ABSTRACT 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.

Ordovician (Hirnantian?) glacigenic deposits are described here for the first time from south-central Eritrea. These deposits rest on an almost peneplained Neoproterozoic basement and define, in Eritrea and Ethiopia, a depositional area measuring at least 200 km in an east-west direction and 170 km in a north-south direction. For this preliminary note, five sections through the glacigenic succession were logged in Eritrea and one in Ethiopia. Facies types are described and interpreted. An ice-proximal facies assemblage is located in the Tigray Province of northern Ethiopia, the type area of the glacigenic Edaga Arbi Beds. These proximal deposits, c. 20 m thick, are characterised by melt-out diamictites, with striated clasts, interlayered with sandstone beds displaying horizontal lamination and normal grading (sand-silt). The horizontal lamination in the section is transitional with climbing ripple beds. Ice rafted clasts in sand-granule grade are common in these sandy beds. This ice-proximal section also exhibits some, minor soft-sediment deformation, such as asymmetrically folded beds, south-dipping reverse faults and glacial grooves suggesting transport to the north. This proximal facies grades laterally into a cross-bedded arkosic sandstone, the Enticho Sandstone, which probably represents deposition on subaqueous outwash fans. Cross-beds in this sandstone dip consistently to the north also in south-central Eritrea. Glacial striae and grooves are observed on top of the Enticho Sandstone in two localities in Eritrea. These proximal facies types are overlain by a distally deposited mudstone-dominated unit, 3-40 m thick, most probably deposited from turbid overflow plumes, although it also contains ice-rafted clasts. Only this unit hosts ice-rafted clasts in Eritrea. In Eritrea it also contains some diamictites. The name Edaga Arbi Beds is adopted for this unit in Eritrea. Icebergs were probably responsible for the deposition of the diamictites in south-central Eritrea. The development of this glacigenic succession was probably related to a regular retreat of the ice margin from north to south. It is also probable, that this succession only represents one cycle of deglaciation, the last of the two (or three) recognised in other parts of North Africa. The post-glacial development is initially represented by the deposition of a probable marine dune complex migrating from north to south. Fossil evidence and trace fossils, particularly Arthrophycus alleghaniensis (Harlan) suggest that the age of these glacigenic deposits is Late Ordovician, probably Hirnantian.

Trace fossils are described here from the Adigrat Sandstone formation of hitherto uncertain Palaeozoic-Mesozoic age in south-central Eritrea. The formation is subdivided into a lower unit, the Adi MaEkheno Member, and an upper informal unit, Member 2. The formation was deposited on the locally mudcracked top of the glacigenic Edaga Arbi Beds, suggesting that these two rock units were formed in a very short time interval. The Adi MaEkheno Member and the lower part of Member 2 contain trace fossils Arthrophycus alleghaniensis (Harlan), Arthrophycus ?brongniartii (Harlan), Didymaulichnus lyelli (Rouault), Palaeophycus tubularis Hall, Taenidium isp., thin winding ridges, winding ridges and furrows, simple cylinders, and ‘stellate’ forms. A. alleghaniensis is distinctively of Ordovician–Silurian (?Early Devonian) age. The trace fossil association belongs to the Cruziana ichnofacies that indicates a shallow marine environment between the normal and storm wave bases. The trace fossil data and stratigraphic relationships indicate that the Adigrat Sandstone formation and the Edaga Arbi Beds in Eritrea are Ordovician–Silurian in age. The Edaga Arbi Beds are correlated with other Upper Ordovician (Hirnantian) glacial units in northern Africa and the Arabian Peninsula, lending these beds the status of a marker unit in the Lower Palaeozoic stratigraphy of the Horn of Africa. The Jurassic “Adigrat Sandstone” in central-west and eastern Ethiopia cannot be correlated with the Adigrat Sandstone formation in its type area and in Eritrea.

The Karoo Basin is a late Palaeozoic to Mesozoic epicontinental foreland basin in South Africa. In its western part the basin fill, the Karoo Supergroup, is composed of the glacigenic Dwyka Group at the base followed by the shallow marine to lacustrine Ecca Group and the fluvial Beaufort Group. The Dwyka Group originated from the contemporaneous glaciation centre in the north. Current direction data suggest that the upper Ecca and lower Beaufort Groups are probably derived from the south and southwest. Petrographical data including heavy minerals from high grade rocks (detrital garnets, biotite and tourmaline) suggest that the likely source is a late Palaeozoic thrust belt and a magmatic arc.

