J.S.V. VAN ZIJL
329
Physical characteristics of the Karoo sediments
and mode of emplacement
of the dolerites
J.S.V. van Zijl
Department of Geology, University of Stellenbosch, South Africa
e-mail: [email protected]
© 2006 September Geological Society of South Africa
ABSTRACT
The mode of emplacement of the dolerites and their relation with the overlying basalts are revisited in the light of new information
provided by a recent resistivity study of the structure of the Karoo basin and augmented by an analysis of age and palaeomagnetic
data. The different styles of intrusion from the bottom to the top are explained in terms of anisotropy, lithology and increasing
upward bending stresses accompanied by a decrease in overburden thickness. The lowermost zone 3 consisting of flat lying
dolerite sills of large extent occurs in well-laminated, homogeneous shales with a high degree of anisotropy. The thick doleriterich middle zone 2 extending from the Upper Ecca Subgroup through the Beaufort Group and partially into higher Formations is
characterised by the occurrence of large basin structures of dolerite. A model, which assumes a central dyke as source and based
on the principle that the propagating magma front always follows the path of least resistance is developed to explain the formation
of a basin structure including the steepening and flattening of the rim. The model also explains the variations in shape and size
such as are encountered in the field by taking into consideration the structure of the nexus of lenticular sandstones intercalated
with shales and mudstones occurring throughout this zone. The dominant style of intrusion of, at least, the upper part of zone 1
that extends from the surface to a depth of about 700 m is that of dykes and steeply dipping sheets. Another striking feature of
this zone is its dolerite-poor nature. An analysis of the magnetic polarities of the dolerites and basalts leads to the conclusion that
the reversely polarized dolerites, which only occur in zone 1 were the feeders to the outpouring of the basalts. The younger
normally polarized dykes that cut through the entire basalt succession are associated with the period of cooling and contraction
following the main phase of dolerite intrusion. The implications, aided by well resolved age determinations are that the duration
of intrusion of the Karoo magmatic event was short. Finally, the significant increase in both the amount of dolerite and the thickness
of the dolerite bearing layer towards the east supports the view of a magma source in that direction.
Introduction
The electrical properties of the sedimentary pile and
the dolerites have provided constraints on their
physical characteristics and therefore on the mode of
emplacement of the latter (van Zijl, (2006)). The use
of the vertical tri-zonal model of Woodford and
Chevallier (2002) as a basis for the interpretation of the
electrical soundings has shown that each zone is
characterized by its own resistivity signature. A summary
of the final model is given in Figure 1.
Zone 3 occurs near the base of the Karoo
Supergroup in well laminated, homogeneous shales with
a high degree of anisotropy and consists mainly of a
relatively low density of flat lying dolerite sills of large
lateral extent.
Zone 2 is a thick, dense network of dolerite mainly
in the form of basin structures interbedded with
anisotropic sediments consisting mainly of a succession
of lenticular sandstone and intercalated shales and
mudrocks.
Zone 1, at the top of the sedimentary pile, is doleritepoor and its presence is manifested by steeply dipping
sheets and dykes occurring from the surface down to a
depth of about 700 m, due to pressure release in a thin
overburden.
Mode of emplacement
In the Karoo basin there is a striking analogy between
the anisotropies controlling the flow of current in
electrical sounding, on the one hand, and the flow of
dolerite magma, on the other (van Zijl, (2006)).
In both cases flow follows the path of least resistance
that is predominantly horizontal due to the pronounced
degree of fissibility in the succession (du Toit, 1920).
Zone 3
The flat lying dolerite sills of fairly low density are
confined to well laminated, homogeneous shales mainly
in the Lower Ecca Subgroup. The large extent of these
sills are well in excess of the total thickness of the Karoo
basin, which implies that tensile bending stresses were
induced in the sediments above them during
emplacement (Sun, 1969). Despite the resulting upward
forces, the intruding sills remained flat, attesting to the
high degree of fissibility in these sediments.
Zone 2
Zone 2 is a thick, dense network of dolerite mainly in
the form of basin structures interbedded with sediments
consisting of a succession of a subordinate amount of
lenticular sandstone and shales/mudrocks. An important
aspect of the basin structures is that their widths are
generally significantly larger than the total thickness of
that portion of the Karoo basin in which they occur,
even allowing for the depth of erosion below the base
of the Drakensberg Group. The criterion expressed by
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PHYSICAL CHARACTERISTICS OF THE KAROO SEDIMENTS AND MODE OF EMPLACEMENT
OF THE DOLERITES
Sun (1969), again applies whereby tensile bending
stresses will be induced in the sediments above the
basin structures at their peripheries. The very fact that
basin structures and sills are widespread in this zone
implies that the bending stresses were not too dominant.
