Torridon Group Research Articles

Microfossils from the Aultbea Formation, Torridon Group, on Tanera Beg, Summer Isles

Synopsis A biota of sphaeromorph and nematomorph cryptarchs has been discovered in a grey shale unit of the Aultbea Formation, on Tanera Beg, Summer Isles. The grey shale unit is 4.9 m thick but microfossils are contained only within very thin fine grained intercalated laminae. The composition and lithological association of the biota suggests sedimentological control of its character.

Distinctive pebbles as indicators of Dalradian provenance

Synopsis Ferruginous chert or jasper pebbles are described from the Jura Quartzite in Argyll. Very similar pebbles have been found in the Torridon Group and the Quartzite may therefore be partially derived from such late Proterozoic sediments. The ultimate source of the pebbles is considered to be an early Proterozoic iron-formation which lay in the area of the present Scotland-Greenland continental margin. The Jura Quartzite and other Dalradian psammites may have been derived from a largely sedimentary source area to the NW of their present outcrop, their mineralogical maturity resulting from sedimentary recycling. In contrast, the Port Askaig Tillite was probably derived from an igneous and metamorphic source area to the SE with the possible implication that the Iapetus Ocean did not start to open until Argyll Group times or later.

Did Iapetus start to open during the Cambrian?

The geological evolution of the British Isles during the Lower Palaeozoic was dominated by the presence of the Iapetus Ocean. Although the timing of the major events in its later history are reasonably well known, its pre-Ordovician evolution is far less clear because of the absence of rocks of that age in the area of the lapetus suture. To deduce the early history of the Ocean, circumstantial evidence from rocks originally deposited on the adjacent continental crust is needed. Petrographical evidence, from such rocks in the Dalradian, is discussed here and implies that lapetus had still not opened by ∼670 Myr ago. A dramatic increase in tectonic instability after this date, culminating in the extrusion of the Lower Cambrian Tayvallich Volcanics, is considered to date the continental rupture that led to the formation of Iapetus. This view is in contrast to the assumption that continental break-up must have preceded the earliest Dalradian sedimentation1 and that the red-bed fluviolacustrine sediments of the Torridon Group, dated at ∼810 Myr, could be a consequence of this initial rifting2.

Palaeosalinity and environmental interpretation of red beds from the late Precambrian (‘Torridonian’) of Scotland

Illite (<1 μm, 1 Md polymorph) separated from red and grey shales in the late Precambrian Stoer and Torridon Groups (‘Torridonian’) has a boron content of about 100 ppm. Boron content of illite does not change through facies transitions, marine? → fluviatile, but increases significantly through an evaporating lake sequence. Illite separated by identical techniques from lacustrine cycles in the Old Red Sandstone has a boron content of 160–200 ppm. Marine shales generally have boron-in-illite contents > 200 ppm. We conclude that the shales in the Stoer and Torridon Groups are non-marine.

Sandstone-boulder conglomerates and a Torridonian cliffed shoreline between Gairloch and Stoer, northwest Scotland

Synopsis A highly irregular and, in places, steep erosion surface overlain by sandstone-boulder conglomerate marks the angular unconformity where the Torridon Group overlies the gently dipping sediments of the Stoer Group. Intermittent exposures suggest a lithosome with an overall linear shape. Locally, infilled embayments and gorges with fossil relief over 30 m occur along the surface of the unconformity. The surface varies from smoothly stepped and gently bevelled with small runnels and channels to strongly bevelled and undercut to near vertical. Using size distribution, roundness and fabric the conglomerate is subdivided into chaotic, boulder and stratified ‘facies’. Size, shape and lithology of the clasts implies local derivation from the underlying Stoer Group and, in places within the chaotic facies where blocks reach 4 by 11 m, very little transport. Imbrication within the boulder facies indicates transport towards the fossil relief. The nature of the unconformity, the apparent linear lithosome, alignment of large blocks along the cliff trace, local clast derivation, vertical and lateral fining away from the unconformity, imbrication and fabric, combined with heavy mineral bands and associated features of the sand portion of the stratified facies indicate erosion and deposition along a cliffed shoreline by a transgressing sea. Some 70 m of sands and grey shales transitional between the conglomerate and the Applecross Formation are interpreted as beach and tidal flat deposits.

