Chapter 1 Stratigraphy and plate tectonic models Stratigraph y and plate tectonic models 1 In North Wales we can identify three geologically thicknesses of muds, now converted to distinct terranes which developed before the slate, which contrast with the coarser Ordovician period: sediments of the Harlech Dome. This is known as the Arfonian terrane. Deformed and metamorphosed rocks of late Precambrian to Cambrian age These early terranes developed separately but forming the north-western part of the were brought together at the end of Cambrian Lleyn peninsula. These include gneiss, times by lateral movements along the major schist and tectonically deformed slump crustal fractures of the Menai Strait Fault Zone. deposits known as mélange. This is known as the Monian terrane. Throughout the Ordovician and Silurian periods, A thick sequence of Cambrian sediments North and Mid-Wales formed a single terrane, dominated by coarse grits and sandstones with similar geological processes operating in the central Harlech Dome and St simultaneously across the whole of the region. Tudwal's peninsula of Lleyn. These rocks Much of the Ordovician was dominated by the were formed in a marine basin, mainly by eruption of volcanics, whilst the Silurian saw a turbidity currents, and are relatively return to the quiet deposition of great thicknesses undeformed. This is known as the of sediment. Megumia terrane. In the following sections, we outline the general Cambrian rocks in the Arfon area of stratigraphy of each of the terrane blocks, then northern Snowdonia containing large discuss plate tectonic models which could account for their origin. Menai Strait fault system Snowdonia Arenig Bala fault Lleyn Harlech Dome Arans COVER TERRANE Silurian Cader Idris Ordovician sedimentary Ordovician igneous BASEMENT TERRANES Monian terrane Megumia terrane Plynlimon Arfonian terrane Figure 2: Terranes of North Wales 5 Geology Field Studies from Lleyn to Plynlimon Monian of the Lleyn peninsula A sequence of heavily deformed sedimentary and unstable conditions. These deposits igneous intrusive and extrusive rocks are found in originated as bedded shelf sediments along a Anglesey and the Lleyn peninsula. All show some continental margin, then slumped into the degree of metamorphism, reaching schist and deep marine trench along a subduction zone. gneiss grade in places. This whole sequence has In contrast to the low grade metamorphic rocks of been termed the Mona Complex. The group was the Gwna Group, a band of high grade gneisses originally considered to be Precambrian, but some and associated igneous rocks crosses central Lleyn, components have now been shown to be where unfortunately the exposure of outcrops is Cambrian in age. poor. These rocks, known as the Sarn Complex, The lowest grade of metamorphism is seen in have been shown by radiometric dating to be rocks of the Gwna Group, exposed in a series of Precambrian in age. They represent the spectacular cliffs and bays along the north-west continental crust of the early island arc of coast of the Lleyn peninsula. Avalonia, below which the later subduction of oceanic crust took place during Gwna Group times. The rocks of the Gwna Group can be divided into two contrasting types: An unusual rock type observed in the Mona Complex of Lleyn is a schist showing a strong Ocean plate stratigraphy: basalt pillow deformation fabric and containing minerals lavas and related deep ocean floor formed under very high pressure. It is thought that sediments, thought to represent the metamorphism took place as sediments were remnants of ancient oceanic crust which dragged downwards by the subducting plate, but were scraped from a descending oceanic later returned to the surface due to buoyancy of plate at a subduction zone. the rock slab. Mélange: disturbed large scale conglomerates showing evidence of deposition under tectonically Nefyn Gwna group, mainly metasediments Gwna group, mainly basalts Schists of the Lleyn shear zone Porth Colmon Sarn complex gneiss and igneous intrusions Ordovician basic intrusions Ordovician, mainly sediments Porth Oer Pwllheli Botwnog Abersoch Uwchmynydd Aberdaron Figure 3: Geological structure of the western Lleyn peninsula 6 Chapter 1 Stratigraphy and plate tectonic models Figure 4: Rocks of the Mona Complex, Lleyn peninsula: (upper left) Basalt pillow lavas of the Gwna Group (upper right) Large scale slump deposits of the Gwna Group, known as mélange. (lower left) Deformed schist rocks of the Lleyn shear zone. (lower right) Mafic gneiss. Cambrian of the Harlech Dome and St Tudwal's peninsula Grits Group at its core. Within this group, the The main Cambrian outcrop lies inland from the massively bedded and resistant Rhinog Grits form coast of Cardigan Bay in a region known as the a range of high mountains. Folding complicates the Harlech Dome, extending from Barmouth in the outcrop pattern to some extent, but there is a south to Porthmadog in the north, and inland to general progression outwards towards the Trawsfynydd (fig. 5). younger Cambrian sediments of the Mawddach During Cambrian times, much of North Wales was Group. These thinly bedded and finer grained occupied by a marine basin into which sediments rocks have been eroded into deep river valleys in were deposited from the surrounding landmasses. the picturesque forested area of Coed y Brenin. The Cambrian rocks in this area can be divided into Typical lithologies of Cambrian rocks are shown in a lower sequence, the Harlech Grit Group, and an fig. 7, representing a general progression in the upper sequence, the Mawddach Group (fig. 6). Harlech Dome from coarse sandstones, often The Harlech Dome has been deeply eroded after containing pebble conglomerates, through bedded uplift, exposing the oldest rocks of the Harlech siltstones, to fine grained mudstones. 7 Geology Field Studies from Lleyn to Plynlimon Blaenau Ffestiniog Figure 5: Geology of the Harlech Dome Maentwrog Porthmadog Trawsfynydd Harlech Rhinog Mesozoic and Tertiary mountains other Ordovician Coed Dol-cyn-afon formation y Brenin Clogau, Maentwrog, Ffestiniog, Dolgellau Barmouth, Hafotty, Rhinog formation Llanbedr formation Dolwen formation Dolgellau Basalt lavas of the Rhobell formation Barmouth intermediate and silicic intrusions mafic intrusions Fairbourne Dolgellau fm. Upper Ffestiniog fm. mudstone Cambrian Mawddach siltstone Group Maentwrog sandstone fm. sandstone Clogau fm. with Gamlan fm. conglomerate Middle volcanic ash Cambrian Barmouth fm. Hafotty fm. Harlech Grits Group Figure 6: Cambrian succession Lower Rhinog fm. in the Harlech Dome Cambrian Llanbedr fm. Dolwen fm. Precambrian Bryn-teg fm. 8 Chapter 1 Stratigraphy and plate tectonic models Figure 7: Cambrian rocks of the Harlech Dome and St Tudwal's peninsula: (upper left) Sandstone and conglomerate of the Barmouth Grit, Barmouth (upper right) Thinly bedded mudstone and siltstone of the Maentwrog formation, Barmouth. (lower left) Mudstones showing slaty cleavage, Dolgellau formation, Barmouth. (lower right) Ripple bedded sandstone, Ffestiniog formation, St Tudwal's peninsula. A small outcrop area of the Cambrian marine basin The succession begins with the Padarn tuff, a sediments also occurs on the southern coast of volcanic ash deposit which has been Lleyn at St Tudwal's peninsula. Some upper radiometrically dated as Precambrian age. Cambrian sediments in this area show The overlying Fachwen formation is a sandstone characteristics of deposition in shallow water, so and conglomerate derived largely from erosion of the coastline of the basin may have been near to the volcanoes which formed the Padarn tuff, and is the west of Lleyn at this time. again of Precambrian age. These beds are overlain The Arfon Basin by the Llanberis Slates formation of definite Cambrian age. Another succession containing Cambrian rocks The muds which produced the Llanberis slates outcrops in the north of Snowdonia, but has very were deposited in a down-faulted marine basin. different characteristics to the sediments of the This basin was probably separated to the north- Harlech Dome and St Tudwal's peninsula. This east of the Harlech basin along the line of the succession, in an area known as the Arfon Basin, Menai Fault zone. The Arfon basin succession is dominated by a large thickness of high quality forms a distinct terrane block which has been slate which has been extensively quarried in moved to its present position by transverse Nantlle, Llanberis and Bethesda. faulting along the fracture zone. 9 Geology Field Studies from Lleyn to Plynlimon Figure 8: Geology of the Arfon basin Bangor Carboniferous Ordovician Marchlyn formation Bethesda Bronllwyd formation Llanberis slates Fachwen formation Padarn tuff Caernarfon Twt Hill granite Llanberis Snowdon Nantlle Figure 9: Rocks of the Arfon basin: (upper left) Padarn tuff, Llanberis (upper right) Conglomerate of the Fachwen formation, Moel Tryfan. (bottom left) Llanberis slate, Llanberis (bottom right) Bronllwyd Grit, Cwm Grainog 10 Chapter 1 Stratigraphy and plate tectonic models The upper Cambrian rocks of the Arfon basin, the Volcanic activity spread progressively northwards Bronllwyd and Marchlyn formations, are to the Arenig and Moelwyn areas, culminating in a composed of coarser sediments. They have some major phase of eruptions in central Snowdonia similarity to the Ffestiniog formation of the during the Caradoc epoch of the later Ordovician. Harlech Dome, so there is a possibility that the The Ordovician volcanic rocks around the south fault movements which brought