Field data from recent geological mapping over a major part of the north-central Scandinavian Caledonides combined with published information give a detailed view of early Cambrian basin successions, comprising the Gärdsjön formation (Gf; Jämtland supergroup) in the Lower Allochthon and autochthonous equivalents (Dividal Group). The Gf comprises ten units of sandstone and siltstone or mudstone (Gf I—X, > 300 m thick). Green siltstones with red layers (Gf VI, c. 521 to 519 Ma) and green–grey siltstones at the top (Gf X, c. 516.5 to 513.5 Ma) are regional key horizons. Gf V, VI, VII, IX, and X deposition may be related to eustatic events. Restoration of Caledonian shortening reveals a major “Hornavan-Vattudal basin” (HVB; > 330 km NW–SE, > 400 km NE-SW) between the Grong–Olden culmination in the S and the Akkajaure–Tysfjord culmination in the N. Published zircon ages imply the latter separated the HVB from those shed from the Timan orogen in the N. The eastern basin margin straddles the present Caledonian erosional margin. Basement highs identified here within the Nasafjället, Bångonåive, and Børgefjellet “basement” windows define the western margin. They separate the HVB from the outer shelf towards the Iapetus Ocean in the W. The onset of sedimentation is time-related with E–W extension at c. 544–534 Ma. NNE–SSW-directed extension occurs after c. 518 Ma, perhaps related with Timan late-orogenic extension. The HVB is distinctly younger (c. 535–513.5 Ma) than Rodinia break-up and Iapetus ocean formation (> 550 Ma), comparable with post-rift basins in inner parts of modern passive margins.

The Ottfjället Dyke Swarm (ODS) is a prominent component of the Ediacaran mafic magmatism associated with opening of the Iapetus ocean, and hosted in the Särv Nappe, Middle Allochthon of the Scandinavian Caledonides. A U-Pb baddeleyite age of 596.3 +/- 1.5 Ma for a thick, well preserved, plagioclase-phyric dolerite dyke in Härjedalen, Sweden, dates emplacement of the swarm. The age represents a robust, inheritance-free reference age for variably deformed and metamorphosed tholeiitic dykes in sandstone-dominated sequences of the lower part of the Middle Allochthon, representing the proximal, rifted Baltoscandian margin preceding the opening of Iapetus. The new age is within the narrow time span between 610 and 595 Ma defined by the most reliable age estimates for mafic dykes in structurally higher nappes (upper part of Middle Allochthon), representing the distal margin during the opening of Iapetus. The Ottfjället Dyke Swarm cuts the Tossåsfjället Group succession, which includes sabkha-related carbonate platform and diamictite couples, one of several correlated Neoproterozoic glaciogenic successions in Scandinavia. The intrusion age of ca. 596 Ma therefore sets a minimum age for the glaciogenic successions. It implies that Neoproterozoic glaciations in Scandinavia predate the ca. 580 Ma Gaskiers glaciation event and are probably part of the ca. 635 Ma Marinoan "Snowball-Earth"-type glaciation.

This study reviews lithological and stratigraphic data, new geological mapping results and microscopic work from the lower and middle Caledonian nappes in central Scandinavia, and suggests a model of their tectonic evolution. The Jamtlandian and Blaik Nappes represent the lower nappes, and are overlain, successively, by the Stalon, Danabergen and Slussfors Nappes. The latter occupies the level immediately beneath the Seve Nappe Complex. Farther north, around Lake Gautojaure, the Akerberget Nappe represents the level between the Stalon Nappe and the Ammarnas Complex. Their hangingwall is the Sadenaive Nappe and the overlying Seve Nappe Complex. The Danabergen, Akerberget, Sadenaive and Slussfors Nappes are formally defined here. In the Blaik Nappe, brittle deformation gave rise to composite thrust systems. The overlying nappes show a more ductile deformation. The arkose sequence of the Stalon Nappe shows fine-grained recrystallization of muscovite. In contrast, finely recrystallized mica minerals form a phyllitic foliation in the Akerberget Nappe. The Danabergen Nappe contains small garnet porphyroblasts. The Ammarnas Complex and the Sadenaive Nappes show garnet with a straight internal foliation (S-i), while the external foliation is refolded. This fabric suggests a time interval after early foliation development, in which garnet grew but no deformation occurred, and subsequent deformation. This may represent an episode of arrest of deformational activity and thrust movement. Spiral-shaped S-i in garnets of the Slussfors Nappe indicate a contrasting and more complex metamorphic and structural evolution, perhaps similar to that of the Seve Nappe Complex, with a well-known, polyphase tectonic evolution.

