Uriconian Group

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The Uriconian Group is an approximately 566 million year old, from the Ediacarian volcanic group of the Avalon or Wrekin Terran of Eastern Avalonia . It is unlocked within the fault system of the Welsh Borderland Fault System that traverses east Wales and west Shropshire in a northeasterly direction.

etymology

The name Uriconian is derived from the Latin uriconio , the name of an Iron Age hill fort ( hill fort ) on the summit of the Wrekin .

stratigraphy

Caer Caradoc with rocks from the Uriconian Group

Due to the isolated outcrops, no continuous stratigraphy, but only a local sequence can be worked out. Up to 1500 meters of tuffs and lavas are pending on the Wrekin. The following sequence can be recognized on the Caer Caradoc (from hanging to lying ):

The volcanic rocks are intruded by dolerites (dikes and storage dikes). At Ercall the Ercall Granophyre penetrated rhyolites of the Uriconian Group 560 ± 1 million years ago, and a quartz porphyry at Cardington Hill .

geology

The Hope Bowdler Hill is built of rocks of Uriconian Group. In the foreground Hope Bowdler, on the left in the background the Caer Caradoc .

The arch of the island , consisting of the Uriconian Group , emerged from a south to south-west facing subduction of oceanic crust of the Ran Sea below East Avalonia. The Ran Sea had formed from the Cryogenium around 750 million years ago after the disintegration of Rodinia through ocean spreading between the continents Laurentia , Baltica and Siberia in the north on the one hand and Amazonia , Avalonia , Florida and Cadomia in the south on the other. Ostavalonia was then part of the northeastern continental margin of Gondwana .

The Uriconian Group had been deposited on metamorphic basement rocks of the Avalon Terran, consisting of schists and gneiss , the Rushton Schists and the Primrose Hill gneiss . The total thickness of the group should have reached more than 1500 meters.

Tectonic evolution

The time of the metamorphosis of the garnet-bearing Rushton Schists and thus the consolidation of the basement was determined to be 667 ± 20 million years ( Cryogenium ). It is not clear whether the Welsh Borderland Fault System (WBFS) already existed at the time and affected the Uriconian Group's tipping. In any case, later movements at the WBFS led to a sagging of the volcanic rocks between the two main fault lines; the resulting accommodation space was then filled with the sediments of the Longmyndian Supergroup .

This was followed by elevation, inclination and leveling of the layers of the Longmyndian Stretton Group . The layers of the Wentnor Group later laid discordant on top of this . Strong sinistral, transpressive movements at the WBFS now narrowed the sediment package lying between the faults synclinal and east-vergent, whereby the underlying Uriconian group was pressed along the two faults. This structure-determining deformation probably took place around 550 million years towards the end of the Ediacarium.

Even before the discordant deposition of the transgressive Cambrian , which began in Shropshire with the Wrekin Quartzite of the Comley Series , the fold structure had been completely leveled .

Petrology

The Uriconian Group, also known as Uriconian Volcanics or just as Uriconian for short , is a mighty volcanic sequence consisting of pyroclastic breccias, agglomerates, tuffs and lavas , the chemical composition of which ranges from basalts to andesites and dazites to rhyolites (with rhyolites predominating ). The basalts are often very rich in bubbles and the rhyolites show flow bands. The tuffs vary from fine-grained ignimbrites to very coarse-grained, bomb-bearing material. The very frequent occurrence of welded tuffs (originating from high-temperature streams of ash or glowing clouds or nuées ardentes ) and the lack of basaltic pillow lavas indicate subaeric eruptions, although some tuffs were probably deposited in shallow water. The coarse breccias and agglomerates suggest that the outbreak center is in the immediate vicinity, but no production chimneys have yet been discovered. It is believed that the eruptions are due to fissure eruptions associated with the faults.

