Precambrian Research 101 2000 341 – 351
High temperature ash flow – wet sediment interaction in the Makwassie Formation, Ventersdorp Supergroup,
South Africa
W.A. van der Westhuizen , H. de Bruiyn
Department of Geology, Uni6ersity of the Orange Free State, P.O. Box
339
, Bloemfontein,
9300
, South Africa
Abstract
The Late Archaean Ventersdorp Supergroup in South Africa constitutes a supracrustal succession consisting of mafic lava at the base that is overlain by a clastic wedge deposit, known as the Kameeldoorns Formation, which
formed in grabens that were the result of extensional tectonics. Overlying these units is an intermediate to felsic ash-flow deposit, the Goedgenoeg and Makwassie Formations, that are now represented by feldspar and quartz-
feldspar porphyries. At T’Kuip in the Northern Cape Province, spherical structures or clasts varying greatly in size occur at the contact of the Makwassie Formation with the underlying sedimentary Kameeldoorns Formation. The
larger clasts have cavities of different shapes and are regarded to have developed as a result of the deposition of a high temperature ash-flow onto water saturated sediments. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Ash flow; Wet sediment; Makwassie Formation; Ventersdorp Supergroup www.elsevier.comlocateprecamres
1. Introduction
The Ventersdorp Supergroup represents a late Archaean early Proterozoic; :2.7 Ga; Arm-
strong et al., 1986 supracrustal succession on the Kaapvaal Craton Grobler et al., 1989; Van der
Westhuizen et al., 1991. This succession is the second in a series of four basins that developed
between 3.2 and 2.1 Ga on the Kaapvaal Craton representing the largest and most widespread se-
quence of volcanic rocks on the craton. It occu- pies
an elliptical
area of
: 300 000 km
2
Pretorius, 1976 that extends for about 750 km along a north-east axis from Sodium in the Brit-
stown District of the Northern Cape Province to Derdepoort in the Northwestern Province with a
width of 350 km at its widest. It consists of a 5100 m thick sequence of sub-aerial volcanic rocks as
well as intercalated sedimentary units that attains a thickness of up to 2900 m Van der Westhuizen
et al., 1991. Current research by Winter and Van der Westhuizen suggests that the Ventersdorp
basin was much greater than the present outline delineated in Fig. 1. The best exposures are found
in the Northwestern Province, Northern Cape Province, Gauteng and southern Botswana.
Corresponding author. Tel.: + 27-51-4012515; fax: + 27- 51-4478501.
E-mail address
:
vdwestwglg.nw.uovs.ac.za W.A. van der Westhuizen
0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 9 9 0 0 0 9 4 - 7
Interesting enigmatic structures were found at the base of the quartz-feldspar porphyry of the
Makwassie Formation in the Britstown District at T’Kuip. They are similar to structures that result
from the interaction of lava with wet sediments and from subaqueous lava flows. Many of these
features are elsewhere described as peperites, but by definition a peperite is the result of the interac-
tion of a lava flow with wet sediment. Numerous examples of basalt to andesite peperites have been
described Schmincke, 1967; Kokelaar, 1982; Walker and Francis, 1987; White and Busby-
Spera, 1987; Kano, 1989. According to Hansen and Wilson 1993 descriptions of rhyolite peper-
ites are rare and restricted to intrusive margins. They mention one example of large-scale interac-
tion between silicic magma and wet sediment to form voluminous peperites Snyder and Fraser,
1963. Hansen and Wilson, 1993 describe an- other occurrence of extensive peperites within a
Jurassic volcano-tectonic rift basin in southern Chile. The structures in the Makwassie Formation
at T’Kuip resemble peperites to a certain extent, but as will be seen from the detailed description,
have characteristics that have not been reported in peperites. The purpose of this paper is to describe
these rare and unique features and to present a case for their possible origin.
2. General geology and stratigraphy of the Ventersdorp Supergroup
