Trans. geol. Soc. S. Afr. 79, 1, 1976, 53 - 57.
PILLOW-LAVAS AND HYALOCLASTITE IN THE ONGELUK
ANDESITE FORMATION IN A ROAD-CUTTING WEST OF
GRIQUATOWN, SOUTH AFRICA
By
N.J. GROBLER and B.J.V. BOTHA
ABSTRACT
In the lower part of the Ongeluk Andesite Formation a road-cutting west of
Griquatown exposed a sequence of lava with very distinctive textures. At the bottom
lies a massive lava of andesitic or basaltic composition. This is overlain conformably
by a unit with large oval fragments of massive lava surrounded by smaller, darkgrey, angular glassy fragments in a fine-grained matrix of angular palagonitised
glass shards. The large fragments of massive lava are interpreted as the remains of
pillows, the outer portions of which were shattered by quenching in water and
which formed the smaller angular fragments surrounding the large ones. The
matrix probably represents aquagene tuff, and the unit as a whole is therefore a
hyaloclastite. Non-vesicular pillow lavas, devoid of concentric zoning, abound in
the top unit. Their dimensions vary from 1 m to 14 m horizontally and less than 1 m to
3 m vertically. Red jasper fills the tricuspate inter-pillow spaces and is probably a
major source of the larger fragments of red jasper rubble on surface.
I. INTRODUCTION
Outcrops of lava of the Ongeluk Andesite Formation of the
Postmasburg Group in the Northern Cape Province are usually too poor to allow a study of its detailed lithology.
However, an excellent exposure has been mapped where the
new tarred road between Griquatown and Groblershoop cuts
through a low hill at a distance of 44, I km west of Griquatown. Along this tarred road the Ongeluk lava occupies a
stretch from a point 20,5 km to another at 48,7 km west of
Griquatown in the so-called Ongeluk-Witwater syncline
[Visser, 1958, p. 19] the axis of which strikes approximately
NNE (Figure 1). Very gentle dips (of the order of 5°) prevail
in this syncline. Local rolls in dip are invoked to
explain the presence of some lower formations in the syncline. In the road-cutting which was mapped the measured dip
was 5° east. This, together with the posi tion of the cutting
near the western extremity of the lava, indicates a lower to
middle horizon in the lava succession.
needles of clinopyroxene in a ground-mass of very fine,
somewhat altered material. Some glass is present. Phenocrysts of a mineral, now completely altered to serpentine, are
scattered throughout the rock and occupy about 10 per cent
by volume. The original mineral could have been olivine or
pyroxene. However, the anhedral shape of the
pseudomorphs would favour original olivine rather than
pyroxene. The texture is therefore microporphyritic and the
rock is probably an olivine-diabase or olivine-basalt. In view
of its highly altered state and the glassy fraction, petrographic work alone cannot decide its composition, and chemical analysis will have to be resorted to for a final answer.
According to Visser [1958, p. 22] an andesitic composition
was established chemically for a specimen of Ongeluk lava
further north. However, that analysis does not necessarily
appl y to the rocks in the road cutting.
The succession is evident from the mapped section of the
road-cutting and its accompanying legend (Figure 2). From
the bottom upwards it consists of a massive andesitic lava
unit concordantly overlain by a hyaloclastite unit composed
of large, massive lava fragments and smaller angular glass
fragments. This unit in turn is overlain along a gently rolling
contact by a unit with prominent pillow lava in which a lens
of massive lava is interbedded. Everywhere a zone of overburden, up to 0,5 m thick, consisting mainly of coarse jasper
rubble, prevails.
B. Hyaloclastite Unit
The massive lava is concordantly overlain by a
heterogeneous unit of volcaniclastic mate.(ial which will be
shown to be hyaloclastite. The basal contact is wavy but the
unit as a whole has a dip to the east, concordant with the rest
of the lava units. Irregular, small masses and lenses of
massive lava, similar in appearance to that of the basal unit,
appear at places in the hyaloclastite unit. In the western part
of the road-cutting a series of faults which strike N-S cause
small up and down displacements of the order of one metre.
