Cambrian Sediments in NW Scotland
(The following text is adapted from an essay on Assynt Geology hosted at earth.leeds.ac.uk and written by Rob Butler, University of Aberdeen.)
After the Torridonian, the next preserved rocks are of Early Palaeozoic, a period of dramatic explosion in life on Earth. The oldest of these rocks are the Cambrian basal quartzites. The quartzites are almost pure quartz probably representing deposition in very shallow seas in which the activity of tides and currents are recorded as cross-bedding. The unconformable junction with the Torridonian below is almost flat, suggesting the Torridonian had been nearly completely eroded following a marine transgression. The top part of the quartzites (the Pipe Rock) contains burrows - the oldest expressions of life in NW Scotland. These pipes, a type of trace fossil called Skolithos, were probably formed by worms burrowing into the sand. The effect of burrowing was to destroy much of the depositional structure in the quartzites and cross-bedding is only rarely preserved in the Pipe Rock. Collectively the quartzites attain a thickness of about 160 metres.
The next rocks are called the Fucoid Beds. These sediments are brown-coloured in outcrop and often have mosses covering them and may look like bits of rotten tree in small outcrops. Irregular features in these rocks are another form of trace fossil – probably a grazing trail of a small beast on the old sea bed. The Fucoid Beds contain lots of different types of trace fossil but are most important for containing rare body fossils – particularly of the Cambrian predators, trilobites. These rocks are about 520 million years old and again record a shallow marine (maybe lagoonal) environment. The Fucoid beds are overlain by clean, coarse-grained quartzites with vertical burrows. The unit contains small (2mm) spiral shells which gives the unit its name – the Salterella Grit. Collectively the Fucoid Beds and Salterella Grit are only about 25m thick.
The grits pass up into limestones and dolomites of the Durness group. The carbonates have a cumulative thickness of over 1 km but in general only the lower few tens of metres are preserved. Although much of the Durness has been recrystallised it is still possible to recognise traces of feeding burrows, algal mats and filaments together with small scours and carbonate grains called ooids. All these features are indicative of deposition in a shallow sea. Fossil evidence suggests that the carbonates began to accumulate towards the end of the early Cambrian but it continued on, with breaks, for a further 50 million years, into the Ordovician period.