GeoMôn

Anglesey Geopark

 

henslow

Above: part of the challenge of Anglesey's geology is that, while the coastal exposure is superb, critical areas inland are sometimes a bit less co-operative!

Above: Greenly's map of Llanddwyn Island, 1919.

Above: John Stevens Henslow - the first pioneer.

Above: zircons in matrix from Norway. Unlike this spectacular mineral specimen, the zircons used for U-Pb dating are often rounded, microscopic grains - but even then they are every bit as useful!  Photo: Rob Lavinsky.

Above: granite clasts in conglomeratic rocks on the Skerries, a small group of islands off the north coast of Anglesey, where several researchers, including a team from Japan, have been active in recent years.

Photo: Brian Windley.

greenlymap skerries3 zircon2 inland

Part 02: The work of many: the Great Precambrian Debate

 

John Stevens Henslow (1796-1861) was educated at St John's College, Cambridge where he graduated in 1818, the year in which Adam Sedgwick became Woodwardian Professor of Geology. He developed a passion, accompanying Sedgwick on fieldwork and studying chemistry under Professor James Cumming and mineralogy under Edward Daniel Clarke. Thus armed, he arrived on Anglesey to investigate aspects of the island's geology, recognising the belts of "chloritic schists" and other exotic rocks, the observations being printed in the first volume of the Transactions of the Cambridge Philosophical Society in 1822.

 

Nearly a century later, Edward Greenly was in the process of preparing his classic, meticulously detailed, account of the stratigraphy of Anglesey's Precambrian and younger rocks. Since that time, the "sequence" has been overturned so many times one could forgive it if it complained of dizziness!

 

One can feel some sympathy here. There are three key problems to any worker looking at these older rocks on the island:

 

1) Exposure is incomplete to poor in many critical inland areas.

 

2) Patchy, post-Arenig (Ordovician, Silurian, Devonian, Carboniferous and Quaternary) cover is present in many areas, obscuring the picture.

 

3) Where exposure is better, and cover absent, the rock units described on page 1 are separated by major tectonic breaks, or, in some cases by hotly debated breaks which, to some, are stratigraphic, while to others they are tectonic.

 

Imagine a jigsaw puzzle, almost complete around the edges (the coast), but with big holes in the middle (inland) - and those few completed bits in the middle all radically different from one another. That's Anglesey's geology in a nutshell, and that's why it magnetically draws so many research teams, to whom the unsolved problems are among the greatest geological challenges left in the UK. They started coming in the early 1800s, and there is no sign of a let-up yet!

 

Greenly's sequence, following, in broad terms, that of Henslow, was said to be stratigraphically continuous, beginning with the Gwna Group, passing up through the New Harbour Group and with the South Stack Group the youngest. The sequence was then recumbently folded and metamorphosed, leaving it in part upside down and with an increase in metamorphic grade towards the south-east where the Blueschists are to be found. The Coedana Gneisses were regarded as ancient basement, with the intrusion of the Coedana Granite a late-stage, post-metamorphic event, finishing the development of his Precambrian Mona Complex.

 

Decades passed and little more was written on the matter. Then, in 1965, Robert Shackleton published his findings, in which the geology was substantially revised. He considered the bedded strata to be the right way up, thus inverting the sequence, with the South Stack Group at the bottom and the Gwna Group the youngest. He considered the Coedana Gneisses to be their high-grade metamorphic equivalent, the culmination of which was granitisation, resulting in the Coedana Granite.

 

During the 1970s, two key papers were published regarding the ultrabasic rocks within the New Harbour Group. In one, Alec Maltman argued the case for their emplacement purely by intrusive processes whilst, in the other, Richard Thorpe argued the opposite, indicating that they had been tectonically emplaced as part of an ophiolite sequence.

 

The 1970s in general saw a lot of activity around the island, but it was 1979 that saw a proliferation of papers that sparked some of the most robust debate ever witnessed in UK geological research. Much of this was centred around a paper published by Anthony Barber and Michael Max, which appeared in the Journal of the Geological Society.

 

Barber and Max restored Greenly's stratigraphy but with an important difference - they considered the three bedded units to be separated by tectonic breaks. Thus, the relatively undeformed South Stack Group was overthrust by an already highly deformed New Harbour Group. The Gwna Group on the other hand was relatively undeformed, and might have been unconformably deposited onto the New Harbour Group. Shackleton's view of the Coedana Gneisses representing high-grade metamorphism of the bedded units was rejected and once again the gneisses were considered to be ancient (possibly Archaean), high-grade basement rocks. The Blueschists in the south were regarded as representing a tectonically complex zone of imbrication from a relatively deep level in a subduction zone.

