Ginkgo forests, sharks and tsunamis in the Swedish countryside
Stephen McLoughlin (Sweden) and Vivi Vajda (Sweden)
Among the rolling green hills of Skåne in southern Sweden lies the valley of Fyledalen. The valley lies about 27km north of Ystad – best known as the setting of the Wallander detective novels and television series. Today, most of the valley has been cleared for agriculture, but a few isolated pockets remain of the tranquil beech and alder forests that covered much of southern Sweden in former times.
Fyledalen lies on the margin of the Vomb Trough, a structural depression that is part of the Sorgenfrei-Tornquist Zone, itself a broad zone of deformed and downthrown rocks stretching from the North Sea across southern Scandinavia and into Poland. This zone marks the southern boundary of the old Fennoscandian Shield that formed the core of the ancient continent called Baltica.
Earth movements since the early Mesozoic have, at various times, allowed accumulation of thousands of metres of sediment in this trough and, at other times, have caused uplift and deformation of those sediments. One consequence of these earth movements is that the rocks at Fyledalen have been tilted vertically and even overturned locally, so that, walking across the landscape, one can traverse a succession of strata that represent steps through time from the Early Jurassic to the Early Cretaceous.
Some of the oldest rocks exposed at Fyledalen are oolitic ironstones of Early Jurassic age. These rocks are poorly exposed, but various attempts at mining the iron ore have been carried out since the early 1800s. The low percentage of iron in the ore (30 to 35% Fe) and the difficulty in removing the unwanted clay and carbonate components meant that all attempts to process the ore proved uneconomic. A concealed adit near the village of Eriksdal is all that remains of the last attempt to mine the ore in the 1930s, but at least this tunnel now provides a home for a local population of bats.
The Middle Jurassic succession is represented by coals, shales and sandstones of the Fuglunda Member (lower Mariedal Formation). Again, the generally thin, laterally discontinuous and shale-rich seams have proved uneconomic to mine.
However, the shales of this unit do contain some of the finest preserved Ginkgo leaf fossils in the world. The most common species, Ginkgo regnellii, is represented by small leaves that are much more finely divided into slender gracile lobes than the single surviving member of this group – Ginkgo biloba (the Chinese Maidenhair tree). At Eriksdal, the fossil Ginkgo leaves can be readily peeled whole from the surface of the rock. After a little processing with acids in the laboratory, the details of their waxy cuticle can be studied to determine the pattern of the epidermal cells and stomata (the pores for gas exchange on the surface of the leaf).
The density of stomata on fossil leaves has been used to infer atmospheric CO2 levels through the course of the Earth’s history. Of course, there are many additional factors that influence the density of pores on leaves, including the evolutionary relationships of the plants and the availability of moisture in the environment. Ginkgo leaves are particularly useful in this respect, because they represent one of the few plant groups that have a fossil record spanning in excess of 250myrs, and they retained a similar shape and appear to have occupied similar climatic and environmental conditions through time. Since the density of stomata on leaves is influenced by CO2 in the atmosphere, and CO2 levels in turn influence global temperatures, the Eriksdal leaves can potentially be used as part of a ‘Ginkgo global thermometer’ stretching back to the Permian.
The fossil leaves of various other plants are preserved alongside those of Ginkgo in the shales exposed on the northern side of the old Eriksdal sand quarry. These include various ground ferns, such as Phlebopteris and Coniopteris, delicate Ginkgo relatives such as Baiera, and rare cycad-like bennettitaleans, such as Anomozamites angustifolium.
These Middle Jurassic sediments are interpreted as having been deposited in a coastal delta plain, where the landscape featured Ginkgo-dominated woodlands with an understory of mostly ferns, although studies of the fossil spores and pollen from the sediments reveal that the Middle Jurassic flora was more diverse than that reflected in the leaf assemblages.
The Eriksdal sand quarry was opened in 1938 to exploit the ‘Glass Sand’ – a unit of very pure quartz sand overlying the Middle Jurassic coals and forming the upper part of the Mariedal Formation.