In contrast with the massive lava, less vertical joints are
present in this unit. However, flattish joints are more abundant; these are often accentuated by quartz veinlets.
A. Massive Andesitic Lava.
This is a uniform, grey-green, dense, fine-grained lava. It
shows no flow-lineation or sign of flow-bedding. It is well
jointed with vertical joints striking 000°,035° and 100°. A
prominent system of flat joints dipping 5° in a direction 080°
is parallel to the bedding in the stratigraphic sequence. Microscopically the rock is composed of a fine-grained felty
mass of unoriented laths of plagioclase and some slender
FIGURE 1
II. LITHOLOGY
29°
Locality map.
Page 54
PILLOW-LAVAS AND HYALOCLASTITE IN THE ONGELUK ANDESITE FORMATION
LEGEND :
The difference in weathering between the massive lava
and the hyaloclastite is conspicuous. Whereas the former has
hardly suffered any weathering, the hyaloclastite is intensely
weathered and crumbles easily. The unit as a whole is characterised by a greenish brown weathering material. Fresh
specimens for petrographic investigation were difficult to
obtain on account of the intensely altered state of the material. The rock as a whole is a completely unstratified, dense,
glassy, angular, chaotic breccia.
It is composed of large fragments surrounded by smaller
glass fragments in a fine glassy matrix. The large fragments
have the same colour, composition and texture as the massive
lava and microscopically cannot be distinguished from it. In
outline the large fragments are rounded to ovoid, often
pointed or pear-shaped. Their long axes, which vary in
"
o
0
v
0
v
(\
0
Overburden
Andesitic pillow lava
Lava breccia (hyaloclastite)
0
v
Massive andesitic lava
f
/
Faul t
f
Joint
2 4
20
6 8
30
metres
o
C)
o
v
f
o
-
f
I
v
o
o
C) ~
0
Iv
60m
o
0
000
f
6CJm
o
o
0
0
120m
C)-_CJ
I
C?OD
f
120m
FIGURE 2
180m
Geological sketch-map of the northern face of a road-cutting west of Griquatown.
length from 15-75 cm, have a random orientation, in some
places perpendicular to the bedding. The large fragments
occupy about 10-20 per cent of the hyaloc1astite unit. A
fine-grained zone, some 2-5 cm wide, is deVeloped along the
outer rim of some of the large fragments and may be interpreted as being a chilled margin. These margins are usually
highly weathered and light grey in colour in contrast with the
bleached yeilow'=-green colour of the inner parts. The chillzone points to rapid cooling such as takes place subaqueously. From the descriptions of Hess and Poldervaart [1967]
and Parsons [1969] (see Discussion) it is clear that the association of these large fragments with the smaller angular glass
fragments identify them as the remains of partly fractured
and granulated pillows. However, their origin as lava bombs
cannot be completely ruled out. The cores of the pillow
remains must have been hollow. These hollows and cooling
cracks are now filled with macrocrystalline quartz, while
quartz veins also appear in other parts of the large fragments.
Typical large fragments are shown in figure 3, while details
of these and the surrounding fine fragments are shown in
figure 4.
The small fragments have a speckled dark-grey to brownblack colour and contrast well with the lighter-coloured large
fragments. They are mainl y glassy, dense and dark in colour,
angular in shape and vary in size from less than 1 cm to 12 cm
in diameter with an average of 1-2 cm. The small fragments
are again scattered in a matrix of still smaller grey-green,
angular glassy material, possibly original aquagene chert as
judged by their degree of palagonisation. Small veinlets of
secondary quartz and calcite are scattered throughout the
Page 55
Trans. geol. Soc. S. Afr. 79, 1, 1976
FIGURE 3
Large, light-coloured fragments in hyaloclastite unit.
