Manchester Geological Association
The Welsh Basin: Some New Thoughts
Saturday 6th December 2008 at 1.00pm, Lecture Theatre LG12
Samuel Alexander Building (South Entrance), University of Manchester
Anglesey Enigmas, Puzzles and Solutions,
Dr. Jack Treagus, University of Manchester
An Event History for the Peri-Gondwanan Margin of Southern Britain,
Dr. David Schofield, British Geological Survey, Keyworth
Coffee Break
The Lower Palaeozoic Welsh Basin: A Window on deep time Climate Change
Dr. Jeremy Davies, British Geological Survey, Keyworth
The most ancient rocks of Wales are found on the Lleyn Peninsula and the Isle of Anglesey
(or, as the Welsh call it, Ynys Mon). On Anglesey itself, the oldest part of the succession, the
Mona Complex, was divided by Greenly into the Monian bedded series resting upon ancient
basement gneisses. The Monian sequence includes the complex folded rocks of the Rhoscolyn
Anticline, and the lavas, tuffs and melange of the Gwna Group. These were overlain by
Arvonian (late Precambrian - early Cambrian) volcanics and intruded by the Coedana Granite.
A variety of Ordovician rocks – basal conglomerates, sandstones, shales and volcanics - occur
throughout the island, resting with high unconformity upon the Mona Complex.
Carboniferous limestones are well exposed on the north east coast of the island, particularly
around Penmon Priory.
On the Lleyn Peninsula the Monian bedded series and gneisses crop out in a narrow strip
along the western coast, and are intruded by the Sarn Granite, thought to be equivalent in age
to the Coedana Granite on Anglesey. Arvonian volcanics occur further along the coast, about
ten kilometres to the northeast. The central and eastern parts of the peninsula are mainly
Ordovician rocks, consisting of sandstones and mudstones, interspersed with accumulations
of lavas and pyroclastic rocks. There are a number of major intrusions in the Ordovician
rocks, mainly along the western coast.
These Ordovician rocks continue eastwards then southwards, covering most of Snowdonia
and encircling the Cambrian rocks of the Harlech Dome. They are, in turn, surrounded by
younger Silurian sediments, which occupy much of Wales.
From the early investigations of Murchison and Sedgwick, ca. 1835-45, continuing work has
shown that this lower Palaeozoic succession collected in a basin, the Welsh Basin, recording
various transgressions and regressions of the sea. Recent work has focussed on deciphering
the palaeogeographical setting and tectonic history of the basin.
Anglesey Enigmas, Puzzles and Solutions,
Dr. Jack Treagus, University of Manchester
There can hardly be another area in Britain, the size of this small island, that exhibits such a
variety of geology. Rocks ranging from Precambrian, possible Cambrian, through to
Carboniferous are seen. There are many puzzling and dramatic features in the superb coastal
exposure.
The first puzzle is the total contrast of many of the rocks with those of the adjacent mainland,
to be described by Dr Schofield. This contrast is in part attributable to a fault of unknown
throw through the Menai Straits.
We will see the effects of four episodes of folding, so it is hardly surprising that untangling
these alone is puzzling. The folds in the supposed-Precambrian quartzites at South Stack and
the Ordovician at Rhosneigr are some of the best exposed in Britain. The effects of some of
the episodes of folding can be recognised in pebbles in later conglomerates so enabling the
episodes to be relatively “dated”.
In the ‘Precambrian’ rocks there are no body fossils but there are, recently recognised, algal
mats in a limestone and worm burrows in a quartzite; both of these give some control with
dating. The ‘Precambrian’ rocks also contain the most amazing mega-conglomerate or
“melange” on the north coast, containing blocks the size of a house; this and spectacular
pillow lavas, at Newborough on the west coast, help us reconstruct the tectonic and
sedimentary environment.
There are some very curious sedimentary features. Some large pipe-like features in a
Precambrian limestone are filled with Miocene rubble, and there are intriguing large fossil
pot-holes in the Carboniferous limestone at Red Wharf. The semi-arid rocks of the Old Red
Sandstone at Lligwy are particularly puzzling, especially for their incongruous folding.
Parys Mountain, previously the site of major copper mining, displays an almost moon-like
scenery with the only exposures of Silurian rocks in Anglesey, and providing a link with those
to be described by Drs Schofield and Davies from the Welsh Basin.
Finally, a unique cross-section through a large drumlin, reveals stones that have travelled
from the Lake District and the Southern Uplands in the Quaternary some 20,000 years ago.
An Event History for the Peri-Gondwanan Margin of Southern Britain,
Dr. David I. Schofield, British Geological Survey, Keyworth, Nottingham, NG12 5GG
The British Geological Survey (BGS) has a long history of research into the geology of the
Lower Palaeozoic successions of the British Isles. Recent surveying in the Lower Palaeozoic
Welsh Basin, commencing in the late 1970’s and continuing up to the present time has
provided a wealth of information on geological relationships, which form the basis of this
presentation.
