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Journal of Volcanology and Geothermal Research 202 (2011) 189–199
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Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
3D relationships between sills and their feeders: evidence from the Golden Valley Sill
Complex (Karoo Basin) and experimental modelling
Christophe Y. Galerne a,⁎, Olivier Galland a, Else-Ragnhild Neumann a, Sverre Planke a,b
a
b
Physics of Geological Processes, University of Oslo, Norway
Volcanic Basin Petroleum Research, Oslo Innovation Park, Oslo, Norway
a r t i c l e
i n f o
Article history:
Received 7 October 2010
Accepted 19 February 2011
Available online 21 March 2011
Keywords:
Feeder dykes
Saucer-shaped sills
Golden Valley
Field observations
Geochemistry
Experimental modelling
a b s t r a c t
In this paper, we address sill emplacement mechanisms through the three-dimensional relationships
between sills and their potential feeders (dykes or sills) in the well-exposed Golden Valley Sill Complex
(GVSC), Karoo Basin, South Africa. New field observations combined with existing chemical analyses show
that: 1) the contacts between sills in the GVSC are not sill-feeding-sill relationships, and 2) there are, however,
close structural and geochemical relationships between one elliptical sill, the Golden Valley Sill (GVS), and a
small dyke (d4). Such relationships suggest that GVS is fed by d4 and that the linear shape of d4 may have
controlled the elliptical development of the GVS.
To test this hypothesis, we present preliminary results of experimental modelling of sill emplacement, in
which we vary the shape of the feeder. In the first experiment (E1) with a punctual feeder the sill develops a
sub-circular geometry, whereas in the second experiment (E2) with a long linear feeder the sill develops an
elliptical geometry. The geometrical relationships in E2 show that the elliptical shape of the sill is controlled
by the linear shape and the length of the linear feeder. The experiment E2 presents strong similarities with the
GVS–d4 relationships and thus supports the proposition that d4 is the feeder of the GVS. Our experimental
results also indicate that the feeders of the other elliptical sills of the GVSC may be dykes.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Extensive sill complexes are common features in large igneous
provinces, for example in the Karoo Basin, South Africa (Chevallier
and Woodford, 1999), the Møre–Vøring Basin off W Norway (Planke
et al., 2005), the Parana Basin, Brazil (Bellieni et al., 1984, and
references therein) and the NW Australian shelf (Symonds et al.,
1998). In these settings, large sills commonly exhibit saucer shapes
(e.g., Polteau et al., 2008b), consisting of axis-symmetrical flat inner
sill connected outward and upward to transgressive inclined sheets,
ending in a flat outer sill (e.g., Planke et al., 2005; Polteau et al., 2008b;
Galland et al., 2009). The relationships between sills (and saucershaped sills) and their feeders are crucial for understanding the
mechanisms of sill emplacement, and the relationship between sill
complexes and flood basalts. However, such relationships are poorly
known because the connections are rarely exposed (e.g., Hyndman
and Alt, 1987). Because of the lack of geological evidence for feederto-sill relationships, the mechanisms of sill and saucer-shaped sill
emplacement are largely debated on the basis of theoretical models.
⁎ Corresponding author at: Now at: Geodynamics/Geophysics, Steinman Institute,
University of Bonn, Germany. Tel.: + 49 228732466; fax: + 49 228732508.
E-mail address: chrisgal@geo.uni-bonn.de (C.Y. Galerne).
0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2011.02.006
Some models propose that saucer-shaped sill intrusions form
along the level of neutral buoyancy of the magma, and the feeders are
expected to be located below the outer sill at one side of the saucer
(Fig. 1a; e.g., Bradley, 1965; Francis, 1982; Chevallier and Woodford,
1999; Goulty, 2005). Other models propose that saucers are fed from
below through a central feeder dyke (Fig. 1b; e.g., Pollard and Johnson,
1973; Hansen et al., 2004; Malthe-Sørenssen et al., 2004; Thomson
and Hutton, 2004; Hansen and Cartwright, 2006; Kavanagh et al.,
2006; Galerne et al., 2008; Goulty and Schofield, 2008; Menand, 2008;
Polteau et al., 2008a, 2008b; Galland et al., 2009). In these models, the
feeders are expected to be situated below the central part of the inner
sills (Fig. 1b).
In both models, sill feeders are considered to be dykes (e.g., Gretener,
1969; Hyndman and Alt, 1987; Kavanagh et al., 2006; Valentine and
Krogh, 2006; Goulty and Schofield, 2008; Menand, 2008). However,
most theoretical models do not account for the three-dimensional
relationships between sills and linear feeders. In two-dimensional
models (e.g., Pollard and Johnson, 1973; Malthe-Sørenssen et al., 2004)
the punctual sill feeders may be either dykes or pipes. In contrast, threedimensional models are mostly axi-symmetrical and consider a central
feeder pipe (e.g., Fialko et al., 2001; Murdoch, 2002; Bunger and
Detournay, 2005; Bunger et al., 2005). In their theoretical studies, both
Pollard and Johnson (1973) and Goulty and Schofield (2008) assumed
that elliptical sills develop from central feeder dykes, but they did not
take the shape of the feeder dyke into account in their mechanical
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C.Y. Galerne et al. / Journal of Volcanology and Geothermal Research 202 (2011) 189–199
Buoyancy Control
1
LNB
2
1: Outer-sill
2: Inner-sill
b
LNB: Level of Neutral
Buoyancy
Horizontal Discontinuity
2
2. Golden Valley Sill Complex, Karoo Basin, South Africa
2
1
In this paper, we use geological and geochemical data on sills and
dykes of the GVSC to investigate the relationships between sills and
their feeders. We subsequently integrate our field observations and
geochemical data with preliminary results of experimental modelling.
Our study demonstrates that 1) the contacts between some sills of the
GVSC do not necessarily imply a feeding relationship, and 2) the major
saucer-shaped sills in the GVSC are fed from below, and that their
feeders are likely to be linear dykes.
