Saucer-shaped Sills Research Articles

Experimental modelling of shallow magma emplacement: Application to saucer-shaped intrusions

Saucer-shaped dolerite and sandstone intrusions are common in sedimentary basins world-wide. We have conducted a series of scaled experiments simulating the process of magma emplacement in sedimentary basins, with particular attention on the formation of saucer-shaped sills. The model materials were (1) cohesive fine-grained silica flour, representing brittle crust; and (2) molten low-viscosity oil, representing magma. The experiments were performed in both homogeneous and layered models. In all the experiments, oil injection resulted in doming of the surface. In the homogeneous models, the injected oil formed cone sheets and sub-vertical dykes. Cone sheets formed for shallow injection (1–3 cm), and vertical dykes formed for deeper injection (4–5 cm). In layered models, the injected oil always formed saucer-shaped intrusions. Our experimental results show that (1) sill intrusion results in the formation of a dome, with melt erupting at the rim; (2) layering controls the formation of sills and saucer-shaped sills; (3) saucer-shaped sills are fed from the bottom and the fluid flows upward and outward; and (4) the diameter of saucer-shaped sills increase with increasing emplacement depth. The systematic relation between domes and sills and the depth-dependence of sill diameters show that saucer-shaped intrusions result from the interaction between a growing flat-lying shallow sill and doming of the free surface. We conclude that saucer-shaped intrusions represent fundamental geometries formed by shallow magma intrusion in stratified basins.

Emplacement mechanisms of sill complexes: Information from the geochemical architecture of the Golden Valley Sill Complex, South Africa

The well-exposed Golden Valley Sill Complex, Karoo basin, South Africa, consists of four large sills (ca. 100 m thick; long axes: 13–24 km), one small sill (55–80 m thick; long axis: 4 km; forming an appendix to one of the large sills), and two large dykes (15–20 m thick; 25 and 70 km long), plus some minor intrusions. Field mapping shows physical connections between the small sill and one of the large sills, but no other connection between the large sills, or connections between the sills and the large dykes. Geochemical variation diagrams for 327 dolerite samples from the different sills and dykes in the Golden Valley Sill Complex showed similar compositional ranges for the dolerite bodies, but some differences in ratios between strongly incompatible elements. These differences were probed by Forward Stepwise-Discriminant Function Analysis using all major and trace elements as variables, and the height major sills and dykes as populations. The Forward Stepwise-Discriminant Function Analysis identified six different geochemical signatures among the sill-dyke populations. The six geochemical signatures were interpreted as representing six magma batches of distinct chemical characteristics that were involved in the formation of the Golden Valley Sill Complex. Four different magma batches gave rise to two of the large saucer-shaped sills and the two dykes. The exposed large dykes thus do not represent feeders to the main sills. A fifth magma batch gave rise to the two other large saucer-shaped sills as well as the smaller one. The statistical analysis also gave information about the connection between different dolerite exposures that could not be obtained by field, and made it possible to reconstruct the different sills through complex areas where several sills meet, and through widely spaced exposures. On the basis the field mapping and statistical analysis of geochemical data the geochemical architecture of the complex was established. Our study gives support to both the main models proposed earlier to explain the emplacement of sill complexes: (1) each sill in a sill complex is fed by different magma batches, and (2) the different sills in a sill complex feed one another, forming a group of nested sills generated by one single batch of magma. Furthermore, our study indicates that the interconnected sills in the Golden Valley Sill Complex are connected along their outermost edges through lateral overflow. The statistical analysis establishes a link between minor dolerite bodies and the main intrusion, thereby improving the geological mapping of the sill complex. The different units in the Golden Valley Sill Complex show strong geochemical affinity to lavas in the Lesotho Formation, the younger of the two formations in the Lesotho lava remnant. This suggests that these sills were emplaced at the same time as the Lesotho Formation, and that magmas with the same chemical characteristics as the Lesotho Formation were emplaced over a significantly larger area than covered by the remnants of the Lesotho Formation.

Implications of simple flexure theory for the formation of saucer-shaped sills

Saucer-shaped dolerite sills have been identified in several sedimentary basins. They are characteristically sub-circular in plan, with an inner sill connected to a steeply inclined arcuate sheet extending upwards from the edge of the inner sill, and in some cases have raggedly developed flat outer rims. By applying elastic plate theory to the overburden, we obtain an expression for the maximum radius of a circular sill in terms of emplacement depth, excess magma pressure, elastic constants of the overburden and the tensile strain at failure. This expression predicts that the radius of the flat base of a saucer-shaped sill increases with emplacement depth, as observed in the western Karoo Basin, South Africa. Some saucer-shaped sills display an elliptical morphology. From consideration of the flexural strains around the periphery of a horizontal elliptical sill, we infer that saucer-shaped elliptical sills have been fed from below by feeder dykes oriented along the major axis of the ellipse. Flat outer rims would have developed at the final stage of each intrusive episode, when the excess magma pressure was decreasing towards equilibrium with the overburden load. Consequently, flexural strains would also have decreased, so continued intrusion might preferentially occur along a bedding plane.

