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,Pretoria0002,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
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1. Introduction
The Palaeoproterozoic, comagmatic Hekpoort and Ongeluk formations represent a major conti-nental flood basalt event on the Kaapvaal craton at 2223913 Ma (Cornell and Schu¨tte, 1995; Reczko et al., 1995). These tholeiitic andesites are thought to have originally covered :500 000 km2
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 km2
(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:[email protected] (P.G. Eriksson)
†Deceased.
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).
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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 Woodlands Sandstones and mudrocks, minor
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 Daspoort Daspoort Sandstones, conglomerates Alluvial marine transgression in E 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 Mudrocks, sandstones Epeiric sea
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
The Hekpoort Formation was studied along a 30 km strike length, where outcrop width varies between 2 and 4 km and the formation dips gently
to the N-NW at about 15° (Fig. 1). A summary of the vertical and lateral arrangement of the major rock types in the study area has been constructed (Fig. 2) based on large- and small-scale maps and field profiles.
The schematic profile, oriented along the NE-SW strike of the Hekpoort Formation, demon-strates that volcaniclastic rocks and lava flows were both important. Although there has been no regional study of the Hekpoort Formation to establish proportions of lava flows and volcani-clastic detritus, Sharpe et al. (1983) suggested that flows predominate. In the east of the preserved Transvaal basin, Button (1973) found that vol-caniclastic rocks only made up about 10% of the formation, and that they were concentrated to-wards its base. The present study area is an exception in that volcaniclastic rocks make up a significant proportion (Fig. 2).
The Hekpoort Formation in the study area is
J.D.Oberholzer,P.G.Eriksson/Precambrian Research101 (2000) 193 – 210 197
for about 5000 m.(Fig. 2). A 400 – 450 m thick volcanic succession follows, composed of lava flows in the southwest, and lenticular volcaniclas-tic rocks in the northeast. Estimated maximum thickness of the NE lens is 450 m and it extends laterally for at least 8 km in the study area (Fig. 2). The volcanic succession is overlain by a thin,
B9 m thick tuffaceous mudrock, followed by 50 – 100 m of volcanic flows, with a laterally re-stricted, 100 – 150 m thick lens of volcaniclastic rocks near the centre of the study area (Fig. 2). The later eruptive event has eroded beneath the extensive tuffaceous mudrock horizon (Fig. 2).
3. Basaltic andesitic flows
Petrographically, the basaltic andesites are composed of plagioclase, quartz and amphibole.
Euhedral to subhedral crystals of plagioclase, with compositions between An35 and An60 are locally
zoned, whereas the amphibole varies from horn-blende to tremolite-actinolite. Amygdaloidal rocks (amygdales make up c. 5 – 25% of the rock) exhibit mostly spherical amygdales, filled with quartz, calcite, chlorite, epidote, clinozoisite and minor chalcopyrite (Oberholzer, 1995). As evidenced by the amphibole replacing original pyroxene in the rocks, the Hekpoort flows are locally highly altered, having been subjected to low grade metamorphism (probably due to in-trusion of the Bushveld Complex in the upper Pretoria Group), hydrothermal alteration and weathering.
The Hekpoort flows are depleted in the alkaline elements (Table 2), which are highly mobile dur-ing metamorphism and alteration (e.g. Reczko, 1994). Classification is therefore based on rela-tively immobile trace elements such as Ti, Zr, Nb, Y and REE (Oberholzer, 1995; Reczko et al., 1995). The Hekpoort lavas may be classified as andesites and basaltic andesites (Nb/Y-Zr/TiO2;
Winchester and Floyd, 1977) or as tholeiitic basalts (modified Jensen (1976) diagram, after Viljoen et al., 1982). Harmer and von Grue-newaldt (1991) also proposed a basaltic andesitic classification.