1: Inner-sill
2: Outer-sill
Fig. 1. Existing models of saucer-shaped sill emplacement mechanisms. Numbers
(1) and (2) indicate the steps of emplacement. a. Model of emplacement controlled at
the level of neutral buoyancy (LNB), Modified from Francis (Bradley, 1965; Francis,
1982 e.g., Barker, 2000). Sills are fed laterally from one part of the outer sills. b. Model of
emplacement along horizontal discontinuity, modified after Malthe-Sørenssen et al.
(2004). Sills are fed radially from the inner sill. See text for more information.
analyses. In addition, most geological observations provide only twodimensional relationships between sills and their feeders (e.g.,
Valentine and Krogh, 2006).
Three-dimensional information on the relationships between sills
and their feeders are essential for a better understanding of sill
emplacement mechanisms. One way of obtaining three-dimensional
information on sill morphologies is through seismic surveys. In the
North Sea outside Norway (Hansen et al., 2004; Cartwright and
Hansen, 2006; Hansen and Cartwright, 2006) and the Rockall Trough
of W Scotland (Thomson and Hutton, 2004; Thomson, 2005, 2007),
recent seismic surveys show that:
1. Large sill complexes show physical connections between individual saucer-shaped sills, i.e. some inner sills are connected with the
inclined sheets of underlying sills. Such connections have been
interpreted as feeding connections, leading to the model of
interconnected sill-feeding-sill networks (Cartwright and Hansen,
2006).
2. Some saucer-shaped sills have developed lobate morphologies
along their outer boundaries. These lobate structures have been
interpreted as indicators of magma flowing outwards and upwards
from a central source (dyke or pipe) beneath the inner sill.
However, the seismic data have not revealed the location or shape
of the feeder, the reason probably being that thin sub-vertical
structures are rarely visible on seismic images.
The best way to constrain the three-dimensional relationships
between sills and their feeders is clearly through detailed, multidisciplinary studies of well-preserved and well-exposed sill complexes.
Recently, much work has been focused on the Golden Valley Sill
Complex (GVSC), Karoo Basin, South Africa (Fig. 2), which is a unique
field laboratory for studying the relationships between saucer-shaped
sills and associated dykes in a large sill complex. The quality of the
outcrops allowed detailed mapping of the three-dimensional structures of the sills. AMS work and macroscopic flow-indicators
characterised the flow pattern of magma within sills (Polteau et al.,
2008a). Additionally, extensive geochemistry aimed to address the
genetic relationships between the sills (Galerne et al., 2008) and the
differentiation processes within single sills (Aarnes et al., 2008;
Galerne et al., 2010). However, none of these studies have addressed
the potential connections between sills and dykes in the GVSC.
2.1. Geology
The Karoo Basin consists of Late Palaeozoic–Early Mesozoic sediments deposited in a foreland setting (e.g., Catuneanu et al., 1998). The
Karoo Igneous Event, which sealed the end of the foreland cycle of the
main Karoo Basin (i.e., prior to tectonic uplift: Catuneanu et al., 1998;
Catuneanu, 2004) resulted in (1) the intrusion of numerous saucershaped sills and dykes all over the basin, and (2) the eruption of the large
Lesotho Flood Basalt Plateau (Erlank, 1984, and references therein). Most
of the Karoo Igneous Event occurred about 180 Ma ago (Jourdan et al.,
2005, 2007); work in progress restricts the age range to b1 Ma, between
182.3 and 183.0 Ma (Svensen et al., 2007; Polteau et al., 2010). During
Mesozoic–Tertiary, the basin was uplifted with the Gondwana Breakup,
and the resulting erosion exposed a large part of the intrusive complexes.
No significant post-Gondwana Breakup deformation or regional metamorphism has affected the Karoo Basin. The exposed sill complexes are
thus exceptionally well preserved and represent a unique field
laboratory to study the emplacement of sill complexes.
The GVSC, located SW of the Lesotho Plateau, consists of four major
elliptic saucer-shaped sills (Fig. 2a; Aarnes et al., 2008; Galerne et al.,
2008; Polteau et al., 2008a). The saucers were emplaced at two
stratigraphic levels: the Morning Sun Sill (MSS) and the Harmony Sill
(HS) at the deeper level, and the Golden Valley Sill (GVS) and the Glen
Sill (GS) at a slightly higher level (Fig. 3). Each sill at the higher level is
located above a sill at the lower level (Fig. 3). The saucers at the lower
level have parallel long axes that trend NW–SE. At the upper level, the
long axes of the GVS and GS trend N–S and NNW–SSE, respectively
(dashed lines in Fig. 2). A minor, circular, relatively flat sill (MV Sill, or
MVS) is in direct continuity with the north-western limb of the GVS
(Fig. 2a–b; Galerne et al., 2008). We consider the MVS to be part of the
outer sill of the major GVS. The GVSC area also includes the major
Golden Valley Dyke (GVD; Fig. 2) located west of the GVS (≤15 m thick,
17 km long) and several small dykes (d1–d4, Figs. 2 and 4a) and short
sill segments (e.g., L2 and L3, Fig. 2).
The sills at the upper level are on average ~ 100 m thick but may be
up to 150 m thick (Galerne et al., 2010). The sills at the lower level are
20 to 50 m thick. Drill-cores from different parts of the Karoo Basin
show several sills at different depths (Neumann et al., in press). This
suggests that, although only two levels of saucers are exposed in the
GVSC area, there may be unexposed saucers at deeper stratigraphic
levels (e.g., Chevallier and Woodford, 1999; Galerne, 2009; Galland et
al., 2009).
2.2. Former studies
This section summarised the main results of previous studies
performed on the GVSC (Galerne et al., 2008; Polteau et al., 2008a;
Galerne, 2009; Galerne et al., 2010). These studies questioned
whether the sills of the GVSC were fed by a single batch of magma
or by several individual batches. The first hypothesis would imply that
the sills fed each other, meaning that the feeders of the sills were
underlying sills, whereas the second hypothesis would imply that the
sills were fed by other conduits that need to be identified.
