Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol101.Issue2-4.2000:

Precambrian Research 101 2000 193 – 210 Subaerial volcanism in the Palaeoproterozoic Hekpoort Formation Transvaal Supergroup, Kaapvaal craton Jacobus D. Oberholzer † , Patrick G. Eriksson Department of Geology, Uni6ersity of Pretoria, Pretoria 0002 , South Africa Abstract Within a 30 km strike length of the Palaeoproterozoic Hekpoort Formation Pretoria Group, Transvaal Super- group, approximately equal proportions of basaltic-andesitic flows and lenses of volcaniclastic rocks are preserved. An overall intracratonic and subaerial setting is inferred for these rocks. A complex volcanic environment is thought to have existed, with relatively quiescent amygdaloidal, massive lava flows and violent pyroclastic eruptions either succeeding each other or, at times, coexisting within the same setting. Identified physical processes of effusive volcanism include coarser and finer pyroclastic flows massive pyroclastic breccia and lapilli-tuff breccia facies, sedimentary reworking of inferred ash-cloud fine material lapilli-tuff facies, distal sheetflood stratified lapilli-tuff breccia facies and debris-flow massive reworked lapilli-tuff breccia facies reworking of pyroclastic debris. Two prominent lenses of volcaniclastic rocks preserved within the study area may lie either side of an eruptive centre and a palaeovalley may have confined lahar deposits. The predominance of pyroclastic flow and lahar deposits in the Hekpoort Formation resembles the succession formed from the 1991 Mount Pinatubo eruption. © 2000 Published by Elsevier Science B.V. Keywords : Subaerial volcanism; Palaeoproterozoic Hekpoort Formation; Basaltic-andesitic flows www.elsevier.comlocateprecamres

1. Introduction

The Palaeoproterozoic, comagmatic Hekpoort and Ongeluk formations represent a major conti- nental flood basalt event on the Kaapvaal craton at 2223 9 13 Ma Cornell and Schu¨tte, 1995; Reczko et al., 1995. These tholeiitic andesites are thought to have originally covered : 500 000 km 2 of the craton Cornell et al., 1996 and are pre- served in two structural basins, Transvaal and Griqualand West Eriksson and Reczko, 1995. The 300 – 830 m thick Hekpoort Formation in the Transvaal basin outcrops over 100 000 km 2 Reczko et al., 1995 and is inferred to have formed in a subaerial palaeoenvironment Button, 1973; Res, 1993; Eriksson and Reczko, 1995; Oberholzer, 1995. Pillow lavas with hyaloclastites and massive flows in the Ongeluk Formation indi- cate subaqueous extrusion in the Griqualand West basin Cornell and Schu¨tte, 1995. Interpre- tation of the Hekpoort – Ongeluk volcanics as mantle plume-related continental flood basalts is Corresponding author. E-mail address : patscientia.up.ac.za P.G. Eriksson † Deceased. 0301-926800 - see front matter © 2000 Published by Elsevier Science B.V. PII: S 0 3 0 1 - 9 2 6 8 9 9 0 0 0 8 8 - 1 supported by their geochemistry and preserved geometry, and processes within a replenished, fractionated, tapped, assimilated RFTA; Arndt et al., 1993 magma chamber have been discussed by Reczko et al. 1995. Stratigraphically, the Hekpoort Formation forms part of the clastic volcano-sedimentary Pre- toria Group of the Transvaal Supergroup for a recent overview, Eriksson and Reczko, 1995. The formation, in many parts of the preserved Transvaal basin, sharply overlies the subaerially emplaced Boshoek conglomerates and sandstones with a sharp contact, and is unconformably suc- ceeded by the Dwaalheuwel continental sand- stones Eriksson et al., 1993. In the central-southern parts of the basin, including the present study area, both the lenticular bounding formations are absent, and the Hekpoort lavas sharply overlie mudrocks of the Timeball Hill Formation, and are similarly overlain by Strubenkop Formation mudrocks Fig. 1 Table 1. Fig. 1. Sketch map of the Hekpoort Formation west of Pretoria. Note study area blocked, and type locality type section along the road south of the town of Hekpoort. Timeball Hill and Strubenkop Formation shales underlie and overlie, respectively, the Hekpoort lavas with sharp contacts. J .D . Oberholzer , P .G . Eriksson Precambrian Research 101 2000 193 – 210 195 Table 1 Stratigraphy of the Transvaal Supergroup after Eriksson and Reczko, 1995 Transvaal basin-formation Lithology Palaeoenvironment Central area includ- West East ing study area Remnant eastern and central regressive Rayton Sandstones and mudrocks, minor Woodlands Houtenbek, Steenkampsberg, Neder- marine basin shoreline; lacustrine tuffs, limestones and andesites horst, Lakenvlei, Vermont basin in west Woodlands Formation Epeiric sea coastline tidal braid- Magaliesberg Magaliesberg Magaliesberg Sandstones deltas Silverton Silverton Mudrocks, lavas Epeiric sea Silverton Daspoort Alluvial “ marine transgression in E Sandstones, conglomerates Daspoort Daspoort Strubenkop Strubenkop Mudrocks, sandstones Lacustrine Strubenkop – Dwaalheuwel Sandstones, conglomerates Alluvial Dwaalheuwel Subaerial volcanism Basaltic andesite Hekpoort Hekpoort Hekpoort Alluvial Boshoek Boshoek Conglomerates, sandstones – Timeball Hill Timeball Hill Timeball Hill Epeiric sea Mudrocks, sandstones Rooihoogte Rooihoogte Conglomerates, sandstones and mu- Rooihoogte Alluvial, locally lacustrine drocks Fig. 2. Schematic profile through the Hekpoort Formation within the study area, drawn parallel to the NE-SW strike of the unit. Note two major volcaniclastic lenses discussed in the text: lower, SW lens and upper, NE and much larger, thicker lens. The latter has a diagonal arrangement of rock types, with massive pyroclastic breccias passing through massive lapilli- tuff breccias into uppermost and most-northeasterly lapilli-tuffs. Note also reworked lapilli-tuff breccia facies in the upper parts of the NE lens, and the widespread mudrock bed, locally eroded at the base of a smaller, laterally restriced volcaniclastic lens upper left-centre of profile. The Hekpoort volcanic rocks are unusual in that most eruptive successions older than 2.0 Ga are preserved from subaqueous settings W.U. Mueller, 1998, personal communication. Previous work conducted on these rocks has entailed either regional mapping e.g. Button, 1973 or geochem- ical research e.g. Sharpe et al., 1983; Reczko et al., 1995. A recent detailed field study of the Hekpoort Formation in its type locality about 60 km west of Pretoria Fig. 1 revealed a complex interplay of subaerial lava flows, pyroclastic de- posits, and their reworked counterparts Ober- holzer, 1995. This paper aims to provide an analysis of the physical processes of volcanism for the subaerial Palaeoproterozoic Hekpoort Forma- tion, based on field observations, thin section petrography, and limited XRF and XRD analysis.

2. General geology of the study area