Spectacular saucer-shaped sill complexes are formed when large volumes of molten basalt ascend through a layered sedimentary basin. Such sill complexes are well imaged on seismic reflection data on the Norwegian continental shelf. Multidisciplinary research combining field and seismic observations with numerical and analogue modelling has provided us with a new understanding of how these intrusive complexes are formed.
Looking for the Karoo sills
A cliff almost 1000 km long cuts across the arid Karoo region of South Africa. This escarpment is situated in the middle of the Carboniferous to Jurassic age Karoo basin, separating a region of layered sedimentary strata to the south from a region of strongly intruded sediments to the north. In 1999, I drove across the escarpment with my South African colleague, Prof. Goonie Marsh from Grahamstown, accompanied by a small group of scientists from Oslo, consisting of two geologists, Bjørn Jamtveit and Ellen Planke, and a physicist, Anders Malthe-Sørenssen.
The previous summer we had seen several large scale maps, produced by Luc Chevallier, at the Council of Geoscience in South Africa, showing outcropping dolerite rocks in most of the Karoo basin. The maps suggested that sill complexes similar to the ones I had studied for years on seismic data in the Norwegian Sea were actually exposed in the Karoo.
Our hope was to find well-preserved outcrops of the dolerite sills and the surrounding metamorphic aureoles in South Africa. Before going, our main concern was over the degree of exposure and topographic relief we would experience. We had heard stories of endless drives across the dry and flat interior of South Africa. But driving across the escarpment at Elandsberg proved that there had been no reason for concern.
The spectacular Golden Valley
The region north of Elandsberg is characterized by table mountains and semi-circular dolerite hills, forming large topographic basins. By far the most spectacular of these basins is the Golden Valley.
This 18 km long and 10 km wide basin is almost perfectly eroded to display the geometry of a saucer-shaped sill. The erosionally resistant 100 m thick dolerite sheet forms the flat floor of the basin. Transgressive sill segments cross-cut the basin sediments, and form the 600 m high elliptical mountain ridges completely encompassing the valley.
The shape of the Golden Valley dolerite body is almost identical to the sill intrusions imaged on 3D seismic data on the Norwegian margin. We realized quickly that nature’s almost perfect excavation allowed us to undertake unique studies of the dynamics of sill emplacement processes at this locality. The Golden Valley was definitely a place worth going back to!
New understanding of sedimentary basins
We visited the Golden Valley several times during field trips in the following years, but this was clearly a geological complex that required much more than these short, intense studies. In 2004 the University of Oslo obtained funding for a four-year project on sill emplacement in sedimentary basins.
The sill project, headed by Prof. Else-Ragnhild Neumann, was based on an innovative and cross-disciplinary approach to investigate massive volcanism in sedimentary basins. The project motivation was to understand emplacement mechanisms which formed the extensive sill complexes seen on seismic data in sedimentary basins on the mid-Norwegian margin. Our approach was to use a combination of field work in world-class natural laboratories with advanced magnetic and geochemical measurements of extensive new sample collections, and through these to develop new numerical and analogue modelling techniques.
Detailed sampling and mapping of the Golden Valley Sill Complex was conducted during two hectic field seasons, and more than 650 magnetic and 375 geochemical analyses have been completed. The field work and analyses reveal that the sill complex consists of connected saucer-shaped sills, formed by distinct magma batches flowing radially outward in sheets and in tube-like structures. The saucer structures have been reproduced in both numerical and analogue modelling experiments, building a new understanding of the magma emplacement processes in sedimentary basins.
One important project result is that porous flow of residual melt usually occurs during the crystallizing stage, strongly influencing the geochemistry and structure of the saucers. This theory is supported by the measured orientation of magnetic minerals, the observed undulating surface structures of the saucers, and the measured geochemical profiles. Numerical modelling shows that large pressure gradients may exist during this stage, leading to porous flow. Our theory is in strong contrast with the conventional convecting magma chamber hypothesis.
