Geology of Anglesey: A
journey through time
Part
1 Part 2 Part 3 Part 4
Part 2: 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.
Go to Part 3
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Above: John Stevens Henslow - the first pioneer.
Above: Greenly's map of Llanddwyn Island, 1919.
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: the South Stack area: is the South Stack
Group conformable with the New Harbour Group or
not? Opinions have differed! Photo: Stewart
Campbell.
Above: who was right and who was wrong? The
debate on the geology of Anglesey was, and has
continued to be, robust!
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.
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