Dalradian Supergroup

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The Dalradian Supergroup is a geological group of strata that was deposited during the Neoproterozoic and Paleozoic Era in Scotland and in the north and northwest of Ireland . The period of its sedimentation is usually given as 806 to 470 million years BP .

etymology

The name for the Dalradian Supergroup, sometimes simply referred to as Dalradian , is derived from the former, Gaelic small kingdom of Dál Riata in Northern Ireland and western Scotland, the extent of which was roughly identical to the distribution area of ​​the Supergroup .

Distribution area

The Dalradian Supergroup is in the Central Highlands Terran between the Great Glen Fault in the northwest and the Fair Head-Clew Bay Line in Northern Ireland or its extension, the Highland Boundary Fault in Scotland. An exception is the Connemara Terran occurrence in Northwest Ireland, which is spatially separated from the main occurrence and represents a so-called outlier . The supergroup can also be found on Shetland .

introduction

The breaking of the Neoproterozoic Super continent (initially Rodinia and Ediacaran then Pannotia ) in cryogenian led to the separation Laurentias of Baltica and Amazonia and to the formation of Iapetus -Ozeans along a three-armed Riftgraben , about the triple point thereof the range of the Dalradian Super Group came to rest. As a result, sediment sequences of a passive continental margin formed on the east side of Laurentia during the crustal expansion lasting from 750 million years BP to the early Ordovician (470 million years BP) .

This rift process is documented in the British Isles in two sediment sequences - in the relatively thin , Cambroordovian shelf sequence in north-west Scotland and in the up to 25,000 meters thick Dalradian Supergroup . The shelf sequence finds very similar counterparts in northwest Newfoundland and eastern Greenland , the Dalradian Supergroup in turn resembles the Fleur de Lys Supergroup in Newfoundland and the Eleonore Bay Supergroup in East Greenland.

The two sediment belts are now roughly parallel to each other, with the shelf sequence being sedimented west of the Dalradian Supergroup on the continental margin of Laurentia. The deposits of the Dalradian Supergroup represent the oceanward area of ​​the opening Iapetus (shelf edge and continental slope). As a result of the later crust shortening of the Grampian Orogeny and the Caledonian mountain formation , the two belts were then transpressively pressed closer together.

Lithostratigraphy

Phyllite of the Ben Ledi Grit Formation , part of the Southern Highland Group, below the summit of Ben Lomond

The Dalradian Supergroup, which is about 25,000 meters thick, is lithostratigraphically divided into four groups (from hanging to lying ):

The sequence consists mainly of marine sands , clays , limestone , glacial pebbles and more rare volcanic inclusions .

Grampian Group

The basic Grampian Group is made up of the following subgroups and formations (from hanging to lying):

The 7,000 to 8,000 meters thick Grampian Group has its main deposit in the Grampian Highlands in Scotland. It also occurs on Islay and Western Ireland, but is absent on Shetland. The Colonsay Group's metasediments on Islay and Colonsay may belong to the Grampian Group. The group is lithological relatively monotonous and consists mainly of micaceous Psammiten and Semipeliten . Despite widespread, ductile deformation and green slate to amphibolite facial metamorphosis , primary sedimentary structures such as grading, inclined stratification , charge and drainage structures, erosion structures ( English scour structures ), clay patty breccias, internal layers and ripple marks can still be recognized. The Ord Ban subgroup, which is not quite 100 meters thick , overlays parts of the Central Highland Migmatite Complex and the so-called Basement Group , the local, metamorphic basement , as a shallow, marine shelf sequence . The nature of the contact - transgression with angular discordance or concordant superposition - can still not be decided. The Corrieyairick subgroup above is turbid in nature and was deposited in deepening rift basins. Significant internal differences in thickness suggest synsedimentary faults . The final Glen Spean subgroup consists of delta and shallow marine tidal sediments that migrated to the southeast and filled the former turbidite basin. The Grampian Group thus shows the transition from rift to thermally initiated subsidence .

