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Transgressive Sills and Lateral Lava Flows: On the Visual Observation of Igneous Sheets in Rugged Mountainous Terrains and the Optical Illusion Factor

Transgressive sills are of common occurrences in rift-related offshore and onshore sedimentary settings worldwide and have been reported in onshore volcanic settings in e.g. flood basalt provinces as well. General geometries of individual sills and of entire sill complexes too are well documented from seismic images in many offshore sedimentary settings of exploration interest, but limitations in seismic resolution may inhibit correct interpretations when it comes to small-scale structures and evidences on sill intrusion modes. Hence, relevant and important details and clues on intrusion mechanisms during emplacement of sheet intrusions, which may be unclear or missing in seismic images from offshore sedimentary basins, can to some degree be clarified by means of thorough visual examination and measurements on exposed onshore sills and sill complexes. In the actual study, relevant visual observations and measurements of the Streymoy Sill and its feeders in particular and to a lesser degree the Eysturoy Sill of the Faroe Islands, as well as some local lava flows, are scrutinised in order to demonstrate the importance of correct and detailed mapping, associated measurements and interpretations at exposed sill margins. The actual study chiefly focuses on potential optical illusion factors, not uncommon for sheet intrusions in rugged mountainous terrain, and potential associated misinterpretations. It is shown that unless due care is taken in assuring that sub-horizontal visual observations from some distance in such terrains are duly accompanied by other observations in the sub-vertical and/or sub-lateral plane at other angles as well, i.e. proper 3-D considerations, noticeable errors could result, when it comes to interpretations on intrusion mechanisms and possible regional stresses that prevailed during emplacement of such sheet-like igneous bodies.

Faroe Islands, Basaltic Rocks, Transgressive Sills, Flood Basalts, Sill Intrusion, Structural Geology

APA Style

Jogvan Hansen. (2020). Transgressive Sills and Lateral Lava Flows: On the Visual Observation of Igneous Sheets in Rugged Mountainous Terrains and the Optical Illusion Factor. Earth Sciences, 9(5), 164-177.

ACS Style

Jogvan Hansen. Transgressive Sills and Lateral Lava Flows: On the Visual Observation of Igneous Sheets in Rugged Mountainous Terrains and the Optical Illusion Factor. Earth Sci. 2020, 9(5), 164-177. doi: 10.11648/

AMA Style

Jogvan Hansen. Transgressive Sills and Lateral Lava Flows: On the Visual Observation of Igneous Sheets in Rugged Mountainous Terrains and the Optical Illusion Factor. Earth Sci. 2020;9(5):164-177. doi: 10.11648/

