Geological Mapping of South Kerrera

Geological Mapping of South Kerrera Laura Briggs Newnham College, University of Cambridge 14/01/2015 View across the south coast of Kerrera, facing east. Word Count: 4,998 Geological Mapping of South Kerrera I declare that the submitted work is my own, except where acknowledgment is given to the work of others or to work done in collaboration. I declare that I have read and understood the Department of Earth Sciences statement on plagiarism. Signed: Date: 1 Laura Briggs Abstract This report describes the outcome of 28 days of geological mapping on the Isle of Kerrera, Western Scotland. The aim was to understand the geological history of Kerrera, particularly the paleoenvironment during the Devonian. Kerrera’s geology consists of basal Dalradian slates (Easdale Slate Formation) unconformably separated from Devonian sediments (Kerrera Sandstone Formation) and lavas (Lorn Plateau Volcanic Formation). Tertiary basaltic dykes oriented northwest are the youngest lithology. The slates show centimetre to kilometre scale folding with a consistent axial orientation to the northeast; the sediments dip gently to the west. The transtensional Gylen Fault on the east coast bounds the Ardchoirc Basalt, emplacing it within the slates. The fault formed under the same stress regime that caused the Great Glen Fault, which runs parallel to the Gylen Fault approximately 10km to the west. The Kerrera Sandstone Formation comprises of a sedimentary breccia, conglomerates, sandstones and siltstones deposited in a continental environment. The patchy sedimentary breccia lies on the unconformity, beneath an alluvial fan conglomerate. Further up the sequence are fluvial sandstones and conglomerates, and lacustrine siltstones which contain significant vertebrate and plant fossil material. Basaltic and andesitic lavas of the Lorn Plateau Volcanic Formation have a calc-alkaline composition and were deposited sub-aerially. Interaction between lava and unconsolidated sediments created peperites; the lavas were emplaced contemporaneously with the sediments. The Easdale slates record deformation from the Grampian Orogeny, whilst sediments allow reconstruction of the Devonian paleoenvironment. Kerrera is therefore an important area for understanding regional events in Scottish geological history. [Word count: 249] 2 Geological Mapping of South Kerrera Contents Section Pages 1. Introduction 4-6 1.1 Geological and Geographical Setting 1.2 Previous literature 2. Stratigraphy 7-16 2.1 Introduction 2.2 Easdale Slate Formation 2.3 Kerrera Sandstone Formation 2.4 Lorn Plateau Volcanic Formation 2.5 Tertiary Dykes 3. Structure 17-26 3.1 Introduction 3.2 Easdale Slates 3.3 KESA Sediments 3.4 Large scale faulting 3.5 Small scale faulting 4. Sedimentology 27-36 4.1 Introduction 4.2 Conglomerates 4.3 Sandstones 4.4 Siltstones 4.5 Interpretation 5. Igneous Geology 37-48 5.1 Introduction 5.2 Composition 5.3 Emplacement 5.4 Timing 6. Synthesis 49-58 6.1 Introduction 6.2 Paleozoic: Easdale Slate Formation 6.3 Devonian: Deposition of the ORS sediments and lavas 6.4 Tertiary: Dykes and faults 6.5 Quaternary: Glaciation References 59-60 Acknowledgements 61 3 Laura Briggs 1. Introduction 1.1 Geological and Geographical Setting The area mapped was the southern half of the Isle of Kerrera, Inner Hebrides of Western Scotland (Figure 1). Kerrera is located 500m offshore from the mainland town of Oban and can only be reached by ferry. 50km Isle of Kerrera 2km m Figure 1. Maps to show location of the Isle of Kerrera. Large scale map shows the position of Kerrera on the west coast of Scotland in the Inner Hebrides. The inset map shows Kerrera’s location between the mainland town of Oban and the Isle of Mull. Maps and scales from Google Maps. The geology of Kerrera comprises a basement of folded Dalradian slates (Easdale Slate Formation) separated from Devonian sediments (Kerrera Sandstone Formation) by a major unconformity. Basaltic and andesitic lavas (Lorn Plateau Volcanic Formation) erupted synchronously with the sediments. The youngest rocks are northwest trending Tertiary basaltic dykes. 