Appin Group

The Appin Group consists of the following subgroups and formations (from hanging to lying):

The approximately 4,000 meter thick Appin Group is dominated by quartzites , limestone, semipelite and pelite, which were deposited on a stable shelf platform . It follows diachronically above the Grampian Group, recognizable by the interlocking exposure relationships. The base locations of the Lochaber Subgroup initiate a new cycle of transgression after the Glen Spean Subgroup of the Grampian Group visibly flattened to the hanging wall. The Ballachulish Subgroup and the Blair Atholl Subgroup are mainly composed of carbonate sequences from the offshore sector and anoxic tone sequences. The sequences indicate low sediment availability while the basin widened and thermally subsidized. Similar to the underlying Grampian Group, the Appin Group was also sedimented in half-trenches rotating to the southeast, recognizable by changes in facies and thickness.

Argyll Group

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

The Argyll Group is up to 9,000 meters thick. The Islay Subgroup starts with the up to 750 meters thick Port Askaig Tillite , a glacial formation that was deposited in the cryogenium between 760 and 720 million years BP during the Sturtic Ice Age . The Tillit contains numerous layers of rubble ( diamiktite ) that can lead to spectacular dolomite blocks of up to 320 meters in size in the Great Breccia on the Garvellach Islands ! Granitic rubble layers are likely to come from the neighboring Rhinns complex . The origin of the tillite, which was left behind either by advancing glacier masses or by calving icebergs , has not yet been clarified .

With the Bonahaven Dolomite above , there was a relatively quick return to warm, arid climatic conditions. The dolomite was sedimented in the shallow marine, intertidal area and contains dome-shaped stromatolites . After the sedimentation of the Bonahaven Dolomites, a marine transgression took place , during which the 5000 meter thick, tidal shelf sands of the Jura Quartzite were deposited. As paleo-current markings show, the sands were carried by a current parallel to the coast in a north to northeast direction. This formation also contains exotic pebbles (hematitic quartzites and cherts ), which are likely to have emerged from archaic or Proterozoic ribbon ores of the Caledonian foreland.

The overlying sediments of the Easdale Subgroup document a rapid expansion of the crust and subsidence, which manifested itself in the construction of deep water basins framed by faults. The Scarba Conglomerate and the Easdale Slates - submarine fan turbidites and deep water clays - have now been sedimented . The deep water basins had filled up again with the onset of the Craignish Phyliite , as this formation again indicates shallow water conditions with tidal sediments. However, when the Crinan Subgroup's Crinan Grits began , deep water conditions came back, probably triggered by tectonic movements.

The topmost section of the Argyll Group consists of the Tayvallich Subgroup , which is predominantly volcanic in origin. The basal Tayvallich Limestone is turbid in nature and was still deposited in the deep basins of the Crinan Subgroup. Above this follows the Tayvallich-Volcanic Formation , which is built up from basaltic cushion lavas , hyaloclastites , tuffs with interposed, turbiditic deep-water limestone and black clays. The tholeiitic volcanic rocks in a keratophyr were 595 ± 4 million years old BP. Volcanism began with thin layers of tufa, followed by basaltic tunnels and tunnels up to 3,000 meters thick , which intruded into the still wet sediment pile from the base.

Southern Highland Group

The following formations can be identified in the Southern Highland Group (from hanging wall to lying):

The up to 4,000 meter thick Southern Highland Group is characterized by the sedimentation of deep water turbidites. Clays, sands and rubble layers were deposited, all of which were later severely deformed. The fine-grained sediments in particular are intensely shed . The sands and pebbles reveal deep-sea fan facies, whose individual sediment packages (with flow channels at the base) reach a thickness of up to 15 meters. The current generally ran to the southeast, subordinate currents parallel to the basin also occur in the northeast-southwest direction. The increasing feldspar pouring towards the hanging wall of the group is striking . At this point in time, the metamorphic basement in the north-western delivery area of ​​the detritus had probably already been eroded and no longer just the sedimentary shell as before. In addition to the usual sedimentation, mafic volcanism occasionally occurs, which left pillow lavas ( Loch Avich Lavas ).

From the beginning of the Cambrian and up to the Lower Ordovician, the Southern Highland Group was located on an outer continental margin with little sediment, on which hemipelagic clays and trilobite- bearing deep-water carbonates ( Leny Limestone Formation ) slowly accumulated. In the MacDuff Slates in north-east Scotland, under deep water sediments, there are even indications of an Upper Cambrian-Lower Ordovician glaciation based on diamictites and dropstones .