Geochemical differences and similarities between the six major
units (MSS, HS, GVS, MVS, GS and GVD) in the GVSC were tested
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Studied dolerite
Dykes
Sills
26 S
SOUTH
AFRICA
Unstudied
dolerite
Lesotho
30 S
Karoo GVSC
Basin
34 S
18 E
26 E
22 E
MSS
30 E
N
0
L8
MV Sill
(MVS)
4
HS
A'''
d2
km
GVD
Minor
dykes
GVS, MVS, GS
300 km
Glen
Sill
(GS)
L7
31.8 (S)
A''
d3
de
d1
31.9 (S)
Morning
ng
Sun Sill
(MSS)
D
ey
all
nV
e(
yk
L2
D)
GV
L3
L4
Harmony
ar
Sill ((HS)
Golden
Valley
Sill
(GVS)
l
Go
A
A'
L4.a
L
B
B'
d4
32.0 (S)
L
L4.b
Fig. 4
26.1 (E)
26.2 (E)
26.3 (E)
26.4 (E)
Fig. 2. Simplified geological map of the Golden Valley Sill Complex (GVSC), Karoo Basin, South Africa, modified after Galerne et al. (2008). The map shows the major sills and the
Golden Valley Dyke (coloured fields), minor dykes d1–d4 (grey circles with orientations), and dolerites ignored in this study (light grey fields). The acronyms and the colours
correspond to chemically distinct magma batches (Fig. 4; Galerne et al., 2008): GVS–MVS–GS (Golden Valley Sill–MV Sill–Glen Sill); MSS (Morning Sun Sill); HS (Harmony Sill); GVD
(Golden Valley Dyke). Straight lines indicate cross sections A–A‴ and B–B′ of Fig. 3. The rectangle locates the map of Fig. 4.
statistically by a Principal Component Analysis (named Forward
Step-Discriminant Function Analysis FS-DFA, StatSoft©, 2007) on the
basis of 46 major and trace elements in 233 rock samples (Galerne et
al., 2008). The statistical analysis showed that the major saucershaped sills and the GVD of the GVSC form four distinct geochemical
groups (shown by distinct colours in Fig. 2; Galerne et al., 2008). The
four groups are (i) the sills at the upper stratigraphic level (GVS–MVS
and GS), (ii) the HS and (iii) the MSS at the lower level, and (iv) the
GVD. The different geochemical signatures imply derivation from
four separate magma batches: the GVS–MVS–GS, the HS, the MSS,
and the GVD magma batch (Figs. 2 and 4; Figs. 10–12 of Galerne et al.,
2008). This result is now confirmed by Sr and Nd isotopic analyses,
which notably show that the sills at the upper stratigraphic level
(GVS–MVS and GS) are derived from a common source (Neumann
et al., in press).
The saucer-shaped sills in the GVSC are locally in physical contact
(Galerne et al., 2008; Polteau et al., 2008a; Galerne, 2009). At the
southern tip of the GVS the southwestern limb of the MSS climbs into
contact with the GVS (L4b in Figs. 2 and 4). The two sills are in contact
from L4b to L4a (Fig. 4a–b) where the MSS disappears. At L4b there is
no chilled margin between the two sills but further west the
superposed sills are separated by a thin chilled margin (L4a, Fig. 4d)
at the transition between the dolerites of MSS and GVS geochemical
compositions (Fig. 4d). These observations clearly show that the
emplacements of the MSS and GVS were not coeval.
At locality L7 (Fig. 2) two sills lie above one another, separated by a
thin zone (a few centimetres thick) of low-grade hornfels; at locality L8
two superposed sills are separated by a ca. 100 m. thick zone of
sedimentary rocks. Their geochemical signatures have identified the
sills at L7 as GS (upper) and MSS, and at L8 as GS (upper) and HS
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A'''
A''
A
A'
MVS
GS
GVS
GVD
a
? ? MSS
B
HS
??
B'
GVS
GVD
b
?
MSS
?
km
0
4
Fig. 3. a. Geological cross section A–A‴. b. Geological cross section B–B′. See Fig. 2 for
colour legend.
compositions, each domain corresponding to one sill. Such transitions in
the chemical profiles show that the magmas of the sills in contact did not
mix (Galerne et al., 2008, 2010).
AMS (Anisotropy of Magnetic Susceptibility) studies combined
with macroscopic magma flow indicators (ropy flow structures and
tube-like undulations) indicate outward magma flow directions in the
different sills, implying that their feeders were located beneath the
inner sill floors (Polteau et al., 2008a). Polteau et al. (2008a) also found
indications in the AMS data of backward flow, but concluded that this
resulted from compaction after the magma supply had stopped.
The existing geological and geochemical data indicate that the sills
of the GVSC were not fed by a single batch of magma, but by several.
This implies that the feeders of the upper level sills were not the sills
of the lower level. Nevertheless, at that stage, it was still not possible
to identify the nature of the feeders of the sills of the GVSC. More
detailed observations are therefore required.
2.3. New observations on minor dykes
(Galerne et al., 2008). Furthermore, in locations where sills appear to be
superposed and in direct contact (L4b), chemical profiles show very
sharp geochemical transitions between two domains of almost constant
The GVSC area includes four minor dykes (d1–d4 in Fig. 2). Dyke d1
crops out ~8 km west of the GVD, striking NW–SE. This ~1-metre thick
Fig. 4. a. Highlight of the geological map of the southern GVSC. The colours, indicating the distinct magma batches involved in the GVSC, are the same as in Fig. 2 (Galerne et al., 2008).
The map indicates the localities L4a and L4b (see text for explanation). b. Panoramic photograph of the southern tip of the GVS (see location on Fig. 2). It shows that the MSS climbing
sheet is in contact underneath the GVS. c. Photograph of the metre thick dyke d4. d. Photographs of the contacts between the underlying MSS to the above GVS at localities L4a and
L4b. At L4a, a chilled margin separates the MSS and the GVS. At L4b, no chilled margin indicates the contact between the MSS and the GVS. The location of the contact between the
two sills was only possible on the basis of different geochemical signatures (Galerne et al., 2010).
