Alum Shale Formation

Short description: Geologic formation in Scandinavia

Black Alum shales intercalated with wellowish to brownish limestone beds. Oslo field Distribution of Alum Shale in Sweden

The Alum Shale Formation is a rock formation of black shale of Miaolingian (Middle Cambrian) to Tremadocian (Lower Ordovician) in age found predominantly in Scandinavia, but also some parts of Estonia and Finland.[1][2] The greatest extent of the rock deposition comprises most of Sweden. It is characterized by organic-rich shales.

The name derives from the ancient mining of the rock to obtain alum. There’s economic interest surrounding these rocks, due to unusually high concentrations of U, V, Mo, P, REE, among others, now considered critical materials for future sustainable development.

## Contents

* 1 Lithology
* 2 Geological history
* 3 Fossil content
* 4 Mining history
* 5 References

## Lithology

The Alum Shale comprises several facies like greywacke, shale, sandstones, and arkose, with varying degrees of organic matter content up to 20%. Clays, quartz, feldspars are the abundant minerals. Clay minerals are dominated by illite, with some smectite, kaolinite, and chlorite. Pyrite is common and calcite veins are usually present.[1][3][4]

The basement is represented by the Paleoproterozoic basement of the Baltoscandian Platform. It is unconformably overlain by Early Cambrian quartz sandstones, which are intercalated with siltstone and shale towards the Caledonies; the upper parts are usually glauconitic and phosphatic, and then is overlain by Middle Cambrian greenish gray siltstones. This transitions to a thin conglomerate, and afterwards the Alum Shale rocks. Above these black shales, glauconitic, phosphatic, and sandy limestones are found. Subsequently, Ordovician limestones and shales sequences were deposited until the Silurian.[1]

## Geological history

Location of the Caledonian/Acadian mountain chains in the Early Devonian Epoch. Present day coastlines are shown for reference.

During Early Cambrian, sedimentation occurred in a shallow-water epicontinental environment, after rising sea-levels allowed the flooding of Baltica, followed by a series of transgressive and regressive events, finishing with a major regression in the Lower Cambrian. During Middle Cambrian, particular conditions such as slow deposition and anoxic environment favored the deposition of the Alum Shale.[3]

Slightly metamorphosed beds of Alum Shale, northern Jämtland.

During the Caledonian orogeny, the Alum Shales layers served as a detachment surface for the allochthonous thrust series. This favored the repetition of Alum Shale beds, increasing its thickness.[5]

## Fossil content

8502 specimens of trilobite-agnostoid fauna were found in the Furongian strata of this formation in Bornholm, Denmark. Most of them are disarticulated sclerites. Described gerena include Ctenopyge, Eurycare, Leptoplastus, Olenus, Parabolina, Peltura, Protopeltura, Sphaerophthalmus, Lotagnostus and Triangulopyge.[6]

Orsten fauna are characteristic of the Orsten lagerstätte, in the Alum Shale Formation at Sweden.

## Mining history

The name “Alum” comes from the ancient mining of alum salt over 350 years ago, which led to the development of a major industry. The mining started in 1637 at Andrarum, in Skåne. The salts were used for several purposes, such as skin preparation, textile coloring, and as astringent in pharmaceutics.[1]