Morarian Orogeny and Grenville Belt in Britain

Now that the nature of the Morarian orogenesis (about 730 Myr BP) is becoming more apparent1,2 it seems appropriate to assess its significance in terms of late Proterozoic history of the northern Atlantic region. It has been argued3 that the event is related to subduction, with a suture partly coinciding with the Great Glen Fault. The event has also been associated with the closing of a Precambrian ocean during the time represented by the unconformity between the Stoer and Torridon groups4. The Proto-Atlantic opened after the deposition of the Torridon Group. In establishing the orogenic character of the Morarian Event several features are important. First, the event is known to have affected Moine rocks in a zone some 20 km wide. If, as seems likely, the Morarian fabric can be traced as far south as the Great Glen Fault then the belt is some 50 km wide after the Caledonian shortening. Until more is known about the geology of Sutherland, however, it is uncertain whether the entire length of the Moine outcrop (about 250 km) is involved. There is no reason to believe that the Morarian fabric is present to the south of the Great Glen Fault: the likelihood is that the Central Highland Granulites (often named the Moines nowadays) represent post-Moine sediments, but the possibility that they are Moines that were not influenced by the Morarian Orogeny cannot be excluded. Until this problem is resolved it would be prudent to revive the name Central Highlands Granulite for the sub-Dalradian rocks occurring to the south of the Great Glen Fault.

Palaeomagnetism of Precambrian Sedimentary Rocks from NW Scotland and the Apparent Polar Wandering Path of Laurentia

Red sedimentary rocks of the Stoer Group (991 My BP) were resampled through their stratigraphic range (600 m at Stoer Bay). The mean direction of magnetization after thermal cleaning at 500° C is 313, +29 (α95= 8°) with a corresponding palaeomagnetic pole at 35° N, 234° E. This result is not significantly different from that obtained previously from studies of natural remanent magnetization. A small collection from the base of the overlying Torridon Group (796 My BP) confirms the general reliability of the earlier work on these rocks also, and shows that the large change between the Stoer and Torridon Groups coincides with the unconformity between them. The magnetization of these sediments originated at the time they were laid down. Since NW Scotland used to be part of Laurentia, the Stoer and Torridon Group poles can be compared with those from the late Precambrian of the Canadian Shield. In order to quantify these comparisons, a system of grading poles on a seven-point scale is introduced, and a generalized polar path for the Canadian Shield drawn. The poles from NW Scotland do not fit onto this path, indicating either that there has been slight rotation of NW Scotland as a marginal block of Laurentia, or that a hitherto unsuspected polar excursion occurred immediately following the Grenvillian orogeny.

Torridonian tourmaline-quartz pebbles and the Precambrian crust northwest of Britain

Exotic rocks found as pebbles in the Torridon Group span the range of Lewisian events (3000 to 1000 my. B.P.) but were formed at higher crustal levels than the present Lewisian. Their variations southward along the present outcrop seem to reflect changes in the basement as it is traced through Greenland towards Labrador. The new evidence is consistent with the views of previous workers. Tourmaline is used as a 'supracrustal index' and confirmed as a valuable dating mineral.

Pre‐Cambrian landscapes in Northwest Scotland

AbstractAn old land surface carved out of basement gneisses is extensively preserved beneath Torridon Group sandstones (deposited ca. 750 m.y. B.P.). Contoured reconstructions show that relief declines westward from 2000 ft (600 m) to almost nothing over a distance of 25 km. North‐northeast trending zones of similar dissection can be traced for 160 km. Weathering up to 2 m deep is locally present in the flat zone. The essentially unweathered land surface forming the mountains probably originated by dissection of a palaeoplain. The flat zone is probably, but not certainly, a relatively young pediplain. All land forms are thought to have developed before, not during, deposition of the overlying sediments.

Origin of disturbed bedding in Torridon Group sandstones

Origin of disturbed bedding in Torridon Group sandstones