This guide provides recommendations on how to introduce formal names for geological units, specifically lithostratigraphic, chronostratigraphic, biostratigraphic, lithodemic, tectonic and tectonostratigraphic units, as well as for other geological features such as deformational structures and bedrock landforms. The recommendations apply to Swedish geology but are crafted to harmonize with international practice in geological nomenclature.

All geoscientists working with the geology of Sweden are invited to join a common effort to review Swedish geological nomenclature and the naming of geological units and other features. A new guide, the Guide for geological nomenclature in Sweden, provides advice for this review process. The Geological Survey of Sweden will set up a new open and freely accessible database for geological names, and a dedicated name committee with mandate to formally approve proposed names will supervise the naming process. The success of this vital long-term commitment will depend on active participation of the geoscientific community at large.

The Langmarkberg Formation (Fm). rests on either the Neoproterozoic rift-related Risback Group or Palaeoproterozoic basement in central Scandinavia. It is always succeeded in stratigraphy by the marine Gardsjon Fm. The Langmarkberg Fm. is a thin, generally less than 50-m-thick, discontinuous, but persistent unit, primarily in the Lower Allochthon; subordinately also in the Autochthon, and Middle Allochthon of the central Scandinavian Caledonides. The formation is composed of a lower diamictite and an upper laminated, lonestone-bearing mudstone. Criteria supporting the glaciogenic origin of the formation are (i) a large regional extension of the formation (>5000 km(2)) and (ii) its fixed position in the regional stratigraphy, (iii) the 'tillite-like' appearance of the lower part of the formation, (iv) the lonestone-bearing mudstone in the upper part, (v) the presence of striated clasts and (vi) striated pavement. It is interpreted as having formed during one single glacial-deglacial cycle. Information on chemostratigraphy and realistic geochronology from this region is missing. The Langmarkberg Fm. has commonly been correlated with the Moelv Fm. in southern Norway and the Mortensnes Fm. in northernmost Norway. The Lower Allochthon in this region also contains an older, potentially glacially influenced diamictite unit at the base of the Risback Group, which is also briefly discussed.

The Tossasfjallet Group, which crops out in the Middle Allochthon of the Scandinavian Caledonides, contains the glaciogenic Lillfjället Fm. Evidence that supports the interpretation of the Lillfjallet formations as glacially related include the presence of diamictite beds, the presence of lonestones, although few, in laminated units, one possible faceted clast, involution-like deformation structures, and the presence of sandstone wedges cutting 2.5-3m down into two different diamictite beds. Similar to the other glaciogenic formations in southern and central Scandinavia (Moelv and Langmarkberg formations), the Lillfjallet Fm. rests on a unit of peritidal, sabkha-type dolomite of the Storan Fm. Poor exposure contributes to the uncertainty about the thickness of the formation. In this account, the Lillfjallet Fm. stratigraphy has been divided into three subunits: (i) a lower diamictite-dominated unit, (ii) a middle unit composed of distinctly laminated grey sandy mudstone and (iii) an upper diamictite-dominated unit. There is no information available from the Tossasfjallet Group concerning palaeolatitudes and chemostratigraphy. Poor isotopic evidence (Ar-40/Ar-39) from the Ottfjallet Dolerites, which cut the Tossasfjallet Group, indicates that the succession is older than c. 665 Ma, so a reliable correlation with global glaciation events is not yet possible.

Trace fossils Broomichnium cf. fliri, Glaciichnium cf. liebegastensis, Planolites isp., ? Undichna isp. and ? Warvichnium ulbrichi have been recognised in early Holocene glacial varved clays near Vittinge, west of Uppsala, Sweden. They are referred to different arthropods and fishes that periodically colonised the lake floor and were important components of the ecosystem. The trace fossils are typical of the Glaciichnium ichnocoenosis within the Mermia ichnofacies, which suggests less stressful conditions in Vittinge than in many other Quaternary varved clays in Europe typified by the Cochlichnus ichnocoenosis.