Scale in the pictures is a 1 nok coin

Shale is formed when clay minerals, particles less than 0.0039 mm in diameter, sets and form a layered rock. If it instead forms a massive stone, then we call it a claystone. Formation of shale can start at the surface where the sediments are deposited. As they are buried deeper and deeper, the temperature and pressure increases. This causes recrystallization and reorienting of the clay flakes. Forcing the length and width to be nearly perpendicular to the compression, presenting a flat side towards the pressure. Rocks that have undergone this low-grade metamorphism and developed a slaty cleavage is called slate. If one breaks slate they prefer to split along the cleavage planes, whereas shale can break in any direction. The transition from shale to slate is a gradual one, so you can find partly developed slate. Even tough slate is a metamorphic rock, I have placed it here with the sedimentary rocks since it hard to separate from shale and the Norwegian naming scheme merges shale and slate in the term “skifer”. 

The term shale is sometimes also applied to other fine-grained, sedimentary, fissile rocks such as fissile mudrocks and fissile siltstones. To be a real “clay”-shale 2/3 of the particles must be of clay size. 

Shale/slate from Elnes Formation found in Sognsveien by Ullevål Hospital. Most likely part of the Håkavik Member (4aα4) described by Owen et.al. (1990).

Alum Shale is a carbon rich shale that can be found in the Oslo Graben, from Porsgrunn-Skien in the south to Hamar in the north. The Alum Shale was formed from marine deposits in anaerobic conditions, on a relatively shallow continental self of the continent Baltica, from the middle of Cambrian to early Ordovicium (ca. 510 Ma – 475 Ma). The anaerobic conditions allowed for a lot of organic matter, that normally would have decomposed, to bee trapped in the sediments. That resulted in a rock with a high carbon content. One test to see if the rock is Alum Shale or not is to make a scratch. Alum Shale makes a coal black streak when  scratched (Owen et.al., 1990). Other shales, with less carbon, leaves a grey to white scratch.

Alum Shale is named for the alum minerals (aluminum sulfates) that can be found in this shale. Alum have been used for making leather from animal skins, and to make dye for textiles. In the 1700, the Alum factory in Ekeberg was actually one of the largest factories in Kristiania (Oslo). Besides Alum the Alum Shale also contains several other minerals that can be of value, in Sweden vanadium and uranium have been mined from the Alum Shale (Dyni, 2006).

With a carbon content of 2-40 % (usually around 15 %), the alum shale was once a source rock for oil and gas. 200+ millions year ago oil and gas likely seeped out of this rock, but today it is overmature. Overmature meaning that the rock has used up the organic matter that could become oil/gas. In Norway all the oil and gas from the Alum Shale have left the rock, but Sweden that is not the case. In Kinnekulle, Sweden, oil was commercially extracted on and off again from the 1890s (Brodin & Wendin). But production was only commercially viable during times when imports from cheaper sources were impossible, like during the world wars. So production was finally shutdown in 1966 (Dyni, 2006).

When it comes to building and construction work, Alum Shale is a big challenge. Much of the Alum Shale contains uranium that decays to radon-222, which is an odorless, invisible, radioactive gas (Løken, 2007). Buildings must therefore be secured so that as little as possible of this gas enters the building, and the buildings must be well ventilated so that the concentration of radon does not become too high. According to the Kreftforeningen (The Norwegian Cancer Society) (visited 2024), there are approximately 300 deaths annually due to lung cancer that are related to radon. The matter is further complicated by the fact that the Alum Shale contains a lot of sulphur, which can weather and create acids that are strong enough to destroy concrete and steel (Løken, 2007).

Relocating excavated Alum Shale is not without its challenges. The first problem is where to dump it. During the construction of the subway in Oslo, some of the Alum Shale that was taken out was used as fill for the main road past Frognerstranda (Løken, 2007). A bad idea, because when Alum Shale weathers it forms salts which can cause the shale to swell and become up to twice as large in volume. Within a couple of years, they had to remake that road. Landfills that are suitable for Alum Shale is also hard to find. If runoffs from a landfill reach a stream, you can end up with strong discoloration of the water, extremely acidic water, release of heavy metals and release of aluminum that is highly toxic to fish (Løken, 2007).

Alum Shale from the bottom of Ekebergåsen in Oslo, probably late-Cambrian age. The yellow areas are a sulfate salt formed by weathering. The darkest areas with wax like shine is likely bitumen. Bitumen is a viscous hydrocarbon that is formed as a by-product in oil production. Bitumen is the main constituent in asphalt, so much so that bitumen is commonly referred to as asphalt. Bitumen is common throughout the Alum Shale Formation.

References

Brodin L. & Wendin, F. (n.d.). Flottans Oljeskifferverk i Kinne-Kleva. https://www.prismavg.se/exhibits/show/oljeskifferverket-i-kinne-klev [Read: 04.09.24]

Dyni, J.R., (2006). Geology and resources of some world oil-shale deposits: U.S. Geological Survey Scientific Investigations Report 2005–5294, 42 p 

Kreftforeningen. (besøkt 2024). Radon og kreft,

https://kreftforeningen.no/forebygging/kreftfremkallende-stoffer/radon-og-kreft/ 

Løken T. (2007). Alunskifer/svartskifer – den forurensende bergarten. VANN, 2007(2).

