Comley Series
The Comley Series is a geological stratification ( series ) from the Lower Cambrian Shropshire . It contains the oldest leading invertebrate fossils in Great Britain.
Type locality
The type locality of the Comley Series is the Comley Quarry at Comley in Shropshire.
Geological overview
In the Lower Cambrian (542 to 513 million years ago) the south-east of Ireland , Wales and England were still at around 60 ° south latitude and formed part of East Avalonia , which, together with Armorica, was off the northern edge of Gondwana - the north-west coast of today's North Africa. Scotland and the north-west of Ireland, however, were part of Laurentia and were at about 40 ° south latitude. The Iapetus Ocean stretches in between .
With the beginning of the Cambrian, a global rise in sea level began , which reached its first maximum towards the end of the Lower Cambrian. Causes therefor were the breaking Rodinia towards the end of Ediacarums and thereby increased incidence mid-ocean ridge, but also an air heating after decay of the Neoproterozoic ice ages. This transition from the icehouse to the greenhouse may ultimately have triggered the remarkable diversification of the marine invertebrate fauna ( Cambrian explosion ).
Within Ostavalonias which formed fault system of Welsh Border country Fault System (WBFS) Terra the boundary between the Cymru-Terran the west and the Wrekin-Terran in the East, probably during the Cadomian Orogenese was applied. On the northern edge of East Avalonia, and indeed on the entire northern edge of Gondwana, a south-facing subduction zone ran from the Cryogenian (from 680 million years ago) to the Ediacarian . The resulting oblique sinistral convergence movements at the WBFS led to the emergence of the Uriconian Group with subsequent deposition of the Longmyndian Supergroup . With the beginning of the Cambrian, the compressions within the two terranos had subsided and Ostavalonia had grown together to form a coherent crustal block.
stratigraphy
The Comley Series, also known as the Lower Comley Group , is discordant over metamorphic neoproterozoic basement rock (slate of the Rushton Schists and gneiss of Primrose Hill Gneisses ) and over volcanic rocks of the Uriconian Group from the Ediacarian. The series consists of three formations (from hanging to lying ):
The series is in turn overlaid discordantly by Central Cambrian sediments of the Saint David's Series , more precisely by the Quarry Ridge Grits of the Upper Comley Group with Paradoxididae .
The 30 to 50 meter thick Wrekin quartzite (Aa) is not a quartzite, but a non-metamorphosed white, quartz-rich sandstone or quartz arenite . It contains a conspicuous conglomerate of transgression at the base with rocks of rhyolites , tuff and granophyr. Often the pebbles are also colored green by chlorite derived from iron-containing minerals . Except for the occasional trace fossils (worm structures), the Wrekin Quartzite is fossilized. Yellow layers with the iron oxide limonite are also worth mentioning .
The overlying Lower Comley Sandstone (Ab1-4 and Ac1) is a 150 meter thick, green-brown sandstone . Its green color is caused by the mineral glauconite , which only occurs in the marine environment.
Above it is the Lower Comley Limestone , which is only 1.80 meters thick . It consists of five thin layers of sandstone or limestone (Ac2, Ac3, Ac4, Ac5 and Ad), some of which contain fossil fuels and are separated from one another by discordances. A distinction is made between the following positions (from hanging to lying):
- Lapworthella Limestone (Ad)
- Strenuella Limestone (Ac4)
- Bellimarginatus Limestone (Ac3)
- Red Callavia Sandstone (Ac2)
- Green Callavia Sandstone (Ac1)
paleontology
Fossil content
In the Lower Comley Limestone, the trilobite Callavia callavei (formerly Olenellus callavei ) was first described by Charles Lapworth in 1888 . In addition to other trilobites, brachiopods and gastropods such as Helicionella subrugosa should be mentioned. In 2001, Klausmuelleria salopensis, a phosphatocopida belonging to the Eucrustacea , was discovered in Lower Comley Limestone in exceptional three-dimensional soft tissue preservation . The fossils are phosphated and show Orsten preservation.
