Abstract
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.
Highlights
Sills may occur in a wide variety of forms, extent and thicknesses
When it comes to sills in rifted or passive sedimentary settings in offshore areas, seismic imaging associated with prospective activity has revealed the wide variety of sills in such settings, including sills as isolated bodies and internally connected sills in entire sill complexes, enabling geologists to estimate/interpret intrusion mechanisms prevailing in such settings [5]
The aim of this work was to assess potential optical effects associated with observation of igneous sheets in rugged mountainous terrains from some distances, with the main focus on potential differences in physical appearances of these according to changes in vertical and/or lateral angles of observation
Summary
Sills may occur in a wide variety of forms, extent and thicknesses. When it comes to sills in the upper crust, these are commonly thought of as either rather simple magma storage champers or simple conduits of magmas ascending through the crust, or they may form part of complex and intricate sill complexes capable of lateral magma transport as well as transport of magmas to storage chambers higher in the crust or to surface magmatism [1,2,3,4,5,6,7]. Sill systems have most commonly been reported in rift-related or passive sedimentary settings, they are not uncommon in volcanic settings, e.g. areas affected by basaltic magmatism such as in flood basalt provinces [3]. When it comes to sills in rifted or passive sedimentary settings in offshore areas, seismic imaging associated with prospective activity has revealed the wide variety of sills in such settings, including sills as isolated bodies and internally connected sills in entire sill complexes, enabling geologists to estimate/interpret intrusion mechanisms prevailing in such settings [5]. Well known examples of examinations of onshore sills in various settings include the Karoo Basin, South Africa [8, 9], Jameson Land, East Greenland [10, 11], Iceland [12, 13], the
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