Regionally, the Hekpoort lava flows are charac-terised by a lower microcrystalline (uncommonly having small amygdales) zone, a medial fine-crys-talline, microporphyritic zone, and an amyg-daloidal upper zone (Reczko et al., 1995). In the study area, flows were identified sporadi-cally, particularly in the southwest of the region, with complex flow patterns (Fig. 3) mak-ing it impossible to determine a consistent flow direction. Microcrystalline, massive lavas characterise the outcrops, with subordinate amygdaloidal zones occurring at certain lo-calities.
Several clasts composed of pre-existing basaltic andesite were also observed within localised lava flows. Such occurrences are in the southwestern portion of the NE lens and presumably reflect interaction between lava flows and pyroclastic processes.
Table 2
Average major plus trace element concentrations for flows (1;
n=7) and volcaniclastic rocks (2; n=22) of the Hekpoort Formation in the study area; clasts and matrix not differenti-ated
1 2
SiO2 56.14 55.95
Fig. 3. Complex flow patterns, locally preserved within the basaltic andesitic lavas.
4. Pyroclastic rocks
Terminology applied to volcaniclastic rocks in this paper follows the usage detailed by Mueller et al. (2000). Volcaniclastic rocks thus include the products of pyroclastic, autoclastic and epiclastic genetic processes. Similarly, the term pyroclast is used for fragments resulting directly from vol-canic action. The well known granulometric clas-sification of Fisher (1961, 1966) for indurated pyroclastic rocks is used here: tuff (B2 mm), lapilli (2 – 64 mm) and block or breccia (\64 mm).
Grain size distribution of the Hekpoort pyro-clastic rocks shows some ordering in the study area. In the southwestern pyroclastic lens (Fig. 2), there is an overall reduction in fragment size away from the thicker medial part of the lens (massive pyroclastic breccia facies, below) and towards its tapering terminations (massive and stratified lapilli-tuff breccia facies, below). In the northeast-ern lens, overall grain size decreases from SW to NE and locally upwards, oblique to the regional stratigraphy (Fig. 2). The massive breccia facies occurs in the southwesterly-lower part of the lens, and passes through the medial massive lapilli-tuff breccia facies into an uppermost and most north-easterly lapilli-tuff facies (Fig. 2). Outcrops
pre-clude observation of whether the transitions between these three facies are sharp or grada-tional. A lens of a massive reworked lapilli-tuff breccia facies, 50 m thick and extending laterally for about 2 km, overlies the thickest pyroclastic breccias in the northeastern lens shown in Fig. 2. The facies are described in detail below.
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Table 3
Volcaniclastic lithofacies in the Hekpoort formation
General setting
Characteristics Origin
Lithofacies
1. Massive pyroclastic 9100–400 m thick; locally interstratified Pyroclastic flow Predominant lava flows with localised breccias with lava flows; clasts 1 cm to 100 cm in volcaniclastic centres. Volcaniclastic
processes included pyroclastic flows and size (average 15 cm), low roundness and
inferred ash-cloud deposition. All these sphericity, and composed mostly of massive
and amygdaloidal andesite, with deposits subjected to reworking by debris-flow and sheetflood processes subordinate mudrocks; clast: matrix ratios
up to 4:1. Secondary silicification common
910–30 m thick; locally interstratified with Pyroclastic flow 2. Massive lapilli-tuff
(1); clasts 4–35 cm in size (average 97 breccias
cm), poorly to well rounded and oval to disc-shaped, composition 94:1 basaltic andesite to clasts of mudrock, sandstone and chert. Secondary silicification common 910–30 m thick; locally interstratified with
3. Stratified lapilli-tuff Reworked from (2), probably by distal breccias (1); clasts 5 mm–10 cm in size (average 5 sheet-floods.
mm–6 cm) and angular to subrounded, compositions similar to (2); graded beds, 25 cm–1.5 m thick
Reworked from ash-cloud deposits 960 m thick; clasts 3 mm–22 cm, angular
4. Lapilli-tuffs
associated with (1)+(2), by debris-flow to moderately rounded and with disc to
and sheetflood sedimentation lenticular geometry. Well bedded, with local
grading
Debris-flow deposition, as coarse and 950 m thick; clastsB1 cm to 120 cm in
5. Massive reworked
size (average 20 cm), moderately rounded,
lapilli-tuff breccias fine lahars
4.1. Massi6e pyroclastic breccia facies
4.1.1. Description
Variation in clast size, shape and roundness (Fig. 5) is substantial in this facies, with no appar-ent systematic trends, either laterally or vertically.
