Torridonian Supergroup
The Torridonian Supergroup is a sequence of predominantly continental, sandy and clayey sediments of the Mesoproterozoic and Neoproterozoic in the Northwest Highlands of Scotland . It lies unconformably on the gneisses of Lewisian ( Archean to Paleoproterozoic ). Their occurrence is limited to the Hebridean Terran .
Etymology and type locality
The Torridonian Supergroup was named after Loch Torridon , the upper section of which it is particularly well exposed. The name Torridon Sandstone for a section of the supergroup was first used by James Nicol .
Occurrence
The Torridonian Supergroup extends in a belt that is no more than 20 kilometers wide and around 200 kilometers long in a north-northeast-south-southwest direction from Cape Wrath to the tip of the Sleat peninsula of Skye . It crosses Caithness , Sutherland , Ross and Cromarty and Skye and Lochalsh . The isolated mountain peaks of Canisp , Quinag and Suilven in the area of Loch Assynt , the Slioch near Loch Maree and other peaks are built by it. The supergroup receives its maximum training in the districts of Applecross , Gairloch and Torridon . It is also present on some islands; B. on Scalpay , which is almost completely consumed by her, on Rùm , Raasay , Soay and on the Crowlin Islands . It can even be found on the seabed in the Sea of the Hebrides , where it is superimposed on Lewisian gneiss.
rocks
The rocks of the Torridonian Supergroup are mainly made up of red and brown sandstones , arkoses and shale clays . The total thickness of the supergroup can reach up to 11,000 meters. At their base, conglomerates occur locally , the components of which consist of the underlying Lewisian gneisses. The main mass of the sediments , however, arose from rocks that are no longer exposed anywhere. The sequence of layers of the supergroup is more or less horizontal above the uneven erosion discordance or has only a very small angle of incidence . In the hanging wall of the supergroup follow white, around 500 million year old quartzites of the Cambrian , which, like for example on the Beinn Eighe , can form distinctive summit structures ( Eriboll sandstone formation ). Some of the quartzites contain fossilized worm structures ( Skolithos and Monocraterion ) and are then referred to as pipe rock .
The landscape formed by the Torridonian Supergroup was strongly overprinted by the effects of water and ice in the last ice ages of the Pleistocene and reduced to an area of erosion.
stratigraphy
At the base of the Torridonian Supergroup is the Stoer Group , which is separated from the overlapping Sleat Group and the subsequent Torridon Group by an angular discordance. Paleomagnetic measurements have shown that this angular discordance corresponds to a pronounced layer gap .
Stoer Group
The Stoer Group, which is more than 2300 meters thick and dipping westward at 20 to 25 °, is open on the Stoer Peninsula near Assynt in Sutherland. It consists of three formations (from the hanging wall to the horizontal ) as follows:
Clachtoll formation
The average of approximately 350 m thick Clachtoll formation is the basis of Torridonian and builds from Fanglomerat - brecciation , whose sometimes very large clast mainly consist of gneiss Lewisian. The gneiss loads occasionally show signs of weathering. The Brecciated were in a Paläorelief poured whose Denivellation can be set, up to 300 meters. Further on to the hanging wall, there are obliquely layered sandstones (with trough oblique layers), which are interpreted as deposits of a meandering river system .
Bay of Stoer formation
The lower section of the Bay of Stoer Formation , with an average thickness of 300 meters, consists of red sandstones with trough sloping layers, whereby there may also be pebbles locally. The lower section is also interpreted as deposits of a meandering river system, but with the pouring direction to the east. The top 100 meters are made up of the Stac Fada member , who is only 10 meters thick, and the Poll-a'Mhuilt member . The minor Stac Fada member is a characteristic horizon, the origin of which is controversial. Facially it occurs as clayey sandstone, the clasts of which consist of up to 30% of vesicular, volcanic glass. Mostly it is interpreted as a volcanic mud or ash stream. The discovery of shocked quartz and biotite suggests an interpretation as the proximal ejection cover of a meteorite impact . The final Poll-a'Mhuilt-Member is a relatively thin sequence of silt and fine-grained sandstones that alternate with clayey sandstones. These sediments are seen as deposits in a silting lake .