## References

1. ↑ 1.0 1.1 1.2 1.3 Andersson; Dahlman; Gee; Snäll, Astrid; Bertil; David G.; Sven (1985). "The Scandinavian Alum shales". Geological Survey of Sweden (56). https://resource.sgu.se/dokument/publikation/ca/ca56rapport/ca56-rapport.pdf.
2. ↑ Zhengfu Zhao, Nicolas R. Thibault, Tais W. Dahl, Niels H. Schovsbo, Aske L. Sørensen, Christian M. Ø. Rasmussen, Arne T. Nielsen (13 April 2022). "Synchronizing rock clocks in the late Cambrian". Nature Communications 13 (1990): 1990. doi:10.1038/s41467-022-29651-4. PMC 9007955. Bibcode: 2022NatCo..13.1990Z. https://doi.org/10.1038/s41467-022-29651-4.
3. ↑ 3.0 3.1 Lecomte, Andreï; Cathelineau, Michel; Michels, Raymond; Peiffert, Chantal; Brouand, Marc (August 2017). "Uranium mineralization in the Alum Shale Formation (Sweden): Evolution of a U-rich marine black shale from sedimentation to metamorphism" (in en). Ore Geology Reviews 88: 71–98. doi:10.1016/j.oregeorev.2017.04.021. https://linkinghub.elsevier.com/retrieve/pii/S0169136815302572.
4. ↑ Snäll, Sven (1988). Mineralogy and maturity of the alum shales of south-central Jämtland, Sweden. Geological Survey of Sweden. ISBN 91-7158-436-6. https://resource.sgu.se/dokument/publikation/c/c818rapport/c818-rapport.pdf.
5. ↑ Gee, David G.; Stephens, Michael B. (2020-01-21). "Chapter 20 Lower thrust sheets in the Caledonide orogen, Sweden: Cryogenian–Silurian sedimentary successions and underlying, imbricated, crystalline basement" (in en). Geological Society, London, Memoirs 50 (1): 495–515. doi:10.1144/M50-2018-7. ISSN 0435-4052. https://www.lyellcollection.org/doi/10.1144/M50-2018-7.
6. ↑ Arne Thorshøj Nielsen, Line Frigaard Andersen (2021). "Furongian (upper Cambrian) trilobites and agnostoids from the Alum Shale Formation of Bornholm, Denmark: revised taxonomy and biostratigraphy". Bulletin of the Geological Society of Denmark 69: 123–213. doi:10.37570/bgsd-2021-69-08. ISSN 2245-7070. https://2dgf.dk/xpdf/bull69-123-213.pdf.

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Geology of Fennoscandia

Paleosurfaces|

* Borsu surface
* Hardangervidda
* Muddus plains
* Paleic surface
* Ostrobothnian Plain
* South Småland peneplain
* 300 m peneplain
* 200 m peneplain
* Strandflat
* Sub-Cambrian peneplain
* Sub-Mesozoic hilly relief

Sedimentary cover|

| Moraines|

* Billingen terminal moraine
* Pulju moraine
* Rogen moraine
* Salpausselkä
* Sevetti moraine
* Skövde terminal moraine
* Trollgarden
* Veiki moraine

|

Sedimentary rock|

* Alum Shale Formation
* Hedmark Group
* Jotnian sedimentary rock
* Reusch's Moraine
* Visingsö Group

Structures|

| Faults and shear zones|

* Burträsk Fault
* Iapetus Suture
* Lainio Fault
* Nordfjord-Sogn Detachment
* Pärvie Fault
* Protogine Zone
* Sorgenfrei-Tornquist Zone
* South Finland Shear Zone

|

Large impact craters|

* Dellen
* Gardnos crater
* Granby crater
* Hummeln structure
* Lappajärvi
* Lockne crater
* Lumparn
* Mien
* Ritland crater
* Siljan Ring
* Sääksjärvi
* Söderfjärden
* Suvasvesi
* Tvären

Morphostrucures|

* Central Swedish lowland
* Gulf of Bothnia rift
* Lake Inari graben
* Lake Ladoga graben
* Lofoten horst
* Muhos Graben
* Norrland terrain
* Omberg horst
* Oslo Graben
* Scandinavian Mountains
* Scanian horsts
* South Swedish Dome
* Vättern graben
* White Sea Rift System

Lesser igneous provinces|

* Alnö Complex
* Central Skåne Volcanic Province
* Fen Complex
* Kattsund-Koster dyke swarm
* Kola Alkaline Province
* Norra Kärr
* Rapakivi granites
* Särna Alkaline Complex
* Satakunta dyke swarms

Provinces and orogens|

* Belomorian Province
* Blekinge-Bornholm Province
* Kola Province
* Scandinavian Caledonides
* Svecofennian Province
* Sveconorwegian Province
* Transscandinavian Igneous Belt
* Timanide Orogen
* Western Gneiss Region

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