Johan Petter Nystuen

Northeastern and northwestern margins of Baltica in Neoproterozoic time: evidence from the Timanian and Caledonian Orogens

Abstract The Neoproterozoic depositional histories of the Timanian and Baltoscandian, orthogonal margins of Baltica show several important differences but also some similarities. The Timanian margin comprises mainly low-grade, terrigenous sedimentary successions with a distinctive, margin-parallel fault zone separating pericratonic and basinal domains. Magmatic rocks are comparatively rare on land, but are common in deep drillcores recovered from beneath the Pechora Basin. Conversion to an active margin occurred in latest Riphean time, ultimately leading to the accretionary and transpressional regime of the Vendian-age, Timanian Orogeny. Along the Baltoscandian margin, successions of low to high metamorphic grade are preserved in diverse Caledonian nappe complexes. Three main types of palaeobasin are distinguished, based largely on sedimentary facies and basin geometry. Magmatic rocks are more common than in the Timanides, ranging from mafic dyke swarms to the voluminous Seiland Igneous Province. This margin remained passive throughout the Neoproterozoic era. The Vendian-dated dyke swarms signify the onset of Iapetus/Ægir ocean opening at precisely the time when the orthogonal Timan margin was being deformed and telescoped during the Timanian Orogeny.

PEDOGENIC MUD AGGREGATES AND PALEOSOL DEVELOPMENT IN ANCIENT DRYLAND RIVER SYSTEMS: CRITERIA FOR INTERPRETING ALLUVIAL MUDROCK ORIGIN AND FLOODPLAIN DYNAMICS

ABSTRACT Mudrocks with well preserved pedogenic mud aggregates in the alluvial Upper Triassic Lunde Formation, northern North Sea, provide information about preservation potential in alluvial sediments, basin infill dynamics, and paleosol development in ancient dryland river systems. Sand-size to millimeter-size pedogenic mud aggregates composed of densely packed clay and silt dominate the floodplain facies. These aggregates are present both in situ in paleovertisol profiles and as reworked aggregates which form thick floodplain units. In situ aggregates are characterized by homogeneous composition, closely associated with pedogenic slickensides and crystallaria, and display evidence of translocation of clays around them. The reworked mud aggregates are heterogeneous and are mixed with reworked carbonate fragments, rip-up clasts, and large, fresh unweathered micas. These pedogenic mud aggregates have survived compaction down to at least 2500-3000 m of burial without any significant overpressure. High rates of sedimentation resulted in only poor to moderate pedogenic overprinting, which could have destroyed the aggregate texture. The preservation of the aggregates was also related to early carbonate cementation and the formation of robust aggregates suitable to resist disintegration during both reworking and burial. The presence of such mud aggregates emphasizes the importance of mud transported as aggregates and not as suspended load in ancient dryland river deposits. Superimposed pedogenic structures on the reworked aggregates indicate a highly dynamic system with pedogenesis, erosion, reworking and redeposition. In floodplain mudrocks, where the texture of reworked aggregates is lost, the mudrock beds attain a massive and structureless morphology. Such units can be confused with more developed paleosol horizons, and in the present study this process is termed pseudo-pedodestratification. Neglecting or overlooking the presence of the mud aggregates also results in the misinterpretation of alluvial mudrocks and paleosol development, and criteria are presented for recognizing in situ and reworked aggregates.

The ‘Sparagmites’ of Norway

The Upper Proterozoic sequences of late Riphaean to Vendian and early Cambrian age in southern and northern Norway and adjacent parts of Sweden (Fig. 18.1) have been studied since the early decades of last century. The successions of this age in Scandinavia became known as the ‘sparagmites’, after the term ‘sparagmite’ was introduced by Esmark (1829) for the dominant, feldspathic sandstone type of these sequences. At present the ‘sparagmites’ are divided into formal lithostratigraphic units, and the old term is only retained as a trivial designation, and in the name ‘sparagmite region’ for the outcrop area of these rocks in South Norway and neighbouring districts of Sweden (Fig. 18.1).