Meall Dearg formation
The Meall Dearg Formation is very similar to the lower section of the Bay of Stoer Formation, but shows planar oblique stratification and is likely to have been set off in a wide alluvial plain in wide meanders of slight incline. The general direction of the discharge was to west-northwest. The upper limit of the Meall Dearg Formation and thus the Stoer Group is nowhere open on the mainland.
Sleat Group
The 3500 meter mighty Sleat Group is on the Sleat Peninsula in the south of Skye . Although it is overlaid by the Torridon Group, the contact should nevertheless represent a discordance. Any contacts with the underlying Stoer Group are nowhere to be seen. It is generally assumed that the Sleat Group is younger than the Stoer Group. However, the view is also expressed that it was deposited in a side basin and with a temporal overlap with the Stoer Group. The Sleat Group is integrated into the Kishorn ceiling and is weakly metamorphosed . As part of the thrust front of the Caledonian it lies in greenschist facies ago. This fact makes correlations with other formations of the Torridonian Supergroup very difficult. The continental sequence of the Sleat Group is predominantly made up of coarse, feldspar- rich sandstones (arcs) of fluvial origin. There are also gray and black, phosphate-rich slates of lacustrine origin. The sediments were generally poured from the west. The sediment clasts are mainly of volcanic origin and come mainly from rhyolite porphyries and rhyodacites .
The Sleat Group is made up of four formations (from hanging to lying) as follows:
Rubha Guail Formation
The discordance at the base of the Sleat Group or the Rubha-Guail Formation cannot be observed on Skye. In Kyle of Lochalsh, however, the sequence begins above the discordance with breccias, the clasts of which come from underlying Lewisian gneisses. The denivellation at the discordance to the basement is several hundred meters here. The remainder of the Rubha Guail Formation is made up of coarse, green sandstones with trough sloping layers. The green color is due to the mineral content of epidote and chlorite . Fine sandstones and siltstones with shrinkage cracks lie between the coarse sandstones . These storages are becoming more and more common for hanging walls. In general, in the Rubha-Guail Formation, a grain size reduction towards the hanging wall can be observed. This trend then continues in the overlying Loch-na-Dal Formation.
Loch-na-Dal formation
The lower, about 200 meters thick section of the Loch-na-Dal formation consists of dark gray siltstones, which can often be phosphatic . Occasionally, coarse to very coarse sandstone layers turn in. These sediments are interpreted as the maximum expansion stage of a lake. The upper section of the formation contains coarse sandstones, sometimes containing pebbles, with trough sloping layers. It is likely to be a result of delta pouring into a former lake.
Beinn-na-Seamraig formation
The Beinn-na-Seamraig Formation is built from coarse, diagonally layered sandstones. Characteristic of the sandstones their deformed stratification ( Engl. Contorted bedding ).
Kinloch formation
The Kinloch formation is very similar in character to the underlying Beinn-na-Seamraig formation. However, the sandstones are generally somewhat finer-grained and have a better developed fine layering of drifting ripples (English ripple-drift lamination). The sediments of the Kinloch Formation are built up cyclically. The 25 to 35 meter thick cycles become finer-grained on the hanging wall, each individual cycle ends with shale clay.
Whether the upper limit of the Kinloch Formation to the overlying Applecross Formation is a discordance or a concordant transition is still controversial. The interlocking between shale clays from the Kinloch Formation and sandstones from the Applecross Formation speak for concordant conditions. Furthermore, there are no more sandstone clasts above the contact and the magnetization of the rocks shows no change.
However, the degree of albitization above the contact changes very significantly, with the rocks of the Sleat Group only partially, while those of the Applecross Formation are completely albitized. Furthermore, there are mostly poorly understood indications of a change in the angle of incidence in the contact area, which indicate a discordance.