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contrast to the other small dykes, dyke d4 shows a simple field
relationship with the GVS (Figs. 2 and 4) and it has a geochemical
composition that is identical to that of the GVS (Fig. 5).
dyke is exposed over a distance of ~8 km. Dykes d2 and d3 are located N
and NW of the GVS. Dyke d2 is ~5 m thick and strikes NE–SW, whereas d3
is only ~1 m thick and strikes NNE–SSW. Also d2 and d3 may be followed
for a few kilometres. Finally, we found a N–S striking 1-metre thick dyke
(d4) exposed in a 5- to 10-metre large outcrop beneath the southern tip
of the GVS, i.e. below the GVS and above the MSS (Figs. 2 and 4).
We did not observe direct physical connections between the sills and
the dykes. However, we noticed that the strikes of (1) the GVD and d1
are parallel to the long axes of the deeper level saucers (MS and HS), and
(2) the d4 is parallel to the long axis of the GVS (Fig. 2). We also notice
that the strikes of the other dykes are different (NE–SW for d2; NNE–
SSW for d3). This suggests that the state of stress at the time of
emplacement of these dykes was almost isotropic. This is in good
agreement with the observation of Catuneanu (2004), who showed that
(1) the Karoo Igneous Event sealed the end of the foreland deformation
of the Karoo basin, and (2) there is no significant regional tectonic
deformation that has affected the Karoo intrusive and effusive rocks.
The chemical compositions of the minor dykes d1–d4 have, so far,
only been published as supplementary material by Galerne et al.
(2008); they have neither been discussed, presented in figures nor
compared to the compositions of the major sills of the GVSC. The
number of analyses for each dyke is small, and the set of analysed
elements is somewhat smaller than for the main group of samples.
This makes the statistical approach used to discriminate the chemical
compositions of the major sills (Galerne et al., 2008) unsuitable for the
minor dykes. We therefore present their compositions through plots
of strongly incompatible element ratios (Fig. 5).
In plots of ratios between pairs of incompatible elements the dykes
d3 and d4 plot consistently within the GVS–MVS–GS field and d2
within the MSS field (Fig. 5). We conclude that the dykes d3 and d4
formed from the magma batch (GVS–MVS–GS) that gave rise to the
upper level saucers, whereas the dyke d2 formed from the MSS magma
batch (or from chemically identical magma batches). The composition
of d1 falls within the overlapping part of the MSS and GVS–MVS–GS
fields (Fig. 5), the dyke d1 thus cannot be clearly attributed to a specific
geochemical signature on the basis of trace elements.
Their geochemical similarities suggest a link between the dyke d1
and the MSS. In addition, Galerne et al. (2008) show that a sill
segment at locality L2 is composed of MSS-type dolerites. The
presence of these units with MSS geochemistry outside the MSS
suggests that the MSS may be part of a nested complex of two (or
more) sills formed from the same (or closely similar) magma batch
(es) (Galerne et al., 2008). Similarly, the locations of the dyke d3 and
the sill segment L3, both with GVS–MVS–GS composition, suggest that
the GVS–MVS–GS sill group continues (or originally continued) to the
west of the GVS (Fig. 2). However, as dykes d1, d2 and d3 are located
far away from the present outcrops of sills with the same chemistry,
these dykes cannot be used to demonstrate feeding relationships. In
90
3. Experimental modelling
3.1. Materials and scaling
In this study, we used the experimental technique developed by
Galland et al. (2009). The model rock consisted of a compacted cohesive
fine-grained silica flour of cohesion C ≈ 350 Pa, and the model magma
was a molten vegetable oil of low viscosity η ≈ 2 × 10− 2 Pa s.
Galland et al. (2009) described in detail and discussed the scaling
of the model. The experiment-to-nature scale ratio is of the order of
2 × 10− 5, so that 1 cm in experiments represents 500 m in nature. As
the GVS was emplaced between 1 and 2 km depth, the depth of
injection in the experiments should be between 2 and 4 cm.
The strength of the solid material is scaled with respect to the
gravitational stress, illustrated by the ratio ρrgD/C, where ρr, g, D and C
are the rock density, the acceleration due to gravity, the depth of
emplacement, and the cohesion of the rock, respectively. Density of
natural rocks is about 2500 kg m− 3 and their cohesion spans between
107 and 108 Pa (Schellart, 2000). Therefore, the gravitational stressto-cohesion ratio for the GVSC ranges between 0.24 and 5. In the
experiments, density of the silica flour is 1050 kg m− 3. Therefore, the
gravitational stress to cohesion ratio is ~ 0.9, which is consistent with
that of the GVSC.
In our experiments, we want to test the shape and size of the
feeder. This leads to the definition of another geometrical scaling
parameter, the ratio between the length (L) of the feeder and the
depth of emplacement. Here we consider two cases. (1) If the aspect
ratio L/D ≪ 1, the feeder is small with respect to the depth of
GVS-MVS-GS
MSS
HS
4
GVD
d2
d1
70
d2
d3
d3
60
d4
50
In the GVSC, we found one minor dyke (d4), which geochemistry,
location and orientation suggests a genetic link to the GVS although
there is no visible connection between the two. The location,
orientation and geochemical composition of the dyke d4 relative to
the GVS give rise to the hypothesis that the three-dimensional
elliptical shape of a saucer-shaped sill and its location are controlled
by the linear shape of its feeder dyke. This hypothesis was briefly
discussed by Pollard and Johnson (1973) and Goulty and Schofield
(2008) on the basis of theoretical aspects, but to our knowledge it has
never been tested experimentally nor supported by field observations.
In order to test this hypothesis, that is to explore how the linear
shapes of feeder dykes control the elliptical shapes of saucer-shaped
sills, we resorted to physical experiments.
10
15
Zr/Nb
20
5
6
d1
7
d4
GVD
3
d4
P/Zr
d3
Zr/Y
Ti/Zr
80
2.4. Summary
8
9
5
d1
d
d2
6
7
8
9
P/Zr
Fig. 5. Plots of strongly incompatible element ratios (Ti/Zr versus Zr/Nb, Ti/Zr versus P/Zr, and Zr/Y versus P/Zr) in dolerites from the GVSC (modified from Galerne et al., 2008). Data
published in Fig. 15 of Galerne et al. (2008, contoured fields) are completed with the data published as supplementary material by Galerne et al. (2008, un-contoured fields). The
coloured fields represent the distinct magma batches involved in the emplacement of major sills and dykes of the GVSC (Galerne et al., 2008; see also Fig. 2). Compositions of minor
dykes (white fields) are also plotted.
