Charnian Supergroup

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The Charnian Supergroup is a volcanic-sedimentary strata sequence in the Neoproterozoic of the Charnwood Terran (part of the Avalon Terran ) in Great Britain . It contains important fossils belonging to the Ediacara biota .

Etymology and Occurrence

The Charnian Supergroup was named after its type locality in the Charnwood Forest near Leicester , where it is found in a basement break in individual, partly isolated occurrences below the angular discordance of the Triassic Mercia Mudstone Group . The English word Charnwood is derived from the Celtic carn ( stone man ) and Old English wudu (forest).

Very similar, rocky tuffs of comparable age were found at Orton (30 kilometers to the southeast) and at Glinton (50 kilometers to the east) below the sediment cover.

stratigraphy

Beacon Hill Formation on Beacon Hill in Bradgate Park

The Charnian Supergroup, which is around 3,200 meters high, consists mainly of volcanic tuff , pelite and greywacke . Subordinate landslide breccias, volcanic breccias, quartz arenites and conglomerates also occur.

The supergroup can be structured as follows (from hanging to lying ):

The Blackbrook Group , which is over 1400 meters thick, and the Maplewell Group , which is up to 2000 meters thick, are of moderate deep marine origin (deposit depth > 50 meters) and are largely dominated by pyroclastics , which were deposited under the sea by tuff and turbid currents near active volcanic centers . The volcanic centers formed part of an arch of calcareous islands resting on juvenile, continental crust . The two groups consist mainly of volcanic agglomerates, tuffs and lavas with interposed breccias .

In fine-grained, volcanoclastic layers, remnants of the Ediacara biota are partially embedded - with fronds from Charnia masoni and Bradgatia linfordensis as well as adhesive discs from Charniodiscus concentricus . The biota can be found in the Blackbrook Group and the Maplewell Group, especially in the Hallgate member of the Bradgate Formation.

Examples of volcanic complexes are exposed at Whitwick and Bardon Hill, recognizable by the intrusion of acidic to intermediate quartz-feldspar porphyries into volcanic breccias and tuffs.

The final brand group is not of volcanic origin and documents the transition to conglomerates and sandstones in the shallow sea area near the beach. The latter are also interpreted as alluvial sediments of fluvial origin.

Blackbrook Group

The Blackbrook Group, which is 1,430 meters thick, is located in the northwestern core of the anticline. It is mainly built up from tuffs in which turbidites and submarine mass flows are switched on. The group includes the at least 820 meter thick Ives Head Formation and the 610 meter thick Blackbrook Reservoir Formation.

The Ives Head Formation is divided into three members. The 238 meter thick Morley Lane Tuffs Member at the base of the exposed sequence of layers consists mainly of coarse-grained, crystalline tuffs of rhyolite composition, dust tuffs and pelitic tuffs. The approximately 550 meters thick, medium to fine-grained Grauwacken of the Lubcloud Greywackes Member are composed of lithic fragments of magmatic origin. The Grauwacken can occasionally take on a coarse-grained character, isolated pelitic tufa layers are also present. The 32-meter-thick South Quarry Breccia Member was created by submarine landslides and is dominated by tuffs - rhyolite dust tuff clasts in a matrix of coarser, weathered rhyolite tuffs.

The Ivesheadiomorpha are the first Ediacara biota to appear in the Ives Head Formation . A total of 49 fossil forms have been discovered so far, including Ivesheadia lobata , Shepshedia palmata, and Blackbrookia oaksi . However, these are not always their own taxa, but often only special, taphonomic special forms.

The Blackbrook Reservoir Formation is not divided into members. It essentially contains tuffy pelite and rhyolite dust tuffs that interlock with thin, coarse-grained rhyolite tuffs. 210 meters above the formation base, a striking, 30-meter-thick layer with very heavily weathered, very coarse-grained tuffs emerges.