In general terms, roundness and sphericity are very low (Fig. 5) and clasts vary from 1 to 60 cm in size, with average values of about 15 cm. Locally, clasts up to 70 by 100 cm occur, with highly angular forms in two dimensions. Clast are mainly composed of massive and amygdaloidal
Fig. 4. Outcrop in the SW termination of the SW lens of pyroclastic rocks, illustrating poor rounding and low sphericity of lapilli within the massive lapilli-tuff breccia facies occurring here (see text for discussion).
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Fig. 6. Scoriae in thin section of the massive pyroclastic breccia facies; width of photograph represents 2 mm and taken under plane polarised light.
andesite, with subordinate mudrocks; a few clasts exhibit a reaction rim.
Generally, these coarse-grained rocks have a high clast content; locally, ratios of clasts:matrix of about 4:1 are attained. The latter pyroclastic breccias are characterised by blocks with maxi-mum dimensions in excess of 62 mm, and they are both poorly rounded and low in sphericity. Sco-riae (Fig. 6) are plentiful, and in thin sections some grains are seen to have corroded margins. The matrix of this subfacies varies from a crys-talline to a lithic material, fine to very fine in grain size (Oberholzer, 1995). Secondary quartz veins are common in the matrix and quartz over-growths often occur on clast surfaces.
4.1.2. Interpretation
The massive nature, lack of any grading and poor sorting of this facies support laminar flow transport and deposition as a volcaniclastic debris flow (Wright et al., 1980). The high clast to matrix ratio suggests that density modified grain flow (Lowe, 1982) rather than pure debris-flow may have operated. Fisher et al. (1980) and Orton (1996) refer to similar deposits as block and ash flows. The quartz veins and secondary over-growths on the Hekpoort clasts, and the common
scoriae, support a high volatile content during and after transport and deposition, as would be found in gas-rich pyroclastic flow deposits. Rounding of grains probably reflects their initially high temperatures (some clasts have reaction rims) and abrasion during transport by locally developed turbulent transport conditions. The combination of generally massive breccias, com-mon scoriae and of predominantly juvenile and accessory basaltic andesite clasts suggests a pyro-clastic flow origin for these rocks (Self and Sparks, 1981; Cas and Wright, 1987).
4.2. Massi6e lapilli-tuff breccia facies
4.2.1. Description
amyg-daloidal) to clasts of mudrock, sandstone and chert (n=100). Clasts are often covered with a thin layer of (secondary) quartz. The andesite clasts are highly altered, with common amphibole, and calcite, epidote, zeolites, magnetite, and clay minerals including both kaolinite and montmoril-lonite (Oberholzer, 1995). Basaltic andesitic sco-riae are common in these breccias and are filled with chlorite, quartz and zeolites (Oberholzer, 1995). Many scoriae have corroded margins, and their generally flattened, elongated shapes possi-bly indicate welding. The matrix consists of fine- to coarse-grained tuff. Clast to matrix ratios vary from a maximum of 2:3 to a minimum of 1:7.
4.2.2. Interpretation
This facies is comparable to the massive pyro-clastic breccias and thus most likely represents finer-grained pyroclastic flow deposits. In the SW lens this facies appears to be distal relative to the central and thicker massive pyroclastic breccia facies, and in the NE lens, these finer pyroclastic flow deposits occur above and to the northeast of basal coarse pyroclastic flow deposits (Fig. 2).
