6.2.1 Torridonian Group
The Torridon Group is sedimentary unit of rocks represented on the Isle of Rum by successions of sandstones, siltstone and shales. This unit totals 2500 meters in thickness and lie uncomfortably on top of the Lewisian Gneiss basement. Radiometric dating put the age of the Torridon Group at 1000 – 950 million years old (Trewin, 2002). Two Formations of the Torridon Group can be located on the Isle of Rum:
1. Diabiag Formation
a. Fiachanis Gritty Sandstone Member
b. Laimhrig Shale Member
2. Applecross Formation
a. Scresort Sandstone Member
b. Sgorr Mhór Member (Not encountered in the mapping area)
Given their closeness to the central complex, these rocks are largely unmetamorphosed. Only those in the immediate vicinity of the complex and minor intrusions, display sight contact metamorphism.
1a. Fiachanis Gritty Sandstone Formation
The Fiachanis Gritty Sandstone Formation exclusively crops out within the Main Ring Fault, and can be clearly observed in the floor of Corrie Dubh between the hydroelectric dam approximately 8 meters below the deer fence and the historic hydroelectric dam in the centre of Corrie Dubh. Another location the Fiachanis Gritty Sandstone can be observed is approximately 30 meters northwest of Príomhs Loch Mòr, where the unit is in direct contact with Lewisian Gneiss, forming an unconformable boundary (Insert figure). This unit of sandstone presents itself at a coarse granular quartzite sandstone that exhibits minor cross and planar bedding. The grain size fines up sequence and finally grade into fine shales and sandstone of the Laimhrig Shale Member. Many of the outcrops in the floor of Corrie Dubh have basaltic dyke cross-cutting them. On one particular outcrop that is located about 12 meters northeast of the Corrie Dubh path under Meall Breac, this outcrop measured 17m x 19m and had two dyke crosscutting perpendicular to each other. Many of these large dykes can be followed for 30 meters in either direction and crosscutting multiple units. The lower boundary of the Fiachanis Gritty Sandstone lies unconfomably on top of the Lewisian Gneiss, as mention before this can be clearly observed northeast of Príomhs Loch Mòr. The upper boundary of this unit is I contact with the Laimhrig Shale Member, this boundary is obscured by vegetation but the approximate boundary occurs below just below the modern hydroelectric dam. The change in lithology can be seen yet the boundary is obscured.
1a. Laimhrig Shale Member
The Laimhrig Shale Member outcrops in one location within the mapping area, which is 4 meters below the modern hydroelectric dam and is best observed in the path leading to Corrie Dubh. The Laimhrig Shale Member is a sequence of shales, mudstone and siltstones. At the location this unit was visible, the rocks showed a very fined grained grey to blue mudstone/siltstone. This outcrop displayed very fine laminations that alternated with grey/blue and cream layers. Within the lamination microstructures such as faulting could distinctly be observed. This microstructure ranged in size from a few millimetres to 10 centimetres (figure).
1a. Scresort Sandstone Member Office1
The Scresort Sandstone Member is one of the most widespread rock units on the Isle of Rum. This unit can be easily observed on the outside of the Main Ring Fault, from the northwest of the island through 225° to the very south of the island. This unit consists of medium to coarse sands in thick beds of 1-3 meters. The Scresort Sandstone Member is pale pink to pale red in colour this is due to prolonged exposure to atmospheric conditions that have caused oxidisation of the iron in the sand. In many areas including in Kinloch Glen and adjacent to the Corrie Dubh path, ripple bed-forms can be observed, especially in the river bed of Allt Slugan a’Choilich, these structures have been interpreted as forming in a paleo-environment of sandbars in a braded river system (reference). This unit gently dips towards the north east, this orientation can be clearly seen on Mullach Mór (figure) and Is best seen at a distance standing in Corrie Dubh looking directly north towards the Isle of Skye. The Scresort Sandstone is a particularly thick unit and has been estimated to be in the order of 2000 ± 500 meters thick. The lower boundary of this unit is marked by medium to coarse arkoses sandstone with thickly bedded and convoluted cross stratification. There is also the appearance of ‘exotic’ pebble which consist of examples of vein quartz’s, volcanic fragments and chert. The upper boundaries of this unit is marked by the dissapearence of the ‘exotic’ pebbles what are considered indicative of the Scresort Sandstone Member.