Torridon Group
The 5,000 to 6,000 meter thick Torridon Group was poured into an uneven topography, with a difference in height that can be up to 600 meters locally (for example at Loch Maree ). It erosively digs into the subsoil, as a result of which it comes to lie at a discordant angle over the Stoer Group and occasionally even over the basement of the Lewisian. Their sediments are mainly composed of red, coarse arkose sandstones of fluvial origin. Within the Torridon Group itself there is also likely to be a clear discordance between the basal Diabaig formation and the overlying Applecross formation.
The geochemistry of the sediments speaks for moist, subtropical deposit conditions, indicated by the predominance of fluvial sediments and the complete absence of caliche , evaporites or aeolian sediments . This is consistent with paleomagnetic measurements that showed palaeobatitudes of 30 to 40 degrees north.
The Torridon Group is made up of four formations (from hanging wall to lying) as follows:
Diabaig formation
The lowest section of the 600 meter thick Diabaig Formation consists of basal, red breccias, the clasts of which stem from the underlying Lewisian. The breccias are thickest in paleo valleys and merge to the hanging wall and sideways into tabular gray sandstones.
These sandstones, formed locally in river valley facies, interlock with gray shale, which in turn contains thin, fine-grained, ripple-leading sandstone layers. The shale is traversed by drying cracks that are filled with overlying sandstone layers.
In the upper section of the Diabaig Formation, massive sandstone layers with a clear lower edge appear for the first time, which become more and more frequent and thick towards the hanging wall. Drifting ripple layers on the upper edge of the sandstone banks indicate eastward transport. This sequence is interpreted as an alluvial fan , which gradually filled the existing profile and built into temporary lakes. The massive layers are interpreted as lake turbidites .
Applecross formation
The roughly clastic, 3000 to 3500 meter thick Applecross formation poured from the northwest abruptly ended the lacustrine Diabaig sedimentation. These are predominantly fluvial arkose sandstones, which were deposited in a wide flood plain of plaited river courses. The coarse, diagonally layered arkose sandstones have both trough and even inclined layers, with the troughs giving the direction already indicated. They are also characterized by a characteristic rubble fraction, which is composed of jasper and porphyry clasts . Most sandstone layers have deformations that were created in the unconsolidated state and possibly indicate seismically caused liquefaction of the sediment.
In the uppermost section of the Applecross Formation, the sandstone sedimentation becomes finer-grained and leads over to the overlying Aultbea Formation.
At Cape Wrath , the lower section of the formation shows fanning out flow directions, which are explained by a huge alluvial fan (with a radius of about 40 kilometers). Its opening point was near the Minch Fault and its catchment area was calculated to be 10,000 square kilometers.
Aultbea formation
The 1500 to 2000 meters thick Aultbea Formation is quite similar to the Applecross Formation, but it is generally more fine-grained (fine to medium-grained) and hardly carries any boulders. It also has deformations of the unconsolidated sediment. The grain sizes of the sandstones are getting finer and finer towards the hanging wall. Coherent flow fans can be found again at Cape Wrath and indicate a Bajada deposit environment in which several fans merged and merged into a pigtail system.
Cailleach head formation
The approximately 800 meters thick Cailleach Head Formation is similar to the Aultbea Formation, but is again much finer-grained. The formation is organized cyclically ( cyclothemes ). The 22 meter thick cycles have an erosion surface at their base, then dark gray slate clays with shrinkage cracks, sandstones with planar inclined stratification and concluding corrugated ripples, and finally mica sandstones with trough inclined stratifications. The cycles are viewed as delta sediments progressing repeatedly into a lake. Missing evaporite minerals indicate that the lake environment had a constant runoff and was therefore well ventilated.
Noteworthy for the Cailleach Head Formation is the discovery of Acritarchen by Teall in 1907. These were the first scientifically described fossils from the Precambrian of Great Britain.
