Timing of natural remanent magnetization in ferriferous limestones from the Swiss Jura mountains

Abstract

Goethite and hematite are the main magnetic phases contained in two ferriferous middle Jurassic beds which are exposed within the Mandach structure, the major tectonic structure of the eastern tabular Jura of northern Switzerland. The iron oxidic phases in a reddish brown sparry limestone known as Spatkalk were formed diagenetically, whereas these phases are weathering products in a stratigraphically slightly younger oolitic bed. The hematite in the oolitic bed was magnetized after the Jura main tectonic event in the Miocene whereas the hematite in the Spatkalk was magnetized before the Miocene. The goethites in the oolitic bed carry a pretectonic and in the Spatkalk a syntectonic magnetization. Physical and chemical data show that the goethite which was resistant to remagnetization contains an Al-substitution of 3.4 mol% resulting in a Néel temperature of 90°C. The remagnetized goethite has an Al-content of 10.7 mol% and a Néel temperature of 70°C. The different goethite properties indicate that the tectonic formation of the Mandach structure occurred under low-temperature conditions below 90°C.