Maplewell Group

The overlying Maplewell Group is horseshoe-shaped to the east, south and west around the core of the Charnwood anticline. It begins with the 1119 to 1347 meter mighty Beacon Hill Formation , which can be divided into four members (from hanging to lying):

The Beacon Hill Formation consists primarily of tuffs of Dazite composition , which have been rebuilt by swell. Furthermore, together with lava flows, there are subaeric and submarine mass flows, all of which originate from a proximal volcanic center. Trace fossils such as the structures of planolites can be seen in the sediments . To the side, the Beacon Hill Formation meshes with the pyroclastics of the Charnwood Lodge Volcanic Formation .

The subsequent, 190 to 530 meters thick Bradgate Formation consists of two members. The Sliding Stones Member at the base is a very distinctive location made up of a landslide breccia. This is followed by the Hallgate Member , which is made up of a coarse tuff breccia and superimposed green and gray fine tufts. The latter contain inclusions of coarse-grained ribbed layers and landslide horizons. At this level, together with horizontal worms, the famous Ediacara biota appear , which is preserved as an epirelief on the layers of fine-grained layers.

Brand Group

The final Brand Group follows the underlying Maplewell Group with a layer gap (hiatus) and is only exposed on the eastern and southern flanks of the Charnwood anticline. It begins with the Hanging Rocks Formation , which begins with a conglomerate up to 47 meters thick, the Hanging Rocks Conglomerate . Thirty meters of dark red to purple pelite with isolated, thin, coarser greywacke layers lie above it. The Brand Hills Formation , also separated by a layer gap , leads the Swithland Camp Member , which is up to 5 meters thick, at its base, which contains red, reconditioned siltstone clasts from the Hanging Rocks Formation, which are embedded in a siltstone matrix with a low degree of maturity. The following stable pit member is up to 30 meters thick. Its composition changes horizontally from a quartz arenite to a greywacke. The first trace fossils of the Arenicolites type appear 5 meters above the base. The Brand Group ends with the Swithland Formation , which is slate-shaped, especially in its upper, clay-rich section. It has purple and green-gray clays, which are replaced by silt layers. The spider structures of the trace fossil Teichichnus are remarkable . Since this Ichnotaxon only occurs from the Cambrian onwards , it can be assumed that the Precambrian-Cambrian border runs within or just below the Swithland Formation.

interpretation

The Charnian Supergroup evidently documents the relatively rapid erosion of an eruption center located in a volcanic arch, the weathering products of which alternate with the arch's basin sediments. A current example of the conditions prevailing at the time is the volcanic island of Montserrat in the Caribbean arc .

Intrusive magmatism

The two magma centers within the Charnian Supergroup with lavas and intrusions into higher floors (so-called porphyroids ) are andesitic to rhyodacitic composition. The more mafic lava series are characterized by an enrichment of the LIL elements potassium , rubidium , barium and thorium and by a depletion of the HFS elements niobium , tantalum , zirconium and yttrium compared to MORB .

After its deposition, the Charnian Supergroup was intruded by two magma series that differ from each other geochemically and structurally. The older North Charnwood diorites in the core area of ​​the Charnwood anticline , predominantly mafic diorites , are geochemically very similar to the upper section of the volcanic sedimentary sequence. The younger South Charnwood Diorites on the southern edge of the anticline are characterized by their granophyric structure . Their geochemical signature is calcareous with a high potassium content ( English high-K calcalkaline). Pharaoh and colleagues (1987) explain this change in magma composition with a thickening of the crust towards the end of the Neoproterozoic.

Comparable, granophyric diorites also intruded the endneoproterozoic volcanic series ( Chaldecote Formation ) of Nuneaton.

Geodynamics

Geological map of Bradgate Park at the southeast end of the anticline

The sequence was deformed and metamorphosed even before the basal sub-Cambrian quartzites , which are exposed at Nuneaton , settled. Regionally, a broad anticline dipping to the south-east was created , on which the individual formations of the supergroup are exposed. Due to the right- shifting transpression, the sediments were folded tightly and shed . The accompanying metamorphosis reached the lower green slate facies . The physical conditions could be determined on the basis of newly formed mica on slate surfaces extending from west-northwest to west-southwest. They reached 350 ° C at a depth of around 10 kilometers or a corresponding 0.35 GPa .