Neoproterozoic glaciations

The following glacial horizons are now known in the Dalradian Supergroup:

  • Scree from Insishowe ( Donegal ) - Loch na Cille (South West Scotland) - Macduff (North East Scotland) from the Southern Highland Group
  • Stralinchy Conglomerate with Cranford Limestone (Donegal) as hat carbonate from the Easdale Subgroup
  • Port Askaig Tillite at the base of the Argyll Group

The Port Askaig Tillite is correlated by Brasier and Shields (2000) with the Ghubrah glaciation in Oman , which Bowring et al. a. (2007) has since been dated 713 million years BP. For the Stralinchy Conglomerate, McCay & Co. a. (2006) assume a correlation with a 635 million year old sequence of the Marino glaciation in Namibia . The scree of Inishowe-Loch na Cille-MacDuff put Bowring u. a. (2007) with the 582 million year old Gaskiers Ice Age .

Timeframe

The middle and upper section of the Dalradian Supergroup is delimited by fossil records and radiometric isotope ages. The lower section with the Grampian Group and the Appin Group is still very difficult to date despite many years of research; the minimum age is usually assumed to be 750 million years BP. It is believed that the sediments of the Dalradian Supergroup transgressed via the basement series of the Dava Succession and the Glen Banchor Succession , which were deformed and metamorphosed during the Knoydartian Orogeny (840 to 800 million years BP) towards 806 million years BP. To what extent the supergroup itself was affected by this geodynamic event cannot be decided. The supergroup can therefore be assigned a maximum age of 806 million years.

The port Askaig Tillite at the base of the Argyll Group can be correlated with the Varanger Ice Age or the Sturtic Ice Age, its age is therefore estimated to be between 750 and 720 million years BP (713 million years BP - see above). However, some editors also correlate it with the Marino Ice Age with an age of only 630 to 590 million years BP.

The only certain age information so far is the age of the Tayvallich-Volcanic Formation in the hanging wall of the Argyll Group, which was dated 594 ± 4 million years BP in the Ediacarian . Felsic tuffs returned 601 million years BP and the associated Ben Vuirich granite returned 590 million years BP. For the Argyll Group, this results in a total sedimentation period of 160 million years (from 750 to 590 million years BP).

The hanging wall of the Southern Highland Group dates from the Lower Paleozoic, as evidenced by Lower Cambrian trilobites from the Bonnia - Olenellus Zone in the Leny Limestone Formation and which are dated 517 to 509 million years BP. In the even younger MacDuff Slates in north-east Scotland, even Lower- Ordovician acritars , assigned an age of 490 to 470 million years BP, have been discovered. However, this find is not yet generally recognized. Overall, the sedimentation of the Dalradian Supergroup should have ended at the latest around 470 million years BP.

This is the point in time at which the Grampian Orogeny (470 to 440 million years BP) encroached on the sedimentary succession from the southeast, folded and metamorphosed it in a complex and multi-phase manner. The south-west of Scotland was less stressed here (green slate facies), whereas in the north-east conditions of 0.9 to 1.2 gigapascals corresponding to a subsidence depth of 25 to 35 kilometers were achieved (amphibolite facies).

Chemostratigraphy

Geochemical analyzes of the δ 13 C carb values in limestone formations by the Dalradian Supergroup show great similarities with other profiles worldwide. They therefore make it possible to narrow down the chronological course of the supergroup more closely.

Four distinct, negative δ 13 C isotope excursions occur within the Dalradian Supergroup . The youngest comes from Girlsta Limestone, which is part of the Southern Highland Group . This very pronounced anomaly ( -11 ‰ V-PDB) can be correlated with the global Shuram-Wonoka anomaly , which is assigned an age of 600 to 551 million years. The next very clear anomaly ( -13 ‰ V-PDB) lies below the glacial Stralinchy Conglomerate of the Easdale Subgroup and a correlation with the 635 million year old Trezona anomaly appears appropriate. The Islay Anomaly (−7 ‰ V-PDB) lies below the Port Askaig Tillite in the Islay Limestone of the Blair Atholl Subgroup and is thus somewhat older than 713 million years BP. The oldest, negative isotope excursion can be found in the Ballachulish Limestone of the Ballachulish Subgroup and should be equated with the approximately 800 million year old Bitter Springs Anomaly (−8 ‰ V-PDB).