4.3. Stratified lapilli-tuff breccia facies
4.3.1. Description
Average grain size varies between 5 mm and 6 cm, with fewer bombs of 8 by 10 cm. The latter exceptionally reach up to 40 cm along their longest dimension and exhibit approximately equal proportions of angular and subrounded forms; scoriae are common. Well developed layer-ing characterises this facies (Fig. 7), and graded beds, 25 cm to 1.5 m thick and with 3 – 5 cm basal lapilli fining upwards into diameters of a few millimetres, are relatively common.
4.3.2. Interpretation
This facies can logically be interpreted as a water-reworked version of the massive lapilli-tuff breccia facies. Generation as a lahar is not sup-ported by the well developed stratification (e.g. Orton, 1996) and graded bedding indicates wan-ing current strength as upward-finwan-ing sediment grains are deposited (e.g. Tucker, 1994). Resedi-mentation of lapilli-tuff breccia deposits by rain-fall, commonly associated with climatic disturbances accompanying eruptive events, is in-ferred for this facies, probably by deposition from distal sheetfloods.
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Fig. 8. Massive reworked lapilli-tuff breccia facies in upper portion of NE lens (see Fig. 2). Note well rounded coarse clasts, concentration of clasts in certain layers and localised disturbance of fine matrix material where large clasts have apparently been subject to turbulent flow within the viscous supporting material. Poorly developed bedding in matrix material separating clasts is a compaction feature.
4.4. Lapilli-tuff facies
4.4.1. Description
About 60 m of lapilli-tuffs are exposed in the northeastern, upper part of the large pyroclastic lens (Fig. 2), exhibiting a local stratigraphy com-prising three units. The lowest unit is character-ised by clasts 1 – 10 cm size (average about 4 cm), irregularly distributed in a fine, siliceous matrix. A few large clasts occur, up to 20×22 cm in size and with disc-like geometry. In general, clast shapes vary from disc-shaped to lenticular and rounding from moderate to highly angular grains. Petrographic investigation of the lower unit re-veals common scoriae, some of them elongated and flattened. Both medial and upper units exhibit well developed layering, defined by basal well rounded but angular (low sphericity) lapilli, 3 – 40 mm in diameter, passing up into tuff. These graded beds are 25 – 30 cm thick.
4.4.2. Interpretation
The massive, matrix-supported character of the lowest unit suggests debris flow deposition, possi-bly as a lahar (Orton, 1996). The upper, graded beds point to fine-grained sheetflood reworking,
distal to the inferred lahar deposits, as already suggested for the stratified lapilli-tuff breccia facies.
4.5. Massi6e reworked lapilli-tuff breccia facies
4.5.1. Description
The basal contact with the massive lapilli-tuff breccia facies (Fig. 2) is nowhere exposed. South-west of the reworked facies, and in its uppermost portion in the northeast, a thin development of much finer reworked pyroclastics is preserved (Fig. 2). The latter contain basaltic andesitic clasts up to 1 cm in size, in a fine matrix, and exhibit fine and convolute laminations, B1 – 2 mm thick. The larger clasts are subrounded and smaller ones subangular, and the clasts are subordinate to and supported by the matrix.
good, the larger clasts being better rounded (Fig. 8). The rock demonstrates subhorizontal bedding, which is probably a compactional feature (Eriksson and Twist, 1986). In addition, clasts are concentrated in definite layers, separated by pre-dominantly matrix-sized material (Fig. 8). Certain of the larger clasts have obviously been subject to turbulent flow, as the long axes of elongated and oval-shaped clasts often do not lie parallel to the subhorizontal bedding; matrix particles around these clasts also show evidence of having been disturbed by their movement (Eriksson and Twist, 1986). In thin section, both matrix and clasts are composed of highly weathered basaltic andesitic material; the preponderance of secondary miner-als such as amphibole, chlorite, micas and clay minerals indicates that alteration is more intense than in the primary pyroclastic rocks (Oberholzer, 1995). There is no evidence of secondary silicification.
