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.
No clasts were found within the mudrock, which consists, petrographically, of disc-shaped
quartz silt particles, clay minerals particularly montmorillonite and minor micas. A few frag-
mentary plagioclase crystals were also observed, in addition to very small possible scoriaceous
clasts. A pyroclastic contribution to this material is thus possible. Sedimentary reworking has oc-
curred, as the mudrock is both horizontally lami- nated 1 – 4 mm thick and exhibits flat-topped,
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.
The continental depositional realm thus indi- cated for the Hekpoort volcanic rocks is also
supported by the ‘cratonic basalt’ tectonic classifi- cation based on lava geochemistry Engelbrecht,
1986; Oberholzer, 1995; Reczko et al., 1995. All previous workers agree that Hekpoort volcanism
was essentially subaerial, there being no evidence in favour of significant subaqueous eruption Vis-
ser, 1969; Button, 1973; Sharpe et al., 1983; En- gelbrecht,
1986; Eriksson
and Twist,
1986; Schreiber, 1990; Res, 1993; Reczko et al., 1995.
Engelbrecht 1986 suggested that subaqueous volcanism may also have played a role, due to the
presence of interbedded thin tuffs and graded chert beds in the west of the Transvaal basin.
Such possible subaqueous deposits may, alterna- tively, indicate localised and intermittent rework-
ing of pyroclastic material in small lakes and in river channels, during a hiatus in volcanic activity,
as is also suggested above for the mudrocks in the present study area.
6
.
3
. Erupti6e and depositional history
6
.
3
.
1
. Fissure eruptions and explosi6e 6olcanism 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.
Clay formation related to rapid weathering was probably responsible for the mudrock lithofacies.
The extensive thin mudrock bed which overlies
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 of 6olcaniclastic 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
