Jotnian

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In north European

intruded by younger diabases

Overview

Gävle sandstone (Swedish: *Gävlesandsten*) a Jotnian sedimentary rock

Jotnian sediments include quartz-rich sandstones, siltstones, arkose, shale and conglomerates.^[3]^[4] The characteristic red colour of Jotnian sediments is due to their deposition in subaerial (e.g. non-marine) conditions.^[5] Jotnian sediments are the oldest known sediments in the Baltic area that have not been subject to metamorphism.^[4]^[5]

Their age is poorly constrained, but generally they are younger than the rapakivi granites and older than Postjotnian^{[note 2]} diabases that intrude the sediments. This means that Jotnian sediments were deposited approximately 1600–1260 million years ago. Some Jotnian sediments are, however, younger than the diabases, meaning they can be younger than 1260 million years.^[2] Jotnian sediments are usually assigned to the Riphean Stage of the Mesoproterozoic Era.^[1]^[2] Jotnian sediments rests on what is known as Subjotnian^{[note 3]} rocks which include weathered rapakivi granites and the Hogland Series.^[6]^[7]^[8] Amantov and co-workers comments about the terms Jotnian and Postjotnian that:^[2]

the terms should be interpreted as more descriptive (i.e. the diabase usually cuts the sandstone) than temporal (all the Jotnian sandstones not necessarily older than all the Postjotnian diabases).

At large scale, Jotnian sediments are classified as being part of a "quasiplatform" group of sediments of the

Västerdalälven-Dalälven system.^[10]

Distribution

Mälaren

Almesåkra [sv]

Tärnö (xenoliths)

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Selection of outcrop localities mentioned in the text.

There is evidence suggesting that Jotnian cover rocks or a Jotnian

preservation potential.^[13] The distribution of some Jotnian sediments is spatially associated with the occurrence of rapakivi granite.^[3]^{[note 4]} Korja and co-workers (1993) claim the Jotnian sediment–rapakivi granite coincidence at the Gulf of Finland and the Gulf of Bothnia is associated with the existence of thin crust at these locations.^[15] At present Jotnian sedimentary rocks are commonly found in half-grabens, narrow grabens, in slightly downfolded (syncline) positions or in mixed graben-synclines.^[2]^[9] Most of the grabens hosting Jotnian sediments are northwest-southeast oriented.^[16] The largest occurrence of Jotnian sediments appear however to occur neither in a rift or a graben and lie in Dalarna, Sweden.^[3]^[7] It has been suggested that the sedimentary basins hosting Jotnian sediments at Dalarna, Gävle, the Bothnian Sea, Satakunta and Lake Ladoga form an alignment of subsidence parallel to the area of inferred Hallandian-Danopolonian subduction, possibly corresponding to an ancient back-arc basin.^[17]

Baltic Sea

Between Finland and Sweden Jotnian sediments are common in the

Stockholm archipelago.^[8]^[11] Jotnian rocks found in the Landsort Basin are red sandstones.^[8] In the Bothnian Sea offshore Pori in Satakunta Jotnian sediments reaches a thickness of thousand metres, in other locations thickness is much less, for example 100 m at the Sylen shoal and yet in other places of the Bothnian Sea Jotnian sediments are absent.^[11] The Sub-Cambrian peneplain cuts across Jotnian rocks in the Bothnian Sea.^[11]

Finland

Muhos

Muhos in the northeastern Gulf of Bothnia is a site of Jotnian sediment occurrence.^[2] At this location Jotnian sediments can be found within the Muhos Graben that has had a downward displacement of about one kilometre. The sediments in the Muhos Graben at the bottom are conglomerates and arkoses whose contact with the underlying metamorphic rocks constitute an unconformity. The pebbles of the conglomerate are made of schist and granite while the matrix is made of arkose. The bulk (90–80%) of the sequence of sediments is made up of siltstones and shales. The colour of the siltstones and shales vary from red and brown to grey to green. The siltstones and shales are intercalated with arkosic sandstone. The red arkosic sandstones of Muhos Graben are comparable to the ones of Satakunta. These sediments are of fluvial origin.^[5]

Satakunta

Southern Satakunta near the shores of the

formation.^[5] Based on the finding of glauconite in the Jotnian rocks of the northwestern part of the outcrop, it has been suggested that at least in that place the diagenesis occurred under water.^[16] It is found in a northwest-southeast trending graben structure^{[note 5]} that has helped to preserve the sediments. This graben has seen a downward displacement of about 650 metres.^[5] The Satakunta sandstone is not younger than 1400–1300 million years. The Satakunta sedimentary pile is at least 600 metres thick but might likely be as thick as much as 1800 metres. 1270–1250 million years old Postjotnian olivine-bearing diabase dykes cuts through the Satakunta Sandstones.^[18]