In this case, the main feature that promoted the
horizontal propagation of magma was the fissibility of
the arenaceous sediments.
Where massive sandstones of large lateral extent
overlie intercalated sandstones and shale one would
expect to find that dykes propagating in the latter
would turn into sills as the sandstone is reached.
Although this is found in the field, sills are found to
prefer the softer partings at the top of sandstones near
the base of a shale horizon (du Toit, 1920). This is due
to the situation, also established by the resistivity results
(van Zijl, (2006)), that the sandstones are generally
neither sufficiently massive nor do they extend far
enough laterally to be detected separately on electrical
sounding curves. This implies that the sandstones occur
mainly as lenses intercalated with shales. The more
homogeneous shale sections within the arenaceous
succession will be more susceptible to sill propagation
as was the situation in zone 3.
The anisotropy in elastic stiffness in such a
succession results in a maximum resistance to magma
propagation perpendicular to the bedding and
a minimum in all directions within a bedding plane. If a
central point source is assumed as feeder and the
resistance in the bedding plane is equal on all sides,
the perimeter of the expanding magma front will be
circular (Gilbert, 1877) as mentioned in Pollard and
Johnson (1973). When the cohesion in a bedding plane
is disturbed by a heterogeneity such as the intervention
of a sandstone lens, the perimeter will no longer remain
circular. The front of the propagating magma will
continue to expand in the undisturbed sector of
the bedding plane until the frictional resistance at the
resulting non-circular front is the same everywhere.
Further propagation around the heterogeneity will then
continue as the magma spreads in all directions in the
bedding plane always fulfilling the condition that
the frictional resistance over the entire magma front
remains equal.
From landsat images it is clear that the shapes of
basin structures vary from circular to rudely circular with
jagged edges. Also, a fairly large proportion of basin
structures are incomplete. In addition, there is a
considerable variation in the diameter of basin structures
at the same stratigraphic level. All these variations from
the ideal basin structure, where it is expected that a
certain diameter at a certain depth below the surface will
correspond to a certain upward bending stress at the
perimeter, can be explained by the large variety of
heterogeneous situations that can occur in the nexus
of lenticular sandstone and shales.
Magma intrudes along the path of least resistance
that is perpendicular to the direction of least
compressive stress and parallel to the tensile strength
Figure 1. The tri-zonal styles of intrusion of the dolerites in the
Karoo basin.
in the direction of propagation (Rubin, 1995).
The preferred horizontal propagation in the Karoo
sediments, as borne out by the greater thicknesses of sill
and basin structures and the bulk amount of dolerite
occurring as sills, rather than as rims, steeply inclined
sills and dykes (du Toit, 1920; van Zijl, (2006)), implies
that the least compressive stress was vertical (v).
The increase in frictional resistance laterally in the
bedding plane effectively causes v to increase in
relation to the larger horizontal compressive stress (h)
thereby decreasing the difference between them. In the
Karoo the lateral increase in frictional resistance was
moderated by several factors:
Firstly, the magma was of low viscosity and brittle
conditions prevailed (du Toit, 1920; Walker and
Poldervaart, 1949).
Secondly, the mechanics of magma intrusion dictate
that it does not have sufficient pressure to occupy the
narrow tip of a propagating crack (Rubin, 1995). Instead,
the gap between the magma and the tip is filled at lower
pressure by volatiles, either from the exsolving magma
or from pore fluids infiltrating from the sediments.
The possibility that the lateral spread of magma could
be facilitated by the formation of water vapour and
other volatiles from the heated sediment, was already
raised by du Toit (1920). Walker and Poldervaart (1949)
have argued that magmatic liquids, mingled with fluxes
from brackish pore fluid of the sediments, could
constitute such a volatile medium. Under such
conditions even a small magma pressure will induce
very large stresses at the periphery of the sill (Chevallier
et al., 2001) with only minimal deformation, if any, of
the sediment above and below the sill (Johnson and
Pollard, 1973).
When the relative magnitude of v has increased to
the threshold value where v = h over the entire
perimeter of the sill, fracturing will take place by tensile
splitting. The wedging action of sheet intrusion
(Anderson, 1938), enhanced by the bending stresses
induced in the sediments above the sill, will initiate the
formation of the rim over the entire perimeter of the sill.