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emplacement, so that it can be considered as punctual. (2) In contrast,
if this L/D ≫ 1, the feeder is long with respect to the depth of
emplacement, and can be considered as linear. In this paper, we
present one experiment of each case.
The viscosity of the magma also needs to be scaled. The
experiments aim to simulate the transport of viscous fluid into a
deforming Coulomb material of cohesion C. The propagation of magma
into the rock depends on the strength and viscosity of the rock and the
magma, respectively. We use the cohesion to scale the strength of the
rock. Thus, the dimensionless number to scale the viscosity of magma
is the ratio of viscous pressure drop within the intrusion relative to the
cohesion of the country rock. For laminar flow of a Newton fluid of
viscosity η flowing at a velocity U, the viscous pressure drop along a
fracture of length l and thickness h is ΔPv = ηlU/h2, so that the suitable
dimensionless ratio is ηlU/Ch2 (Galland et al., 2009).
If we assume that usual magma velocity U ranges between 0.1 and
1 m s− 1 in 1-metre thick dykes (e.g., Spence and Turcotte, 1985;
Battaglia and Bachèlery, 2003; Roman et al., 2004), the magma
velocity in 20- to 200-metre thick sills is expected to range between
5 × 10− 4 and 5 × 10− 2 m s− 1. In this paper, we only consider common
magma types such as basaltic to rhyolitic magma, for which the
viscosities range between 100 Pa s and 108 Pa s (Dingwell et al., 1993;
Romano et al., 2003). In basins, saucer-shaped sill radii are typically
5 km. Therefore, the viscous drop to cohesion ratio in nature ranges
from 6.3 × 10− 11 to 6.3 × 10− 1, so that viscous stresses are small
compared to the strength of the country rock. In the experiments, the
Fig. 6. a. Schematic drawing of the experimental apparatus, after Galland et al. (2009). b. 2-dimensional sketch representing the relationship between the inlet, the flexible net and
the intrusion. c. Example of excavated model intrusion (from experiment E1). It exhibits a typical saucer shape. d. Details of the injection setups used in experiments E1 (left) and E2
(right). In E1, the injection inlet was punctual. In E2, the inlet was linear, like a dyke.
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injection flow rate was 40 ml min− 1, within fractures of typical radii
about 7 cm and typical thicknesses of 1–3 mm; the viscosity of the oil
is 2 × 10− 2 Pa s at 50 °C (Galland et al., 2006). Thus, the values of the
viscous drop to cohesion ratio in experiments are 4.23 × 10− 4 to
1.2 × 10− 2. Such values are in the same range as those in nature and
much smaller than 1, which means that viscous stresses are small
with respect to the strength of the country rock.
In their paper, Galland et al. (2009) also discussed the flow regime
into the sills by considering the Reynolds number. They show that the
Reynolds number in both nature and experiments is small, so that the
magma flow is laminar.
The buoyancy of the magma is acquired by integrating the body
forces along the entire magmatic system, i.e. from the source to the
emplacement level (e.g., Hogan et al., 1998; Gerya and Burg, 2007). In
the experiments, we consider only the very superficial part of a
magmatic system. Whether the magma is buoyant or not depends on
the deep parts of the magma path, but not on shallow portions
simulated by the experiments. In addition, Galland et al. (2009)
showed that the buoyancy forces generated at the scale of an
intruding sill are negligible. The same conclusion was drawn by Lister
and Kerr (1991) and Menand (2008). Therefore, we will not consider
the buoyancy forces at the scale of intruding sills any longer.
A critical parameter to scale is the rheology contrasts between the
distinct strata in sedimentary basins. In their experiments, Galland et al.
(2009) showed that a horizontal mechanical layering was required to
simulate sills and subsequent saucer-shaped sills. In order to simulate
layering, they used a flexible net located right at the top of the inlet.
Without this layering, the resulting intrusions were vertical dykes and
cone sheets, whereas the layering controlled the horizontal spreading of
the oil. Although such layering is crucial for the formation of sills, the
mechanical effect of the flexible net is hardly quantifiable, and so hard to
scale properly. Nevertheless, the mechanical effect of such a layering is
strong enough to control the horizontal transport of the oil in the
experiments, such as in basins. On this basis, we assume that the
layering used in the experiments is suitable to simulate the mechanical
effect of strata in nature.
3.2. Experimental setup
Our apparatus is a modified version of that developed by Galland
et al. (2003, 2006, 2007, 2009). The models lay in a square box, 40 cm
wide and with variable thickness (Fig. 6a). The silica flour is
mechanically compacted to (1) reduce the porosity of the flour and
thus the percolation of the oil, (2) control the density of the flour, and
(3) control its cohesion (C ≈ 350 Pa; Galland et al., 2006). The
preparation procedure consisted of measuring a mass of silica flour
that we compacted using a high frequency compressed-air shaker
sold by Houston Vibrator (model GT-25). Such a procedure allows a
homogeneous, repeatable, and fast compaction of the silica flour.
We wanted to take into account the mechanical layering to simulate
the sedimentary strata. To prepare the experiments with the net
(Fig. 6a), we poured a first layer of flour; then we shook the box to
flatten the surface of the flour. Subsequently, we placed the net and a
second layer of flour. We shook the box a second time until the density
of the upper layer was 1050 kg m− 3. Such a procedure induced a density
contrast of less than 5% between the lower and upper layers. We
consider that this difference does not affect our experimental results.
In all experiments, we used the same square net of dimensions
30 × 30 cm (Fig. 6a). We chose the size of the net so that it was much
larger than the expected inner sills. Therefore, the size of the net did not
influence the size and shape of the inner sills. We also chose a net slightly
smaller than the box to avoid any interaction between the net and the
walls of the box during the preparation of the experiments (Fig. 6a).
Because there is no evidence of active tectonic deformation coeval
with the Karoo Igneous Event, and because the observed dyke
population exhibit very different orientations, we infer that the state of
195
stress at the time of emplacement of the GVSC was isotropic. Therefore,
neither deformation nor load was applied in our experiments.
