The upper part of the Platberg Group is repre- sented by the Rietgat Formation. This is a succes-
sion consisting of mafic volcanic rock, tuffs as well as tuffaceous siltstone and stromatolitic
cherty limestone that developed during the subsi- dence stage after the volcanic emanations Grob-
ler et al., 1989.
The upper part of the Ventersdorp Supergroup is made up of the Bothaville Formation; a sedi-
mentary unit comprised of conglomerates and quartzites. Another mafic to intermediate volcanic
succession, the Allanridge Formation, overlies the sedimentary Bothaville Formation and represents
the culmination of the Ventersdorp volcano-sedi- mentary pile.
3. The geology of the T’Kuip Hills
The geology of the basal part of the Venters- dorp Supergroup at T’Kuip Hills between Brit-
stown and Prieska in the Northern Cape Province is depicted in Fig. 3 and a profile is presented in
Fig. 4.
The Platberg Group is generally absent from the northeastern region of the depository and
outcrops inconsistently over the rest of the area, where large variations in thickness occur. The
thickness of the Makwassie Formation can reach up to more than 1000 m in the Northwest
Province, but it is only between 200 and 300 m thick at the T’Kuip area. Here the Makwassie
Fig. 3. The geology of the Ventersdorp in the study area.
Fig. 4. Simplified stratigraphic profile of the Ventersdorp succession in the study area modified after Grobler et al., 1989.
Formation directly overlies Kameeldoorns For- mation. Numerous layers of tuffaceous material
occur in the latter unit. The tuffaceous material is intercalated with argillaceous material derived
from the granitic basement that forms the base- ment to the Ventersdorp Supergroup rocks in the
study area.
Deposition of the lower Ventersdorp Super- group took place in a half-graben at this locality
Grobler et al., 1989. The basal member of the Kameeldoorns Formation, previously described
as the Ongers River Arkose Formation Grobler et al., 1989, consists of poorly sorted, matrix-sup-
ported boulder beds with granitic boulders of up to 1.4 m in diameter Grobler et al., 1989. Up-
ward in the succession, the boulder size diminishes and tuffaceous layers are frequently encountered.
Raindrop imprints Fig. 5 and desiccation cracks Fig. 6 occur in the tuffaceous material and
cross-bedding as well as graded bedding are fea- tures of this succession Van der Westhuizen et
al., 1989. The tuffaceous beds are abundant at
Fig. 5. Raindrop imprints on argillaceous horizon in the Kameeldoorns Formation of the Ventersdorp Supergroup at
T’Kuip Hills. Fig. 8. Agglutinated unit at the base of the Makwassie Forma-
tion below Fig. 7.
this locality in contrast to other Kameeldoorns exposures. Laterally, towards the northeast of the
main outcrop, tuffaceous material becomes domi- nant and raindrop imprints as well as desiccation
cracks occur Van der Westhuizen et al., 1991. Close to the contact with the Makwassie, sheared
material resembling shales andor tuffaceous ma- terial was observed. Very little disturbance of the
underlying material was observed, but the lack of outcrop immediately above the contact between
the sediment and the quartz-feldspar porphyry hampers a more detailed investigation.
The occurrence of raindrop imprints and desic- cation cracks suggest shallow water deposition
and periodical exposure, the argillaceous material was probably deposited during floods, as sup-
ported by the presence of upward-fining cycles and graded bedding.
The basal unit of the Makwassie Formation varies from 1 to 2 m in thickness and consists of
quartz-feldspar porphyry clasts of different sizes distributed in a matrix of altered rhyolitic mate-
rial Fig. 7.
Another primary feature of the basal Mak- wassie unit is depicted in Fig. 8 and consists of
what resembles agglutinated material. Accretion- ary lapilli were also observed along strike in the
basal unit. This forms the base of the unit and is overlain by the thinly banded tuff of between 1
and 1.5 m thick as depicted in Fig. 7.
The banding is mesoscopically visible as alter- nating lenses of lighter and darker material, the
Fig. 6. Dessication sracks in an argillaceous horizon in the Kameeldoorns Formation of the Ventersdorp Supergroup at
T’Kuip Hills.
Fig. 7. Base of the Makwassie Formation at T’Kuip-note lower 1.5 m with spherical structures and the overlying banded
unit.
latter possibly being fiamme that are strongly flattened. The thickness of these lenses is between
1 and 10 mm. Both the light and dark bands have visible phenocrysts of quartz and feldspar, al-
though the percentage of phenocrysts is lower in the darker ones than the lighter ones.
Also present are occasional lithic fragments of mainly basement granite and irregular, though
elongated isotropic bodies that may represent remnant fiamme in the rhyolitic tuff. Similar fea-
tures in rocks from the Ritchie area, which are presently stratigraphically correlated with the
Makwassie Formation, were described as fiamme in eutaxitic ignimbrites Potgieter and Lock,
1978. The bulk of the Makwassie Formation, however, consists of a unit of massive quartz-
feldspar porphyry, with phenocrysts of quartz, feldspar and cryptocrystalline material. The latter
is probably a devitrification product of volcanic glass, visible as distinct patches of differing grain
size. Apatite, zircon, sphene and opaque minerals are present in accessory amounts, whereas sericite
flakes and chlorite are present as alteration prod- ucts. The feldspar phenocrysts in the clasts of the
basal unit show a high degree of alteration, whereas higher up in the Makwassie succession,
they are in a better state of preservation. The contact between the basal unit with the clasts and
the banded unit is sharp, whilst the latter unit grades into the upper massive rhyolite Fig. 9.
4. Structures in the basal unit