4 Geological Mapping of South Kerrera The Gylen Fault is the only fault of notable displacement on Kerrera, and has emplaced the Ardchoirc Basalt amongst the slates on the east coast. The fault formed under a transtensional regime, allowing strike-slip and dip-slip components of motion. The aim of this project was to make a geological map of the southern half of Kerrera in order to understand the geological history of the area and how it relates to the regional geology of Scotland. In particular, this report will focus on the depositional environment during the Devonian and the interaction between the Kerrera Sandstone Formation and Lorn Plateau Lavas. Kerrera is geologically important because it provides evidence for many regional events in Scottish geological history, such as the Grampian Orogeny and the opening of the North Atlantic. Whole specimens of previously unknown vertebrate species have been found in the siltstone Fish Beds of the Kerrera Sandstone Formation, allowing biostratigraphic dating and helping reconstruct the paleoenvironment during the Devonian. The terrain of Kerrera consists of steep hills of variable exposure (maximum elevation 189m), separating marshy lowlands with poor exposure. Vegetation is scrubby and inland access variable due to lack of paths; coastal areas have well exposed beach sections, but few cliff sections. I mapped in a group of three, with Krishan Patel (Homerton College) and Holly Zhang (Emmanuel College). In this report I will first describe the stratigraphy, before outlining the structures in the area. I will then discuss the sedimentology, followed by an analysis of the igneous geology. Finally, I will summarise the geological history of the area, drawing on the evidence recorded during the project. 5 Laura Briggs 1.2 Previous literature The first summary of Kerrera’s geology and palaeontology was the 1925 memoir of the Geological Survey (Lee and Bailey, 1925), where it is first suggested the Fish Beds were deposited in lake environments, formed in topographic hollows. Interest in the Fish Beds continued after discovery of further specimens in 1962-3, and it was suggested the fish inhabited brackish water in a tidal basin (Tarlo and Gurr, 1964). This was not accepted and instead there is now general agreement the Fish Beds were deposited in a lacustrine environment. Trewin et al (2012) suggest lakes formed by temporary damming of the fluvial system which was depositing sandstones and conglomerates. Durant (1999) describes the interaction between the Devonian sediments and lavas, which show evidence of eruption into unconsolidated sediment and sub-aerial flow. 6 Geological Mapping of South Kerrera 2. Stratigraphy 2.1 Introduction Kerrera is based in the Easdale Slate Formation, unconformably separated from the Devonian Kerrera Sandstone Formation (KESA). Basaltic and andesitic lavas of the Lorn Plateau Volcanic Formation (LPVO) were also emplaced during the Devonian. Tertiary basaltic dykes are the youngest lithology. Figure 2 demonstrates the age relationships between the lithologies. As the sequence and thickness of the sediments is variable across the island, the stratigraphic column is generalised. In this section I will describe the appearance and features of each lithology in turn. Figure 2. Stratigraphic column to show age relationships between lithologies of Kerrera. Beds of the KESA Formation are laterally discontinuous and therefore bed thicknesses and their sequence are schematic, as are lavas emplaced within the sediments. 7 Laura Briggs 2.2 Easdale Slate Formation (ME); Argyll Group, Dalradian Supergroup The Easdale Slate Formation outcrops on the east coast, Eilean Orasaig and Barnabuck. The description in Lee and Bailey (1925) matches the field appearance, so the lithostratigraphic name is used. Neoproterozoic and Early Cambrian forms have been found in the Easdale Slates, dating them as of Vendian age (Downie, 1971). The type section for the Easdale Slates is the east coast. The slates are dead black and pyritous with distinct slaty cleavage, breaking the rock into 5-10mm sheets. Elsewhere, the slates have a silvery phyllitic sheen, but the formation was not sub-divided. The slates are very fine grained; the only visible mineral is pyrite of size 1-10mm (Figure 3). A thin section (Figure 4) of phyllite shows crenulation cleavage formed by white mica, with porphyroblasts of quartz, plagioclase, mica and pyrite following the fabric. Figure 3.Pyrite crystals showing characteristic cubic morphology, 3-6mm in length. This example is from Eilean Orasaig [79501 26673] and shows the golden colour expected for pyrite. Elsewhere on Kerrera, the pyrite found was often tarnished and blackened. 8 Geological Mapping of South Kerrera Figure 4. Thin section description of phyllite of the Easdale Slate Formation. The top half of the diagram shows appearance of minerals under crossed polars, whilst the bottom is in plane polarised light. Prominent cracks both with the fabric and across it would have transported fluid, and may have been re-opened whilst making the thin section. The thin section displays the crenulation cleavage marked by the micas in the groundmass. Sample shared with mapping group. The slates (beds 20-50cm) are interbedded with black limestone (beds 10-80cm), which often has prominent quartz veins but no allochems. Both slate and limestone weather with a brown colouration, and the limestone with honeycomb weathering. The formation is

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