Explaining mass extinctions
Five of the big mass extinctions during the last 250 Ma are associated with periods of global warming synchronous with the formation of large igneous provinces. Most of the controversy surrounding igneous provinces as the cause of dramatic environmental changes involves the duration of volcanism, and synchronicity between mass extinction and the main volcanic phase.
Our group has proposed a new mechanism for understanding the relationship between volcanism and global warming (Svensen et al., 2004). The key element in this hypothesis is that intrusion of magma into organic rich sedimentary basins leads to the formation of very large volumes of greenhouse gases that are transported rapidly into the atmosphere through hydrothermal vent complexes. The hypothesis was originally proposed to explain the Paleocene-Eocene Thermal Maximum (PETM) occurring simultaneously with intrusive volcanic activity offshore mid-Norway, but it is equally relevant for the Early Jurassic Toarcian global warming and Karoo volcanism, both of which occurred about 183Ma ago.
New U/Pb zircon dating of dolerites from the Karoo basin indicates that the intrusive activity took place at the time of Toarcian Early Jurassic global warming. Analyses of organic carbon in the sediments around the dated intrusions show that sufficiently large volumes of greenhouse gases to create global warming were generated in the Karoo basin. The presence of thousands of breccia pipes in the host rock sediments finally documents that the gases were rapidly transported away from the production zone around the sills.
The saucer-shaped sills in the Karoo basin form prominent structures that can be observed on satellite imagery. A similar strategy can be applied in the search for saucer-shaped intrusions in space exploration programs by using the Earth as analogue.
We have begun the search for extra-terrestrial analogues of saucer-shaped intrusions on planets with known volcanic activity, such as Venus. Coronae are thought to be present on Venus and none of the current models can explain their formation. We propose that they represent surface deformations caused by the emplacement of a saucer-shaped intrusion at depth.
Saucer-shaped sill formation
Field observations, combined with numerical and sand-box modelling, show that saucer-shaped sills represent a fundamental shape. When fluids are injected into a layered, elastic medium the formation and propagation of sub-horizontal fractures is initiated. The fractures are subsequently filled and inflated by the fluids, causing doming of the overburden and high stress concentration at the fracture tips. The initially symmetric stress field at the fracture tips becomes increasingly asymmetric during uplift and bending of the overburden. The alteration in the stress field facilitates a sudden change in the propagating fracture direction, forming inclined sheets. The stress asymmetry is formed as a consequence of the emplacement process, and the saucer-shape is not governed by the presence of fractures or vertical weakness zones.
The illustrations show 3D visualization of a saucer-shaped sill in the Møre Basin (top); an excavated saucer-shaped sill formed by intrusion of heated oil into a silica powder (middle); and a 2D discrete-element numerical model showing the stress field around a saucer-shaped intrusion (lower).
Image courtesy of PGP (Stephane Polteau, Olivier Galland, and Anders Malthe-Sørenssen).
Volcanic basins are sedimentary basins with a significant amount of primary emplaced volcanic rocks.
Sills are tabular igneous intrusions that are dominantly layer parallel. They are commonly sub-horizontal. Sills may locally have transgressive segments, crosscutting the stratigraphy.
Dykes: Magma sheets crosscutting the strata, generally sub-vertical.
Contact metamorphic aureoles are the volume of rocks heated beyond 100ºC following the sill emplacement. As a rule of thumb, the thickness of the metamorphic aureole is comparable to, or greater, than the sill thickness on both sides of thick (>50 m) sills intruded into shales.
Breccia pipes are vertical chimney-like structures consisting of fragmented and brecciated thermally altered rocks located in a relatively undeformed sedimentary sequence. They are filled by mixtures of magmatic and sedimentary rock fragments, and represent the deep parts of hydrothermal vent complexes.
Svensen et al: Eocene global warming. Hydrothermal vents prompt methane release; Nature, June 3, 2004.
Written by Sverre Planke