A Bathonian age (roughly 165myrs) has been attributed to the Glass Sand, but its age is poorly constrained because the only fossils it contains are U-shaped Diplocraterion invertebrate burrows. This unit consists of remarkably pure quartz sand (99.8% silica) of even texture. It was deposited in shoreline environments, in which the sands were well sorted by longshore currents and wave action.
These sands were never deeply buried or consolidated, so they proved easy to exploit with mechanical shovels and dredges. Silica sand was mined at Eriksdal until 2008, when the operation was deemed no longer economic and some local residents had complained of noise and silica dust emanating from the processing plant. Nevertheless, the Eriksdal quarry supplied silica for 70 years towards the manufacture of glass, porcelain, fillers, abrasive pastes, tool mouldings and even the paint that makes up the white lines on Swedish roads. The silica was also sold internationally to markets as far away as the Middle East – not a bad effort, since selling sand to Arabia must be like selling ice to the Inuit.
Above the Glass Sand lies a series of Late Jurassic to Early Cretaceous clay and sand-rich units (Fyledal Clay, Nytorp Sand and Vitabäck Clay members) that have no economic value, but locally contain fossils of sharks’ teeth, bivalves, ostracods, foraminifera and the tiny calcified reproductive structures of charophytes (stoneworts: green algae) that were deposited in a mix of freshwater, brackish and marine environments.
One of these units – the Vitabäck Clay – is significant in that it potentially records traces of one of the most dramatic events in Scandinavian geological history. Around 145mya, an asteroid slammed into the continental shelf off northern Norway creating the 40km wide Mjølnir crater on the sea floor.
As this impact occurred in the sea, it probably generated a mighty tsunami that rolled southward through the shallow seaways that traversed Western Europe at that time.
On the southern flank of Baltica, the waves crashed ashore, ripping up coastal sediments, soils and vegetation and re-depositing them in distinctive cycles, as successive waves and backwash surges took place. The Vitabäck Clay at Eriksdal preserves beds with scoured bases containing jumbled marine bivalves, wood fragments and organic soil clasts that likely represent the deposits of that event. Fossil spores, pollen and algae have helped date these deposits to the same age as the impact event.
Similar deposits can be found as far away as France from this time.
A thick sheet of glacial till overlies the Mesozoic sediments throughout much of Fyledalen.
Such till deposits are typical features of the Nordic landscape and are a constant reminder to geologists that the great Scandinavian ice sheet retreated from southern Sweden just 15,000 years ago.
Although the Eriksdal sand quarry has closed, the old minesite remains an important locality for university geological fieldwork and has been on the itinerary of several major geological conference excursions. Although nature is slowly reclaiming this significant site, expeditions from Lund University and the Swedish Museum of Natural History over the past five decades have recovered collections of several hundred fossil samples from Eriksdal that will remain a valuable research resource for palaeontologists for centuries to come.
Stephen McLoughlin is a senior curator of palaeobotany at the Swedish Museum of Natural History, Stockholm; Vivi Vajda is Professor of Geology and Royal Swedish Academy of Sciences Research Fellow at Lund University in Sweden.
Further reading
Norling, E., Ahlberg, A., Erlström, M. & Sivhed, U. 1993. Guide to the Upper Triassic and Jurassic geology of Sweden. Sveriges Geologiska Undersökning, Serie C 82, 1–71.
Tralau, H. 1968. Botanical investigations into the fossil flora of Eriksdal in Fyledalen, Scania 2. The Middle Jurassic microflora. Sveriges Geologiska Undersökning, Serie C 633, 1–185.
Vajda, V. & Wigforss-Lange, J. 2006. The Jurassic-Cretaceous transition of Southern Sweden – palynological and sedimentological interpretation. Progress in Natural Science 16, 1–38.
Vajda, V. & Wigforss-Lange, J., 2009. Onshore Jurassic of Scandinavia and related areas. GFF 131, 5–23.
Yang, X.Y., Friis, E.M. & Zhou, Z.Y. 2008. Ovule-bearing organs of Ginkgo ginkgoidea (Tralau) comb. nov., and associated leaves from the Middle Jurassic of Scania, South Sweden. Review of Palaeobotany and Palynology 149, 1–17.