However, this early deformation before the Lower Cambrian sedimentation is now being called into question. Carney and colleagues (2008) were able to show that the foliation was created between 420 and 416 million years ago during the final phase of the Caledonian orogeny ( Acadian Orogeny ) towards the end of the Silurian . The deformations are therefore generally in the context of the subduction of the Iapetus Ocean under the eastern continental margin of Laurentia , which ended at the Silurian-Devonian border with Avalonia's docking with Laurentia. This collision united the land masses of southern Great Britain with Scotland .

The basement was during the Permian period and the beginning of the Triassic regional to a Paläorelief leveled (with some fairly steep Paläotälern) and then by the Mercia Mudstone Group Keuper transgresses.

Age

A minimum age of 603 ± 2 million years BP ( Ediacarium ) could be determined at Nuneaton on an intrusive diorite for the supergroup. Boreholes at Orton returned an age of 612 ± 21 million years BP and 616 ± 6 million years BP at Glinton for felsic tuffs.

Compston and colleagues (2002) found zirconia ages around 1600 million years BP, which suggest that the parent rocks ( Mesoproterozoic and Paleoproterozoic ) were still much older for the Charnian Supergroup.

More recent dating by Noble and colleagues (2014) on zircon populations using the U-Pb method for the base of the supergroup (Ives Head Formation) resulted in a maximum age of 611 and a minimum age of 569.1 ± 0.9 million years BP. The authors dated the Hanging Rocks Formation following in the hanging walls of the Maplewell Group with 604 and 557 million years BP. You therefore consider the interval 569 to 557 million years BP as the period of the Blackbrook Group and Maplewell Group deposition. For the classical horizon of the Ediacara biota (Bradgate Formation) they give 561.9 ± 0.9 million years BP.

This much younger age also confirmed the result of Compston and colleagues (2002), who were able to determine a comparable age of 559.3 ± 2 million years BP for tuffs in the Beacon Hill Formation. The authors also showed that the South Charnwood Diorites must be younger than the Beacon Hill Formation - a result that calls into question the previously accepted 603 million years BP for the correlable diorites from Nuneaton.

Individual evidence

  1. ^ A b Nigel Woodcock and Rob Strachan: Geological History of Britain and Ireland . Blackwell Science, 2000, ISBN 0-632-03656-7 .
  2. ↑ a b McIlroy, D. et al .: The Proterozoic-Cambrian transition within the 'Charnian Supergroup' of central England and the antiquity of the Ediacaran fauna . In: Journal of the Geological Society, London . tape 155 , 1998, pp. 401-411 .
  3. ^ Carney, JN: Geology of the Thringstone, Shepshed and Loughborough districts (SK41NW, SK41NE and SK51NW) . In: British Geological Survey Technical Report WA / 94/08 . 1994.
  4. ↑ a b Boynton, HE and Ford, TD: Ediacaran fossils from the Precambrian (Charnian Supergroup) of Charnwood Forest, Leicestershire, England . In: Mercian Geologist . tape 13 , 1995, pp. 165-182 .
  5. ^ A b Moseley, J. and Ford, TD: A stratigraphic revision of the Late Precambrian Rocks of Charnwood Forest, Leicestershire . In: Mercian Geologist . tape 10 (1) , 1985, pp. 1-18 .
  6. ↑ Pharaoh, TC et al .: Geochemical evidence for the Tectonic Setting of the Late Proterozoic Volcanic Suites in Central England . In: Geological Society, London, Special Publications . tape 33 , 1987, pp. 541-552 .
  7. ↑ Carney, JN et al .: 40 Ar- 39 Ar isotope constraints on the age of deformation in Charnwood Forest, UK . In: Geological Magazine . tape 145 , 2008, p. 702-713 .
  8. ↑ a b Compston, W. et al .: Dating the Late Precambrian volcanicity of England and Wales . In: Journal of the Geological Society, London . tape 159 , 2002, pp. 323-339 .
  9. ^ Noble, Stephen R. et al .: U-Pb geochronology and global context of the Charnian Supergroupn UK: Constraints on the age of key Ediacaran fossil assemblages . In: Geological Society of America Bulletin . 127 no. 1-2, 2014, p. 250-265 , doi : 10.1130 / B31013.1 .