As h becomes the least compressive strength the edges
of the sill will steepen. The steepening often takes place
as a series of steps (Rubin, 1995) as described for the
Karoo by du Toit (1920), Walker and Poldervaart (1949)
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and Bradley (1965) unless a steep fracture in the
sandstone is encountered which the propagating magma
will then follow. As the rim steepens, the relative
increase in _v due to the increasing frictional resistance
will cause a decrease in the difference between v and
h and the thickness of the rim will gradually become
smaller provided that the bending stresses due to the
decreasing distance to the free surface are not dominant.
Such ascending branches may even peter out as v and
h become equal as has been demonstrated by the
drilling results in the western Karoo (Chevallier et al.,
2001).
Alternatively, if a bedding plane is encountered, the
thickness of the rim will increase as v again becomes
less than h and the rim will flatten and evolve into a sill
higher up in the succession. If the bedding plane
encountered is weak, the flattening can also take place
rapidly as drilling results in the western Karoo illustrate
(Chevallier et al., 2001). The thinning of a rim has
frequently been seen in the field by the author along the
associated row of koppies where the normally more
resistant thicker and flatter portion of the rim higher up
has been eroded away leaving a topographically lower,
outcropping, thinner segment.
A striking example of the steepening of a horizontal
sill due to increasing vertical stress at depth over a large
lateral and vertical distance is shown on a record of a
seismic reflection profile just north of the southern limit
of the dolerites east of Somerset East. (Fatti, 1972).
The north-south seismic section shows the course of the
dolerite sill from a relatively small distance above
the Dwyka Tillite at a depth of more than one kilometre,
southwards to near its outcrop, a lateral distance of more
than 35 km. For the last 30 km the sill progressively
steepens attaining a dip of 17° close to its termination at
the surface. In this instance, the steepening can be
attributed to the increase in the vertical stress
presumably due to the effects of folding in the Cape
Orogen.
For convenience of explanation of the model a
theoretical central plug was assumed as the origin for
a basin structure. However, a dyke or segment of a rim
can equally give rise to such a structure. There is a
plethora of dyke intrusions with a typical length of
several kilometres (Chevallier and Woodford, 1999;
Marsh et al., 1997), which would be suitable candidates
as a feeder. The linear dimensions of the dyke or rim
segment dictate that the form of the propagating sill at
the onset will be ellipsoidal becoming progressively
more circular with increasing distance from the foci of
the ellipse, due to the confocal nature of the spreading
magma. The model can explain a large variety of detail
in the forms of basin structures and their connections
with one another throughout the nexus of lenticular
sandstone and shales making up zone 2, by applying the
principle that magma propagation adheres to the path
of least resistance at all times and thus that the spread of
even the regional magma front, maintains the equality
of frictional resistance as propagation progresses.
331
This model differs from that of Chevallier and
Woodford (1999) which proposes that the feeder dykes
are a series of sub-vertical dykes of different orientations
which cross one another to form segments of the rim
and the often seen sharp edges of a basin structure.
Each of the series of adjoining dykes then adopts a
double curvature leading to a trumpet-shaped intrusion.
In each of the segments the flattening of the rim causes
a sediment updrag with a resultant opening of a fracture
at a lower level. Magma then intrudes inwards in
each segment forming the unified flat lying basal sill.
Their elaborate model tailored around the identification
of an east to west dextral shear zone and northnorthwest trending dykes in the western Karoo has two
obvious shortcomings. As stated, the model does not
explain why a linear sub-vertical feeder dyke would
curve both along strike and at depth, nor does there
seem to be any inducement for crossing sub-vertical
dykes to develop a centripetal bond that would lead to
a basin structure. Flattening of the rim could just as well
result in a dome structure.
The model also differs from the laccolith model for
dolerite intrusion (Burger et al. 1981; Botha et al., 1998),
which requires the centre of the basal sill above the
feeder to be sufficiently thick to cause upwarping of
the overlying sediments, allowing fracturing to take
place at the perimeter. A prerequisite for such a model
is a viscous magma. Du Toit (1920) and Walker and
Poldervaart (1949) have effectively discounted the
laccolith model by providing overwhelming evidence in
favour of a low viscosity of the Karoo magma.
A comparison of the contrasting styles of intrusion
between a classic laccolith terrain, that of the Henry
Mountains of Utah and the dolerite basin structures of
the Karoo (Table 1) provides further grounds for
discounting a laccolith model.