Structure and emplacement of high-level magmatic systems: introduction

Despite the continual round of annual conferences, special sessions and symposia that provide ample opportunity for researchers to get together and talk about igneous processes, the origin of laccoliths and sills continue to inspire and confound geologists. In one sense this is surprising. After all, don’t we know all we need to know about these rocks by now? As testified by the diverse range of topics covered in this Specical Publication and elsewhere (Breitkreuz & Petford 2004), the answer is clearly no. This Special Publication contains 13 papers that cover a diversity of perspectives relating to the geology and emplacement of sills, dykes and laccoliths that together help advance our understanding of their formation. Ablay et al. describe a new fracture-mediated intrusion model that attempts to resolve the sequence of magma and rock displacements comprising felsic magma systems coupled with a thermal model for the lower crust, arguing that the system is driven fundamentally by partial melting at source. Thomson & Schofield report on the relationship between sills, dykes, laccoliths and pre-existing basin structure in the NW European Atlantic margin. Using three-dimensional (3D) seismic data, they interpret the sills as predominantly concave-upwards in shape with flat inner saucers connected to an outer rim by a steeply inclined sheet structure. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. Magma emplacement below the level of neutral buoyancy would allow sill inflation and country rock deformation. Fracturing of country rock allowed magma to move upwards and feed shallower-level intrusions. In a study of sills in the Ferrar large igneous province, Leat surmises from field observations and geochemical relationships (consistent with fractional crystallization during magma flow) that magma may have been transported laterally over large distances, possibly in excess of 3000 km. No feeder dyke swarm has been identified and the implication is that the Ferrar sills are some of the longest subterranean lateral magma flows on Earth. Nemeth et al. describe magma–water interactions from Palhaza, NE Hungary, and show that the rock succession is made up of a complex association of Miocene rhyolitic shallow intrusions, cryptodomes and endogenous lava domes emplaced into and onto soft, wet pelitic sediment in a shallow submarine environment. Nemeth & Cronin have examined pit craters and high-level magma feeding systems of a mafic island-arc volcano in the South Pacific. One of the Marum craters, Niri Taten, exposes portions of solidified lava lakes and magma pods that fed spatter cones, small shallow-level intrusions and larger sills that connect through a complex network of dykes to the surface. These features show that shallow-level infiltration of degassed and low-viscosity melts into pyroclastic deposits can play an important role in the growth of scoria and spatter cones. Cone collapse results in lateral escape of magma to form lava flows. Vinciguerra et al. present the results of experiments to determine melt concentration and strain distributions around basalt dykes determined from image analysis and chemical profiles. Melt migration is enhanced by porosity of the microstructure and by the loading conditions. Highest melt concentrations (and presumably highest stress concentrations) occur at the dyke tip. Matrix deformation appears to be controlled by granular flow, but dilatancy occurs near the tip of the dyke, implying coupled magma transport and granular flow. Bunger et al. present the results of analogue experiments and quantitative analysis designed to gain better insight into the mechanics of formation of saucer-shaped sills. In their experiments, fractures that govern final sill geometry are seen to undergo three separate transitions. Each transition is governed by a characteristic timescale relating to viscous (magma) flow and energy dissipation, the time lag between fracture and magma propagation fronts, and the time for a sill to grow as long as it is deep from the surface. Mazzarini & Musumeci have studied sheet-like intrusions emplaced close to the surface on Elba Island, Italy. Field data show that sill and dyke emplacement was controlled by a combination of host-rock fracturing and magma overpressure resulting in hydraulic fracturing. Analysis of the

Evolution and morphology of saucer-shaped sills in analogue experiments

Abstract The growth of shallow sills is studied in analogue experiments performed in polymethyl methacrylate (PMMA) and glass. The experimental fractures curve towards the surface to become saucer-shaped, which is consistent with many field observations of dolerite sills. The curvature of the saucer is shown to decrease as the in situ stress acting parallel to the surface increases relative to an estimate of the strength of the fracture-induced stress field. The initially circular fractures also elongate in plan view to become egg-shaped, a tendency that decreases with increasing importance of viscous dissipation in the growth process. Sill emplacement is further examined mathematically by considering a shallow, circular, fluid-driven fracture propagating in a homogeneous brittle elastic material. The fractures are shown to undergo three transitions related to the mechanics of sill growth. Each transition is associated with a characteristic time that is derived from analysis of the governing equations using scaling methods. These characteristic times provide an estimate of how long viscous flow is the dominant energy dissipation mechanism, how long significant lag between the fluid and fracture fronts is expected to persist, and how long the sill will take to attain an extent that is of the same order as its depth.

