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Llanbadrig Point RIGS Site
NRW RIGS no. 317
GeoMôn Global Geopark original webpage
RIGS Statement of Interest:
The rocks of Llanbadrig Point RIGS Site on the steep northern coast of this headland offer the finest view of the Gwna Melange in Anglesey. The melange consists of a jumble of blocks of rock, ranging in size from several tens of metres to a few centimetres across, mainly quartzite, limestone, and phyllite. The melange is thought to be the product of submarine slumping. As Greenly 1919 remarked “the many-coloured melange is really indescribable and must be seen in the field to be envisaged”. Apart from the three major rock types, the matrix includes a bewildering mozaic of rock types one within another. Grey limestone over 200 m across forms the largest olistolith in the mélange. This site is a good place to view the Ogof Gynfor unconformity between the Precambrian Gwna Group schists and quartzites (Greenly 1919) and the overlying Ordovician (Arenig) conglomerates of the Porth Cynfor Formation. Porth Wen, a short distance to the West exhibits a similar unconformity (see Graig Wen & Porth Wen RIGS). The unconformity is important in understanding the palaeogeography of this area in late Precambrian to early Ordovician times, a time span of around 100 million years and proves that during the intervening period some great earth movements occurred (orogeny) where the mélange which had accumulated in a deep sea trench was uplifted prior to the area being transgressed by the sea and new sediments being deposited at the onset of the Ordovician Period. (RIGS 317)
Geological setting/context: The Precambrian basement rocks of Anglesey and south-west Llŷn can be divided into several discrete groups, all of which were juxtaposed along a series of steep, brittle and/or ductile faults and shear zones (e.g. Dinorwic and Aber-Dinlle faults; Berw, Central Anglesey and Llŷn shear zones) collectively referred to as the Menai Strait Fault System (MSFS). First, the Monian Supergroup consists of a thick sequence of polydeformed metasediments and meta-igneous rocks, comprising the South Stack, New Harbour and Gwna groups, the latter representing the type example of a large-scale submarine debris flow or mélange said by some researchers to be of Lower Cambrian age. Ongoing research, however, may suggest a much older date for the Gwna Group with possible Cambrian ages being put forward for the South Stack metasediments. Second, the Coedana Complex of central Anglesey comprises high-grade metasediments, amphibolites and gneisses, and low-grade, thermally metamorphosed hornfelses adjacent to a granite (Coedana Granite), which has recently yielded a late Precambrian zircon age of 614 ± 4Ma. Third, a belt of schists and metabasites displaying blueschist facies grade of metamorphism lies within the MSFS. The metabasites exhibit a strong mid-ocean ridge basalt signature and have yielded ages of 580–590Ma. Fourth, the Sarn Complex in Llŷn comprises metagabbros and granite rocks which occur to the south-east of the Llŷn Shear Zone (LSZ), a continuation of the MSFS, which separates these igneous rocks from low-grade Monian mélange to the north-west. A late Precambrian zircon magmatic age of 615 ± 2Ma has been obtained from a metagabbro (LSZ). Fifth, on the mainland of north-west Wales, the Arfon Group comprises a thick sequence of tuffs and volcaniclastic rocks, dated at 614 ± 2Ma, which are conformably overlain by late Lower Cambrian siltstones. Correlatives of the Arfon Group may occur as isolated outliers on Anglesey and, if proven, would provide an important potential lithostratigraphical link across the MSFS. The stratigraphical correlation between the various units has proved highly controversial. The recent recognition of mylonitic rocks, for example in the LSZ, emphasises the presence of tectonic contacts and indicates that each component may represent a so-called ‘suspect terrane’ which was transported laterally into position along the major faults and shear zones. Ongoing unpublished research suggests, that Anglesey’s Precambrian rocks accumulated in accretionary prisms, providing a tectonic sequence rather than a stratigraphic sequence which was formerly accepted. This new research would reverse the accepted stratigraphic order established for the island. This Precambrian basement later formed the north-west margin of the Lower Palaeozoic Basin, the initiation of which was contemporaneous with Arfon Group volcanism. The timing of the inferred fault displacements has also been the subject of debate. Investigations on Llŷn have demonstrated that assembly of the basement terranes was completed at least by early Ordovician times since an unconformable Arenig overstep sequence has been identified at several localities such as Wig Bach, Parwyd and Mountain Cottage Quarry. The Arenig sequence of Anglesey and Llŷn is considerably less deformed and metamorphosed than the underlying basement, although this distinction is not everywhere obvious.
