Late Ordovician-Early Silurian facies development and stratigraphy of Jämtland, central Sweden
Abstract: This thesis is based on studies of Lower Palaeozoic sedimentary successions within the central Scandinavian Caledonides. These deposits have been transported by considerable nappe displacement (several kilometres), and are today relatively isolated from the nearest coeval outcrops (Dalarna 250 km, Oslo Region 350 km). As a consequence, and due to the previously limited biostratigraphical control the investigated area have received little attention the last decades. Therefore, a detailed investigation, using several untried geological tools was performed. During the study it became clear that the strata reflected substantial changes in depositional environment and complexity in lateral facies relationship during the Late Ordovician?Early Silurian. This was a time characterised by global environmental changes such as glacio-eustasy and climate change, in particular during the end-Ordovician (Hirnantian) glaciation. Facies and sequence stratigraphical analysis of two key units (the Ede Quartzite and the Kyrkås Quartzite) and their preceding and succeeding units resulted in reinterpretations and refinement of the stratigraphy. The new data implied that the successions could be linked to known Hirnantian sea-level fluctuations. This correlation was subsequently supported, and our understanding of the timing of changes was improved, by biostratigraphical evidence (conodonts, graptolites, and brachiopods). This revealed a ca 5 myr stratigraphical gap, within the Ede Quartzite, spanning the Ordovician?Silurian boundary. It is concluded that the interaction of allocyclic changes (sea level and climate) were the overriding controls on deposition during the Late Ordovician?Early Silurian in the Jämtland basin. The contrasting sedimentary architecture of the Ede Quartzite to the west and the partly coeval Kyrkås Quartzite to the east remained enigmatic. Provenance studies (radiometric dating of zircons) tentatively indicate different dominating source areas, but further studies are needed to solve their relationship.
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