2022
Ediacaran to early Cambrian weathering of the Kautokeino Greenstone Belt in Finnmark, northern Norway
102

Accepted manuscript

During exploration for copper and gold in the Bidjovagge area north of Kautokeino, northern Norway, in the Kautokeino Greenstone Belt, more than 20 holes were drilled through 10–20 m of Cambrian marine sandstones and shales, 5–8 m of lower Cambrian fluvial sandstones, and into the underlying Precambrian metamorphic basement, which appeared weathered. The drillholes start in marine Cambrian sedimentary rocks and continue with fluvial sandstones. The basal conglomerate of the fluvial sandstone is ’floating’ in a 10 to 15 m-thick Ediacaran–early Cambrian saprolite. The weathering profile provides information about the environmental conditions in a particularly fascinating period of the Earth's history, the late Ediacaran to early Cambrian. The fluvial sandstone between the basement and the marine sediments has protected the weathered basement from marine erosion. Newly formed clay minerals (kaolinite, smectite, illite and mixed layer minerals) can be found down to 10–15 m below the sub-Cambrian peneplain, and hematite can be found down to more than 100 m on faults and fractures.

One of the clay-rich samples from the saprolite profile yielded an age of 541 ± 6.8 Ma using the K–Ar method. This age confirms that the weathering process in the sub-Cambrian peneplain took place before the marine transgression in early Cambrian (i.e., at approximately 520 Ma) and also before the deposition of the fluvial sandstone. The types of clay minerals, such as kaolinite and smectite, support a model with a humid and warm climate in the Ediacaran with increasing oxygen content in the atmosphere in early Cambrian reaching a maximum at 520 Ma, based on Se/Co ratios in marine pyrite.

An abundance of hematite has been found precipitated in faults and fractures down to more than one hundred metres depth. It is suggested that this hematite was crystallised from iron-hydroxides. The hematite is clearly related to tectonic structures and is suggested to have formed at a later stage than the clay minerals.

The composition of the fluvial sandstone indicates movements of the shear zones in early Cambrian time related to the later stages of the Timanian collision (Arctida–Baltica) at around 540 Ma. Such a tectonic event can cause seismic pumping of oxygen-rich surface water deep into the basement and may explain the deep occurrences of hematite.

arne.bjorlykke[at]ngu.no