Deformation History of Anglesey: The deformation history of the Anglesey rocks is controversial. In the absence of reliably dated fauna or of radiometric dating, it is not clear which, if any, of the deformation phases is truly Precambrian, rather than Caledonian. At Rhosneigr, basal Arenig conglomerates contain clasts of New Harbour-looking pelites that exhibit small-scale folds; similarly at the faulted junction of the Ordovician and Mona Complex south of Point Lynas, at Porth Corwgl, the clasts in the Caradocian rocks have been reported to contain clasts of New Harbour Group aspect (or their equivalent, the Amlwch Formation of the north coastal area) with pre-existing folds. On the other hand, at Ogof Gynfor and Porth Wen, clasts in the cleaved Caradocian siltstones and sandstones above the unconformity with the Monian, contain no evidence of deformation that pre-dates that of the clasts and their matrix. The Monian and adjacent Ordovician rocks at all the above localities share a common, single-phase deformation history, with open to tight (depending on lithology) folds, trending NE–SW, upright , but verging somewhat to the SE. The geometry of the folds and cleavage in the Ordovician is very similar to that of the dominant deformation in the South Stack Group; the only exception to the general similarity of deformation style in the Monian and in the Lower Palaeozoic rocks is in the polyphase nature of the deformation of the New Harbour Group. If the correlation of the Anglesey outcrops of the Arfon Group with basal Cambrian sequence of the mainland is correct, their accumulation appears to have post-dated any strike-slip movements on the Dinorwic fault, which separates Anglesey and the mainland. Moreover, if a profound unconformity separates Anglesey Arfon Group from the underlying Monian, then at least those Monian rocks must have been deformed in the Precambrian or very earliest Cambrian.
Llanbadrig Point Coast RIGS: The cliff that backs the beach on the west side of Porth Padrig contains a phyllitic rock containing numerous quartz and limestone blocks and more rarely several jasper pits which are not associated with Pillow lavas as at Llanddwyn Island. Although, clearly, the Gwna Group rocks were deformed by tilting, and perhaps gentle folding, before the deposition of the Ordovician, on the evidence seen here there appears to be no reason to attribute this to a major orogenic event, associated with folding or cleavage-formation. Both rock units were affected by the Caledonian deformation, resulting in upright E-W folding and axial-planar cleavage associated with the horizontal shortening of clasts (about 33%) and their sub-vertical elongation. It should be emphasised that the identity and age of the supposed Gwna Group rocks at this site, in relation to those elsewhere in the Mona Complex, is not certain.
To select RIGS to demonstrate the Precambrian evolution of Anglesey and Llŷn, three separate networks were devised. These are: 1. Precambrian stratigraphy and structures. This category includes two sub-sets: a) Precambrian sedimentary structures; and b) tectonic structures, such as folds, faults and unconformities, which may have occurred during a tectonic event in Precambrian times or even later, for example, during the Caledonian Orogeny; 2. Precambrian palaeontology which includes any life-form and trace fossil, such as stromatolites, sponge spicules, worm burrows and bioturbated metasediments. Current research suggests that some of these fossils may be Cambrian or even Ordovician in age, but as these life-forms were previously held to be Precambrian in age, they have been included in this category. 3. Precambrian reference sections. These aim to represent all the important Precambrian rock types found in Anglesey and Llŷn. They include the major units mapped by Greenly (1920). The aim is to provide the best and most accessible exposure of the rock type. These can be considered as ‘type sections’. Where there is a relevant mineralogical, sedimentary, structural or other change across an outcrop, several representative sites have been chosen.