Individual evidence

  1. ^ Soper, NJ: Neoproterozoic sedimentation on the NE margin of Laurentia and the opening of Iapetus . In: Geological Magazine . tape 131 , 1994, pp. 291-299 .
  2. ↑ McKie, T .: Tidal and storm influenced sedimentation from a Cambrian transgressive passive margin sequence . In: Journal of the Geological Society, London . tape 147 , 1990, pp. 785-794 .
  3. ^ Anderton, R .: Sedimentation and Tectonics in the Scottish Dalradian . In: Scottish Journal of Geology . tape 21 , 1985, pp. 407-426 .
  4. ↑ Harris, AL et al. a .: The Dalradian Supergroup in Scotland, Shetland and Ireland . In: Gibbons, W. and Harris, AL, A revised correlation of Precambrian rocks in British Isles (Eds.): Geological Society, London, Special Report . tape 22 , 1994, pp. 33-53 .
  5. ↑ a b c d e Woodcock, N. and Strachan, R .: Geological History of Britain and Ireland . Blackwell Science Ltd, Oxford 2000, ISBN 0-632-03656-7 .
  6. ↑ Condon, DJ and Prave, AR: Two from Donegal: Neoproterozoic glacial episodes on the northeastern margin of Laurentia . In: Geology . tape 28 , 2000, pp. 951-954 .
  7. ↑ Macdonald, R. and Fettes, DJ: The tectonomagmatic evolution of Scotland . In: Transactions of the Royal Society of Edinburgh: Earth Sciences . tape 97 , 2007, p. 213-295 .
  8. ↑ Condon, DJ a. a .: Neoproterozoic glacial rainout intervals: observations and implications . In: Geology . tape 30 , 2002, pp. 35-38 .
  9. Jump up ↑ Brasier, MD and Shields, G .: Neoproterozoic chemostratigraphy and correlation of the Port Askaig glaciations, Dalradian Supergroup of Scotland . In: Journal of the Geological Society London . tape 157 , 2000, pp. 909-914 .
  10. ↑ Bowring, SA a. a .: Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman . In: American Journal of Earth Science . tape 307 , 2007, p. 1097-1145 .
  11. ↑ McCay, GA et al. a .: Glacial trinity: Neoproterozoic Earth history within the British-Irish Caledonides . In: Geology . tape 34 , 2006, p. 901-912 .
  12. ↑ Halliday, A. et al. a .: The depositional age of the Dalradian Supergroup. U-Pb and Sm-Nd studies of the Tayvallich Volcanics, Scotland . In: Journal of the Geological Society, London . tape 146 , 1989, pp. 3-6 .
  13. ↑ Dempster, TJ et al. a .: Timing of deposition, orogenesis and glaciation within the Dalradian rocks of Scotland: Constraints from U-Pb zircon ages . In: Journal of the Geological Society, London . tape 159 , 2002, pp. 83-94 .
  14. ↑ Fletcher, TP and Rushton, APA: The Cambrian fauna of the Leny Limestone, Perthshire . In: Earth and Environmental Science Transactions of the Royal Society of Edinburgh . tape 98 , 2007, p. 199-218 .
  15. ^ Knoll, AH: Learning to tell Neoproterozoic time . In: Precambrian Research . tape 100 , 2000, pp. 3-20 .
  16. ↑ Prave, AR and a .: Global C cycle perturbations recorded in marbles: a record of Neoproteroizoic Earth history within the Dalradian succession of the Shetland Islands, Scotland . In: Journal of the Geological Society, London . tape 166 , 2009, p. 129-135 .
  17. ↑ Melezhik, VA et al. a .: The Shuram-Wonoka event recorded in a high-grade metamorphic terrane: insight from the Scandinavian Caledonides . In: Geological Magazine . tape 145 , 2008, p. 161-172 .
  18. ↑ Halverson, GP et al. a .: Towards a Neoproterozoic composite carbon isotope record . In: Geological Society of America Bulletin . tape 117 , 2005, pp. 1181-1207 .
  19. ^ Hill, AC and Walter, MR: Mid-Neoproterozoic (830-750 Ma) isotope stratigraphy of Australia and global correlation . In: Precambrian Research . tape 100 , 2000, pp. 181-211 .
  20. ↑ Prave, AR and a .: A composite C-isotope profile for the Neoproterozoic Dalradian Supergroup of Scotland and Ireland . In: Journal of the Geological Society . tape 166 (5) , 2009, pp. 845-857 .