Copyright © 2020 Authors retain the copyright of this article.
This article is an open access article distributed under the Creative Commons Attribution License ( which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Thomson, K. 2007. Determining magma flow in sills, dykes and laccoliths and their implications for sill emplacement mechanisms. Bulletin of Volcanology, 70, 183–201.
2. Muirhead, J. D., Airoldi, G., Rowland, J. V., White, J. D. L. 2012. Interconnected sills and inclined sheet intrusions control shallow magma transport in the Ferrar large igneous province, Antarctica. GSA Bulletin. 124 (1/2), 162-180.
3. Hansen J. 2015. A numerical approach to sill emplacement in isotropic media: Do saucer-shaped sills represent ‘natural’ intrusive tendencies in the shallow crust? Tectonophysics, 664, 125–138.
4. Jerram D. A. and Bryan, S. E. 2015. Plumbing systems of shallow level intrusive complexes. In: Breitkreuz, C. H. and Rocchi, S. (eds) Physical Geology of shallow magmatic systems. Advances in Volcanology. Springer, Berlin, 1-22,
5. Buntin, S., Malehmir, A., Koyi, H., Högdahl, K., Malinowski, M., Larsson, S. Å., Thybo, H., Christopher Juhlin, C., Korja, A. and Górszczyk, A. 2019. Emplacement and 3D geometry of crustal-scale saucer-shaped intrusions in the Fennoscandian Shield. Scientific Reports, 9, 10498.
6. Magee, C, Hoggett, M, Jackson, CA-L and Jones, S. M. 2019. Burial-Related Compaction Modifies Intrusion-Induced Forced Folds: Implications for Reconciling Roof Uplift Mechanisms Using Seismic Reflection Data. Fronters in Earth Science, 7, 37. doi: 10.3389/feart.2019.00037.
7. Magee, C., Muirhead, J. D., Karvelas, A., Holford, S. P., Jackson, C. A. L., Bastow, I. D., Schofield, N., Stevenson, C. T. E., McLean, C., McCarthy, W. and Shtukert, O. 2016. Lateral magma flow in mafic sill complexes. Geosphere, 12 (3), 809-841. doi: 10.1130/GES01256.1.
8. Chevallier, L. and Woodford, A. 1999. Morpho-tectonics and mechanism of emplacement of the dolerite rings and sills of the western Karoo, South Africa. South African Journal of Geology, 102, 43–54.
9. Polteau, S., Mazzini, A., Galland, O., Planke, S. and Malthe-Sørenssen, A. 2008. Saucer-shaped intrusions: Occurrences, emplacement and implications. Earth and Planetary Science Letters, 266, 195–204.
10. Larsen, H. C. and Marcussen, C. 1992. Sill-intrusion, flood basalt emplacement and deep crustal structure of the Scoresby Sund region, East Greenland. In: Storey, B. C., Alabaster, T. & Pankhurst, R. J. (eds) Magmatism and the Causes of Continental Break-up. Geological Society, London, Special Publications, 68, 365–386.
11. Eide C. H., Schofield N., Jerram D. A. and Howell J. A. 2017. Basin-scale architecture of deeply emplaced sill complexes: Jameson Land, East Greenland. Journal of the Geological Society, London, 174, 23-40. Doi: 10.1144/jgs2016-018.
12. Burchardt, S. 2008. New insights into the mechanics of sill emplacement provided by field observations of the Njardvik Sill, Northeast Iceland. Journal of Volcanology and Geothermal Research, 173, 280-288.
13. Gudmundsson, A., Pasquarè, F. A., Tibaldi, A., 2014. Dykes, Sills, Laccoliths, and Inclined Sheets in Iceland. Advances in Volcanlogy. 10.1007/11157_2014_1.
14. Geoffroy, L., Bergerat, F. and Angelier, J. 1994. Tectonic evolution of the Greenland–Scotland ridge during the Paleogene: new constraints. Geology, 22, 653–656.
15. Hansen, J., Jerram, D. A., McCaffrey, K. J. W., Passey S. R., 2011. Early Cenozoic saucer-shaped sills of the Faroe Islands: an example of intrusive styles in basaltic lava piles. Journal of the Geological Society, London, 168 (1), 159-178. doi: 10.1144/0016-76492010-012.
16. Hansen, J., Davidson, J. P., Jerram, D. A., Ottley, C. J. and Widdowson, M. 2019. Contrasting TiO2 compositions in Early Cenozoic mafic sills of the Faroe Islands: an example of basalt formation from distinct melting regimes. Earth Sciences, 8 (5), 235-267. doi: 10.11648/
17. Walker, R. 2016. Controls on transgressive sill growth. Geology. doi: 10.1130/G37144.1.
18. Galland, O., Spacapan, J. B., Rabbela, O., Maira, K., Sotod, F. G., Eikene, T., Schiuma, M. and Leanzag, H. A. 2019. Structure, emplacement mechanism and magma-flow significance of igneous fingers-Implications for sill emplacement in sedimentary basins. Journal of Structural Geology, 124, 120–135.
19. Hansen J., 2011. Petrogenetic Evolution, Geometries and Intrusive Styles of the Early Cenozoic Saucer-Shaped Sills of the Faroe Islands. Doctoral thesis, Durham University.
20. Gudmundsson, A., 2011a. Deflection of dykes into sills at discontinuities and magma-chamber formation. Tectonophysics, 500, 50-64. doi: 10.1016/j.tecto.2009.10.015.
21. Hatcher, R. D. 1995. Structural Geology, Principles, Concepts, and Problems. Prentice Hall, Englewood Cliffs, New Jersey, 07632. pp. 525.