Porsangerfjord, Mid-Finnmark

This region is defined as Porsangerfjord and Laksefjord. Most of the discussion is restricted to Porsangerfjord, where I undertook surveys.

1.0 Geological Overview

The geology of inner Porsangerfjord (Figure 1) can be divided into three main structural units: 1) the Precambrian gneiss basement complex at the head of the fjord, 2) the Gaissa Nappe, consisting of sedimentary formations extending from the inner to mid-fjord areas, and 3) the Kalak Nappe, a metasedimentary unit in the outer fjord area. The Porsanger Dolomite Formation, the main focus here, is part of the Gaissa Nappe. South of Laksefjord, the Porsanger Dolomite Formation is overlain by the tillite and sandstones of the Vestertana Group. See Føyn 1938, Føyn, Chapman and Roberts 1983, Roberts (1971), and White (1968, 1969) for more details. Figure 1 provides a simplified geological map.

Figure 1. Geological Map of Porsangerfjord. Click on map for a larger version.

2.0 Porsanger Chert

Chert is found in the Porsanger Dolomite Formation (PDF), which is exposed in a relatively narrow belt extending from the Kolvik area of western Porsangerfjord to the eastern side of the fjord at Børselv, including several islands in the middle of the fjord. One of the most prominent features of the PDF is its extensive deposits of stromatolites (White 1969, Tucker 1977). Stromatolites are fan-shaped columnar or mat-like structures formed by the interaction of microbe activity and sediment accumulation. Cyanobacteria (blue-green algae) trap sediment particles and calcium carbonate precipitation may preserve the resulting structures. In the PDF the stromatolites have been strongly chertified and chert bands and nodules are found in various places throughout the formation.

Chertified columnar stromatolites do not provide a raw material suitable for tool manufacturing, but the chertified laminated algae mats may provide more useful materials. The latter occur as thin bands, mostly less than 5 cm thick, but bands of 10-20 cm are found occasionally. The veins are usually of restricted lateral extent; the largest observed extended only 10-15 meters.

Porsanger chert is sometimes extremely fine-grained and isotropic, but usually it exhibits a very block structure related to its stromatolite precursor and is therefore of poor quality for tool manufacturing. Colours range from light gray to dark gray/black, with less frequent orange variants. In some cases the biostrome structure is visible when the chert is held up to a light source.

2.1 Source Areas

In 1985 I undertook surveys on the west side of Porsangerfjord in the vicinity of Kolvik. Cherts were encountered in several of the lithological units described by White (1969), but little of tool quality was identified and no traces of prehistoric quarrying were observed. The vertical extent of the chert deposits ranges from less than 10 m to 120 meters above sea level, so they were available throughout the Stone Age.

In 1988 I surveyed the Børselvnes area on the east side of the fjord. Areas with chertified columnar stromatolites and laminated algal mats were identified 200 meters west of Nummedal's (1929) Early Stone Age site (Figures 2 and 3). The greatest lateral extent of the laminated cherts is ca. 20 meters, while their thickness ranges from 6 to 30 cm. Chert colour ranged from light to dark gray and biostrome structure was generally visible. The material was uniformly blocky in texture and of poor manufacturing quality. Nevertheless, the chert is present in considerable quantities as flakes in the Børselvnes Early Stone Age site.

Overall, the quality of the Porsanger chert observed on the field surveys was relatively poor. Thin veins and blocky texture inherited from the stromatolite structure limit its viability for tool manufacturing. While it is poorly suited for bifacial reduction it may still have been useful for the production of expedient flake tools and the small retouched flake tools used during the Early Stone Age.






Figure 2. Børselvnes, Porsangerfjord. Chert occurs as a rust coloured bed in the dolomite.




Figure 3. Børselvnes, Porsangerfjord. Porsanger chert often occurs as silicified stromatolite structures, visible here as convolutions on the surface of the dolomite.

2.2 Examples

Figures 4 to 6 provide examples of Porsanger chert. Hand specimens of Porsanger and Kvenvik chert (Alta region) can appear very similar. In some cases it is impossible to distinguish them, but an experienced eye can detect subtle differences. Some Porsanger chert tends to have a more pronounced luster (shiny surface). Porsanger chert frequently exhibits traces of biostrome structure, either as "cracking" (Figure 5)
or as small globular inclusions visible along flake edges when held up to a light source. Some Kvenvik chert may be associated with stromatolites, but this appears to be rare. Petrographic and geochemical analysis provide stronger grounds for distinguishing the materials.

Figure 4. Porsanger Chert. Note dolomite cortex.

Figure 5. Porsanger Chert, Børselvnes. Note "cracked" texture caused by stromatolite structure.

Figure 6. Porsanger chert, West Porsanger. Dark gray-black variant.

3.0 Slate

No systematic field surveys were undertaken for slate, but in the course of the chert investigations at west Porsanger the Igeldas Shale of the Stabbursnes Formation (underlying the PDF) was inspected briefly. These shales are red with occasional green banding or mottling. The material is probably usable for ground tool manufacturing but the shaley cleavage makes it inferior to the Nyborg formation slates of Tanafjord.

Red slate also occurs at Laksefjord in outcrops of the Nyborg and Stappogiedde Formations. Bjørnar Olsen (pers. comm.) reports white-banded materials of good quality for tool manufacturing.

4.0 Quartzite

Andreassen (1988:24-25) reports a quartzite source at Iford in the Laksefjord region. A large vein of coarse green quartzite was found beside the highway and a bifacial point and debitage of the same material were found at the foot of the vein.

References Cited

Andreassen, R.
1988 Arkeologiske undersøkelser i Leirpollen, Ifjord, Lebesby kommune, Finnmark. Tromura, kulturhistorie 9.

Føyn, S.
1938 The Eo-Cambrian Series of the Tana District, Northern Norway. Norsk Geologisk Tidsskrift 17.

Føyn, S., T.J. Chapman and D. Roberts
1983 Adamsfjord og Ul'lugai'sa: Beskrivelse til de begrunns geologiske kart 2135I og 2135II, M-1:50,000. Norges Geologiske Undersøkelse 381.

Nummedal, A.
1929 Stone Age Finds in Finnmark. H. Aschehoug, Oslo.

Roberts, D.
1971 Geology of South and East Porsanger, Finnmark. Norges Geologiske Undersøkelse 269:307-313.

Tucker, M.
1977 Stromatolite Biostromes and Associated Facies in the Late Precambrian Porsanger Dolomite Formation of Finnmark, Arctic Norway. Palaeogeography, Palaeoclimatology, Palaeoecology 21:55-83.

White, B.
1968 The Porsanger Sandstone Formation and Subadjacent Rocks in the Lakselv District, Finnmark, Northern Norway. Norges Geologiske Undersøkelse 255:59-85.

1969 The Stabbursnes Formation and Porsanger Dolomite Formation in the Kolvik District, Northern Norway: The Development of a Precambrian Algal Environment. Norges Geologiske Undersøkelse 258:79-115.