Llanbadrig Point Coast RIGS belongs to category 1b and is classed as a tectonic structure. The Precambrian Gwna Mélange rocks were formed as the result of a mixture of sea bed sediments and great chunks of rock sliding down into a deep sea trench at a destructive plate margin.
References:
BARBER, A.J. & MAX, M.D. 1972. A new look at the Mona Complex (Anglesey, North Wales). Geological Society of London 136, 407–432. B
BATES, D.E.B. 1972. The stratigraphy of the Ordovician rocks of Anglesey. Geological Journal .8,29–58.
GREENLY, E. 1919. Geology of Anglesey. Memoir of the Geological Survey of Great Britain. 2 volumes, London.
GREENLY, E. 1920. 1:50,000 and 1 inch to 1 mile Geological Map of Anglesey. Geological Survey of Great Britain, Special Sheet No. 92 and 93 with parts of Sheets 94,105 and 106.
Section B
Practical considerations:
Accessibility: Take the minor road that leaves the A5025 a short distance from the turn off to Cemaes town Centre on a lane signed to Gadlys Hotel and Llanbadrig Church. Opposite the church is a small car park for about 6 cars.
Safety: The path along the cliff and along Porth Padrig beach allows easy access to all relevant exposures and crosses the headland which is grazed by cattle. The normal precautions of working on cliff exposures and in coastal terrain should be observed, and the state of the tide should be monitored.
Conservation status: Currently, there is a GCR designation in the area covered by this RIGS, however, as yet, it has not been notified as a SSSI. It is in the Anglesey A.O.N.B. and the entire site is within an area owned by the National Trust.
Condition, use & management:
Present use: Porth Padrig and Llanbadrig Point Headland are not difficult to access, so it is ideal for students and the general public to examine a mélange, an unusual rock type which, although sedimentary in type, owes its origin to a tectonic process. Apart from students, the headland and beach are used by tourists, particularly in the summer, and by walkers who cross the site on the Anglesey Coastal Path. The land on the headland is used mainly for grazing purposes.
Site condition: The site is generally in good condition. The small quarry on the headland is associated with a derelict lime kiln, both of which are in good condition with no loose blocks of rock. In time, the lime kiln could become unsafe and this feature should be monitored.
Potential threats: Other than the potential deterioration in the condition of the limekiln and the rugged terrain down steep cliffs to the sea, there are no other obvious threats to the site. The fossils on the headland, included in another RIGS within the area, Ffynnon Badrig RIGS, are not obvious to a general passer-by and should not therefore attract unscrupulous collectors to the area.
Site Management: Ideally, a management plan would benefit from some form of interpretation to include all the RIGS on this headland.
Site development:
Potential use (general): This outstanding example of mélange, the unusual Precambrian ironstone and the fossiliferous limestone, all RIGS on this headland, and the unconformity on the hillside, a short distance to the west, could be part of a trail around the Anglesey coast. It is an ideal place for informing the general public of such matters and could be combined with a look at the historic church of Saint Badrig (Patrick) and the well, attributed to this saint. If the well is to be included in a trail, it would need steps incorporated in the cliff and some remedial repairs made to the roof of the cave to make the site safe for the general public.
Potential use (educational): The site is suitable for the general public and for school children as well as older students.
Other comments: Research notes This description starts at the eastern end of the peninsula and progresses along the top of the northern coast. As Greenly (1919, p. 306) remarked “the many-coloured melange …is really indescribable and must be seen in the field to be envisaged”. Similarly, the map shown here merely indicates the three major blocks of outcrop, which are characterised east to west by limestone, phyllite and quartzite; it makes no attempt to delineate the bewildering mozaic of rock types one within another. Grey limestone forms the principal rock type that makes up the eastern block of the melange, from
Site geometry: