From Garnets to Gannets, exploring the Lonely Isles
- 20 Jul 2022
- Start time
- 2:30 PM
- Tempest Anderson Hall
- Dr Anna Bird, University of Hull
From Garnets to Gannets, exploring the Lonely Isles
Dr Anna Bird, University of Hull
Project Team: Anna Bird and Eddie Dempsey, Department of Geography, Geology and Environmental Sciences, University of Hull
Tim Armitage and Bob Holdsworth Department of Earth Sciences, Durham University
The Lonely Isles are series of well exposed, yet extremely remote islands that give a unique and previously unknown insight into the ancient geology of northern Scotland. These rocks are part of the Precambrian basement that underlies much of northern Scotland, including Orkney and Shetland, and parts of Greenland.
These Precambrian rocks also underlie the Clair Oil Field, which is situated to the west of Shetland, and represents the largest hydrocarbon resource in the UKCS and Europe. The hydrocarbons of the Clair field are found within Devonian-Carboniferous sandstones sitting on a ridge of this Precambrian metamorphic basement as well as within fractures in the basement rocks themselves. Despite the economic and scientific importance of these rocks, our understanding of the basement geology in these regions is extremely limited. Recent work that has been undertaken basement core samples retrieved from over several hundred km2 beneath the Atlantic Ocean suggest that the basement rocks here are Neoarchaen in age (ca 2-7-2.85 Ga) and that they lack the widespread Proterozoic “Laxfordian” event (ca. 1.6-1.75 Ga) seen in mainland Scotland and Outer Hebrides. This requires the presence of a northern “Laxford Front’ somewhere N of the Scottish mainland.
The Lonely Isles are the only place where these basement rocks outcrop above sea level providing a rare opportunity to examine and understand these enigmatic rocks. As such, understanding the geology of these islands is key to understanding the location and nature of this theorised terrane boundary between the Scottish Mainland and the basement rocks found under the Clare Oil Field.
In November 2018, a team of geologists from University of Hull and Durham University went to the Flannan Isles, North Rona and Sula Sgeir. The team, Dempsey, Bird and Armitage, (funded by Prof Holdsworth of Durham University) are the first geologists to examine the Flannan Isles since 1933 or North Rona since 1958. Many geological fundamental concepts have been developed or refined (including plate tectonics) in this time. This talk will show the highlights of the expedition and some of the initial findings of this study as well as future research and fieldwork plans.
This lecture was held in the Tempest Anderson Lecture Theatre in the Yorkshire Museum, York at 2.30pm and included the announcement/presentation of the 2022 John and Anne Phillips prize to a third year geologist from the University of Hull.
The main focus of the lecture was the findings of a very short geological expedition in November 2018, from the Universities of Hull and Durham. The expedition studied the ancient geology of the Hebridean Terrane, that outcrops only in the Lonely Isles off the Northwest of Scotland. Brief visits were made to Eilean Mor of the Flannan Islands, North Rona, and Sula Sgeir – all small uninhabited islands in the region of interest. The fieldwork was supplemented by aerial photographs taken by drones, and by laboratory analyses of the samples obtained.
However, the lecture was far-ranging, and also covered the expedition’s boat and air transport challenges; the short time window due to the bird nesting season; the bad weather; the archaeological evidence of earlier inhabitants; and the wildlife of the islands – including the gannets of the title (the garnets were used to date the last time the rock was heated to over 800 deg C).
Surprisingly, there was little earlier off-shore geological data in this area, though bore-holes had established ages of the various strata. Also, previous aerial photography had identified metamorphic rocks and had suggested ancient tectonic events in the area, and hydrocarbons had been found in the Rona Ridge. Detailed Digimaps had been produced, and proved valuable in the planning for the 2018 expedition.
The main aim of the Hull/Durham expedition was to obtain additional data pertinent to the Clair Oil Field – the largest hydrocarbon resource in Europe – which is underlain by the c. 2.7-million-year-old Lewisian gneiss complex, a suite of Precambrian basement rocks that stretches from NW Scotland to Greenland. The fieldwork studied the nature of the rocks, linear features and their orientation. Later lineament analysis of the data studied the linear structures of faults folds and fractures for indications of ore, oil and groundwater. Deep fractures are the sources of hydrocarbons and also opportunities for carbon capture and storage, so are of great economic importance.
From a geological perspective, the most interesting island studied was North Rona. Pegmatites, which contain potentially valuable minerals such as lithium, caesium, tantalum, niobium etc, gave evidence of high temperatures in the past. Other earth resources included copper, gold and silver. Mafic gneiss contained many garnets. Metasediments with many garnets were found in the north, and metasediments with whale-back folds were found in the south. Dr Bird showed photographs of many geological faults that provided evidence of geological phenomena such as foliation, thrusts, fold hinges, lineation of minerals, and later brittle structures. It concluded that there were different periods of compression and of tensile extensions, but that the island geology was dominated by a NNE-SSW compression. The field work was later supplemented by extensive laboratory studies of the many mineral samples taken. Microscopy of thin sections identified garnets, zircon, and titanite. Mass spectroscopy measured the levels of many isotopes, and U-Pb dating confirmed the ages of the strata.
The team produced a detailed geological map of the island, and concluded that there had been episodic mountain building over a very long time period. Their work provided valuable data for developing the Clair Oil Field, and proved much more cost effective and timely than the alternative approach of using geological survey ships.
Rod Leonard/Paul Thornley