A new chapter has been written for the geological development of southern Finland. The research study utilized unconformities, old sedimentary rocks and impact craters to estimate erosion through the last 1.5 billion years. The ancient bedrock of Finland has been alternately exposed to erosion and meteorite impacts and then buried in sedimentary rocks over many cycles lasting 100s of millions of years. Buried valleys in Ostrobothnia may be the some of the oldest valleys on Earth.
We know that the rocks in Finland are very old. The rocks at the core of the Baltic shield in Karelia are Archaean in age, extending back to 3.5 Ga (billion years) But how old is the land surface, the ground beneath our feet? A group of GTK’s researchers, together with Adrian Hall of Stockholm University and Marko Holma of Arctic Planetary Science Institute, have been searching in deep time to try to find out. Their geological journey began with discoveries of ancient buried valleys in Ostrobothnia and the collection of scattered records of sedimentary rock in the region. The scope then expanded vastly across the enormously long-time span for the evolution of the Fennoscandian shield. The project group has now described the development of the craton in deep Finland in the journal Precambrian Research article “Ultra-slow cratonic denudation in Finland since 1.5 Ga indicated by tiered unconformities and impact structures”.
The first step back into geological time is easy. Everywhere in southern Finland we see the imprint of scouring by ice sheets over the last million years (1 Ma). The glaciers removed a few tens of metres of rock and formed shallow basins now filled by a thousand lakes and swamps. Seeing further back is much more challenging. Across much of Finland the loose sediments from the Ice Age rests directly on the ancient, hard basement gneisses and granites, a gulf in time of at least 1.5 Ga.
But around the fringes of southern Finland, in the Bothnian Sea, in Estonia and around Lake Ladoga, we find layers of sedimentary rocks which once covered much wider regions and which bridge significant parts of that time gap. The oldest quartz sandstones on the island of Suursaari in the Gulf of Finland date back to 1.7 Ga, Jotnian sandstones at Satakunta date from 1.5 Ga, marine mud and sand from 941 Ma fill a fault zone near Helsinki and more sandstones and limestones began to accumulate at 550 Ma, eventually burying all of southern Finland in the Early Palaeozoic. The contacts between overlying sedimentary rocks and the basement are called unconformities and each marks the end of a long period of erosion and the onset of an even longer period of burial.
Old buried valleys in Kurikka
Yet southern Finland is a lowland and so the unconformities span a narrow height range. Consider Kurikka in Ostrobothnia, where recent drilling for groundwater has shown that deep valleys in the basement hold patches of 1.5 Ga old sandstone beneath thick glacial sands. These valleys were once entirely buried by the sandstone; only in the geologically recent past were the sandstones eroded away. So the Kurikka valleys are extremely old features, amongst the oldest valleys on Earth. Nearby at Lauhanvuori we have a flat unconformity surface which is overlain by 540 Ma old Cambrian sandstone. Glacial erosion of that sandstone in the Ice Age has re-exposed the basement around Lauhanvuori. The vertical stacking of the three unconformities – from 1.5 Ga, 541 Ma and 1 Ma – makes clear that over the last 1.5 Ga only about 100 m of the basement rock has been eroded from Ostrobothnia. That erosion is ultra-slow when compared with erosion rates over recent geological time which start at 10 m per million years on flat lowlands like Amazonia and which can exceed 1 km per million years in mountain ranges like the Himalaya.
Meteorite craters – the key to the Earth’s crust archives
Did erosion really go that slow in Finland? As a check, the researchers turned to impact structures. Finland has more of these than any other country in the world – we will ask why shortly. Some, like Söderfjärden in Vaasa, still have crater form, despite an age of about 541 Ma. Others have been more deeply eroded, like the larger Keurusselkä structure that dates from 1.2 Ga. The depth of post-impact erosion can be approximated by using the geometries of young, well-preserved impact craters on Earth and applying those geometries to older, eroded structures in Finland. The key parameters are the present diameters of the structures and the remaining depth of impact breccias in the structure, the rocks on the crater floor that, on impact, were shattered, heaved upwards and outwards and partly melted.
In Finland, there are several small craters from asteroid collisions between 710 Ma to 85 Ma. Some craters hold remnants of the sedimentary rocks that covered the basement at the time at impact; others were filled with sediments soon after impact. Piecing together the erosion histories of each impact structure allowed the researchers to build up a broader picture of when and where the ancient basement of Finland was either exposed to erosion or it was protected by burial or lay on the seabed.
Earth’s slowest erosion behind the unique story
Results for the impact structures support ultra-slow erosion in basement and sedimentary cover at rates of <2.5 mm per thousand years – as slow as anywhere on Earth today but continued over timespans of many millions of years in Finland. The debris that buried Finland came from mountain belts in Sweden and Norway, since eroded away, but these sedimentary blankets were thinner than previously thought. A key event in the Early Palaeozoic was the development of the Cambrian unconformity, a remarkably flat basement surface, and its burial beneath shallow water sandstones and limestones. The planar surface of the Cambrian unconformity, now gently inclined and roughened by Ice Age glaciers, is still apparent on the land surface as it rises out of the Bothnian Sea and the Gulf of Finland.
The Cambrian unconformity developed slowly after 580 Ma. During this long period of exposure to bombardment by asteroids the surface of southern Finland became pock-marked with impact craters. Each crater was filled and covered by Early Palaeozoic sediments, preserving it from erosion. At Söderfjärden the fine state of preservation of the crater rim indicates that this crater has been only recently exhumed by glacial erosion of soft Cambrian clays from the crater floor. The remarkable concentration of impact structures in Finland is a result mainly of re-exposure of the Cambrian unconformity.
Projection of the plane of the Cambrian unconformity across central Finland shows that later minor tectonics raised the land into a low dome, similar to that found in southern Sweden. Erosion over the last 50 million years or so has sliced the top off that dome, removing 100-200 m of rock. A similar thickness of rock, mainly sedimentary, is missing from above the Lappajärvi impact dated to 78 Ma. Through the latest geological period of relatively enhanced erosion, the erosion rate has still remained below 2.5 m per million years.
The preservation of ancient valleys, closely-stacked basement unconformities and small impact structures show that the depth of erosion in the basement in southern Finland through geological time has been remarkably small. Only a few of the most stable parts of the Earth’s crust in places like Australia have similar histories of inertia. Thus, the valleys of Ostrobothnia are among the oldest in the world, if not the oldest.
The ultra-slow erosion in basement is a result of prolonged burial of Finland, spanning about 1 billion of the last 1.5 billion years, and the fundamental tectonic stability of the ancient crust – Finland is indeed an old land.
Senior Scientist Niko Putkinen, +358 29 503 5220, firstname.lastname@example.org
Geologist Satu Hietala, +358 29 503 3005, email@example.com