Biozones
The Comley Series spans three fossil zones. The first zone without trilobites ( English Non-trilobite Zone ) includes the Wrekin Quartzite and the lower 60 meters of the Lower Comley Sandstone. The fossils here include worm structures , small shelly fauna (SSF) of the Camenella baltica biozone, brachiopods and problematic drugs. The following Olenellid Zone ( Olenellid Zone ) extends over the rest of the Lower Comley Sandstone and the lower section of the Lower Comley Limestone. The Protoleniden-Strenuellid Zone ( Protolenid-Strenuellid Zone , named after the trilobites Protolenus and Strenuella including Lapworthella ) comprises the upper section of the Lower Comley Limestone. It is characterized by the disappearance of the Olenellidae and the appearance of Eodiscidae and ophisthopic trilobites. The microfossils and trilobites allow a correlation with Newfoundland .
In detail, the following structure results for the Comley Series (from hanging to lying):
- Protoleniden-Strenuelliden-Zone - 2nd series, 4th stage
- Cephalopyge -Biozone, Ad
- Orodes -Biozone, Ac5
- Strenuella sabulosa -Biozone, Ac3 and Ac4
- Olenelliden Zone - 2nd series, 3rd stage
- Callavia Biozone, Ab4, Ac1 and Ac2
- Fallotapis biozone (with Eofallotaspis ), Ab2 and Ab3 - first appearance of trilobites
- Non-trilobite Zone - Terreneuvium, 2nd stage
- Camenella baltica -Biozone, Aa and Ab1 - SSF
In the hanging wall, the Kiskinella biozone is just touched, which already belongs to the 3rd series and 5th stage of the Cambrian. The following Eoparadoxides harlani biozone, which leads to the Upper Comley Group, has fallen prey to erosion.
Age
So far, the Comley Series has a robust age of 514.45 ± 0.36 million years, established with the uranium-lead method (ID-TIMS) on zircons , only for the hanging wall of the Lower Comley Sandstone (Ac1) - corresponding to that 3rd stage hanging walls (521 to 515 million years ago) from the 2nd series of the Cambrian. Harvey and colleagues were also able to time the rest of the series by dating the base of Upper Comley Sandstone (Ba1) to 509.10 ± 0.10 million years.
The stratigraphically lowest trilobite - very likely Kjerulfia - appears 60 meters above the base of the Lower Comley Sandstone (in Ab3, in the Fallotaspis zone at the base of the 3rd stage of the Cambrian) and is thought to be around 519 million years old. The base transgression of the Wrekin Quartzite can only be estimated in terms of time. Rushton and colleagues (2011) place it in the upper section of the 2nd stage of the Terreneuvian (528 to 522 million years) and assign it an approximate age of 523 million years.
The Comley Series therefore represents the period from 523 to 510 million years.
Occurrence
The Comley Series occurrences follow the two faults of the Welsh Borderland Fault System - the Church Stretton Fault and the Pontesford-Linley Fault . Well-known occurrences can be found at Lilleshall ( Lilleshall Hill ), at Wrekin , at Ercall Hill , at Lawley , at Caer Caradoc , around the type locality Comley and at Cwms . Outside of Shropshire, the Comley Series also performs in northern Herefordshire , the Malvern Hills , Nuneaton and even Bristol . The series may have been spread across the entire Midlands earlier , but was removed by elevation and erosion shortly before the beginning of the Ordovician . This is nice to see at Hope Bowdler or at Hazler Hill , where Ordovician sediments with a clear discordance come to lie directly on a deeply eroded land surface from volcanic rocks of the Uriconian Group and the Comley Series was eroded away.
The Comley Series as a stratigraphic unit
In Great Britain, the term Comley is generally used to describe the Lower Cambrian, with distribution areas also outside Shropshire, for example in the north-west of Gwynedd , at the Harlech Dome in north Wales, in the south-west of Dyfeds and at Nuneaton on the Midland Platform in England ( Hartshill Formation ).