4.5.2. Interpretation
These rocks reflect aqueous gravity flow deposi-tional processes, a common feature in volcanic environments (Laznicka, 1988). The generally high level of rounding of the clasts in these rocks is compatible with reworking, as is the greater alteration (compared to the breccias discussed above) of the constituents, both clasts and matrix. The poor sorting and good rounding of the large clasts, in contrast to the more angular finer clasts points to a gravity flow deposit (Lowe, 1982). A debris-flow origin is supported by the massive, matrix-supported nature of this facies, by polymictic clast compositions, rounding of frag-ments and an absence of evidence for hydrother-mal silica. An alternative intepretation, as pyroclastic surge deposits, is not supported by the massive nature and lack of evidence for extensive turbulent flow conditions (Sparks et al., 1973). However, subordinate turbulent flow may occur within debris-flow systems (Shultz, 1984), as sug-gested by the elongated or oval-shaped clasts in this facies which lie at an angle to the subhorizon-tal fabric of the rock.
The poorly defined horizontal arrangement of larger clasts in specific layers, separated and sup-ported by matrix material, is compatible with
laminar flow of a fine-grained, high viscosity tuff-water matrix (Lowe, 1982), supporting large clasts in a gravity flow or coarse-grained lahar (Ober-holzer, 1995). The high water content and ele-vated temperatures typical of lahars (Fisher, 1982) would also have promoted alteration of the re-worked pyroclastic debris, compared to the pyro-clastic flow deposits. Fisher (1982) proposes that lahars form due to eruption of pyroclastic debris into crater lakes, snow or ice, due to heavy rain-fall during eruption, or to warm pyroclastic mate-rial flowing into streams. The lack of evidence in the present study area for rapid cooling or of secondary silica (i.e. hydrothermal fluids) suggests that rainfall associated with eruption is a likely explanation. Finer reworked pyroclastic debris, which overlies locally the coarse lahar deposit, and also overlies pyroclastics breccias in the SW of the northeastern lens (Fig. 2), was most likely laid down distal to the coarse mass-flow sedi-ments; deposition out of suspension from a water column is indicated by the laminated nature of these sediments.
5. Mudrocks
Mudrocks occur in two horizons in the study area: (1) a thin lenticular bed in the northeast, at about the same stratigraphic level as the south-western pyroclastic lens, and (2) a much more persistent bed, 9 m thick, sharply overlying the northeastern pyroclastic lens (Fig. 2). Southwest of the central study area, a third pyroclastic lens, of similar character to the two already described, has a discordant base and cuts through the persis-tent mudrock horizon (Fig. 2), indicating an ero-sive event.
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straight-crested current ripples, indicating trans-port towards the southeast. Some of the ripple marks are covered by a thin siliceous layer about 3 mm thick, which has aided their preserva-tion.
6. Discussion: processes of volcanism and depositional setting
6.1. Pyroclastic or autoclastic origin
As has been established in Section 4, there are no facies preserved in the Hekpoort volcaniclastic rocks which support either surge or air-fall de-posits. This, combined with the low viscosity of basaltic andesitic lava, the spatial relationship of volcaniclastic rocks to flows (Fig. 2), the predom-inantly massive nature of the volcaniclastic rocks and their commonly angular to subrounded clasts support a possible autoclastic origin. In contrast, grain size changes observed in the two main vol-caniclastic lenses described here, their widespread distribution over the study area, evidence for hy-drothermal alteration (secondary silica), local sco-riae and abundant vesicular clasts support a pyroclastic origin. Secondary silica is pervasive in the two most abundant facies, massive breccias and massive lapilli-tuff breccias, either as veins in the matrix or as partial grain over-growths. This suggests that the rocks were still warm when deposited, as also indicated by reac-tion rims present on some coarse breccia frag-ments. The large scale grain size changes are marked in the study area (Fig. 2) and also rather support a pyroclastic genetic model. Possible welding features (e.g. Section 4.2) are compatible with pyroclastic rather than autoclastic genera-tion. As autoclastic flow breccias tend to be monogenetic, deriving most clasts from the parent magma (Lajoie, 1984), the mixture of andesitic and sedimentary compositions in the Hekpoort volcaniclastic rocks also supports a pyroclastic interpretation. Sedimentary reworking, to pro-duce the stratified lapilli-tuff breccia or the mas-sive reworked lapilli-tuff breccia facies could apply equally to pyroclastic and autoclastic de-posits.
