Giant trilobites and biotite nodules in Portugal

Peter Perkins (UK)

The generally accepted reason for the fame of Arouca is Princess Mafalda, born 1195, who was responsible for the convent becoming Cistercian. Here is an interesting story – she was beatified in 1793. However, I won’t go into that now, but it is well worth investigating. For this article, there are other reasons for its fame, at least among geologists.

Arouca is 38km to the south east of Oporto, in northern Portugal, and gives its name to one of two geoparks in Portugal. In Arouca Geopark (Fig. 1), which has an area of 330km2 (just a little smaller than the Isle of Wight), there are two quite remarkable geological features, one palaeontological and the other concerning igneous petrology.

A geopark is an area of significant size that has a particular geological heritage, with a certain number of sites of special importance – scientific quality, rarity, aesthetic appeal and educational value. It must also have a sustainable strategy for development to be accepted as a member of the worldwide network of geoparks. There are 42 in Europe, in 16 countries. The other Portuguese Geopark is Naturtejo, through which the River Tagus flows.

There are nine geoparks in the British Isles, for example, NW Highlands (Scotland), Copper Coast (Ireland), Fforest Fawr (Wales) and the English Riviera.

The website, www.europeangeoparks.org, gives website addresses for all.

The geology of Portugal is very complex. There are no strata younger than Triassic, except for Holocene deposits in the valleys. The pre-Permian sediments have been metamorphosed and intruded by igneous rocks and there is also a considerable amount of mineralisation.

During the Lower Palaeozoic, there was an ocean environment with fine sediments accumulating on the floor. In this ocean lived a range of invertebrates. The animals that are the concern of this article are the trilobites, but there were also orthocone nautiloids, gastropods, bivalves, brachiopods, echinoderms and graptolites. There are also outstanding trace fossils.

The trilobites predominate and are what make the locality at Canelas exceptional. These fossils are found in the slates of the Valongo Formation – a fossiliferous succession of shales and siltstones, including slate beds giving material of roofing quality. This correlates with the Dapingian and Darriwilian of the Middle Ordovician, 471.8 to 460.9mya.

Complete specimens of some species reach up to 70cm in length and there seems to be a pattern to their occurrence – either isolated specimens or in clusters of up to thousands of specimens. While it is reasonable to suppose that these trilobites had a similar life style to the Horseshoe Crab (Limulus polyphemus), which number many thousands when they spawn on the beaches of Delaware Bay at full moon in Spring and Summer, the fact that the fossils are complete indicates that they died as a result of a catastrophic event such as a volcanic eruption. (In a natural death, the carapace would soon break apart along sutures, as it does when it moults as part of growing larger.) The asaphid, Ogyginus forteyi (Fig. 2), is abundant, but the most common trilobites are the calymenoids, Neseuretus and Bathycheilus, and the illaenid, Ectillaenus giganteus. These all indicate shallow water conditions.

At Canelas, in the northeast of the park, there is a working quarry extracting slate. The firm has been supportive of the palaeontological research that has been done and recognises the need to encourage educational opportunities. A small information centre (Fig. 3) has been established – Centro de Interpretação Geológica de Canelas.

This comprises a meeting room, where there is the opportunity to watch a video – mainly devoted to life in the Palaeozoic seas. A second building houses the museum, where the remarkable giant trilobites are on view (Fig. 4).

In the south of the park is the village of Castanheira, where a small (1,000m x 600m) and very peculiar granite can be seen. It is a medium-grained granite in which both muscovite and biotite are important – a type described as a ‘two micas granite’. (The granite of the Cornubian intrusion, extending from Dartmoor to Scilly Isles, is also a two-micas granite.) What is very rare, if not unique, is the presence of nodules of biotite, ranging from 1 to 12cm in diameter. These nodules are discoidal and show alignment (Figs. 7 and 8).

There is a popular legend that attributes magical properties to these nodules in terms of female fertility. As a result of differential expansion and dilation during hot summers, the dark lumps pop out spontaneously from the light coloured granite. As a result, the rock has been given the popular name of “Rocks delivering stones” (Pedras parideiras = ‘the stones that give birth’).

It is assumed that this ‘blob’ of granite magma rose slowly through the crust due to its low density compared with the surrounding rocksf and is related to the underlying Serra da Freita pluton. A close study of the nodules shows that they are zoned (Fig. 9).

The nucleus has a massive granular texture and consists of quartz and oligoclase (a potassium-rich plagioclase feldspar). Directly in contact with the nucleus is a zone of quartz and biotite, with the next zone made up almost entirely of biotite (and scarce muscovite). The outer layer is composed entirely of biotite. The micas, which contain potassium, have made K-Ar dating possible and ages of 320 to 310Ma have been obtained. (This equates to the Bashkirian stage, the lowest of four subdivisions of the Upper Carboniferous, and is approximately equal to Upper Namurian.)

What else to see? There are many features in the park of a geological and geomorphological nature. I would pick out the following:

• At Ragoufe are the remains of a wolfram and tin mine, opened in 1915 but now abandoned (Fig. 11).

• ‘Corn bread’ rocks – polygonal weathering of two large granite boulders – at Junqueiro (Fig. 12).

• Ichnofossils (trace fossils) at Vilarinho village.
• The contact between metamorphosed sedimentary rocks and granite, with metamorphic aureole and staurolite crystals near Mizarela.

GPS coordinates for all of these and other special features are given at http://www.geoparquearouca.com. The vast majority of visitors to the geopark are walkers – there is a network of marked walks (percursos pedestres) ranging in duration from 1½hrs to 6hrs.

Getting there
The journey by road can be done on your way to the Algarve from Santander. There are flights to Oporto from Gatwick, using Easyjet and AirPortugal.

Acknowledgements

The author wishes to thank Dr Artur Sá, who provided images for Figs. 5, 11 and 12, and for other helpful advice. Peter Perkins is a retired teacher of geology and geography, and is now a U3A tutor in Diss, Norfolk.