Provenance and rift basin architecture of the Neoproterozoic Hedmark Basin, South Norway inferred from U–Pb ages and Lu–Hf isotopes of conglomerate clasts and detrital zircons

The Neoproterozoic Hedmark Basin in the Caledonides of South Norway was formed at the western margin of the continent Baltica by rifting 750–600 Ma ago. The margin was destroyed in the Caledonian Orogeny and sedimentary basins translated eastwards. This study uses provenance analysis to map the crustal architecture of the pre-Caledonian SW Baltican margin. Conglomerate clasts and sandstones were sampled from submarine fan, alluvial fan and terrestrial glacigenic sedimentary rocks. Samples were analysed for U–Pb isotopes and clast samples additionally for Lu–Hf isotopes. The clasts are mainly granites c . 960 Ma and 1680 Ma old, coeval with the Sveconorwegian Orogeny and formation of the Palaeoproterozoic Transscandinavian Igneous Belt (TIB). Mesoproterozoic (Sveconorwegian) ages are abundant in the western part of the basin, whereas Palaeoproterozoic ages are common in the east. Lu–Hf isotopes support crustally contaminated source for all clasts linking them to Fennoscandia. Detrital zircon ages of the sandstones can be matched with those from the granitic clasts except for ages within the range 1200–1500 Ma. These ages are typically found in the present-day Telemark, SW Norway. The sandstones and conglomerate clasts in the western part of the Hedmark Basin were sourced from the Sveconorwegian domain in the present SW Norway or its continuation to the present-day NW. The conglomerate clasts in the eastern part of the Hedmark Basin were sourced mainly from the TIB domain or its northwesterly continuation. The Hedmark Basin was initiated within the boundary of two domains in the basement: the TIB and the Sveconorwegian domains.

The Sub-Cambrian Peneplain in southern Norway: its geological significance and its implications for post-Caledonian faulting, uplift and denudation

The Sub-Cambrian Peneplain in southern Norway formed in the Cryogenian–early Cambrian. It was transgressed in the Cambrian–early Ordovician, and subsequently broken up by Caledonian thrusting, post-Caledonian normal faulting and regional uplift. In southern Norway the Sub-Cambrian Peneplain is generally expressed as an angular unconformity or nonconformity surface with no apparent deep weathering. In some localities the basement is characterized by a weathered top. Furthermore, the basement may have a mineralized top in some cases. The peneplain is commonly overlain by in situ regolith, and/or Cryogenian glacial diamictite, Lower–Middle Cambrian siliciclastic sediments, Lower–Upper Cambrian black shale or Lower Ordovician limestone. It has commonly mildly tectonized contacts, but occasionally displays contacts that have been completely destroyed by Caledonian thrusting and reworked into fault rocks. Tracing the peneplain across southern Norway reveals a fault-affected and fault-delineated plateau with a sloping eastern margin. Onshore cumulative offsets record vertical movements of > 2000 m. A 1–5° dip below Caledonian nappes suggests a flexural component related to tectonic overburden. As the oldest identifiable and dateable surface covering much of Scandinavia the peneplain constitutes the reference surface for studies of the younger regional morphotectonic evolution. Its shape supports the interpretation that Cenozoic uplift combined with faulting, not smooth flexural doming, caused the Southern Scandes high in South Norway.

