The Geology of the Lakeland Finland Area
Total Page：16
File Type：pdf, Size：1020Kb
THE GEOLOGY OF THE LAKELAND FINLAND AREA Jari Nenonen and Anne Portaankorva Geological Survey of Finland 2009 2 TABLE OF CONTENTS 1. FINLAND’S GEOLOGICAL HISTORY ……………………………………………….. 3 2. BEDROCK GEOLOGY OF FINLAND AND THE NEED-PROJECT’S REGION…… 4 3. THE BEDROCK OF THE NEED-PROJECT’S REGION ……………………………... 9 4. SURFICIAL DEPOSITS IN THE NEED-PROJECT REGION ……………………….. 13 4.1. Glacial deposits ……………………………………………………………………. 18 4.1.1. Basal till ....…………………………………………………………….…... 18 4.1.2. Ablation moraine – hummocky moraine ………………………………….. 19 4.2. Glaciofluvial deposits ….…………………………………………………………... 20 4.2.1. Eskers ……………………………………………………………………... 20 4.2.2. Ice-marginal formations …………………………………………………... 21 4.3. Aeolian deposits ………………………………………………………………….... 23 4.4. Peat deposits ……………………………………………………………………….. 24 5. EVOLUTION OF THE LAKES ……………………………………………………….. 24 6. GEOLOGICAL SITES IN NATURE TOURISM ……………………………… …….. 29 7. LITERATURE …………………………………………………………..……………... 31 3 1. FINLAND’S GEOLOGICAL HISTORY A special feature in Finnish geology is the very old bedrock and the much younger surficial deposits lying on top of it. Finland’s geological history is extremely long. The history of bedrock goes up to 3,5 – 3,7 Ga ago when the old Archaean basement was developed(Fig. 1). The oldest stone in Finland and in the whole of the EU is Siurua gneiss (3,5 Ga years old) near the town of Pudasjärvi in northern Finland (Fig. 2). Surficial deposits have developed during the last two million years, during the so called ice age. Glaciers eroded the bedrock beneath them and deposited loosened material upon it as quaternary deposits. Bedrock, together with quaternary deposits and landforms, builds up the variable landscape complex which can be seen today. Figure 1. Two scales illustrating the historical development of Finnish geology by Tapani Tervo and Harri Kutvonen, GTK. 4 2. BEDROCK GEOLOGY OF FINLAND AND THE NEED-PROJECT’S REGION THE ARCHEAN BASEMENT The bedrock of Eastern and Northern Finland consists of ancient rocks dating back to more than 2500 million years, the detailed origins of which are not accurately known. Their development would seem, however, to have been controlled by events in which large quantities of the earth’s crust melted and then crystallized slowly to form various granitoids and gneisses (Fig. 3). These were interspersed with sequences of schists, depicted in a darker shade on the map, which represent the remnants of rocks that had once been deposited at the surface, mostly from thick, widely distributed lava flows. This Archean area is, in effect, an ancient continent that served as a starting point for the second stage in the development of the Finnish bedrock (Map 1). Figure 2. The Siurua gneiss (age 3,5-3,7 Ga) Map 1. The Archean basement Figure 3. Gneiss 5 THE KARELIDIC BEDROCK The creation of the Archean bedrock was followed by more than 500 million years of erosion and deposition that caused clay, sand and gravel to accumulate on top of the ancient continent and at its edges. These gave rise in time to the quartzites, conglomerates, mica schists, mica gneisses and granulites of the Karelidic belt (Fig. 4, 5). During this period the earth’s crust was broken up by numerous rift valleys bounded by fault zones. The faults were filled with molten rock, or magma, some of which crystallized deep in the crust, while some was discharged at the surface to form sequences of volcanic rocks. These are marked in dark green on map 2. Then, about 2000 million years ago, the ancient continent split in two and the blocks drifted apart causing the fault to develop into an ocean. It is not known what happened to the western block. Figure 4. Ripple marks in quartzite Map 2. The Karelidic bedrock Figure 5.Granulite 6 THE SVECOFENNIAN BEDROCK Already about 1920 million years ago, the present area of Eastern Finland was an ancient continental land mass covered by sedimentary rocks, with an ocean to the west of it. As a result of movements of the tectonic plates, a number of arcs of volcanic islands arose that were thrust eastwards by flows taking place within the earth, colliding with each other and eventually with the old Archean basement. By the time this collision phase had come to an end around 1870 million years ago, the area of Finland was occupied by a range of high mountains( Map 3). The processes of folding that gave rise to these mountains, had caused the volcanic islands and their related sedimentary rocks to be piled up against the ancient continental mass in the east. This meant that some of the sedimentary rocks were thrust down deep into the earth’s crust, where they melted and then slowly crystallized to form various plutonic rocks (Fig. 6, 7). Figure 6. The Nattanen granite tor formations Map 3. The Svecofennian bedrock Figure 7. Koli quartzite polished by