Concrete-like till: A scientific and economic problem

Abstract

In Finland, extremely compact and hard, concrete-like basal till, a type rare to the country, occurs in the district of Haukipudas on the coast of Northern Ostrobothnia. The cause of cementation is not its age but rather the composition of the underlying bedrock, which consists of pyrrhotite- and pyrite-bearing black schists. During intense erosion under the ice sheet, a large amount of material derived from this was released into the overlying basal till. Rapid oxidation of sulphide grains in the till began as a consequence of uplift of the area above sea level and the lowering of the groundwater surface. Sulphide was oxidised to sulphate and ferrous iron became ferric, which precipitated as ferric hydroxide. As this initially water-rich gel-like ferric hydroxide formed, it enclosed the till clasts and mineral grains. With water loss it hardened and cemented the till into a concrete-like mass.

Keywords

• till
• lodgement till
• black schists
• sulfides
• oxidation
• iron
• hydroxides
• cementation
• water table
• drawdown
• Haukipudas
• Finland