In north-west Gwyned, the Comley is very powerful at over 2,000 meters and consists of 1,200 meters of sandstones and conglomerates from the coastal area, the 800-meter-thick, open marine Llanberis Slates with turbidite connections and the sandstones of the Cymffrwch Grits .
The sequence at the Harlech Dome is made up of 725-meter-thick sandstones and shale clays from the delta and pre-delta area (also contains inserted volcanic rocks), followed by the 600-meter-thick sandstones from the Harlech Grits Group .
In south-west Dyfed, the Comley is made up of the 300-meter-thick Caerfai Group - conglomerates and sandstones from the coastal area as well as open marine shale with volcanic layers.
At Nuneaton, the Comley begins with the 270 meter thick, coastal Hartshill Quartzite , which is overlaid by more than 200 meters thick shale of the open marine area.
literature
- Peter Toghill: Geology in Shropshire . Swan Hill Press, Shrewsbury 1990, ISBN 1-85310-090-0 .
Individual evidence
- ^ Torsvik, TH, Smethurst, MA and Meert, JG: Continental break-up and collision in the Neoproterozoic and Paleozoic - a tale of Baltica and Laurentia . In: Earth Science Reviews . tape 40 , 1996, pp. 229-258 .
- ↑ Brenchley, PJ, Rushton, AWA, Howells, M. and Cave, R .: Cambrian and Ordovician: the early Palaeozoic tectonostratigraphic evolution of the Welsh Basin, Midland and Monian terranes of eastern Avalonia. Eds .: Brenchley, PJ and Rawson, PF The Geology of England and Wales. Geological Society, London 2007, p. 25-74 .
- ↑ Patchett, JP, Gale, NH, Goodwin, R. and Humm, MJ: Rb-Sr whole rock isochronous ages of late Precambrian to Cambrian igneous rocks from southern Britain . In: Journal of the Geological Society . tape 137 . London 1980, p. 649-656 .
- ^ Cocks, LRM, Fortey, RA and Rushton, AWA: Correlation for the Lower Palaeozoic . In: Geological Magazine . tape 147 , 2010, p. 171-180 .
- ^ David J. Siveter, Mark Williams, and Dieter Waloszek: A Phosphatocopid Crustacean with Appendages from the Lower Cambrian . In: Science . tape 293 , 2001, p. 479-481 .
- ↑ Brasier, MD: The succession of small shelly fossils (especially conoidal microfossils) from English Precambrian-Cambrian Boundary Beds . In: Geological Magazine . tape 123 , 1986, pp. 237-256 .
- ↑ Brasier, MD: Towards a biostratigraphy of the earliest skeletal biotas . In: Cowie, JW and Brasier, MD The Precambrian-Cambrian Boundary (Eds.): Oxford Monographs on Geology and Geophysics . tape 12 , 1989, pp. 117-165 .
- ^ Thomas HP Harvey et al: A refined chronology for the Cambrian succession of southern Britain . In: Journal of the Geological Society . tape 168 . London 2011, p. 705-716 , doi : 10.1144 / 0016-76492010-031 .
- ↑ Peng, S.-C. and Babcock, LE: Cambrian Period. Ed .: Ogg, JG, Ogg, G. and Gradstein, FM The Concise Geologic Time Scale. Cambridge University Press, Cambridge 2008, pp. 37-46 .
- ^ Rushton, AWA, Brück, P., Molyneux, SG, Williams, M. and Woodcock, NH: A revised correlation of the Cambrian rocks in the British Isles . In: Geological Society, London, Special Report . tape 25 , 2011.
- ^ Cowie, JW, Rushton, AWA, and Stubblefield, CJ: A Correlation of the Cambrian Rocks of the British Isles . In: Geological Society, London, Special Reports . tape 2 , 1972.