continental ice 7 YOUNGER FORMATIONS The folding that created the range of mountains was followed by a long period of erosion, so that only a few isolated features of the Finnish bedrock have been preserved from that time. These rapakivi granite areas are indicated in red on the map 4 (Fig. 8), while the yellow areas show the presence of sandstones and argillites dating back 1400-600 million years. The events that gave rise to the Caledonides in Norway about 400-450 million years ago also caused a bed of younger sedimentary rocks to thrust into the north- western parts of the Enontekiö region of Finland. A number of kimberlite pipes of igneous origin that contain diamonds, have been discovered in Northern and Eastern Finland (Fig.9) , and the hills of Iivaara and Sokli are formed by alkaline igneous rocks aged 370-360 million years. Figure 8. The Rapakivi granite, GTK. Map 4. Young bedrock formations. Figure 9. Diamonds and side minerals. GTK Development of Finnish bedrock presented on same time scale. 8 NEED-region 9 3. THE BEDROCK OF THE NEED-PROJECT’S REGION In the area under study, the bedrock is divided into three bedrock units of different ages. The northernmost is the Archean gneiss area (age 3200-2700 Ma), in the center is theEarly Prorerozoic schist and plutonic rock area (age 1900-1800 Ma) and in the south the Middle Proterozoic rapakivi area (age 1650-1570 Ma) (Map 5) (Vaasjoki ym. 2005). The northernmost, Archean area, belongs to Finland’s oldest bedrock area. Typical types of rocks in that area are gneisses with greenstone periods. The Archean mountain folding was followed by hundreds of millions of years of erosion, which involved strong faulting and volcanism. The bedrock were splitting strongly, forming sedimentation basements and there was also active volcanism at that time. At the end of the splitting phase major climate changes occurred, the most significant being the increase in the amount of oxygen ca 2100 million years ago to the current level (Karhu 1993). About 2400 million years ago the mountain ridge was eroded into a flat peneplain. Thick sediments were deposited on top of it about 2200 - 1970 million years ago. Sedimentation was associated with several periods of volcanism which resulted in diabase veins cutting the Archean basement and the sediments about 2450 - 1970 million years ago (Fig. 10) (Vaasjoki ym. 2005). The long sedimentation phase was followed by the folding of the Svecokarelian mountains about 1900 – 1800 million years ago. The sediments were folded and metamorphosed as crystalline schists like quartzite, phyllite and mica schist. Primary structures like ripple marks in the Koli quartzites can still be seen in the rocks (Fig 11). 10 Map 5. The bedrock of the NEED region, GTK. 11 Figure 10. A series of illustrations showing the development of eastern Finland and the Koli area’s bedrock. T. Tervo, GTK 12 Figure 11. Quartzite with ripple marks. A. Portaankorva, GTK. Tectonic movements in the earth crust’s ended about 1800 million years ago, and was followed by a long phase of erosion. In the southernmost part of the area fine grained rapakivi rocks can be found. This rapakivi bedrock is aged about 1640 – 1635 million years. (Simonen & Tyrväinen 1981) During the Precambrian eon many meteorites crashed in to the earth. The big meterorite crater, Paasselkä, can be seen today as a rounded lake in the eastern part of the area (Fig. 12) (Kuivasaari ym. 2000). Figure 12. Paasselkä, meteorite crater. A. Portaankorva. 13 Tectonic movements created fault and fracture zones in the bedrock. The Kolvanauuro and Korkiakoski gorges are situated in these kind of fault zones (Fig 13, 14). Later the ice age and melting waters cleaned the loosened stone and earth material out from the gorges and made them clearer. The fractured outlook of the bedrock is clearly visible in the mosaic like landscape of Saimaa – Pielinen lake. Erosion is active all the time and different kinds of erosion forms can be seen in the area, e.g. boulder fields and caves (Kuvaanniemi cave). Figure 13. Kolvananuuro gorge . Figure 14. Korkeakoski gorge and rapids. Photos: A. Portaankorva, GTK 4. SURFICIAL DEPOSITS IN THE NEED-PROJECT REGION Finland is the daughter of the ice age. During the last 2,5 million years, several glaciations have shaped today’s landscape. Eastern Finland’s quaternary deposits were deposited during the late- Weichselian glaciation. The Scandinavian ice sheet reached its maximum size about 20 000 years ago (Lunkka 2007), then as a result of global warming, the glacier began to melt, and its edge reached the southern coast of Finland about 13 100 years ago (Donner 1995). (Fig. 15) During the melting phase, the continental ice sheet was divided into ice lobes which in turn again diminished into smaller, independently operating sub lobes. During the last glaciation there were two ice lobes, Lake-Finland and the North Karelian lobes.