6.2. Bounding formations and subaerial setting
The bounding formations to the Hekpoort vol-canic rocks support a subaerial setting. Although regionally, across much of the Transvaal basin, these volcanics overlie the alluvial Boshoek For-mation sharply and are succeeded erosively by the alluvial Dwaalheuwel sandstones (Eriksson and Reczko, 1995; Table 1), these two continental units are absent within the study area. Here, shales of the Timeball Hill Formation precede the volcanic rocks, which are sharply overlain by further shales of the lacustrine (Eriksson et al., 1998) Strubenkop Formation. Although the Timeball Hill Formation is inferred to reflect epeiric marine deposition (Eriksson and Reczko, 1998), tectonic deformation, erosion and conti-nental Boshoek deposition preceded eruption of the Hekpoort volcanics (Eriksson and Reczko, 1995). Therefore, in the context of the general development of the Pretoria Group basin, Hek-poort volcanism was both preceded and followed by the alluvial deposits of the Boshoek and Dwaalheuwel Formations (e.g. Eriksson and Reczko, 1995).
6.3. Erupti6e and depositional history
6.3.1. Fissure eruptions and explosi6e6olcanism
Although a basin-wide study of the proportion of lava flows to volcaniclastic rocks in the Hek-poort Formation is lacking, previous studies sup-port a preponderance of flows (Button, 1973; Sharpe et al., 1983; Engelbrecht, 1986; Schreiber, 1990). This suggests that quiet fissure eruptions predominated during deposition of the Hekpoort Formation (Reczko et al., 1995). The present study area, with approximately equal proportions of volcaniclastic rocks and flows (Fig. 2) would thus appear to be an exception to the regional character of the formation. Presumably, the partly basaltic character of the Hekpoort lavas and the concomitant low volatile content was responsible for the apparent lack of pyroclastic rocks on a regional scale. The genesis of the more calc-alka-line andesitic magmas has long been a subject of debate (e.g. Kushiro, 1974; Ringwood, 1975), but with strong evidence in support of differentiation from basaltic magmas accompanied by modifica-tion through fluids and volatiles (Fisher and Schmincke, 1984). The basaltic andesitic Hek-poort lavas would thus allow for varying propor-tions of flows and pyroclastic rocks to have formed, as observed in this paper. A greater volatile content may have promoted a more ex-plosive volcanic style; there is field evidence of quartz-rich hydrothermal fluids in the study area (Section 4). Secondary quartz is a common filling of amygdales in the Hekpoort andesites and of porous scoriae in the pyroclastics; in addition, Oberholzer (1995) reports small quartz-filled veins and joints as a relatively common feature of the flows preserved in the study area.