Owen, AW, Bruton, D.L., Bockelie, J.F. & Bockelie, T. (1990). The Ordovician successions of the Oslo Region, Norway. Norsk geoogisk. undersøkelse Special Publication 4, 3‑54.

Nodular limestone from the Brønnøya Bed, Myren Member of Solvik formation. (Approximately 443.8 Ma) The dark areas is shale and the light areas is lime nodules. The nodules is coated by a thin layer of lime due to weathering. In reality the nodules consists of a light grey lime rich clay. The Brønnøya Bed is among others described by Bockelie, Baarli & Johnson (2017).

Nodular limestone and shale with lime nodules is common sedimentary rocks in the Oslo Graben. Both consists of lime nodules in a darker shale matrix and are especially common in the Ordovician rocks of the Oslo Graben, where the rocks alternates frequently between limestones and shales. Nodular limestone and shale with lime nodules are similar, if the lime nodules are very close or even form a continues layer the rock is called nodular limestone further spacing makes it a shale with lime nodules. 

Several authors have discussed the formation of the nodular limestones of the Oslo Graben. Bjørlykke (1973, 1974) argues that the nodular limestones represents ordinary continues limestone layers that have partially eroded away before burial. The minerals in limestone will dissolve in water that is unsaturated on carbonate. That combined with the fact that larger particles will dissolve slower than small and evidence of burring animals would lead a continues limestone layer to become nodular. Sramek, J. (1974) and other have criticized that hypothesis, instead interpreting the nodules as lime concretions made early in the burial process of the sediments. Concretions are formed from mineral precipitation in pores in the sediments when water passes through them before the sediments forms a rock. 

References

Bjørlykke, K. (1973). Origin of limestone nodules in the Lower Palaeozoic of the Oslo Region. Norsk Geologisk Tidsskrift, Vol. 53, pp. 419-431.

Bjørlykke, K. (1974). A reply. Origin of limestone nodules in the Lower Palaeozoic of the Oslo Region. Norsk Geologisk Tidsskrift, 54, 397-399.

Bockelie, J.F., Baarli, B.G. & Johnson, M.E. 2017: Hirnantian (latest Ordovician) glaciations and their consequences for the Oslo Region, Norway, with a revised lithostratigraphy for the Langøyene Formation in the inner Oslofjorden area  Electronic Supplement – Lithostratigraphy . Norwegian Journal of Geology 97, 119–143. https://dx.doi.org/10.17850/njg97-2-01.

Sramek, J. (1974). A comment. Origin of limestone nodules in the Lower Palaeozoic of the Oslo Region. (Nor. Geol. Tidsskr. (53) 419-431). Norsk Geologisk Tidsskrift, 54, 395–396.

Sandstone is a clastic sedimentary rock made of sand grains (0.0625 to 2 mm). As sandstones are made from pieces of other rocks, they can assume all colors that other rocks have. However, the most common colors are grey, brownish and red. 

Sandstones can be made from materials from all other rocks, so it is not surprising that it is difficult to make a comprehensive classification for them. Folk (1980, p. 123) even says: “The perfect classification for sandstones does not now and never will exist.”. Because of that, there exists a plethora of classification system for sandstone. Some focuses on the mineral content, others try to make it easy to determine the depositional environment in which the rock was made and some focuses on ease of use in the field. All depends on the background and interest of the geologist that made the given system. One of the most commonly used system is probably Folk’s (1980) QFR triangle diagram. 

Where a sandstone plots gives it its name; if its primary quartz grains it is a quartzarenite, if it contains sufficient feldspar it is an arkose and a lot of rock fragments makes it an litharenite. Litharenite is however not meant to be a name to be used, but rather a grouping. If the rock fragments cannot be identified the term litharenite is used. If the rock fragments can be identified names will be based on that instead. If the majority of the rock fragments is from volcanic rock fragments the stone is dubbed volcarenite, if it is metamorphic rock fragments it is called phyllarenite and sedimentary rock fragments form sedarenite. If the rock fragments can be determined, the name is changed to convey as much information as possible. 

In Folks classification scheme, the grains determine the base name, or clan name. But there can be up too four other parts of the name. If the rock contains any significant other mineral, either due to quantity or rarity of the mineral, it can be add in front of the base name. In front of that a tag describing the textural maturity of the sandstone. Divided in four categories going from poorly rounded fragment with varying sizes to round grains with homogenous sizes: immature, submature, mature and supermature. In front of that the mineral that cements the rock. And in front the grain size: 0.0625 – 0.125 mm is very fine sand, 0.125 – 0.25 mm is fine sand, 0.25 – 0.50 mm is medium sand, 0.50 – 1.0 mm is coarse sand and 1.0 – 2.0 mm is very coarse sand. Full name: (Grain size): (chemically precipitated cements) (textural maturity) (other important minerals) (base name). Unnecessary or undetermined part of the name is cut. 