Dating
The maximum age of the Torridonian Supergroup can be limited based on the last tectonic or metamorphic movements in the Lewisian basement, whose ages are all concentrated in the period 1200 to 1100 million years BP ( stenium ). The minimum age results from the age of the discordant overlying quartzite of the Cambrian, which was dated to 544 million years BP.
Radiometric ages of the Torridonian Supergroup showed 1200 million years BP for the Stoer Group and 1000 to 950 million years BP for the lower section of the Torridon Group. There is thus a gap in time of at least 200 million years between the deposition of the two groups. Paleomagnetic measurements also come to the same conclusion.
Zirconia ages can also help further limit the age requirements in question. The Stoer Group and the lower section of the Sleat Group have zirconia ages that come primarily from Scourian gneisses and to a lesser extent from Laxfordian gneisses, all of which are older than 1,700 million years. The upper section of the Sleat Group predominantly has an age of the Laxford orogeny (around 1700 million years BP), archaic ages are almost nonexistent. The lower limit of the zirconia ages encountered is 1200 million years BP. An age cluster appears in the Diabaig Formation at 1100 million years BP, the time of the Grenville Orogeny . In the Applecross Formation and in the Aultbea Formation, Grenville ages also occur, but also ages that are slightly below 1000 million years. It follows that the Stoer Group and the Sleat Group were deposited before the Grenville orogeny, ie 1200 million years ago BP, but the Diabaig formation at the base of the Torridon Group after orogeny (after 1090 million years BP). The younger sections of the Torridon Group were not sedimented until after 1060 million years BP.
A very precise direct dating could be carried out on autogenous feldspars of the Stac-Fada-Member of the Stoer Group with 1177 ± 5 million years BP, which began to grow immediately after the ejecta was placed.
Tectonic position
The tectonic position of the Torridonian Supergroup is still controversial. Changes in thickness and lithology suggest that the supergroup was deposited in a rift .
Seismic reflection measurements in the Minch suggest that the Minch Fault was constantly in motion during the supergroup's sedimentation. This is in good agreement with the generally western provenance of the pebbles of the Applecross Formation, which suggests a continuously renewing sediment source in this direction.
Although the rift character of the sediments of the Stoer Group and the Sleat Group is evidently generally recognized, more recent work suggests that the extent and continuity of the Torridon Group, especially the Applecross and Aultbea Formations, are more likely for the deposit in a molasse basin of the Grenville orogeny is likely to speak.
Individual evidence
- ↑ Parnell, J .; Mark D., Fallick AE, Boyce A. & Thackrey S .: The age of the Mesoproterozoic Stoer Group sedimentary and impact deposits, NW Scotland . In: Journal of the Geological Society . tape 168 (2) , 2011, pp. 349-358 .
- ^ A b c Stewart, AD: The Later Proterozoic Torridonian Rocks of Scotland: Their Sedimentology, Geochemistry and Origin . In: Geological Society of London Memoir . No. 24 , 2002.
- ↑ Amor, K .; Hesselbo SP, Porcelli, D., Thackrey S. & Parnell J .: A Precambrian proximal ejecta blanket from Scotland . In: Geology . tape 36 (4) , 2008, pp. 303-306 .
- ↑ a b c Kinnaird, TC, Prave, AR, Kirkland, CL, Horstwood, M., Parrish, R., Batchelor, RA: The late Mesoproterozoic - early Neoproterozoic tectonostratigraphic evolution of NW Scotland: the Torridonian revisited . In: J. Geol. Soc. tape 164 , 2007, p. 541-551 .
- ↑ a b c d e f g Park, RG, Stewart, AD, Wright, DT: The Hebridean Terrane . In: Trewin, NH (Ed.), The Geology of Scotland, 4th ed.Geological Society of London, 2003, pp. 45-61 .
- ^ A b Nigel Woodcock and Rob Strachan: Geological History of Britain and Ireland . Blackwell Science Ltd, 2000, ISBN 0-632-03656-7 .
- ↑ Teall, JJH: The petrography of the Torridonian formation . In: Memoirs of the Geological Survey of Great Britain . 1907, p. 278-290 .