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Reconstructing a high paleolatitude Mesozoic paleoenvironment from a truncated and deeply buried regolith, Norwegian North Sea

Riber, Lars, Driese, Steven G., Stinchcomb, Gary E., Dypvik, Henning, Sørlie, Ronald
Palaeogeography, palaeoclimatology, palaeoecology 2019 v.528 pp. 60-77
Cretaceous period, acid soils, cations, chemical bases, endomycorrhizae, eroded soils, feldspar, fungi, kaolinite, leaching, microstructure, pH, paleoecology, regolith, saprolite, sediments, smectite, soil horizons, soil quality, subhumid zones, weathering, North Sea
The paleoregolith in this study was unconformably overlain by lower Cretaceous (Valanginian) marine deposits of the Åsgard Formation and buried below two kilometers of sediments, on the Utsira High, Norwegian North Sea. The upwards intensification of chemical alteration, manifested in the replacement of feldspars by clays, leaching of base cations and reduced mechanical rock strength, in addition to the presence of terrestrial biotitic features, suggested weathering during subaerial exposure of the Utsira High during the Mesozoic. The paleoregolith can be divided into three weathering facies: altered coherent rock, saprock and saprolite, whereas the absence of true soil horizons suggested erosional truncation of the regolith before burial. Possible remnants of the eroded soil were identified as pedogenic mud aggregates in the overlying Åsgard Formation, bearing similar compositional characteristics as the saprolite.Clay mineralogy, bulk geochemical composition, micromorphology and depth of biotic features were used to reconstruct the paleoenvironmental conditions that prevailed during the formation of the regolith in the Mesozoic North Sea. Our results point to a regolith formation under periodically water-restricted conditions, promoting the co-existence of interstratified illite-smectite (beidellite) and kaolinite, deep presence of endomycorrhizae (the fungal extension of the root structure) and precipitation of iron oxide nodules. Application of recently developed geochemical climofunctions indicated that the regolith formed in a mesic (10–12 °C) and subhumid (1157–1406 mm/yr) environment, and under strongly acidic soil conditions (pH 5.2–5.6).