Synthesis of these data from the Late Neoproterozoic through to Early Devonian geological
record of southern Britain has revealed an ‘event stratigraphy’ that provides a powerful tool
for local and regional tectonic interpretation. This talk provides an overview of significant
‘events’ recorded by deformation, development of unconformities and major sedimentological
changes that have affected southern Britain, with particular emphasis on the Lower Palaeozoic
Welsh Basin. Together, these track the evolution of the continental margin of West
Gondwana, which culminated with the Caledonian Orogeny and its assembly with the
palaeocontinent of Laurentia.
This overview account will also act as an introduction to Dr. J. R. Davies who will speak on
the detailed evolution of the type area of the Llandovery Series and will provide a vehicle for
a brief description of our ongoing activities in Wales. The main events considered in this talk
are summarised as follows:
Comparison of the disparate Late Neoproterozoic geological successions of southern Britain
reveals a number of common events, which together preserve an overall geological history
similar to other Avalonian terranes and suggest that they preserve a collage of decoupled
upper-plate fragments formed during subduction along the outboard margin of West
Gondwana.
The Cambrian to Tremadocian succession is marked by subsidence in a number of discrete
basins and formation of a Tremadoc arc (<489 to >478 Ma) illustrating attenuation of the
Gondwanan margin in response to both the onset of Iapetan subduction and opening of the
Rheic ocean. Palaeogeographic affinities of these basins are poorly understood although
inversion in the Harlech Dome (<489 to >466 Ma), low-grade metamorphism within the
Arfon Basin (490 Ma) and folding in the Monian Supergroup attest to tectonism analogous to
Penobscottian accretionary events on the Gander margin of the northern Appalachians.
Renewed subsidence recorded in the Ordovician record of the Welsh Basin is interrupted by
intrabasinal uplift during Sandbian times (460 Ma) associated with development of volcanic
edifices in the north and margins of the basin. Following a Katian highstand (>445 Ma),
glacioeustatic regression that reached its acme during Hirnantian times is interrupted by
localised deformation in the basin margin (‘Shelvian Orogeny’) that may record the distant
effects of Avalonia/Baltica collision.
Good preservation of Silurian strata within the Welsh Basin and adjacent shelf area has
enabled more detailed sequence stratigraphic analysis in which anomalous high frequency
lowstands can be interpreted with respect to intrabasinal tectonics. Events include an intraTelychian event (<436 to >428 Ma), an Early Wenlock event (<428 to >426 Ma), enhanced
subsidence during Pridoli times (<418 to >416 Ma) and final, Early Devonian penetrative
deformation (400 Ma) that together record closure of Iapetus and the possible influence of
Rheic ocean subduction.
The Lower Palaeozoic Welsh Basin: a Window on deep time Climate Change,
Dr. Jeremy Davies, BGS Keyworth
A twenty-year multidisciplinary study by the British Geological Survey and its collaborators
has elucidated the complex sedimentary architecture and geological structure of the intracratonic Lower Palaeozoic Welsh Basin. The basin was the site of deep-water deposition
throughout much of Ordovician and Silurian times and the mixed sand-mud succession is
dominated by material deposited by slumps, debris flows and turbidity currents (turbidites).
Thin, either burrowed or laminated mudstones preserved between these resedimented units
represent material deposited from suspension (hemipelagites) either during periods when the
basin’s bottom waters were well oxygenated (oxic), or when stagnant anaerobic (anoxic)
conditions prevailed.
Biostratigraphical work, including the study of abundant fossil graptolite remains, has allowed
the basin’s sedimentary fill to be dated in detail and revealing complex lateral changes in
thickness and sedimentary facies. Periods of rapid subsidence during the late Llandovery and
early Wenlock were characterised by high rates of sediment accumulation and intra-basinal
tectonism. These contrast with intervals of slower sedimentation which record the influence of
extra-basinal processes including global changes in sea level in the late Ordovician, early-mid
Llandovery, late Wenlock and Ludlow. More recent investigations of the succession on the
adjacent shelf have provided further insight into these contrasting processes and events and
have underpinned detailed comparisons with successions of the same age preserved elsewhere
in the world.
Such comparisons reveal the influence of glacial events on the palaeo-supercontinent of
Gondwana. Periods during which oxic bottom conditions prevailed across the Welsh Basin
floor equate with episodes of Gondwanan ice advance and global sea level fall; anoxic
conditions were introduced and sustained by the elevated sea levels and warmer waters
associated with interglacial episodes. The transitions between oxic and anoxic facies in the
Welsh Basin therefore provide a record of repeated and linked changes in Lower Palaeozoic
climate and oceanic processes and of their environmental impact. The detailed study of these
key intervals allows the rate and impact of ancient global warming episodes and their value as
analogues of modern climate change to be assessed.
Early Palaeozoic Timescales
Period
Epoch
Pridoli
Ludlow
Silurian
Wenlock
Llandovery
Late
Ordovician
Middle
Early
Age/Stage
Ludfordian
Gorstian
Homerian
Sheinwoodian
Telychian
Aeronian
Rhuddanian
Hirnantian
Katian
Sandbian
Darriwilian
Dapingian
Floian
Tremadocian
Starts at (Ma)
418.7
421.3
422.9
426.2
428.2
436.0
439.0
443.7
445.6
455.8
460.9
468.1
471.8
478.6
488.3