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 B 2 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.
At the outcrop scale, clast size variation in the Hekpoort pyroclastic rocks is relatively chaotic.
Vertical variation was only observed at the two terminations of the SW lens, and at one locality in
the NE lens. Generally, lapilli and breccia-size clasts exhibit low sphericity and poor rounding
Fig. 4, and are set within a fine matrix. Clasts are predominantly composed of basaltic andesite
90, with lesser mudrock, chert and recrys- tallised, quartzitic sandstone. The basaltic andes-
itic
clasts are
massive and
commonly amygdaloidal, and some clasts exhibit internal
stratification Oberholzer, 1995. Geochemically, there is no significant difference in the composi-
tion of the Hekpoort flows and the various vol- caniclastic rocks discussed below, taking into
account that mobile elements e.g. MgO are more depleted in the more highly altered volcaniclastic
rocks Table 2 Oberholzer, 1995; Reczko et al., 1995. Volcaniclastic lithofacies identified in the
study area are detailed below, and are sum- marised in Table 3.
J .D
. Oberholzer
, P
.G .
Eriksson Precambrian
Research
101 2000
193 –
210
199 Table 3
Volcaniclastic lithofacies in the Hekpoort formation General setting
Characteristics Origin
Lithofacies 1. Massive pyroclastic
9 100–400 m thick; locally interstratified
Predominant lava flows with localised Pyroclastic flow
breccias volcaniclastic centres. Volcaniclastic
with lava flows; clasts 1 cm to 100 cm in 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
9 10–30 m thick; locally interstratified with
Pyroclastic flow 2. Massive lapilli-tuff
1; clasts 4–35 cm in size average 9 7 breccias
cm, poorly to well rounded and oval to disc-shaped, composition 9 4:1 basaltic
andesite to clasts of mudrock, sandstone and chert. Secondary silicification common
9 10–30 m thick; locally interstratified with
3. Stratified lapilli-tuff Reworked from 2, probably by distal
breccias sheet-floods.
1; clasts 5 mm–10 cm in size average 5 mm–6 cm and angular to subrounded,
compositions similar to 2; graded beds, 25 cm–1.5 m thick
Reworked from ash-cloud deposits 9
60 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
9 50 m thick; clasts B1 cm to 120 cm in
5. Massive reworked size average 20 cm, moderately rounded,
lapilli-tuff breccias fine lahars
and spherical to elongated oval forms, compositions as for 1 and 2.
Subhorizontal compaction fabric. Clasts concentrated in distinct layers, with
evidence of local turbulent flow. No secondary silicification
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.
Fig. 5. Outcrop of massive pyroclastic breccia facies in the lower, SW portion of the NE pyroclastic lens see Fig. 2. Note large variation in clast sizes, and the generally poor rounding and low sphericity of the fragments. Clast margins have been outlined in
chalk for clarity.
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 These breccias exhibit clast populations with a
high degree of variation in grain size, shape and composition. Most clasts fall within the 4 – 7 cm
size interval, with isolated blocks up to 20 × 25 cm. Larger clasts are moderately to well rounded
rounding indices of 0.5 – 0.7; Pettijohn, 1957 and are oval to disc-shaped sphericity indices of 0.3 –
0.7; Pettijohn, 1957. Lapilli are poorly rounded, with angular forms being quite common Fig. 4.
Clast compositions are in a proportion of 4:1 basaltic
andesite massive,
commonly 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-fining 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.
Fig. 7. Unit of stratified lapilli-tuff breccia facies, SW termination of SW lens of pyroclastic rocks. Note well developed layering and the disturbance of the layering around the block at lower right.
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, B 1 – 2 mm thick. The larger clasts are subrounded and smaller ones
subangular, and the clasts are subordinate to and supported by the matrix.
The coarse reworked lapilli-tuff breccias con- tain clasts varying in shape from spherical to
elongated, oval forms. Clast compositions are mainly basaltic-andesitic, with subordinate sedi-
mentary fragments, mainly of quartzitic sand- stone. Clast sizes vary between B 1 and 80 cm,
with one clast up to 120 × 50 cm; average grain size is about 20 cm. Rounding is moderate to
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