Differences in permeability, parallel versus perpendicular to bedding, in e.g. a stratovolcano, direct the major part of the ground water flow to the near-surface deposits, where it may be collected in local, perched aquifers. Percolation of meteoric water down to the basal ground water aquifer is slow, but is enhanced by steep fault surfaces and dykes. In much of the present literature in hydrogeology it is assumed that the ground water flow takes place by seepage, if tunnels such as lava tubes are not available. The presented data, from the slopes of Tenerife and a table mountain, Blafjall, in northern Iceland, demonstrate that ground water flow is able to create its own tunnel network in the near-surface deposits of a volcano slope in order to enhance the drainage of the meteoric recharge. As the volcano grows, new tephra beds are deposited on top of the older units and a new gallery of tunnels will probably be formed in the new near-surface deposits. The older galleries receive less water and the tunnels are filled by stratified subsurface sediments; eventually the tunnels will become abandoned. The groundwater tunnels are a part of the regulating system enhancing the drainage of a volcano; they also contribute to the erosion of the volcano. Volcanoes are important aquifers in different parts of the world and are used as reservoirs far drinking water and irrigation. A ground-water tunnel system preferentially drains the perched aquifers and as such their role is in conflict with the interests of the users of ground water. On the other hand, they prevent overflow in the perched aquifers, which otherwise might release devastating debris-flows down the volcano slopes.

The Ammeberg Zn-Pb ore district constitutes the southern margin of the Bergslagen mining district in south-central Sweden. It is hosted by a succession of Paleoproterozoic supracrustal units belonging to the Svecofennian domain and was considered as a genetically distinct ore district within Bergslagen already at the beginning of this century. The most prominent ore deposit belonging to this district is Zinkgruvan which has been a major zinc producer for more than 100 years. In this paper, the results from an investigation of the stratigraphy, geochemistry, and U-Pb and Sm-Nd isotope systematics are reported from the ca. 10-km-thick volcanosedimentary succession which forms the country rocks in the ore district, informally named the Emme group.The Emme group may be subdivided stratigraphically into eight lithological units and generally into four parts. The lowermost part (the Mosjon and Gokberget units and the Igelfors formation) is dominated by felsic volcanic rocks and contains the subeconomic Zn-Pb-(Cu) deposit at Marketorp. The lowermost part of the group also contains intrusions of subvolcanic rhyolitic porphyries (U/Pb age 1901 +/- 18 Ma). The lowermost volcanite-dominated part grades upward into homogeneous, medium-grained arkoses (the Narkesberg formation) of continental derivation. Two selected single zircons from these arkoses yielded Pb-207/Pb-205 ages of 1959 +/- 52 and 2703 +/- 178 Ma, respectively, indicating derivation from both Paleoproterozoic and Archean sources. This arkosic unit is overlain by an upper part containing felsic volcanic rocks (Mariedamm volcanic unit and its lateral correlatives, the Godegard volcanic unit; U/Pb ape 1889(-24)(+35) Ma) which locally display strong hydrothermal alteration and which are associated with deposition of the Zinkgruvan Zn-Pb-Ag ores and related carbonate rocks (the Hoksjon limestone). Tholeiitic subvolcanic intrusions are common in the upper part of the Emme group. The uppermost part of the Emme group is composed of a succession of sanely to muddy turbidites (the Vintergolen formation).The epsilon(Nd) (1.9 Ga) values from the volcanogenic units range from 0.1 to 1.1 and are comparable with other felsic volcanic rocks elsewhere in Bergslagen. The epsilon(Nd) values for the arkosic units range from -2.8 to -1.9 and for the turbidites from -2.6 to +0.9. This demonstrates a rapid dilution of Archean detritus with material from younger sources in die uppermost part of the Emme group. The epsilon(Nd)(1.9 Ga) values for the mafic subvolcanic intrusions range from 1.4 to 4.6 and are among the highest epsilon(Nd)(1.9 Ga) values reported from Bergslagen. These data indicate that a primitive rift component was introduced in the upper part of the Emme group before the formation of this volcanosedimentary succession was completed. The occurrences of these mafic rift-related intrusions are considered to be genetically related to the deposition of the Zinkgruvan ore, partly by providing a separate metal source and partly by providing more heat to the convecting hydrothermal cells.The deposition of the Emme group demonstrates an interplay between voluminous, felsic volcanism and the input of large quantities of continentally derived arkosic sediments. The probable depositional site was along an active continental margin, where subsidence of the basin varied from slow and regular to very fast.All sulfide ores in the Ammeberg district were previously interpreted to constitute a characteristic ore type in Bergslagen. The present investigation demonstrates that at least two different ore-bearing levels (Zinkgruvan and Marketorp), constituting two distinct volcanosedimentary environments, can be distinguished in the Ammeberg area. It is suggested that the sulfide ores at the Zinkgruvan level represent a distinct ore-forming environment for Bergslagen, located along an active continental margin and deposited in a marine environment. The particular environment characterizing the Zinkgruvan ores appears to extend all along the southern margin of Bergslagen.