Geological background
Approximately 1.5 to 1.8 billion years ago, the bedrock in the Kurikka region experienced fractures as the Svecofennian mountain range formed. This process eventually led to the formation of one of the world’s oldest valleys in Kurikka. Although detailed information from that era is scarce, rare sedimentary rocks found in the region, along with data from other parts of Southern Finland, help us understand the geological development of that time.
After the valley’s formation, the bedrock was buried multiple times under layers of sediment over hundreds of millions of years. These sediments were repeatedly eroded in continental conditions, and when the area sank below sea level again, new sedimentary layers began to form.
Developmental history of the area
Key events in the region’s geological history include the formation of fracture zones in the Earth’s crust during mountain formation and the development of ancient valleys. A significant phase occurred about 40 million years ago when the area rose from the sea. Following this, under tropical conditions, the land eroded on the surface, exposing the valley before the climate of the Nordic region began to cool about 2.6 million years ago. This cooling period led to repeated glaciation in the area (Gibbard & Levin 2016).
The layers currently visible were primarily formed during the most recent ice ages. The uppermost layers align with the glacial retreat of the last deglaciation, positioning Kurikka between two active ice streams in what is known as the Suupohja passive ice zone. This location, at the junction of the ice lobes, resulted in varied sediment deposits in these convergence areas.
Determining the age of the valley
The valley is estimated to be over 1.5 billion years old, making it one of the oldest valleys in the world. Buried valleys revealed in GTK’s groundwater drilling indicate an extremely slow erosion rate of less than 2.5 millimeters per thousand years, ranking among the slowest erosion rates in the world.
Determining the valleys’ age has been challenging due to multiple transformations of sediment layers. For this purpose researchers developed a narrative for the valley. The narrative takes us to a journey back in time.
The Kurikka region contains geologically significant unconformities that separate rock layers of different ages: 1.5 billion years, 540 million years, and 1 million years. This suggests that only about 100 meters of bedrock has eroded from the Ostrobothnia region over the past 1.5 billion years, highlighting the exceptional stability of the Earth’s crust.
Meteorite craters further support the study of erosion and valleys’ age determination. The analysis of the craters show that Finland’s bedrock has been buried under sediments for long periods, protecting it from weathering. By examining impact structures and sediment remnants, researchers have estimated when the region was subjected to erosion or covered by sediments.
For more detailed information on the story and age determination of the Kurikka Valley, see Hall et al. (2021): “Ultra-slow cratonic denudation in Finland since 1.5 Ga indicated by tiered unconformities and impact structures.”
Formation of the deep groundwater reservoir
A unique characteristic of the valley is its groundwater formation. The water in the aquifer does not come from rain that falls directly above it but rather from rainfall in surrounding higher areas. Water flows down slowly into the deepest soil layers due to gravity. The fractured surfaces of the bedrock play a significant role in this process, conducting water as effectively as soil. This explains why the valley has such abundant groundwater resources.