Zone 1
Basin structures become less prevalent and smaller in
zone 1 (Botha et al., 1998; Woodford and Chevallier,
2002), as the thickness of the overburden decreases.
The decrease in overburden thickness and the
concomitant increase in upward bending stresses have
resulted in a marked change of style of intrusion, at
least, in the upper part of this zone where soundings
were carried out. Dykes and steeply inclined sheets are
the predominant form of intrusion while lithology no
longer plays any significant role (Figure 1). This zone is
dolerite-poor with no dolerite detected in the majority of
sounding curves in the area to a depth of 700 m, which
marks the base of zone 1. From the resistivity results the
amount of dolerite is evaluated at less than 0.5%. It is
evident from a consideration of the pertinent elevation
data that the vertical distance between the present
erosional surface and the average level of the base of the
basaltic lavas is less than about 500 m. This finding
supports the view of Winter and Venter, (1970) that
dolerite sheets “terminate at a critical distance of a few
thousand feet below the basalts” and confirms the
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PHYSICAL CHARACTERISTICS OF THE KAROO SEDIMENTS AND MODE OF EMPLACEMENT
OF THE DOLERITES
Table 1. Comparison of intrusion styles between the laccolith and basin structure terrains of the Henry Mountains and the Karoo
respectively.
Entity
magma
sills
laccoliths
Parameter
Henry Mountains
Karoo
viscosity
composition
terminations
high
diorite
blunt
0.5 – 1
3.3 – 200
low
dolerite
sharp
0.025 – 0.25
—
—
sandstones + shales
cataclastic near contacts and terminations
semi – ductile
sheared phenocrysts
0.03 – 1.2
sandstones + shales
along rims
brittle
—
thickness/area
basin structures
sediments
nature
fracturing
intrusions
deformation
widespread opinion of most geologists about the origin
of the feeders for the lavas.
A greater understanding of the emplacement of zone
1 and its relation to the overlying basalts as well as the
order of intrusion of the dolerites can only be obtained
from palaeomagnetic and geochronological studies.
The palaeomagnetic investigations of van Zijl et al.
(1962), confirmed by Kosterov and Perrin (1996) and
Hargraves et al. (1997), have firmly established the
occurrence of a magnetic reversal in the succession of
lavas of the Drakensberg Group. The lowermost section
of 300 m is reversely magnetized followed by a 200 m
thick transitional section with intermediate polarities and
an upper section of 900 m, which is normally
magnetized. Although the dolerites are mostly normally
magnetized a polarity reversal also occurs (Graham and
Hales, 1957; Graham et al., 1962; Hargraves et al., 1997).
In the areas covered by the palaeomagnetic studies the
reversed polarities are confined to a belt between
Swaziland and Lesotho. The elevations of the measuring
sites vary but they are all within 700 m of the average
level of the base of the basalts. The reversed polarities
in the dolerites are thus in zone 1. Du Toit (1954) has
noted that dolerites in the basalts consist almost entirely
of occasional narrow dykes. The palaeomagnetic
measurements that have been made reveal only narrow
normally magnetized dykes cutting through both the
older reversed and the younger normally magnetized
basalt sections (van Zijl 1961).
These findings provide a strong indication that the
reversely polarized dykes were the only feeders to
the outpouring of the lavas. The thin younger normally
polarized dykes that have intruded through the entire
lava succession are probably linked to those that also cut
through the dolerites lower down in the Karoo
succession. The latter are coupled with a general period
of cooling and contraction following the main phase of
dolerite intrusion (du Toit, 1920; Scholtz, 1936; Walker
and Poldervaart, 1949). If the propagation of the magma
was from the bottom to the top as is implied by
the model, the normal and reversed polarities in both
the dolerites and the basalts cannot be used as a
criterion for their contemporaneity. Instead, the normally
magnetized dolerites would then belong to an older
period of normal magnetization. Such a suggestion is in
line with the proposed model of dolerite emplacement
with intrusion taking place from the bottom to the top.
As has already been mentioned the model is based on
the premise that the regional magma front maintains the
equality of frictional resistance as the propagation
progresses throughout the entire Karoo Supergroup.
The gradual but continuous upward intrusion would
then also explain the observation made by du Toit
(1920) that lower dolerite sills are not cut by
later intrusions with the exception of the distinctly later
narrow dykes that have already been referred to. Until
additional, reliable age data on normally polarized sills
lower down in the succession become available this
view remains speculative. The only reliable radiometric
ages available for the dolerites were obtained on the
New Amalfi sheet, which acted as a feeder to the basalts
(du Toit, 1920). The highly resolved age of 183 ± 1 Ma
(Encarnacion et al., 1996; Duncan et al., (1997) does not
differ significantly from that of the lava sequence.
