The Torridonian

Suilven, Cul Mor, Cul Beag, Canisp, Coigach and Quinag - all these mountain massifs are formed from the Torridonian rocks. These are the oldest sedimentary rocks in the British Isles, deposited upon the Lewisian and preserved in the Caledonian foreland of NW Scotland. They tell of the erosion of long-gone mountain ranges and of ancient landscapes. Back in the middle 19th century the Torridonian was considered to be of Cambrian age. We now know it to be significantly older - indeed they are different ages. Grouping these sediments together as the Torridonian is itself misleading. In fact they are at least two distinct packages of sediment. The main sequence is called the Torridon Group that attains an estimated thickness in excess of 6 km. The lower part of this Group (the Applecross Formation, named after its type area in Wester Ross) is the classical pebbly sandstone that forms the cliffs and scarps of those great hills. But beneath the Torridon Group there are older sedimentary rocks, classically lumped in with the rest of the Torridonian. This is called the Stoer Group. The Stoer reaches thicknesses in excess of 3 km.

There are other Torridonian Rocks found within the structures of the Moine Thrust Belt. These include material that matches the main Torridon Group units together with older sequences, termed the Sleat Group. There have been various attempts to correlate these units with the Stoer Group and interpretations placing it as a basal part of the Applecross. But there is little information at present to demonstrate which of these ideas, if either, is correct.

Where and how were the Torridonian Rocks deposited?

Work by Stewart through the latter part of the 20th century has gradually established that the Torridonian Rocks were deposited within sedimentary basins controlled by normal faults. The sediments were deposited dominantly by rivers but other continental environments are also recorded by the deposits. One of the most dramatic features of much of the Torridonian outcrop is the basal unconformity with the Lewisian basement. This represents elements of the ancient landscape and preserves low hills and valleys. Up to 600m of topographic relief in the ancient landscape is preserved in Wester Ross.

The Torridon Group has been extensively studied to determine the palaeo-flow direction of the rivers that deposited the sediments, together with evidence for the likely source of the material. The data suggest that the Torridon Group was deposited on very large fans, perhaps 50 km across, fed from source areas to the west, beyond the modern Outer Hebrides. It is likely that much of the detritus was derived from a combination of Laurentian shield material (now buried on the continental shelf) together with recycled older sediment.

The pattern of sediment transport during the deposition of the Stoer Group was more complex than for the later, Torridon Group. Different parts show palaeoflows from the east and from the west, suggesting the basins were filled from their sides. In general much of the material was derived from the surrounding Lewisian basement and delivered into the basin by rivers and flash floods. The Stoer Group also contains shaly units and thin limestones - some with desiccation cracks. These features are interpreted as representing local lake and swamp environments. Rather mysteriously the Stoer also has deposits of volcanic ash - although the source volcanoes have never been found.

The ages of Torridonian rocks

Dating sediments is particularly difficult - especially when they are essentially unfossiliferous. We know that the Stoer is younger than the basement that they overlie - which makes it younger than about 1400 million years old. Thin limestones within the Stoer Group have yielded radiometric ages of about 1200 Ma (based on Pb-Pb geochronology) - thought to give the time of diagenesis (alteration soon after deposition).

The Torridon Group sediments are younger than the Stoer Group and older than the oldest Cambrian strata, placing them in the interval of 1200 to about 520 million years old. This rather wide age range can be narrowed if we consider the ages of some of the detritus within the Torridon Group sediments. Some robust minerals called zircons have yielded an age of about 1050 million years old. At this time the particular zircon must have been deeply buried within some basement unit - so the sedimentary rocks that now contain the zircon must be significantly younger. The best guess is that the Torridon Group was deposited about 1000 million years ago.

In summary, the Torridon Rocks are actually two distinct sequences separated by an unconformity representative of a significant time gap - perhaps as much as 200 million years. Support for this notion comes from palaeomagnetic data. This method can determine the palaeolatitude of the sediments at the time of deposition. For the Stoer Group the palaeolatitude is estimated as having been about 15 degrees from the equator. The Torridon Group on the other hand records palaeolatitudes of about 42 degrees from the equator. A significant time period is needed to account for the plate motion to take what was eventually to become NW Scotland through this distance.

Further information

Check out this wonderfully detailed yet accessible account of the Torridonian:

Stewart, A.D. 2002. The Later Proterozoic Torridonian Rocks of Scotland: their Sedimentology, Geochemistry and Origin. Geological Society Memoir 24, London, pp. 130.

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