The three-dimensional geometry and growth of forced folds above saucer-shaped igneous sills

Forced folds formed at the seabed immediately overlying shallow (<1 km) saucer-shaped sills along the NE Atlantic Margin during the early Paleogene. Examples of this sill-fold relationship are exceptionally well imaged by high-resolution 3D seismic datasets from the NE Rockall Basin. The forced folds are domal in shape, 2–4 km in diameter, exhibit a structural relief of up to 350 m, and comprise sediment volumes of ca. 1 km 3. A comparison of the thickness distribution across and volume of a saucer-shaped sill with a high intrusion diameter to depth ratio and the structural relief and volume of its associated forced fold shows a remarkable equivalence. This has the important implication that the structural relief on intrusion-related forced folds can be used as an estimate of the thickness of the underlying sill. The analysed forced folds are interpreted to have formed through three continual growth stages that are directly linked to the mechanical emplacement of the underlying saucer-shaped sills. Their growth was associated with an increase in faulting of the overlying strata and influenced coeval or subsequent development of polygonal fault systems within the overburden. These structures represent a new type of four-way dip closed hydrocarbon trap.

Saucer-shaped sill with lobate morphology revealed by 3D seismic data: implications for resolving a shallow-level sill emplacement mechanism

We use 3D seismic data to describe the 3D geometry of a large igneous intrusion, the Solsikke Compound Sill, and address a number of issues related to sill emplacement. The Solsikke Compound Sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer-shaped depressions and linear discontinuities. One of the saucer-shaped sub-elements of the compound sill, the Solsikke Lobate Sill, has a previously unrecognized morphology. It has a basal feeder and consists of a bifurcating network of interlinked lobe-shaped sill segments. We propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments. The lobate pattern indicates that the Solsikke Lobate Sill was fed at its deepest point and adopted its geometry through outwards and upwards propagation. The feeder location is coincident with a fault intersection, suggesting that magma transport from the underlying source exploited the zone surrounding the intersection.

Formation of saucer-shaped sills

Abstract We have developed a coupled model for sill emplacement in sedimentary basins. The intruded sedimentary strata are approximated as an elastic material modelled using a discrete element method. A non-viscous fluid is used to approximate the intruding magmatic sill. The model has been used to study quasi-static sill emplacement in simple basin geometries. The simulations show that saucer-shaped sill complexes are formed in the simplest basin configurations defined as having homogeneous infill and initial isotropic stress conditions. Anisotropic stress fields are formed around the sill tips during the emplacement due to uplift of the overburden. The introduction of this stress asymmetry leads to the formation of transgressive sill segments when the length of the horizontal segment exceeds two to three times the overburden thickness. New field and seismic observations corroborate the results obtained from the modelling. Recent fieldwork in undeformed parts of the Karoo Basin, South Africa, shows that saucer-shaped sills are common in the middle and upper parts of the basin. Similar saucer shaped sill complexes are also mapped on new two- and three-dimensional seismic data offshore of Mid-Norway and on the NW Australian shelf, whereas planar and segmented sheet intrusions are more common in structured and deep basin provinces.

Sill complex geometry and internal architecture: a 3D seismic perspective

Abstract Seismic volume visualization techniques demonstrate that saucer-shaped sill complexes consist of a series of radiating principal flow units rising from an inner saucer and fed by principal magma tubes. Such flow units contain smaller scale secondary flow units, each being fed by a secondary magma tube branching from the principal magma tube. This pattern is repeated down to scales of approximately 100 m with successively smaller flow units being fed from magma tubes repeatedly branching from higher order tubes. The data demonstrates that each sill complex is independently fed from a centrally located point source, that sills grow by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature. These features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening.

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Europa's surface contains numerous quasi-elliptical features called pits, domes, spots and small chaos. We propose that these features, collectively referred to as lenticulae, are the surface expression of saucer-shaped sills of liquid water in Europa's ice shell. In particular, the inclined sheets of water that surround a horizontal inner sill limit the lateral extent of intrusion, setting the lateral dimension of lenticulae. Furthermore, the inclined sheets disrupt the ice above the intrusion allowing the inner sill to thicken to produce the observed relief of lenticulae and to fracture the crust to form small chaos. Scaling relationships between sill depth and lateral extent imply that the hypothesized intrusions are, or were, 1–5 km below the surface. Liquid water is predicted to exist presently under pits and for a finite time under chaos and domes.