Jotnian
In north European geology, Jotnian[note 1] sediments are a group of Precambrian rocks assigned to the Mesoproterozoic Era (Riphean), albeit some might be younger.
Stratigraphically, Jotnian sediments overlie the rapakivi granites and other igneous and metamorphic rocks and are often intruded by younger diabases.
[3][4] The characteristic red colour of Jotnian sediments is due to their deposition in subaerial (e.g. non-marine) conditions.
[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.
[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 East European Platform not being metamorphosed enough to be part of the "protoplatform" and not being undeformed enough to be part of the "cataplatform" category.
[9] This surface actually exists but its extent is very limited, represented as small valley plains at Sveg and along the Västerdalälven-Dalälven system.
[11][12] The limited geographical extent of Jotnian sediments at present is indebted to their erosion over geological time.
[15] At present Jotnian sedimentary rocks are commonly found in half-grabens, narrow grabens, in slightly downfolded (syncline) positions or in mixed graben-synclines.
[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.
[8] Fully within Swedish waters there is a Jotnian sediment occurrence at the Landsort Basin between Gotland and Stockholm archipelago.
The sediments in the Muhos Graben at the bottom are conglomerates and arkoses whose contact with the underlying metamorphic rocks constitute an unconformity.
[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.
[11][19] Gritstone, sandstone and conglomerate are the sedimentary components of the Salmi suite which further includes mafic volcanic rocks.
[19] As with Satakunta and Muhos in Finland, the sediments at Lake Ladoga are located in a graben of Late Proterozoic age.
[20] Jotnian sediments are exposed only as small outcrops on land but exposures are larger at the lake bottom.
[2] The Rybachy Peninsula in northern Murmansk Oblast hosts Jotnian sedimentary rocks dated to have sedimented 1126±50 million years ago.
[22] Geologists Eva-Lena Tullborg and co-workers consider the Almesåkra Group as originating from first phases of erosion of the uplifted Sveconorwegian region to the west.
[24] Near Brevik in Eksjö Municipality and Röjda [sv], there are diabase dykes hosting numerous rounded clasts of red Jotnian sandstone including both quartz arenite and arkose.
The conglomerate which contained the rounded Jotnian sedimentary rock clasts has only been deduced to exist and has not found in the proximity of the diabases in question.
The unconformity, believed to have been formed by subaerial erosion, was the surface of a peneplain at the time of deposition of the Dala sandstone.
[27] The Dala sandstone has an imprint of low grade burial metamorphism of the pumpellyite type, meaning they must once have been buried beneath several kilometres of sediments.
[29] In the Nordingrå area, spanning from Malmön to Hernön, Jotnian sediments rest on a complex of gabbros, monzogranite, granites and anorthosites.