Chapter 59 Neoproterozoic glaciation of South Norway: from continental interior to rift and pericratonic basins in western Baltica

Abstract Neoproterozoic glacial deposits of South Norway comprise the Moelv and Koppang formations. The former occurs on Baltican crystalline basement in autochthonous position at the Caledonian erosional nappe front, on basement windows and basement thrust sheets in the Caledonian nappe region, and in thick sedimentary successions in the allochthonous Hedmark and Valdres rift basins. The Koppang Formation (Fm.) occurs on top of platform carbonates in the allochthonous pericratonic Engerdalen Basin. The glacial deposits are dominated by diamictite interpreted as basal till from warm-based grounded ice, whereas stratified successions of diamictite beds, sandstone and laminated siltstone with outsized stones represent local ice-margin deposits and/or subglacially infilled water bodies, and the final glaciomarine stage. Palinspastic reconstruction of Caledonian nappe complexes carrying the glacial formations indicates that the glacial deposits were deposited over a wide area by a large western Baltoscandian ice sheet, probably during the Gaskiers (c. 580 Ma) glacial event (or events), but the age of the glaciation in South Norway needs to be better constrained.

The Oligocene succession in the eastern North Sea: basin development and depositional systems

The Oligocene sedimentary succession in the eastern North Sea is revised and re-interpreted by applying new state-of-the-art reflection seismic data integrated with new bio- and Sr-stratigraphy data from three key wells in the study area. The Oligocene succession in the eastern North Sea is divided into four transgressive–regressive (T-R) sequences, characterized by non-accretional and/or aggradational transgressive systems tracts and prograding regressive systems tracts. Detailed studies of three wells, including biostratigraphy and Sr analysis, constrain the age relationships between the T-R sequences. Internal clinoform geometry indicates that the sediments were sourced from the present southern Norwegian mainland to the north of the depositional area. The direction of progradation shifted from being SE-directed in the earliest Rupelian (early Oligocene) to S- and SW-directed during Chattian time (late Oligocene). Rapid basin subsidence is indicated by the development of non-accretionary transgressive systems tracts, with subsequent progradation into water depths of hundreds of metres. The creation of accommodation space was out of phase relative to eustatic sea-level changes, and mainly controlled by regional-scale differential vertical movements where uplift and exposure of landmasses of the hinterland (southern Norway) occurred concurrently with basin subsidence. Halokinesis had an intra-basinal influence on the main sediment transport direction, but probably did not contribute much in creation of accommodation space.

Seismic stratigraphic subdivision of the Triassic succession in the Central North Sea; integrating seismic reflection and well data

The Triassic sedimentary succession in the Central North Sea has been investigated to establish a broader understanding of the Triassic Period, from the combined interpretation of seismic reflection data and well data. The Triassic succession has been subdivided into four seismic units, where unit boundaries are characterized by regional seismic amplitude anomalies, reflecting changes in gross sedimentary facies or rock properties. A successful correlation between sedimentary facies, interpreted within the well sections and distinct seismic reflection patterns, allowed a thorough mapping of the gross palaeoenvironment throughout the Triassic. The method presented of subdividing a continental sedimentary succession into seismic units should be applicable elsewhere in other basins. The main source area during the Triassic was Scandinavia to the north, and sediment transportation was mainly along north–south- and NE–SW-trending lineaments, which are at present located onshore southern Norway, and in the Åsta Graben and the Varnes Graben offshore. An uplifted Skagerrak Graben area acted as source area in the Early and early Middle Triassic, with sediment dispersal to the south and SW. High relief existed for a longer period in western Scandinavia than in eastern Scandinavia, which supports an asymmetric shape of the Scandinavian mountains during the Triassic. Accommodation space in the Early Triassic was mainly controlled by the relief inherited from a Late Carboniferous–Permian rift phase. Although thermally induced regional subsidence continued in the Middle and Late Triassic, creation of local accommodation space was mainly limited to halokinesis, including redistribution and withdrawal of salt from the subsurface. The Upper Triassic succession is eroded across the western and central parts of the study area, although the Upper Triassic unit is preserved in synforms adjacent to salt structures. In the western part of the study area, dry, playa conditions prevailed during the Early Triassic, although fluvial systems supplied long-transported sandy detritus southeastwards in the late Early Triassic. More sandy detritus was transported into the sedimentary basin in the Middle and Late Triassic, concurrently with a gradually wetter climate. Supplementary material: Uninterpreted seismic sections are available at www.geolsoc.org.uk/SUP18726.