unravelling the Precambrian….
Setting the scene: the Precambrian….
Anglesey has an amazing diversity for such a small area of well exposed and easily accessible geological formations and is perfect for a geopark and a centre for geological exploration, education and enjoyment. A large area of the island consists of exposed Precambrian rocks – or at least rocks that have been considered to be of Precambrian age though accurate dating was impossible until very recently. This group, known as the Mona Complex (Mona, the ancient name for Anglesey), comprises green and blue schists, gneisses, pillow lavas and associated fragmentary rocks, the Coedana granite and gneisses as well as unspecified meta-sediments and coarse quartzites; it probably underlies most if not all of the island. Boundaries with adjacent rocks are in most places obscured and field relationships have to be inferred, permitting a range of interpretations which only adds to the interest. This is the largest area of Precambrian rocks in England and Wales.
Geologists have been debating Anglesey’s geology since the early 19th Century, when the first attempts to describe the island’s rocks were made. Nearly a century later, the island was mapped, privately, on a 6 inches to the mile scale, by Edward Greenly; on seeing his superb maps, the Geological Survey decided to publish his work as a one-inch map and a 2-volume Memoir, these being printed in 1919.
GeoMon has published a detailed and lavishly illustrated account of the history and local geology of the island including 14 geotrails. Available from the e-shop for £30 plus p&p.
Campbell, S. Wood, M & Windley, B. (2014) Footsteps through time; the rocks and landscape of Anglesey explained. Isle of Anglesey County Council 193pp.
The controversy since Greenly’s time has been the interpretation of the pre-Arenig rocks on the island. Thought at one time to be entirely Precambrian, it is now known that they straddle the period between late Precambrian and late Cambrian times, between about 700 and 500 million years ago.
These old rocks, consisting mostly of turbidites, debris-flow deposits, metabasites, granites and gneisses, with metamorphic grades varying from blueschist to greenschist to amphibolite facies, clearly represent parts of an ancient destructive plate margin. The problem lies in the interpretation of how each unit is related to the others, for they are typically separated by major structural breaks, steep zones of shearing and mylonite development, indicating that they have arrived in their current juxtaposition by tectonic movements. The trouble is, we don’t know how far they have travelled to get where they are now.
Precambrian (4,600 – 542 Ma)
The Precambrian interval of geological time covers the immense time span between the birth of planet Earth, about 4.6 billion years ago, to the start of the Palaeozoic Era (‘interval of ancient life’) at around 542 Ma. Anglesey has the most extensive tracts of Precambrian strata in England and Wales and its geological story extends back an incredible 1.2 billion years! Many different plate tectonic processes and settings were associated with closure of an ancient seaway – the Iapetus Ocean – and were responsible for assembling the island’s Precambrian rocks. These range from constructive to destructive plate margins. The constructive-margin rocks include the internationally famous pillow lavas of Llanddwyn Island and Newborough Forest in the southwest. Anglesey is also the world type area for mélange – which occurs both on Llanddwyn Island and in northern Anglesey where it contains huge limestone blocks that preserve the oldest fossils in England and Wales – stromatolites dated at around 860 Ma. The 614 Ma Coedana Granite occupies a large tract in the centre of the island. The country rock that it injected, a gneiss, provides a link with the supercontinent Rodinia, and extends Anglesey’s geological record back beyond a billion years.
Gwna Group: this unit consists of greenish silty and sandy layers, but it features an extraordinary and spectacular mélange. A mélange is the product of a major undersea debris-slide of catastrophic proportions, and this one, cropping out not only on Anglesey, but also on northern Llyn from Nefyn to Bardsey Island, lives up to the name. Within that green silty matrix there are clasts, ranging from pebble size up to rafts over a kilometre across, of a variety of rocks, all jumbled together. The diverse clasts include stromatolitic limestone [discovered by Margaret Wood], basaltic pillow-lavas, bedded cherts, red mudstones, white quartzite and – rarely – granite.
Skerries Group: this is a weakly deformed, bedded succession of pebbly sandstones, conglomerates and basalts, the pebbles being derived from a sub-volcanic granitic to felsic volcanic source. The presence of abundant granitic and volcanic clasts suggests probable derivation from an island arc, the location of which is yet to be identified.
Coedana Gneisses and Granite: cropping out in central Anglesey is a muscovite-granite, intruded into obviously high-grade acid and basic gneisses. Both appear to be related to ancient island-arc magmatism, and as such may represent the exhumed roots of an arc complex (sometimes referred to as the Older Arc).
Blueschists: forming a linear belt in the south-eastern part of Anglesey, these rocks include metabasites containing a metamorphic mineral assemblage that shows that they have been subjected to unusually high pressures, such as those found deep within a subduction zone