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Mel Drury
This memoir and the 1:50 000 Series geological map that it describes are the products of a mapping contract between the Natural Environment Research Council and Cardiff University. The interpretations presented are those of the authors.
The authors acknowledge the supervision and encouragement provided by R A B Bazley (BGS), the excellent editing support provided by T Charsley, and valuable discussions in the field with D Bate (BGS), M J Branney (University of Leicester), T Druitt (University of Clermont Ferrand) and M F Howells (formerly BGS). Professor M Brooks (Cardiff Univeristy) provided help with the seismic surveying and as Head of Department was highly supportive of this mapping contract. P T Leat and R Thorpe (Open University) kindly lent thin sections of rocks with published analyses. G Tegerdine (Production Geoscience Ltd.) and A Beckly (British Petroleum) loaned their maps of Arenig outcrops. H Ivimey-Cook facilitated examination of the Matley fossil collection in the BGS. The authors also acknowledge discussions with A W A Rushton and J Zalasiewicz (BGS) on aspects of Ordovician biostratigraphy, and with D J Hughes (Portsmouth) on igneous geochemistry. W Gibbons would particularly like to thank G M Power (Portsmouth) and A J Reedman (BGS) who together initiated and supported both his early work on Llŷn and the subsequent preparation of BGS Memoirs 133 and 134. The memoir was edited by A A Jackson. Figures were produced by R J Demaine, P Laggage and G Tuggey, BGS Cartography, Keyworth.
This memoir and the new 1:50 000 Series geological map Sheet 134 Pwllheli that it describes are the products of a mapping contract awarded to Cardiff University by the Natural Environment Research Council. The funding provided allowed for the continuation of fieldwork from the adjacent Aberdaron sheet. The contract stems from a NERC policy of encouraging academic researchers to transfer their data and information into the public domain. In addition to the map and memoir, the detailed field maps and other material will be lodged in the National Geosciences Record Centre at BGS Keyworth.
The Llŷn Peninsula with its rocky coastline along the north coast of Cardigan Bay and views towards Snowdonia and the mountains of central Wales, the Rhinogs and Cadair Idris, is one of the most beautiful parts of Wales. The geology described here includes Precambrian mlange, Cambrian sedimentary rocks, Ordovician volcanic centres and the well-exposed Quaternary sediments that record past ice movement from both the north and the east; it provides interest for both academic and amateur geologists.
The Pwllheli district lies on the Llŷn Peninsula in north-west Wales (Figure 1), (Figure 2). The coastline includes both wide, sand and gravel beaches backed by Quaternary glacial deposits, and low rocky shores and high cliffs that expose the Lower Palaeozoic succession. Inland, exposure is poor, low hills and drift plains are mainly cultivated, and only the higher hills remain as rough pasture. The main towns, Pwllheli and Abersoch, are heavily dependent on the tourist industry for employment.
The Mynydd Tr-y-cwmwd to Garn Fadryn range divides the district into two catchment areas. West of the hills drainage is via the incised valleys of Nanhoron, Nant Llaniestyn and Sarn Meyllteyrn into the Soch as it swings eastwards across the plain behind Porth Neigwl. Despite being only 800 m from the coast near Llanengan, the Soch flows north and then eastwards to eventually reach the sea at Abersoch (Figure 2). East of the Fadryn range of hills, drainage is mainly via Afon Penrhos, Afon Rhyd-hir and Afon Erch into Pwllheli harbour, although the Cors Geirch drains north-westwards towards the north coast of Llŷn.
This memoir extends and develops several of the observations and interpretations on both solid and drift geology instigated during the remapping of the adjoining Aberdaron area by Gibbons and McCarroll (1993).
During late Ordovician times, major volcanism developed across much of North Wales, with magmatic activity being recorded in Llŷn as three successive magmatic centres through late Soudleyan to Woolstonian times, and each of these has left some record in the Pwllheli district. These centres are referred to, in order of decreasing age, as Llywd Mawr (in the north-east), Upper Lodge (in the north), and Llanbedrog (centred on the Pwllheli district). The lowermost Caradoc strata in the district are conglomeratic sandstones (Cwm Eigiau Formation) and ashflow tuffs (Pitts Head Tuff Formation), both associated with the development of the Llwyd Mawr centre. Above the Pitts Head Tuff Formation, the upper part of the Cwm Eigiau Formation comprises early Longvillian marine siltstones with subordinate basic tuffs and sandstones (Figure 3).
In the north-west of the district, the lowermost Caradoc strata rest unconformably upon the Nant Ffrancon Subgroup and comprise basaltic trachyandesites (Upper Lodge Formation) erupted from a transitionally alkaline volcanic centre, thought to have been located near Nefyn. These volcanic rocks are interpreted as coeval with a prominent ashflow tuff (Allt Fawr Rhyolitic Tuff Formation) which extends farther east across the Pwllheli district. Both of these volcanic units are overlain by shallow marine siltstones and sandstones (Dwyfach Formation), of Longvillian age (Figure 3).
A further phase of intermediate volcanism (Penmaen Formation), was the forerunner of a second major igneous centre of mildly alkaline nature, this time centred on Llanbedrog. This centre is of Woolstonian age, developed in the area south of the Efailnewydd Fault, and produced thick volcanic deposits which dominate the upper Ordovician geology of the district. In the western and south-western parts of the district, the basaltic trachyandesitic and dacitic lavas and tuffs of this formation are up to 300 m thick, and are overlain by the more acidic tuffs and lavas of the Foel Ddu Rhyodacite Formation, which has a maximum thickness exceeding 250 m. This trend of increasingly evolved volcanic products was continued by the eruption of two major rhyolitic tuffs, the Nant-y-Gledrydd and Bodgadle members of the Carneddol Rhyolitic Tuff Formation. These tuffs together are up to 500 m thick between Llanbedrog and Madryn, an area interpreted as lying within a Llanbedrog caldera, but are represented by only up to 45 m of rhyolitic tuff at Pwllheli, outside the caldera. The volcaniclastic sedimentary apron to the Llanbedrog volcanic centre, the Yoke House Formation, passes progressively to the north-east into the fine-grained clastic sediments of the upper part of the Dwyfach Formation (Figure 3).
Following the Carneddol Tuff eruption there was regional subsidence, and graptolitic mudstones were deposited across the district (Nod Glas Formation). These mudstones have intercalated basaltic volcanic rocks associated with the east-west Efailnewydd Fault, running approximately along the northern margin of the former Llanbedrog volcanic centre. In contrast to the earlier more alkaline volcanicity, these basic rocks are markedly tholeiitic. Evidence for the age of the youngest parts of the Nod Glas Formation is poor, and no definite linearis Biozone faunas have been recorded. The district appears to have been starved of sediment, and rocks of latest Caradoc and early Ashgill age are probably not represented.
The youngest Ordovician rocks in the district are the calcareous siltstones and mudstones of the Crugan Formation, which were deposited in a relatively deep-water outer-shelf environment. This formation defines the base of the Powys Supergroup of Woodcock (1990). There was probably a considerable lapse of time between the deposition of these sediments (the base of which is of Rawtheyan age) and the underlying Nod Glas Formation. The renewal of sedimentation may have been associated with a major reactivation of the fault systems of the Welsh Borderlands, and probably elsewhere in the Welsh Basin, recorded in mid-Ashgill times.
The Caradoc volcanic rocks were contemporaneous with a wide variety of intrusive igneous rocks. The basic rocks are represented by numerous dolerites, particularly around the town of Pwllheli itself, and by the layered igneous complex at Rhiw, which is probably also of this age. Acid intrusive rocks include the Mynydd Tr-y-cwmwd Granophyric Microgranite, the Wyddgrug Porphyritic Microgranite, a peralkaline suite (Nanhoron Suite) including the Mynytho Common, Foel Gron and Nanhoron granophyric microgranites, and an alkaline suite (Carn Fadryn Suite) including the Garn Bach Dacite, the Glynllifon Trachydacite and the Carn Fadryn Quartz-microdiorite.
The large-scale features of the Pwllheli district landscape, particularly the steep-sided hills, probably predate the Pleistocene glaciations, but their origin and the timescale involved in their development is the subject of debate. The classical interpretation involves marine erosion of a Cainozoic (Tertiary) peneplain, but a more recent interpretation suggests a much longer history of Mesozoic and Cainozoic differential weathering under tropical conditions following emergence as early as the Triassic.
The oldest rocks of the Pwllheli district crop out in the north-west, from around Tudweiliog [SH 237 368], past Mynydd Cefnamwlch [SH 230 340], to north-west of Mynydd Rhiw [SH 225 296] (Figure 1), (Figure 2). These exposures form a small part of a broader outcrop of similar rocks that runs for 23 km south-westwards from Nefyn to Bardsey Island on Llŷn, and continues over a much larger area on Anglesey (the Mona Complex of Greenly, 1919). On Llŷn these rocks are better exposed in the adjacent Bardsey district (Gibbons and McCarroll, 1993) where they comprise the Gwna Mlange and the Sarn Complex, these being separated by the near-vertical Llŷn Shear Zone which is interpreted as a transcurrent mylonitic terrane boundary (Gibbons, 1983, 1987). In the Pwllheli district, only Gwna Mlange and Sarn Complex rocks are exposed, but the Llŷn Shear Zone is assumed to occur beneath the drift cover.
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