During the experiments, the pump injected the oil at constant flow
rate of 40 ml min− 1. Each experiment typically lasted for 1 min. After
the end of the experiments, the oil solidified within about half an hour.
Then, the intrusion was excavated (Fig. 6b), and its top surface
digitalized using a moiré projection technique developed by Brèque
et al. (2004).
In this paper, we compare two experiments with different feeder
sizes and geometries. In experiment E1, the inlet was circular and
0.5 cm in diameter; the inlet was located right underneath the flexible
net, at 4 cm depth (Fig. 6c). In E1, the inlet represented a pipe-like
feeder and could be considered as punctual. This inlet would simulate
a 500-m wide magma pipe, which is in the range of commonly
observed magma pipes in nature (from 100 m to several kilometres,
e.g. Odé, 1957). Experiment E1 has been described by Galland et al.
(2009) and corresponds to a background experiment. In contrast,
experiment E2 is new and is used to test the effect of linear feeder
dykes on the three-dimensional shape of the sills. The length of the
inlet is 12 cm and its thickness is ~ 1 mm; it was located at 3 cm depth.
The thickness/length aspect ratio is thus b10− 2, which is a realistic
value for dykes (e.g., Rubin, 1995). Simulating a 1-m thick dyke such
as the dyke d4 would require a linear inlet of 1/500 cm, which is
technically hard to achieve. However, as long as thickness/length ≪ 1,
the thickness of the linear feeder has negligible effect on the final
result.
3.3. Experimental results
Although E1 has been already described by Galland et al. (2009),
we briefly describe it in this manuscript in order to compare it with
the new experiment E2. In experiment E1, the punctual inlet was
small with respect to the depth; using its diameter as “L”, the feeder
size-to-depth ratio is L/D = 0.17. During experiment E1, oil injection
resulted in smooth doming of the model surface; the dome was
circular, and a network of sub-radial open cracks developed. At the
end of the experiment, the oil erupted at the rim of the dome. The
resulting intrusion was saucer-shaped, with a horizontal inner sill,
steep inclined sheets and flatter outer sills (Fig. 7a). The inner sill
formed along the net and its shape was sub-circular with a diameter of
4.5 cm. The centre of the inner sill was off centre with respect to the
inlet, and the inclined sheets were asymmetric (Fig. 7a).
In experiment E2, the linear feeder was long with respect to the
depth, such that the inlet size-to-depth ratio was L/D = 4. Also during E2,
oil injection resulted in smooth doming. However, in contrast to E1, the
dome in E2 was elliptical, and an open crack developed at the surface of
the dome along its long axis. In addition, the long axis of the dome was
parallel to the linear inlet. At the end of the experiment, the oil erupted
near the intercept between the edge of the dome and the short axis of
the dome. The intrusion in E2 was also saucer-shaped, but in contrast to
that of E1, it exhibited a strongly elliptical shape (Fig. 7b); the long and
short axes of the inner sill were ~11 and 5 cm long, respectively.
The long axis of the inner sill was directly above the linear inlet.
The linear inlet was slightly longer than the long axis of the elliptical
sill, as the linear inlet extended further away than the upper tip of the
inner sill (Fig. 7b). In contrast, we noticed that the lower tip of the
inner sill was located right above the tip of the linear inlet (Fig. 7b).
4. Discussion
4.1. Sill shape vs. feeder shape
The experiment E1 (Galland et al., 2009; Fig. 7a) showed that
injection through a pipe-like inlet in a model with layering always
results in a sub-circular saucer-shaped sill. In similar experiments
from Galland et al. (2009), a sub-circular sill shape was obtained for
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Fig. 7. a. Topographic map view of the intrusion of experiment E1 (punctual inlet). The white straight line locates the cross-section underneath. The black circle locates the circular
feeder below the inner sill. The intrusion is a saucer-shaped sill with sub-circular inner sill (white dashed line). The scales of the map and colour bar are in millimetres. b. Topographic
map of the intrusion of experiment E2. The horizontal white straight line locates the cross-section underneath. The vertical bold black line locates the linear feeder. The intrusion is a
saucer-shaped sill with elliptical inner sill (white dashed line); its long axis located directly above the feeder dyke. In both cases, the horizontal net was a much larger square
(30 × 30 cm) than the intrusions, so that we did not report it in this figure.
any depth of emplacement. This suggests that the horizontal state of
stress was isotropic in the experiments.
Experiment E2 is used to test the effect of the linear shape of a
feeder dyke on the three-dimensional development of sills and
saucer-shaped sills. The experiment E2 suggests that when the linear
feeder dyke is long, i.e. the inlet size-to-depth ratio is large (L/D = 4),
the resulting intrusion is an elliptical saucer-shaped sill (Fig. 7b). In
addition, the long axis of the elliptical saucer in experiment E2 was
parallel to, and located directly above, the linear inlet (Fig. 7b). Thus,
experiment E2 represents strong evidence that a linear feeder with a
large size-to-depth ratio controls the ellipticity of the sill intrusion.
Moreover, in E2, the position of one tip of the inner sill directly above
one tip of the linear inlet (Fig. 7b) strongly suggests that the feeder
also controls the exact location and extent of the sill.
In more detail, the elliptical sill produced from experiment E2
exhibits an asymmetric appendix rooted at the tip of the short axis of
the elliptical sill. Pollard and Johnson (1973) and Goulty and Schofield
(2008) showed that stresses generated around elliptical sills are
higher at the tips of short axis than at the tips of the long axis of an
elliptical sill. This leads to the formation of lateral inclined sheets
rooted at the tips of the short axis of the elliptic sills, such as in E2. In
addition, the theoretical study of Bunger (2005) shows that when
near-surface fractures grow, they reach a critical size from which they
become unstable and develop asymmetrical shapes. We suggest that
this asymmetrical appendix formed as a result of such mechanical
instabilities.