The volcanic rocks of the Uriconian Group are bimodal, calcareous and consist of basic, but also of intermediate to acidic rocks. Geochemically, they mainly have intraplate signatures, but subduction signatures have also been found. Further petrological work on the volcanic rocks could show that they were deposited in a marginal basin behind an island arch . The arch of the island was created by an oblique subduction and was subject to an overall transient tension regime.

Age

Rhyolites from the Uriconian Group were dated 566 ± 2 million years ago. As a result, they were formed at the same time as the Warren House Formation in the Malvern Hills , which has very similar basalts and basaltic andesites. At about the same time, the Coomb Volcanic Formation in South Wales, the South Charnwood Diorites in the Charnwood Terran , the Fachwen Formation and the Minffordd Formation in the Arfon / Lleyn area in North West Wales and the Aethwy Terrane Blueschists , which were metamorphosed in the blue slate facies , in the southeast of Anglesey .

Occurrence

The type locality The Wrekin , looking west (Wales)

The Uriconian Group occurrences are associated with either the Church Stretton Fault or the Pontesford-Linley Fault and occur along the course of these two faults. The faults strike predominantly in a north-northeast direction and turn further north in the northeast direction. The Church Stretton Fault occurrences begin in the north 3 kilometers south-southwest of Newport at Lilleshall ( Lilleshall Hill ) and end approximately 50 kilometers further southwest 3 kilometers west of Craven Arms (Craven Arms Inlier). An isolated occurrence can be found near Old Radnor further south in Powys (Wales). Along the Pontesford-Linley Fault, Uriconian Group volcanic rocks appear at Plealey and Pontesbury in the north, at Earl's Hill and Pontesford Hill near Pontesford and southeast of the Stiperstones and again at Linley Hill near Linley 18 kilometers to the south.

The Uriconian Group's type locality is The Wrekin .

literature

  • Peter Toghill: Geology in Shropshire . Swan Hill Press, Shrewsbury 1990, ISBN 1-85310-090-0 .
  • Carney, JN et al .: Precambrian Rocks of England and Wales . In: Geological Conservation Review Series . tape 20 . Peterborough 2000, ISBN 1-86107-487-5 .
  • Nigel Woodcock and Rob Strachan: Geological History of Britain and Ireland . Blackwell Science Ltd, Oxford 2000, ISBN 0-632-03656-7 .

Individual evidence

  1. ^ PJ Brenchley and PF Rawson: The Geology of England and Wales, 2nd Edition . 2006.
  2. ^ Greig, DC et al .: Geology of the country around Church Stretton, Craven Arms, Wenlock Edge and Brown Clee . In: Memoir of the Geological Survey of the United Kingdom . 1968.
  3. ^ A b Tucker, RD and Pharaoh, TC: U-Pb Zircon ages for Late Precambrian - early Palaeozoic plate tectonics evolution of England and Wales . In: Journal of the Geological Society of London . tape 148 , 1991, pp. 435-443 .
  4. ↑ Thorpe, RS et al .: Crustal growth and late Precambrian-early Palaeozoic plate tectonic evolution of England and Wales . In: Journal of the Geological Society . tape 141 . London 1984, p. 521-536 .
  5. ^ Carney, JN et al.: Precambrian Rocks of England and Wales . In: Geological Conservation Review Series . tape 20 . Peterborough 2000, ISBN 1-86107-487-5 .
  6. ^ Pharaoh, TC and Gibbons, W .: Precambrian Rocks in England and Wales south of the Menai Straight Fault System . In: A Revised Correlation of the Precambrian Rocks in the British Isles . 1987.
  7. ^ W. Compston, AE Wright and P. Toghill: Dating the Late Precambrian volcanicity of England and Wales . In: Journal of the Geological Society . tape 159 , 2002, pp. 323-339 .
  8. ^ Pauley, JC: A revision of the stratigraphy of the Longmyndian Supergroup, Welsh Borderland, and its relationship to the Uriconian Volcanic Complex . In: Geological Journal . tape 26 , 1991, pp. 167-183 .