 

The discussion following the paper is an instructive read, not least because it reveals the passion with which the various theories of key workers in the field were held. The late Dennis Wood, a geologist with a lifelong association with the island's rocks, was strident in his view that the South Stack and New Harbour groups were conformable and it was their lithological differences that had led to different textures produced by the same deformation. He considered the bedded units to pass with increasing metamorphic grade into the Coedana Gneisses - and these into the Coedana Granite.

 

Meanwhile, in the same issue of the same journal, Thorpe, writing with R.D. Beckinsale, presented the results of Rb-Sr whole-rock isotopic analyses for the Coedana Gneisses and the Coedana Granite. These gave evidence for a late Precambrian age (around the 600 million years ago mark) for gneissification and granitisation and reinforced Shackleton's and Wood's view of their origin.

 

The occurrence of fossils in the limestone clasts of the Gwna Mélange also came to light in the 1970s, with Margaret Wood and G.D. Nicholls describing stromatolites (algal mats) from the north coast in 1973 and M.D. Muir and colleagues describing microfossils from the same clasts in 1979. Neither were as useful in terms of stratigraphy as more recent fossils are, such as the graptolites occurring in Lower Palaeozoic rocks. The stromatolites could be late Precambrian or Lower Cambrian, whilst the microfossils could be early Cambrian, but without 100% certainty!

 

Old controversy attracts new research, and the last decades of the Twentieth Century saw much of the latter. Studies included work on the Coedana Complex and the Blueschist Belt by Jana Horak, Wes Gibbons and co-workers. By 2000, it was widely considered that Anglesey was made up of three individual terranes, the Monian Supergroup (the bedded units), the Coedana Complex (gneisses and granite) and the Blueschist Belt, collectively being referred to as the Monian Composite Terrane. These terranes were thought to have widely differing metamorphic histories, and had docked together before becoming accreted to the rest of England and Wales (or Avalonia, to use its geological name) at the major shear-zone that bounds the Blueschist Belt along the Menai Strait.

 

Progress was also made regarding the age of some of the rocks. Ar-Ar radiometric dating of the metabasites of the Blueschist Belt gave two ages - the oldest, 580-590 million years ago being considered to mark sea-floor metamorphism and the youngest, 550-560 million years ago, representing the metamorphism in the subduction zone. Thus, the Blueschists were definitely late Precambrian.

 

In 2004 there came the confirmation that at least one of the bedded units was, however, considerably younger. U-Pb data, obtained by Alan Collins and Craig Buchan from detrital zircons in the South Stack Group, indicated a maximum age for these rocks of 501+/-10 million years, or very solidly Cambrian in age. This is a useful dating method, since zircons crystallise in igneous rocks and, obviously, can only be eroded, transported and deposited in sediments after that crystallisation has happened. So if a zircon is 501 million years old, the sedimentary rock into which it has become incorporated cannot be any older than that. The results throw doubt on the earlier view that the Coedana Gneisses are the highly metamorphosed equivalent of the bedded units. They can't be - they're demonstrably over 100 million years older!

 

Up to the future, and in recent years a team of Japanese scientists, working with Brian Windley of Leicester University, has been undertaking a variety of studies. Japan of course lies on a subduction zone, and this team has spent many years studying such features in their local area. Some valuable new data and interesting interpretations are emerging: perhaps a key conclusion is that, for all its tectonic dismemberment, Anglesey is in fact a classic example of an ancient accretionary orogenic belt, with an exhumed blueschist, a late Precambrian mélange with many oceanic components, an ophiolite sequence and a passive margin sedimentary basin sequence of Cambrian age.

 

So, we go into the future with new interpretations going into publication, which again will attract great interest and inspire new generations of research geologists to come over to Anglesey and continue the study of its extraordinarily complex, controversial, inspiring and sometimes frustrating rocks. Let's take all of the latest conclusions, and try to piece together the story of the evolution of the island, in Part 3.

southstack1a

Above: the South Stack area: is the South Stack Group conformable with the New Harbour Group or not? Opinions have differed!

Photo: Stewart Campbell.

arguments2

Above: who was right and who was wrong? The debate on the geology of Anglesey was, and has continued to be, robust!

Geology of Anglesey: A journey through time