FIGURE 6 Thin section of angular glass shards in very finegrained matrix. Central fragments with green centres, others with
brownish centres. Note light-coloured outer rims of secondary
quartz or chalcedony (x 30) .
FIGURE 4
unit.
Detail of large and small fragments in hyaloclastite
FIGURE 7 Thin section of angular glass fragments. Note distinct rim (of quartz) and annular texture. Radiating, fibrous
pyribole appears in the centre of the fragment in the lower lefthand corner (x 30).
FIGURE 5 White veinlets of quartz and calcite scattered
throughout the smaller fragments and matrix of the hyaloclastite
unit. Note also the two larger fragments.
smaller fragments and matrix and act as cement (Figure 5).
Veinlets of jasper prevail in certain areas.
Thin sections show that the small fragments are composed
mainly of volcanic glass (Figure 6) . The larger of these
fragments are a conglomeration of smaller brown, angular
glass fragments cemented by chalcedony and quartz. In the
smallest range of fragments the angularity persists and the
centres are composed of green glass surrounded by
(a) a regular to irregular brown, annular zone and that in tum
by a narrow, annular rim of quartz or chalcedony of
secondary origin, acting as cement
(b) a colourless outer annular rim (as in a) without any
brown glass.
The different colours of glass probably represent different
compositions as was shown by Muffler and others [1969] by
microprobe analyses of similar volcanic glass. The matrix is
composed of similar, smaHer, angular shards of green glass
together with some secondary microcrystalline quartz .
Various stages of devitrification have been observed: at
the one extreme hardly any devitrification has taken place,
while in the more advanced stages only few glass fragments
or portions of glass fragments remain as such in a mass of
pyribole and alteration products (chlorite) and quartz, some
of which at least is of secondary origin. Radiating, brown,
fibrous pyribole crystals are shown in figure 7. Other minerals, too fine-grained to identify , appear as matrix between the
fragments. The annular texture of the devitrified fragments is
still recognisable as layers of crystals of more or less the same
size and optical orientation and can best be distinguished
under crossed nicols . Perlitic textures are visible under the
microscope.
c.
Pillow-lava
The hyaloc1astite unit is overlain by a unit of conspicuous
pillow-lava (Figures 8 and 9) . Again the lower contact is
wavy. Further east along the road-cutting a lens of massive
lava is intercalated with the pillow-lava unit. Near-vertical as
well as flattish joints of the zone of pillow-lava occur,
although the smaller pillows are usually free of these joints.
Individual pillows vary in length (horizontal dimension)
from 1 m to 14 m while the vertical dimension varies from
less than 1 m to 3 m. The downward protruberances indicate
a normal succession. Vertical, irregular fissures, most
probably cooling-joints, often appear in individual pillows.
Page 56
FIGURE 8
PILLOW-LAVAS AND HYALOCLASTITE IN THE ONGELUK ANDESITE FORMATION
Pillow-lavas with distinct inter-pillow material.
FIGURE 9 Three-dimensional view of a pillow-lava showing the
wrinkled outer "skin".
These fissures are always filled with vein-quartz. In the cores
of many pillows, openings up to 70 cm long, have been filled
with white vein-quartz and epidote and, in isolated cases, by
microcrystalline jasper. Near the base of the pillow zone the
material is rather fresh but higher up weathering is intense.
The pillows are composed of dense, massive, grey-green
lava devoid of amygdales or any internal structure. A
lighter-coloured chill-zone, up to 3 cm wide, envelopes
individual pillows. Except for the change in colour along the
chill-zone the pillows are remarkably uniform and no textural
differences could be observed. Macroscopically and microscopically the pillow-material resembles that of the massive
lava bed. The chill-zone has a higher proportion of glass and
matrix than the interior material of the pillow.