The presence of only one reversal in the 1400 m
thick basalt sequence during a period where the average
time between reversals was about 0.7 Ma (Gradstein
et al., 1994) and the absence of weathering between
flows (Walker and Poldervaart, 1949) requires a very
rapid rate of extrusion. This is in accord with the wellresolved age determinations on the basalt sequence
obtained by Duncan et al., (1997) which do not differ
significantly from the top to the bottom.
Source of the magma
Since the Karoo magmatic event is compositionally
linked to and contemporaneous with that of the Ferrar
in Antarctica over a distance of more than 3000 km, it
has been suggested that they have a common heat
source (Encarnacion et al., 1996). The main candidate is
a giant, circular mantle plume (Burke and Dewey, 1973;
Cox, 1978; White and McKenzie, 1989; Encarnation
et al., 1996; Duncan et al., 1997; Storey and Kyle, 1997;
White, 1997), with its centre off the present east coast of
South Africa opposite the Lebombo Monocline with its
south-western edge close to the thick dolerite
occurrences in East Griqualand (Scholtz, 1936; Walker
and Poldervaart, 1949). A plume below a thinned
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J.S.V. VAN ZIJL
lithosphere at a depth of about 50 km (White, 1997)
would be conducive to lateral propagation of magma in
the fissile Karoo basin.
A similar site for the source of the magma has been
postulated (Chevallier and Woodford, 1999; Woodford
and Chevallier, 2002) from the identification of an east to
west dextral shear zone and north-northwest trending
dykes in the western Karoo which resemble a transform
fault between two rifts (Courtillot et al., 1974), one off
the west coast and the other a failed rift system east of
East London. The latter is assumed to be the source
of the magma.
The resistivity structure of zone 2 shows a significant
thickening by at least a factor of 5 of the dolerite bearing
layer with a dolerite content of about 45% towards the
east (van Zijl, (2006)). This finding supports a lateral
source of magma to the east.
Summary and conclusions
The mode of emplacement of the dolerites and their
relation with the overlying basalts are revisited in the
light of new information provided by a recent resistivity
study of the structure of the Karoo basin and augmented
by an analysis of palaeomagnetic data.
The different styles of intrusion of the dolerites from
the bottom to the top are explained in terms of
anisotropy, lithology and increasing upward bending
stresses accompanied by a decrease in overburden
thickness.
The lowermost zone 3 consisting of flat lying dolerite
sills of large extent occurs in well laminated,
homogeneous shales with a high degree of anisotropy.
The thick dolerite-rich middle zone 2 extending from
the Upper Ecca Subgroup through the Beaufort Group
and partially into higher Formations is characterised by
the occurrence of large basin structures of dolerite.
A model which assumes a central dyke as source and
based on the principle that the propagating magma front
always follows the path of least resistance is developed
to explain the formation of a basin structure including
the steepening and flattening of its rim. The model also
explains variations in shape and size, such as are
encountered in the field, by taking into consideration
the structure of the nexus of lenticular sandstones
intercalated with shales and mudstones occurring
throughout this zone.
The predominant style of intrusion of, at least, the
upper part of zone 1 which extends from the surface to
a depth of about 700 m from the surface, is that of dykes
and steeply dipping sheets. Another striking feature of
this section is its dolerite-poor nature.
An analysis of the magnetic polarities of the dolerites
and basalts leads to the conclusion that reversely
polarized dolerites which only occur in zone 1 were the
feeders to the outpouring of the basalts and that younger
normally polarized dykes associated with the period
of cooling and contraction following the main phase of
dolerite intrusion, cut through the entire basalt
succession.
333
The implications, aided by well resolved age
determinations, are that the duration of intrusion of the
Karoo magmatic event was short but more age
determinations are needed on the normally magnetized
sills lower down in the succession.
The significant increase in both the amount of
dolerite and the thickness of the dolerite bearing layer
towards the east supports the view that the source of the
dolerite was in that direction.
Finally, this conclusion together with well-resolved
age data leads to the inference that the Karoo magmatic
event and the Ferrar of Antarctica are contemporaneous.
Acknowledgements
The help of Stefan van Zijl, Stefan Kruger and Annatjie
Haumann in preparing the manuscript is greatly
appreciated. I also thank Luc Chevallier and Reinie
Meyer for critically reviewing an earlier version of this
manuscript.
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