6.3.2. Pyroclastic flows,sheetfloods, coarse and
fine lahars
Interpretation of the facies (Section 4) pre-served within the Hekpoort study area suggests that pyroclastic flows and sedimentary reworking of volcaniclastic debris predominated during de-position. Examination of Fig. 2 indicates that most of the volcaniclastic rocks are pyroclastic flows, with evidence for proximal-distal arrange-ments of massive pyroclastic breccia and massive
lapilli-tuff breccia facies within the SW and NE lenses. Such variation in grain sizes is well known from pyroclastic flow deposits (e.g. Wright and Walker, 1977; Druitt and Sparks, 1982). The rela-tive arrangement of these inferred more proximal and more distal facies in the Hekpoort Formation (Fig. 2) suggests the possibility that an eruptive centre lay between the SW and NE lenses, an idea originally proposed by Oberholzer (1995). The fine-grained ash-cloud deposits that could be ex-pected to have accompanied these subaerial pyro-clastic flows are often limited in volume or poorly preserved (e.g. Fisher and Heiken, 1982; Boudon et al., 1993), but they may have been reworked to form the lapilli-tuff facies described in Section 4. Considerable reworking of primary pyroclastic flow deposits (massive pyroclastic breccia and massive lapilli-tuff breccia facies) appears to have characterised the Hekpoort volcanic palaeoenvi-ronment. From Fig. 2 it appears that these more distal and reworked deposits lie to the NE of the pyroclastic flow deposits; this may reflect a north-easterly palaeoslope, or possibly, the presence of a palaeovalley (Oberholzer, 1995) which confined re-sedimentation of primary volcaniclastic de-posits. Sheetfloods are inferred to have formed the stratified lapilli-tuff breccia facies and the massive reworked lapilli-tuff breccia facies is interpreted as coarse lahar deposits. In the absence of vegeta-tion during the Precambrian, torrential rainfall due to disturbances of the weather patterns from volcanic eruptions would easily have reworked volcaniclastic debris in the Hekpoort palaeoenvi-ronment; under such conditions, the formation of both debris-flows (e.g. massive reworked lapilli-tuff breccia facies) and sheetfloods (e.g. stratified lapilli-tuff breccia facies) would have been facili-tated (e.g. Mueller and Corcoran, 1998). Erosion of pyroclastic deposits after explosive generation commonly produces high density mass-flow de-posits (White and Robinson, 1992). In addition, aggressive weathering conditions in the early Pre-cambrian (Corcoran et al., 1998) would also have favoured both debris-flow and sheetflood deposi-tional processes.
J.D.Oberholzer,P.G.Eriksson/Precambrian Research101 (2000) 193 – 210 207
the large northeastern pyroclastic lens, passes southwestwards into lavas, where it indicates a break in volcanic flows (Fig. 2). The apparently partly tuffaceous petrography of this mudrock, allied to evidence for normal sedimentary pro-cesses such as horizontal laminations and current ripple marks, permit an origin due to weathering from coarser volcaniclastic debris and reworking by shallow aqueous depositional processes.
6.4. Sources of6olcaniclastic fragments
The geochemical similarities of the Hekpoort flows and volcaniclastic lithologies (Table 2) and the predominance of basaltic andesite clasts within the latter rocks support strongly that pre-existing lava flows were the main source of acces-sory pyroclasts. Subordinate accessory clast compositions of mudrock and sandstone would sensibly have been derived from the Timeball Hill Formation (comprised of such lithologies) under-lying the Hekpoort volcanics (Oberholzer, 1995). The source of the minor chert clasts within the pyroclastics is probably to be found deeper within the pre-Hekpoort Transvaal sedimentary pile. Cherts are a common although subordinate rock type within the Malmani Dolomite Subgroup, a unit that underlies the Pretoria Group (Eriksson and Reczko, 1995). The chert clasts, derived as accessory clasts from the vent systems, were pre-sumably brought to the surface and erupted with the juvenile basaltic andesitic pyroclasts.
6.5. Volcanic history of the study area
Examination of Fig. 2 suggests that Hekpoort volcanic activity within the study area commenced with relatively subdued lava flows, followed by a hiatus and localised explosive eruption to form the SW volcaniclastic lens. The latter was re-worked by sheetfloods, whereas weathering and resedimentation of resulting clay minerals was probably responsible for the thin tuffaceous mu-drocks in the northeast of the study area. A number of blocks and smaller lava clasts within localised lava flows in the SW lens suggest juve-nile pyroclastic debris being expelled from an active eruptive centre and falling into flowing lava from the same volcanic system.