In experiment E1, the punctual feeder was located off the centre of
the circular inner sill (Fig. 7a). This was also the case for the
experiments described by Galland et al. (2009), for which L/D ≪ 1; in
some of them, the feeder was even close to the rim of the sill. In these
experiments, the resulting intrusions were always circular although
the location of the feeder and the ratio L/D varied. This shows that
when L/D ≪ 1, the feeder was so small with respect to the depth of
injection that its location, size and shape had no influence on the final
shape of the sill. Therefore, if a small dyke feeds a deep sill, (L/D ≪ 1),
the resulting sill may develop a sub-circular shape. We thus conclude
that the occurrence of sub-circular saucer-shaped sills does not imply
that their feeders are pipes, but that they can be dykes which lengths
are smaller than the depth of emplacement. The feeders of the subcircular saucer-shaped sill observed in the Karoo Basin (see Fig. 1 of
Galland et al., 2009) may thus be small dykes.
4.2. Implications for the GVSC
We showed above (see also Galerne et al., 2008; Polteau et al.,
2008a) that some sills of the GVSC are in contact (GVS and MSS at
location L4, Fig. 8a; GS and MSS at location L7; GS and HS at location L8;
Fig. 2). However, the geochemical data show that the magmas of these
sills originate from different magma batches, implying that these
contacts are not feeding connections. Therefore, we do not have any
evidence of sill-feeding-sill relationships in the GVSC. Instead, the AMS
analyses and macroscopic flow indicators of the major sills in the GVSC
(Polteau et al., 2008a) imply that the magma flowed outward from the
centre of the sills. The feeders of the sills were thus located underneath
their inner sills. The feeders could either be pipes, the inclined sheets of
an underlying sill (Polteau et al., 2008a), or dykes.
Our experiments show that in an isotropic state of stress a long linear
feeder (dyke) gives rise to an elliptical sill. This suggests that the
elliptical sills of the GVSC are fed by linear feeders. In addition,
experiment E2 shows that the tips of the linear feeders are likely to be
located below the tips of the inner sills, though these former can extend
further (Fig. 7b). Therefore, if the feeders of these elliptical sills can be
observed in the field, we expect to find them at the vicinity of the tips of
the sills. Indeed, we did find a dyke (d4) that fulfils the criteria for being
the feeder of the GVS: it crops out underneath the southern tip of the
GVS, it strikes parallel with, and is located directly below the long axis of
the GVS (Figs. 2 and 4a–b), and its geochemical composition is identical
to that of the GVS dolerites (Fig. 5). There is thus good evidence that the
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197
Fig. 8. a. Geological cross section of the southern part of the Golden Valley Sill and the underlying Morning Sun Sill (along B–B1 in Fig. 3). Although there is a physical contact between
the two sills, their different geochemistry shows that the MSS was not the feeder of the GVS. b. Example of seismic image showing contact between two saucer-shaped sills,
interpreted as feeding relationship (modified after Hansen et al., 2004). The comparison between this image and the geological cross section of the GVSC show remarkable
similarities. Because our study shows that contacts between sills are not obviously feeding relationships, we propose another interpretation where each sill (highlighted in red and
purple) may represent a distinct batch of magma. Thus, in order to infer the nature of these contacts on seismic images, more criteria are needed.
dyke d4 is the feeder to the GVS and that the elliptical shape of the sill is
controlled by the strike and extent of d4.
One major concern is that there is no evidence of potential feeder
dykes at the tips of the other elliptical sills of the GVSC. Nevertheless,
our experiment E2 shows that a feeder dyke is essentially located
under the inner sill, although it can extend merely longer than the
long axis of the inner sills (Fig. 7b). Because the inner sills in the GVSC
are not eroded, their feeders remain mainly hidden. This explains why
we did not observe potential feeders for the other sills of the GVSC.
Another major concern is the small size of the dyke d4 (~1 m thick;
Fig. 4c). Can this dyke feed a sill as large as the GVS? Experiment E2
suggests that the length of the long axis of the inner sill is almost the
same as the length of the feeder dyke. The exposed part of the dyke d4
at the southern end of the GVS may represent only the tip of the feeder
dyke. Thus, d4 may be much wider beneath the central part of the sill.
If we assume that d4 is as long as the long axis of the GVS its length
should be 20 km. Magma velocity estimates for mafic dykes range
between 0.1 and 1 m s− 1 (e.g., Spence and Turcotte, 1985; Battaglia
and Bachèlery, 2003; Roman et al., 2004). Given such parameters, the
time for a 1 m thick dyke such as d4 to feed the GVS, which length,
width and thickness are 20 km, ~ 10 km, and ~100 m, respectively, is
between ~12 and ~ 115 days. These values are typical for the duration
of eruptions in basaltic shield volcanoes, e.g. at Piton de la Fournaise
Volcano, Réunion Island, France (see review by Peltier et al., 2009), or
at Etna, Sicily, and Mauna Loa, Hawaii (Bebbington, 2008).
Experiment E2 shows that a linear feeder dyke can generate an
elliptical sill with a feeder size-to-depth ratio L/D = 4 (Fig. 7b). In the
GVS, the estimated depth of emplacement is 1 to 2 km (Polteau et al.,
2008a). If we assume that d4 is the feeder to the GVS, the feeder L/D
ratio ranges between 10 and 20, which is larger than the value in E2,
and so more favourable for elliptical sill formation.
The identical geochemical compositions of the GVS and the GS
suggest that their magmas derived from a common source. Galerne et al.
(2008) proposed that these sills were connected laterally by their outer
sills. However, the integrated results of this study suggest that each
elliptical sill of the GVSC was fed by an individual linear feeder located
below. This implies that they are fed from a common reservoir at depth.
Our dataset does not allow us to decipher whether such a reservoir is a
mid- to lower-crustal reservoir or a shallow reservoir, e.g. a deeper
saucer-shaped sill. Thus, we cannot definitely rule out a sill-feeding-sill
type of connection.
It is well known that anisotropic horizontal stresses control the
orientation and the shape of magma conduits (e.g., Hubbert and
Willis, 1957; Odé, 1957; Muller and Pollard, 1977; Vigneresse, 1995).
