence of rafted pebbles, however, argue for
deposition of the material under water, largely from a
buoyant ice mass. Some diamictite beds, especially those inte:bedded with the carbonates, probably represent slumped tIll
derived from the basin margin.
The clayey sandstone/subgraywacke, siltstone and conglomerate occur not only as a basal unit of the sequence, but
are also interbedded at regular intervals in the diamictite.
They could either be explained as till partly reworked by bottom currents or subaqueous mass flow deposits. The presence
of slump structures and graded bedding in the beds, and
intrabasinally derived fragments in the conglomerate rather
point to slumping of material and turbidity flows into the
basin during periodic retreats of the ice.
The deposition of carbonate beds in peri-glacial ~~­
vironments during warmer periods does occur as the solubilIty of CaC03 decreases with an increase in temper~tur~. The
solubility is dependent on the pH of the water which IS controlled by the CO2 content. There is also evidence that the
Campbell Group acted as a source with the result that during
the cold spell, waters would have been almost saturated with
bi-carbonate and calcium ions. As the non-clastic beds occur
mainly towards the south it implies tha~ the deeper an~ q~ieter
portion of the basin lay in th~t direction. The as.soclatlOn of
silica, iron and manganese With the carbonates IS, howeve:,
unique. Two possible explanations could be suggested for thiS
phenomenon: (1) The underlying Gamagara Formation could
have acted as a source for the glacial sediments as the
presence of ferruginised quartzite grains indicate that
ferruginization had already started in the Gamagara Formation at that time. Colloidal iron particles could thus have
been fed into the basin and deposited together with the
calcium carbonate; (2) Volcanic ash particles were foun~ in
the diamictite indicating that the initial phases of volcamsm
giving rise to the overlying Ongeluk la~a we~e cont~m­
poraneous with the last stages of glaCial sedl~entatlOn.
Weathering of this volcanic ash could have Yielded the
necessary silica, iron and manganese in the basin. Both these
processes could have supplemented each othe: .i~ yiel~ing the
siliceous and ferruginous deposits, but later SIlIcificatIOn and
ferruginization, especially along the unconformity at the base
7
of the glacial beds, also played an important role. Although
these explanations seem plausible for deposition of the nonclastic beds in the Northern Cape, striated dropstones indicative of a glacial period, are described from Upper
Proterozoic iron fromations of the Rapitan Group, Canada
(Young, 1976, p. 137). Thus there seems to be a relationship
between Precambrian, non-clastic, iron-bearing deposits and
a cold climate, but the conditions controlling deposition are
not yet fully understood. Cloud (1976, p. 27) suggests that
organisms could have played an important part in the deposition of banded iron formations. Rapid temperature changes
between glacial and interglacial periods could thus have
triggered the proliferation of micro- and macro-organisms.
In general, the following sequence of events during deposition of the glacial beds is envisaged:
(i) Glaciation started on land in the far north. The glacial
debris was transported southwards and laid down as immature sandstones, largely by turbidity currents in a
water body to the south and south-west.
(ii) As the cold climate continued the ice lobe advanced
into the water body and glaciomarine/glaciolacustrine
deposits resulted.
(iii) The Gamagara Formation acted as source and the ice
removed large parts of the deposit by erosion; hence the
arenaceous character of the basal diamictite.
(iv) Periodic retreats of the ice resulted in the build-up of a
sedimentary pile along the margin of the basin, which,
under the effect of gravity, slumped and flowed into the
basin resulting in the interbedded clayey sandstones and
subgraywacke.
(v) In the south during quiet interglacial periods non-clastic
deposits, mainly iron-rich carbonates, accumulated.
The carbonate sedimentation was frequently disturbed
by the turbidity currents- and part of the carbonate
deposits was eroded and picked up by the flows.
(vi) As parts of the Gamagara Formation were removed by
erosion in the source areas, the underlying Campbell
Group was exposed to glacial scouring. A more
argillaceous diamictite with scattered chert clasts
resulted.
(vii) Volcanism started in remote areas in the basin and
volcanic ash and fragments became mixed with the
glacial sediments. This volcanic episode coincided with
the waning stages of the ice period.
(viii) Deposition of clastic sediment, stopped completely
when the Ongeluk lava covered the entire area. The
presence of pillow lava (Grobler and Botha, 1976, p. 57)
indicates outpouring before the basin was filled with
sediment.
ACKNOWLEDGMENTS
This paper is published with the kind permission of the
Management of ISCOR. The authors wish to express their appreciation to their colleagues, Drs. F. E. Malherbe and J. G.
D. Steyn for reading the manuscript and others who have
contributed to this work.
REFERENCES
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Research and Process Development
ISCOR
PRETORIA
Geology Department
University of the Orange Free State
BLOEMFONTEIN 9301
Accepted for publication by the Society on 10.3.1977