6.3.1 Corrie Dubh Breccia
The Corrie Dubh Breccia is a sedimentary breccia formed of mainly Torridon Group rock, predominantly the Fiachanis Gritty Sandstone Member, but in rare cases clasts of Lewisian Gniess and igneous clast can be found. This unit can be both clast and matrix supported, the matrix is comminute sandstone. Good exposure of outcrops can be found in the floor of Corrie Dubh, where they display angular and sub-angular clast ranging in size from a few millimetres up to several centimetres wide. Although most clast range between 5-20cm wide. Bedding within the unit is very poor and in most cases non-existent, but within the clasts themselves it is clear to see original laminations and bedding from their parent rock. The rocks colour is fairly homogenous light grey throughout. Yet below Cnapan Brecca, north east of the silted up lochan there is bands of dark grey ‘beds’ (figure). The matrix of these ‘beds’ is almost black, the clasts remain a homogenous light grey. As well as the obvious change of colour the ‘beds’ there is well-defined normal faults. The Corrie Dubh Breccia was formed by debris avalanching into the caldera during the first stage of caldera building. the breccia can be found in Corrie Dubh and in the immediate area surrounding the Corrie, for instance the valley between Meall Breac and Am Màm. The Corrie Dubh Breccia is in sharp contact with the Fiananis Gritty Sandstone and this can be distinctly seen in at the top of the fissure where Allt Slugan a’Choilich runs through. The upper boundary can be found on the flanks of Cnapan Brecca and Meall Breac where the unit passes up into tuffs and pale cream epiclastic sandstones. The epicalstic sandstone can be most easily observed at the bases of Cnapan Brecca, the most likely expaination for these rock is reworking of the debris in the caldera or the reworking of country rock fragments during explosive events of volcanism. The age of this unit will coincide with the emergence of the caldera itself. This has been estimated to approximately 60 ± 0.5 Ma (reference).
The rhyodacite on the Isle of Rum only occurs in the Northern Marginal Zone. The outcrops of rhyodacite can be easily found on the hills of Cnapan Brecca, Meall Brec and Am Màm. Within the area of Corrie Dubh both intrusive and extrusive examples of rhyodacite can be found. The rhyodacite is porphyritic as it contains easily observed phenocryst of plagioclase. The matrix of the rhyodacite is very fine. The extrusive example of the rhyhodacite can be found in several places including Meall Brec and Cnapan Brecca. On the south-eastern edge of Meall Brec, fiamme can be seen. Fiamme can also be observed on Cnapan Brecca. The fiamme represent the presence of ignimbrite, which is a poorly sorted pyroclastic rock. It is known that the rhyodacite phase of the eruption would have been explosive and would have produce heavy ash fall and pyroclastic density currents. Areas of intrusive rhyodacite can be found in Corrie Dubh itself where it cuts through the Corrie Dubh Breccia at NM 393 979. Although the margins of the plug are not clear, fragments and rubble of Rhyodacite can be found freely lying on the ground, as well as some intact and for the most part what seems to be in situ rock can be seen.
6.3.3 – Long Loch Granite (microgranite) ????? may be older than the rhyodacites further research.
Within the mapping area the Long Loch Granite occurs along the north-eastern shore of Long Loch. This unit is confined within the boundaries of the Long Loch fault and the Main Ring Fault and has contacts with peridotites to the south, has minor contacts with Lewisian Gneiss to the south east and south west and with Gabbro to the east. This unit is not the best exposed unit on the Isle of Rum, yet outcrops can be seen intermittently over its expanse. The microgranites are pale cream/brown in colour, phenocryst of feldspar can be identified. The outcrops are highly weather, making mineral identification difficult, but due to the colour of the rock being pale cream/brown it would be fairly likely that the unit will contain mafic minerals. The age of this unit is similar in aged to that of the rhyodacite, it may even be older than the locally occurring rhyodacites.
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