The QFR diagram, after Folk (1980), have three poles, or corners, and a sandstone is placed dependent on the percentwise composition of the sand grains in the sandstone. A sandstone with a lot of quartz grains will be close to the Q, quartzite, pole. The F pole is the feldspar pole, both k-feldspars and plagioclase is placed here. Fragments of rocks that are made from feldspars, such as granites and gneisses, is also counted towards the F pole. Finally, the R pole is rock fragments, meaning everything else.  

References

Davies, N.S., Turner, P. & Sansom, I.J. (2005). A revised stratigraphy for the Ringerike Group (Upper Silurian, Oslo Region). Norwegian Journal of Geology, 85, 193-201. 

Dons, J. A.; Györy, E. (1967). Permian sediments, lavas, and faults in the Kolsås area W of Oslo, Norsk geologisk tidsskrift 47 (1), 57-77 

Folk, Robert L. (1980). Petrology of Sedimentary Rocks. Austin, Tex: Hemphill Pub. Co. 

The picture is of an Oolitic Limestone from Pilodden Member of Langøyene Formation. The rock is described by Bockelie et.al. (2017) as an Oolitic limestone with millet-seed quartz grains. A rock that mainly consists of calcite ooids. Millet-seed is spheroidal grains of quartz formed from wind and is typical of desert conditions. The grey lower part of the picture shows a fresh cut in the stone and the true coloring. The upper white part is weathered rock, where the white is a coating of lime. 

Oolite is a clastic sedimentary rock made out of small spheres that is less than 2 mm called ooids. Oolitic Limestone is formed in shallow ocean areas, less than 3 m, where the water is in movement and is supersaturated with calcium carbonate (CaCO3). Ooids form when sand or small shell fragments are lifted into the calcium carbonate saturated water by a current. There calsium precipitates unto the sand/shell particles. As these particles are suspended in the water, the calcium can for an even sphere around particle at the core. Abrasion with the seafloor smoothen out any unevenness, and ooids can grow further next time they are lifted off the bottom (Donahue, 1969; Hill, 2007). 

Oolite is reminiscent of a bunch of fish eggs, so it is also called “egg stone”. The name Oolite is also based on the old greek word “òoion” meaning egg. If the ooids exceed 2 mm the rock is instead called Pisolite, after the Hellenic word for pea. 

References

Bockelie, J.F., Baarli, B.G. & Johnson, M.E. 2017: Hirnantian (latest Ordovician) glaciations and their consequences for the Oslo Region, Norway, with a revised lithostratigraphy for the Langøyene Formation in the inner Oslofjorden area. Norwegian Journal of Geology 97, 119–143. https://dx.doi.org/10.17850/njg97-2-01.

Donahue, J. (1969). Genesis of oolite and pisolite grains: an energy index. Journal of Sedimentary Petrology, 39(4):1399-1411. 

Hill, J. (2007). Ooid Formation. GeologyRocks. https://web.archive.org/web/20130620192517/http://www.geologyrocks.co.uk/tutorials/ooid_formation 

Conglomerates are just like sandstones, sedimentary rocks. The only thing that distinguishes a conglomerate from a sandstone is the size of the deposited material. According to Folk (1980)'s classification, the divide is set at a grain size of 2 mm. Smaller sizes of the sediments are coarse sand and thus become coarse-grained sandstone and larger granules, fine gravel, become granule conglomerate. There is no upper limit to how big the stones in the conglomerate can be and the matrix, the fine-grained groundmass that glues the larger grains together, can have much smaller grains. 

The sizes for conglomerates are: 2-4 mm; granule, 4-8 mm; fine pebble, 8-16 mm; medium pebble, 16-32 mm; coarse pebble, 32-64 mm; very coarse pebble, 64-256 mm; cobble and >256 mm; boulder. 

The formation process for conglomerates is very similar to that of sandstones, but the larger and heavier granules/stones mean that conglomerates forms in other places than the sandstones. Heavier stones sink more easily than the light clay and sand particles, which means that in environments with more movement in the water such as rivers, beaches and places with underwater currents, only large sediments will remain. So a conglomerate would indicate that the sediments were deposited in an environment with movement in the water, and larger stones in the conglomerate would be a sign of greater velocity in the water. 

Coarse pebble: quartz conglomerate from the Tanum Formation, The Asker Group (308 Ma – 301 Ma) 

The rock is described by Dons and Györy (1967) as an oligomictic quartz conglomerate. Oligomictic meaning that he conglomerate is dominated by single type rock fragment, in this case quartz. Rock fragments of quartz mica schist, limestone, and shale have also been found. The rock fragments varies in size from “pea” to “apple” size. The matrix consists of varying amounts of quartz and calcite. The rock is some places dyed red from hematite.