Following these deposits, flows began again, characterising the southwestern half of the study area, whereas massive pyroclastic flows formed in the northeast (Fig. 2). Fine ash-cloud debris ac-companying the pyroclastic flows was probably reworked to form the lapilli-tuff facies, whereas the pyroclastic flows themselves were subject to local reworking to form coarser and finer lahar deposits. Aggressive weathering formed a second mudrock layer. Finally, the uppermost part of the Hekpoort succession in the study area comprises predominant flows and a small lens of volcaniclas-tic rocks, similar in character to those already discussed above. This lens does, however, have a discordant contact with the extensive, upper mu-drock layer and its underlying lavas (Fig. 2), indicating that a violent eruption disrupted the stratigraphy of pre-existing volcanic deposits in the study area.
Smith (1991) identified syn-eruptive sequences comprising primary volcaniclastic deposits and immediately (in the context of geological time) reworked sedimentary products, and inter-erup-tive sequences formed as a result of sedimentary reworking in the absence of significant volcanic activity. Within this framework, the volcaniclastic facies identified in the Hekpoort Formation clas-sify as essentially syn-eruptive, with only the mu-drock facies reflecting an inter-eruptive character. The succession resulting from the 1991 eruption of Mount Pinatubo provides a modern analogue to the Hekpoort deposits. The primary volcani-clastic Pinatubo deposits were predominantly py-roclastic flows, with subordinate air-fall tephra; about one third of the preserved strata record lahar reworking processes (Pierson et al., 1992).
7. Conclusions
lithologies. Although the pyroclastic rocks are relatively highly altered, their geochemistry shows no significant differences in immobile elements to that of the flows.
Volcaniclastic rocks occur in three lenses in the study area (Fig. 2). About 100 m of basal lava flows were succeeded by a break in this effusive activity, during which a southwestern lens of mas-sive pyroclastic breccias and lapilli-tuff breccias was deposited; these pass laterally into stratified lapilli-tuff breccias. Origin by pyroclastic flow and distal sheetflood processes is inferred. Pyroclastic debris was predominantly composed of juvenile and accessory clasts of Hekpoort lava, with sub-ordinate accessory sedimentary clasts, derived from underlying Transvaal formations.
A second and much larger lens of volcaniclastic rocks in the northeast of the study area passes southwestwards into basaltic andesitic lavas (Fig. 2). These explosive-eruptive rocks comprise mas-sive pyroclastic flows, fining both upwards and northeastwards into massive lapilli-tuff breccia flows. Inferred ash-cloud fine debris was reworked to lapilli-tuffs, and coarse to fine lahars formed from debris-flow resedimentation of pyroclastic flow debris. An extensive, thin, partly tuffaceous mudrock succeeded the NE lens, with reworking of this fine weathering detritus by normal, shallow water sedimentary processes. Finally, uppermost flows with a small lens of pyroclastic flow deposits complete the preserved Hekpoort Formation in the study area.
Acknowledgements
Financial support from the University of Preto-ria and the Foundation for Research Develop-ment is acknowledged. Dr Martin Sharpe of Rock Labs c.c. performed the geochemical analyses. Messers H. Pretorius, W. Prinsloo and L. Visser kindly allowed access to properties under their control. Kobus Oberholzer, the first author of this paper, died tragically in an underground mining accident on 9th September 1997. The second au-thor has prepared the manuscript based partly on Kobus’ excellent field work and from his own data. The paper is thus dedicated to the memory
of a good friend and fine colleague. Wulf Mueller and Patricia Corcoran are thanked for very thor-ough and fair reviews, which contributed signifi-cantly to improving the original manuscript. Subeditor Wulf Mueller is also acknowledged for his support and encouragement throughout preparation and correction of the paper.
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