Such stresses may have influenced the orientations of some dykes in
the GVSC area, as well as the shape of the elliptical sills (Trude, 2004;
Goulty and Schofield, 2008). However, the Karoo Igneous Event seals
the end of the foreland deformation of the Karoo basin, and there are
no significant regional tectonic deformation that has affected the
Karoo intrusive and effusive rocks (Catuneanu, 2004). Furthermore,
many of the Karoo saucers are circular (see Fig. 1 of Galland et al.,
2009). Therefore, we consider the regional tectonic setting to have
been negligible at the time of the Karoo Igneous Event, i.e. the state of
stress was essentially isotropic. This statement is in good agreement
with the scatter of the orientations of minor dykes in the GVSC
(Fig. 2). We infer that the elliptic shapes of the sills of the GVSC were
not controlled by any anisotropic regional state of stress but by
different orientations of their feeder dykes.
4.3. Implications for sill emplacement mechanisms
The existing models for saucer-shaped sill emplacement imply
distinct magma flow patterns (Fig. 1). In the buoyancy-controlled
emplacement model, the feeders are expected to be located beneath
the outer sheets, resulting in lateral magma flow (e.g. Francis, 1982;
Goulty, 2005). In contrast, in the laccolith model controlled by
discontinuities of the country rock, the feeders are expected to be
located below the inner sill, resulting in lateral upward and outward
magma flow (e.g. Pollard and Johnson, 1973; Malthe-Sørenssen et al.,
2004; Galland et al., 2009). In the GVSC, flow indicators show that
magma flow pattern was radial, implying that the feeders of the large
sills were located beneath their inner floors (Polteau et al., 2008a).
This is consistent with the location of the dyke (d4) identified as the
feeder of the GVS. In contrast, we have no evidence of feeders located
below the outer limbs of the saucer-shaped sills. Thus, our study
supports the laccolith model for sill emplacement. However, we have
no evidence in favour of the “neutral buoyancy” model.
As cited above, some authors argue that large sill complexes
represent interconnected sill networks (Hansen et al., 2004; Cartwright and Hansen, 2006). This assumption mostly results from the
interpretation of seismic 3D imaging, in which sills emplaced at
different stratigraphic levels appear to be physically connected,
forming sill-feeding-sill networks (Hansen et al., 2004). In the GVSC,
we observe similar relationships at locations L4, L7 and L8 (Figs. 2–4
and 8a). In a seismic 3D image these sill–sill contacts might be taken
for sill-feeding-sill relationships (Fig. 8b). However, the combination
of detailed geological observations and geochemical analyses of
Galerne et al. (2008, 2010) are robust evidence that there is no
feeding relationship between these pairs of sills. Our study does not
provide any evidence of interconnected sill networks. Instead, our
study strongly suggests that the GVSC sills were fed by dykes (GVS by
the small dyke d4) located underneath the long axes of their inner
sills. Such small dykes would most likely be invisible on seismic
profiles. The results from the GVSC have important implications for
seismic interpretation of large sill complexes: contacts between sills
do not necessarily represent feeding relationships (Fig. 8b).
In contrast to the sill-feeding-sill model, three-dimensional lobate
morphologies imaged on seismic data have been interpreted as
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channels where magma flows outwards from central feeders, some of
which have been identified as dykes (Thomson and Hutton, 2004;
Thomson, 2007). The observations collected in the GVSC are in good
agreement with this interpretation: (1) the radial undulations
observed in the limbs of the GVS, interpreted as magma channels,
are parallel to other flow-indicators (Polteau et al., 2008a); and (2)
the radial undulations of the GVS indicate a radial flow of magma,
which is consistent with a central feeder (dyke d4). Therefore, the
data collected in the GVSC validate the use of lobate morphologies and
undulations in sills as magma flow indicators.
5. Summary and conclusions
The main conclusions of our field observations, geochemical
analyses and experimental modelling on the three-dimensional
relationships between saucer-shaped sills and their feeders are
summarised in the following points:
1. In the GVSC, we identified physical contacts between some of the
major sills (the roof of one sill in contact with the floor of the
overlying sill). However, geochemical contrasts show that these
contacts do not represent sill-feeding-sill relationships. Our study
thus shows that a physical connection between two sills does not
necessarily imply a sill-feeding-sill relationship.
2. Magma flow indicators in the GVSC sills, i.e. (1) radial undulations of
the limbs of the GVS, and (2) AMS data (Polteau et al., 2008a),
indicate that the magma flowed radially from the centres of the sills.
3. We identified 4 minor dykes in the GVSC. One dyke, d4, has
identical chemical characteristics to the Golden Valley Sill (GVS); it
is located right below the southern tip of the GVS, and is parallel
and superimposed on the long axis of the GVS.
4. The geometrical and geochemical relationships between the GVS
and the dyke d4 suggest that this dyke was the feeder of the GVS.
5. To test this hypothesis, we resorted to experimental modelling. The
models show that a sill fed by a punctual feeder develops a subcircular saucer shape, whereas a sill fed by a long linear feeder (a
dyke) develops an elliptical saucer geometry.
6. The relationships between the elliptical sill and its linear feeder in
experiment E2 are very similar to those between the GVS and the
dyke d4. This shows that d4 may be the feeder of the GVS. The
experimental results also suggest that the other elliptic sills of the
GVSC were fed by dykes.
7. The consistency between the radial magma flow indicators in the
GVS, based on AMS data and on the orientation of magma channels
(Polteau et al., 2008a), and the central location of the feeder dyke
d4 confirms that the lobate and tubular structures imaged on
seismic data (see e.g., Thomson, 2007) are good criteria for
determining sill feeding processes.
Acknowledgements
This study was supported by a Centre of Excellence grant from the
Norwegian Research Council to PGP. This work was also partly funded
by the Norwegian Research Council (NFR project 159824/V30
“Emplacement mechanisms and magma flow in sheet intrusions in
sedimentary basins”) and by the Norwegian Young Outstanding
Researcher (YFF grant 180678/V30 “Processes in volcanic basins and
the implications for global warming and mass extinctions”). We thank
B. van Wyk de Vries, J. Cartrwight, G. Norini, T. Menand, O. Merle, A.
Guerer, and V. Acocella for their constructive reviews.
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