No abstract

Cryoturbation and slump fold‐like sedimentary structures in ca. 1.9 Ga old dacitic metavolcanic sediments in West Bergslagen, Central Sweden, are recognized as a lowland periglacial environment. This type of environment is comparable with present day tundra in Siberia. Ice‐wedge casts and cryoturbation, together with polygonal frost patterns, are typical geomorphological structures above permafrost in this type of environment. The sedimentary environment could be interpreted as periglacial, broadly comparable to present day tundras. Intensive cryoturbation of the formation and close structural analogy with Quaternary ice‐wedges suggests a cold and humid environment. This discovery is corroborated by a previous report of glacial sediments and structures from NW Australia of ca. 1.8 Ga age. Both occurrences developed at low geographical latitudes, at locations far apart in the Late Palaeoproterozoic supercontinent Columbia. Either suggest the existence of a ca. 100 Ma long epoch of extreme, though possibly intermittent glaciations during the ca. 1.4 Ga long ‘Proterozoic gap’ (∼2.2–0.77 Ga) from which no convincing glacial deposits were previously known.

A number of discrete slate belts of limited size occur in the Palaeoproterozoic volcano-sedimentary Bergslagen Group of western Bergslagen in the Fennoscandian Shield of south-central Sweden. The Grythyttan Slate Belt (GSB), studied for more than a century, forms a single basin with the nearby Saxån Slate Belt (SSB). We use the lithostratigraphy of the Grythyttan Slate Formation of the Grythyttan belt, based on basin-scale sedimentary facies associations of the volcaniclastic sediments in time and space, as a method to interpret overall tectonic structure. Contrary to traditional views, we reconstruct the GSB as a single overturned limb of a km-scale anticline with horizontal axis in the hanging wall of an east-vergent thrust fault, reactivating a listric extensional fault. The fold connects the GSB to the neighboring SSB. Folding and thrusting were related to tectonic closure of a volcanic back-arc or intra-arc basin. The early folds were subsequently affected by strike-slip shearing and folding around vertical fold axes, which partitioned preferentially into the least competent lithologies (slates and marbles), significantly modifying the map appearance of the slate belt. The late shearing and folding resulted from accretion of Bergslagen onto the Fennoscandian continental margin during the late, Svecobaltic phase of the Svecofennian orogeny. The GSB forms a thin, intraformational wedge in the Bergslagen Group and represents a relatively short interlude with a conglomeratic alluvial fan and turbiditic volcaniclastics followed by more felsic volcanic rocks resembling those of the Bergslagen Group. We suggest that the GSB, and by inference the other slate belts, stem from calderas, as either terrestrial volcanic lakes or shallow submarine eruption centres. The conglomerates are pre-orogenic, not a post-orogenic molasse as traditionally conceived, and their clast fabric has resulted from sedimentary processes only. Newly determined SIMS U-Pb zircon ages constrain the age of the GSB at c. 1895 Ma.

The Älvestorp conglomerates, deposited in an alluvial fan setting, form part of the Svecofennian orogenic belt in west Bergslagen, south central Sweden and are estimated to be as old as c. 1.85 Ga. Reaching a thickness of one kilometre, their architecture and form suggest an alluvial origin. Along sections, massive conglomerates often grade into pebbly mudstones and greenschist facies slates, while pure slates with dolomite concretions and olistolites occur in dark slatey mudstones on the eastern shore of lake Brunnsjön. Inner fan trenches are filled with massive, clast-supported conglomerates that contain more than 80 percent epiclastic tuffaceous material. The Älvestorp conglomerate is therefore classified as the product of a Proterozoic stream-flow channel and debris flow, or alluvial fan. The Grythyttan Basin to the north originated by extension after the first of two orogenic stages of Bergslagen.

This paper reports on an ice‐wedge pseudomorph that formed and is preserved in metavolcanic host material that was later transformed to metamorphic solid bedrock. It has been dated to 1,895 ± 5 Ma by U–Pb geochronology of zircon in the bedrock, an Early Proterozoic age. Detailed observation of the deformation structures of the wedge points to an ice‐wedge pseudomorph based on typical downbending around the wedge and vertical lamination in the inner part of the wedge due to slumping into the wedge after the ice melted, along with a few remains of lateral pressure structures (such as folds and upturned strata) in the adjacent host sediment. The interpretation of the wedge structure as an ice‐wedge pseudomorph confirms previous work on this topic. This ice‐wedge pseudomorph demonstrates for the first time the existence of permafrost at c. 1.9 Ga. It indicates that permafrost and associated conditions were present in lowlands at low latitude at discrete time intervals early in Earth’s history. Although some caution should be applied, mean annual air temperature appears to have been slightly below the freezing point at that time.