Justyna Kowal
Katarzyna Hlek
Paweł Mikuś
Palaeozoic Succession of the Fennoscandian Shield
The Fennoscandian Shield is located in the territories of Sweden, Finland, Karelia and Kola Peninsula. In this area occure Precambrian rocks, only partly covered by Quaternary. From the Proterozoic the Fennoscandian Shield was uplifting. Only in the older Paleozoic, sedimentary deposits covered the shield (but it is eroded mostly).
Cambrian successions on the Baltica Platform are dominated by stable condensed successions. The various episodes of transgression and regression may be related to local crustal instability, but it appears that at least some of the major episodes coincide in time on both sides of Iapetus and may be of eustatic origin, perhaps related to periods of faster and slower sea – floor spreading. The thin Cambrian successions are dominated by Lower Cambrian light – coloured cross – bedded sandstones and grey – green shales (<200m), overlain by Middle and Upper Cambrian grey and black shales together with stinkstones (<100m, usually 20m).
The Ordovician sedimentary succession over the platform is thin (commonly <200m) and represents deposition rates of 2-3 mm per 1000 years. Successions of the platform are situated in belts indicating a westward deepening; those of the Oslo Region, however, show a shallowing in this direction.
The early Ordovician history of the platform is one of the transgression but a late Arenig regression was eustatically superimposed on the transgressive trend.
The middle and upper Ordovician of the platform shows increasing derivation of sediments and (essentially in the middle Ordovician) bentonite horizons from the Iapetus margin. Basement faulting, possibly triggered by events in the fold belt produced local gaps in the platform sequence.
During the Caradoc and Ashgill there is evidence of three major regressive events on the platform.
Silurian rocks of Scandinavia preserve a fragmentary record of the end of phases of Caledonian ocean closure and the complementary infilling of the adjacent Baltoscandian epicontinental sea. Dark, clastic muds were deposited in most of the region through the early Llandovery, and mark an interval of tectonic stability, although terrigeneous input remained too high to permit carbonate deposition.
Along western areas of the Platform, marine facies continued until latest Llandovery times, when the marine sediments were overlain conformably by distal Old Red Sandstone molasse deposits derived from the advancing nappes.
Major carbonate deposition (with reefs) over Gotland and the eastern Baltic commenced at this time.
In many of the outcrop areas pre-uppermost Silurian marine deposits form the youngest bedrock preserved, but in those areas in which marine facies are succeeded statigraphically by non-marine rocks there is evidence that the change in regime took place well within the Silurian, supporting the tectonostratigraphical interpretation of a mid-late Silurian culminating phase of orogenesis.
By the end of the period marine deposition had ceased over the whole region; rocks of provable Devonian age are entirely of late- to post-orogenic terrestial facies
Devonian was the time of Old Red Continent - a result of Caledonian evolution.
Old Red Sandstones are a thick sequence of Devonian rocks that are continental rather than marine in origin and occur in northwestern Europe, Scandinavia, Greenland, and northeastern Canada.
The rocks were deposited in structural basins between the ranges of the Caledonian Mountains, which were also formed during the Devonian period after a section of northwestern Europe collided with a landmass made up of parts of present-day North America and Greenland.
The sediments are poorly sorted and quite variable: red, green and gray cross-bedded sandstones, mud-cracked siltstones and gray shales. In this rocks occurs also raindrop impressions. Occurrences of Old Red deposits are discontinuous.
In south Wales, SW Norway and in the Holy Cross Mountains are the best Old Red sequences.
Reference:
Gee, D. G., Sturt B. A. (eds.), 1985. The Caledonide Orogen – Scandinavia and Related Areas. – John Wiley & Sons, Chicherster, 1266pp.
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