In some cases the pillows are stacked closely together so
that inter-pillow material is absent. However, in most instances the tricuspate inter-pillow spaces are filled by white
or red cherty material Uasper) (Figure 10). The jasper bodies
are up to 40 cm thick. Some of the chert/jasper appears in a
massive form with uniform red colour, but a speckled red and
white, apparently brecciated form, predominates. However,
FIGURE ~O Close-up view of tricuspate inter-pillOW space (to
left and nght of hammer) filled with red jasper.
in spite of the brecciated appearance cataclastic textures are
absent.
Under the microscope the inter-pillow jasper usually
shows a banded texture, being composed of streaks and
elongated lenses of light-coloured chert and red-brown jasper
together with black ore and grey-black opaque material
(glass and fine ore?) of the same shape. However, in some
thin sections the jasper and chert fragments are less elongated, but fine streaks of chalcedony still wrap around the
larger ovoids to form a banded texture. In yet other types
elongated but unoriented patches (ovoids) of microcrystalline quartz (chert) appear, each patch with its own distinguishing grain size. Such textures appear to have been formed
from the original unoriented fragments of glass: the orientation of the chert patches probably took place under the weight
of the overlying rock. Apart from the lenses of ore material,
euhedral to rounded ore minerals form an abundant constituent (5-10 per cent) in some specimens. Epidote in the
form of fine needles and bundles of needles is often present as
a secondary mineral.
In other road-cuttings, quarries and outcrops at points
32,6, 31,8 and 30,5 km west of Griquatown only massive
lava exists, while in a road-cutting at a point 37,5 km west of
Griquatown massive and pillow-lava were seen. Volcaniclastic material as well as massive lava appear in a roadcutting at a point 37,7 km west of Griquatown. It is noticeable that coarse jasper rubble is confined to the western and
central parts of the Ongeluk Andesite Formation along the
new tarred road.
III. DISCUSSION
The unit composed of large and small fragments undoubtedly is some form of volcaniclastic deposit, particularly
some form of volcanic breccia. It then remains to establish
the mode of formation: do they represent debris from the
explosive ejection of lava fragments (bombs and lapilli) or is
the breccia of hydrovolcanic origin (hyaloclastite)? Following the criteria for the recognition of volcanic breccias [Parsons, 1969, p. 263-304] the following characteristics point to
a hydrovolcanic origin:
1. The chilled selvages of the large fragments of lava and
their shape (although not their orientation) point to them
as portions of pillows.
2. The angular, palagonitised smaller glassy fragments and
the even finer-grained palagonitised glass shards are typical of hyaloclastites.
3. Pillow-lavas proper overlie the hyaloclastite. Higgins
[1971 , p. 321-331] has described a similar association of
a sub-unit of pillow-lavas with a sub-unit of mainly
broken pillow-lavas, isolated pillow-breccia and
aquagene tuff, overlain by a sub-unit of mainly aquagene
tuff with some isolated pillows. The latter zone resembles
the hyaoclastite described above.
4. Descriptions of hyaloclastite formations by various authors e.g. Hess and Poldervaart I1967, p. 30-31], and
Parsons [1969, p. 263-304] agree very well with the
observed breccia.
5. The presence of the lens of massive lava in the
hyaloclastite-pillow lava sequence is a feature which is
commonly associated with hyaloclastite as described in
literature such as quoted above.
The origin of the ubiquitous coarse jasper rubble over the
western half of the Ongeluk lava exposure west of Griquatown was never explained satisfactorily. In localities
along the tarred road west of Griquatown jasper appears as
amygdales in the lava. However, these amygdales seldom
measure more than 2 cm in diameter, while the jasper rubble
Page 57
Trans. geol. Soc. S. Afr. 79, 1, 1976
over the pillow lava usually exceeds 10 cm in size. The
amygdales can therefore not be the main source of the coarse
jasper rubble. Visser [1958, p. 21] mentions the development of lenses of "interbedded bluish chert and bright-red
jasper" nearly 15 m (50 feet) thick along the western part of
the outcrop. These chert-jasper beds together with the
pillow-lava units should be major sources of the jasper rubble.
