Longmyndian Supergroup

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The Longmyndian Supergroup is a 6,000-meter-thick geological formation in Shropshire that was deposited during the Ediacarian . It contains primitive fossils , precursors of the Ediacaran fauna .

etymology

The Longmyndian Supergroup is named after the Long Mynd , a mountain range in western Shropshire in England near the border with Wales . Long Mynd ( elongated mountain ), Welsh Mynydd Hir , is a combination of the English long (long) and the British Mynd (mountain).

geology

The valley Ashes Hollow on the eastern slopes of the Long Mynd, west of Little Stretton . The valley cuts through rocks of the Synalds Formation. Fossil finds were made along its north side.

The Longmyndian Supergroup is wedged between the Pontesford-Linley Fault in the northwest and the Church Stretton Fault in the southeast. These two faults strike northeast-southwest to north-northeast-south-southwest and form what is known as the Welsh Borderlands Fault System , a significant crust line within East Avalonia that forms the boundary between the Cymru Terran in the west and the Wrekin Terran in the east. The sedimentary basin of the Longmyndian Supergroup created by these two fault systems was probably located in the immediate vicinity of a volcanic island arc , the Uriconian Volcanic Arc , and it may also form part of this arc.

The arch of the island arose 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 .

Towards the end of the Ediacariums incorporated between the two warp systems basin sediments were then deformed synclinal steep provided and closest proceeding under sinistral transpression verfaltet . The north- northeast trending Synkline dips slightly to the south, its axis now at Wentnor a little west of the main ridge of the Long Mynd.

stratigraphy

Carding Mill Valley on the eastern edge of the Long Mynd with the Lightspout Formation and the Portway Formation in the rear section.

The Longmyndian Supergroup is divided into two groups , the Stretton Group on the recumbent and the Wentnor Group on the hanging wall . Their total thickness reaches a good 6,000 meters. The deposited sediments generally document the transition from open marine sedimentation with turbidites to shallow marine and clastic continental facies, so it is a regressive sequence . The sedimentation basin, which was elongated to the northeast and located between the two fault systems, was mainly filled from the southeast, as flow indicators show. Rare slate clasts and garnet detritus indicate a metamorphic delivery area, roughly comparable to the Rushton Schist .

Stretton Group

The Stretton Group des Lying documents a regressive sequence of layers from marine basin clays via turbidites and shallow marine delta sands to sandstones and siltstones of an alluvial alluvial plain. In shallow marine, low-energy sediments, sediment structures of possible biogenic origin can be observed. The Stretton Group produces partly pyroclastic deposits, consisting of lapilli and layers of ash. The volcanic rocks likely come from the neighboring Uriconian Group .

The Stretton Group is structured as follows (from hanging to lying):

Wentnor Group

The Wentnor Group of the hanging wall is of sandstones and occasional conglomerates dominated alluvial Zopfströme. It consists of the fluvial bridges formation in the hanging wall . The lying form is the plait sediments of the Bayston-Oakwood Formation .

Age

Townbrook Valley with Burway Hill and Caer Caradoc in the background . Looking east over the Burway Formation.

The Longmyndian Supergroup includes layers of lapilli and bentonite that have been radiometrically dated using the uranium-lead method on zirconia. For example, the Felsic Batch Volcanics within the Stretton Shale Formation are 566 ± 2.9 million years old BP . The Lightspout Formation itself could be dated to 555.9 ± 3.5 million years BP.

Fossils

The Longmyndian Supergroup contains numerous hemispherical structures that are only several millimeters in size. These structures form small mounds on the undersides of the layers and holes on the tops of the layers. Their organic origin is now considered certain, even if they were often seen as raindrop holes in the past. The structures are now assigned to the following taxa: Beltanelliformis brunsae , Beltanelliformis minutae (sp.nov.), Intrites punctatus and Medusinites asteroides . Along with the hemispheres, linear, net-like structures can also be found. The linear structures called Arumberia banksi are likely to represent pseudofossils. McIlroy and Walter (1997) explain it by microbially caused sticking of sediment surfaces ( microbially induced sediment structure or MISS for short ). But they could also be of tectonic origin.

Fossils first appear in the Stretton Group's Burway Formation. However, the Wentnor Group is fossiler. The common occurrence of simple trace and body fossils together with microbial mats is typical for the outgoing Neoproterozoic , but the same association can still be found in the Phanerozoic .

Larger body fossils, such as the Rangeomorpha so typical of the Ediacaran fauna , have not yet been discovered in the Longmyndian Supergroup. Nevertheless, the fossil finds that were made by John William Salter in 1856 (but were still misinterpreted by him at the time) are historically extremely important, as they provided the first evidence of life in the supposedly Azoic layers of the Precambrian.

Note: In a more recent study Menon and colleagues (2016) come to the conclusion that all the taxa listed are pseudofossils. They are based in their design yet all on the presence of microbial mats ( English microbial mats ) on the sediment surface. These clogged the sediment and therefore forced the pore water in these very special structures to rise and drain in an unusual way.

Individual evidence

  1. ↑ Brenchley, PJund Rawson, PF: The Geology of England and Wales (2nd ed.) . 2006.
  2. ^ Nigel Woodcock and Robert Strachan: Geological History of Britain and Ireland . Blackwell Science, Oxford 2000, ISBN 0-632-03656-7 .
  3. ^ 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 (2) , 1991, pp. 167-183 , doi : 10.1002 / gj.3350260209 .
  4. ↑ Compston, W., Wright, AE and Toghill, P .: Dating the Late Precambrian volcanicity of England and Wales . In: Journal of the Geological Society . tape 159 (3) , 2002, pp. 323-339 , doi : 10.1144 / 0016-764901-010 .
  5. ↑ McIlroy, D. and Walter, MR: A reconsideration of the biogenicity of Arumberia banksi, Glaessner and Walter . In: Alcheringa . tape 21 , 1997, pp. 79-80 .
  6. ^ Duncan McIlroy, T. Peter Crimes and John C. Pauley: Fossils and matgrounds from the Neoproterozoic Longmyndian Supergroup, Shropshire, UK . In: Geological Magazine . tape 142 (4) , 2005, pp. 441-455 , doi : 10.1017 / S0016756805000555 .
  7. ^ Salter, JW: On fossil remains in the Cambrian rocks of the Longmynd and North Wales . In: Quarterly Journal of the Geological Society . tape 12 , 1856, pp. 246-251 .
  8. ↑ Latha R. Menon, Duncan McIlroy, Alexander G. Liu and Martin D. Brasier: The dynamic influence of microbial mats on sediments: fluid escape and pseudofossil formation in the Ediacarian Longmyndian Supergroup, UK . In: Journal of the Geological Society . 2016, doi : 10.1144 / jgs2015-036 .