The lack of concentric internal structure in the pillowlavas rules out the possibility that these may be sections of
pahoehoe lava toes. Furthermore, figure 9 which gives a
three-dimensional view of these structures clearly define
them as pillows. Some of the pillows depicted by figure 9
also display the roughly hexagonal cracking of the skin so
common in pillows.
IV. CONCLUSIONS
The succession mapped in the road-cutting probably represents lava which was degassed either in the vent or while the
lava was emplaced terrestrially and flowed towards a body of
water. This explains the lack of vesicles. The massive lava
might have cooled on dry land. As the later lava flows
reached a body of water granulation of the lava front took
place. Granules accumulated at the toe of the flow in a bank
having an angle of repose of the tough interlocking grains.
Occasional trickles of liquid lava formed sheets (hence the
interbedded massive lens) or elongated pillows on the surface
of the bank. Some pillows became detached and might have
suffered partial break-up as a result of further granulation so
that they landed up as irregular large fragments among the
smaller glassy fragments. The smaller glassy fragments were
palagonitised and subsequently suffered complete devitrification in some cases.
Upwards in the succession pillows proper were formed
without attendant granulation. The inter-pillow spaces were
probably filled by fragments of fine-grained tuff and partly
by material which crystallised from an iron-rich silica gel.
Petrographic work indicates the deposition of lenses of
siliceous and iron-rich material and that portions of the
inter-pillow material assumed regular layering probably
under the influence of the weight of the overlying material.
Although Rogers [1905, p. 182] described some "volcanic breccia" in the area, the presence of different lava
flows was established and some silicified tuff associated with
greyish chert and red jasper was mentioned by Visser [1958,
p. 20-21], the lithology of the lavas is generally poorly
known and no pillow-lava was ever described in the area.
Understandably such larger pillows as described above will
be most difficult or even impossible to see in the type of
rubble-strewn blocky outcrops typical of the Ongeluk Andesite Formation. Apart from the thick lenses of chert and
jasper quoted by Visser [1958, p. 21] it is concluded that the
inter-pillow material must be a major source of the usual
jasper rubble of a size exceeding 10 cm. In a general way,
therefore, the abundant presence of coarse rubble could be
used as a possible indication of pillow-lava interbeds.
V. ACKNOWLEDGMENTS
Sincere thanks are due to the CSIR for assistance and Dr G.J.
Beukes who identified the serpentine after olivine by means
of X-ray diffraction techniques.
REFERENCES
Hess, H.H. and Poldervaart, A. (Eds) [1967]. Basalts, Vol. 1. Interscience
Pub!', New York, 482 pp ..
Higgins, M.W. [1971]' Depth of emplacement of Games Run Formation
pillow basalts, and the depth of deposition of part of the Wissahickon
Formation, Appalachian Piedmont, Maryland. A mer. J. Sci., 271,
4,321-332.
Jones. J.G. [1969]. Pillow lavas as depth indicators. Amer. J. Sci., 267, 2,
181-195.
Muffler, P.L.J., Short, J.M., Keith, T.E.C. & Smith, V.c. [1969].
Chemistry of fresh and altered basaltic glass from the Upper Triassic
Hound Island Volcanics, Southeastern Alaska. Amer. J. Sci., 267,2,
196-209.
/
Parsons, W.H. [1969]. Criteria for the recognition of volcanic breccias
(Review). Geol. Soc. Amer., Mem. 115, 263-304. -304.
Rogers, A. W. [1905]. Geological survey of parts of Hay and Prieska, with
some notes on Herbert and Barkly West: Ann. Rep. geol. Comm.
e.G.H.
Visser, D.J.L. (Compiler) [1958]. The geology and mineral deposits of the
Griquatown area. Cape Province (An explanation of sheet 175,
Griquatown). Geol. Surv. Dept. Mines, 72 pp.
Department of Geology
University of the Orange Free State
BLOEMFONTEIN.
Accepted for publication by
the Society on 21/4/1976.