干旱区生态环境知识资源中心

The European continental plate drifted northwards in Late Triassic to Early Jurassic time. The change in plate position gave rise to a long-term shift from arid/semi-arid to humid climate in the North Sea region, in addition to a global warming effect of the igneous activity in the Central Atlantic Magmatic Province. In the Tampen Spur area in the northern North Sea, this climatic change profoundly influenced the development of alluvial successions represented by the Lunde Formation (Norian-early Rhaetian) and the overlying Raude and Eiriksson formations (Rhaetian-Hettangian) in the Statfjord Group. Arid to semi-arid climate during the Norian and early Rhaetian is expressed in the lower part of the Lunde Formation by ephemeral braided stream sandstones, reddish-brown entisols/inceptisols and floodplain mudstones characterised by diverse clay mineral suites and high feldspar contents. A change towards semi-humid climate in the late Rhaetian is reflected in the upper part of the Lunde Formation by fluvial sandstones representing meandering rivers, moderately developed cumulative vertisols and higher contents of smectite and kaolinite resulting from increased chemical weathering. Increased production of clays in the hinterland, lowered relief and decreased rate of sediment supply gave rise to a fining-upward trend with an increasing degree of palaeosol development in the upper part of the Lunde Formation and the lowermost part of the Statfjord Group. Increasing humidity during the latest Rhaetian and Hettangian is witnessed in the lower part of the Statfjord Group by a change from reddish-brown to greenish-grey colour in mudstones, disappearance of calcrete nodules and smectite as the dominating clay mineral formed on wet floodplains. Quartz-arenitic braided stream sandstones and kaolinite-dominated floodplain mudstones in the upper part of the Statfjord Group reveal that humid and warm climate conditions prevailed, with intense chemical weathering in the hinterland. Monsoonal rainfall gave high surface runoff, producing sediment-laden discharge within well-drained alluvial fans or plains. Coupled with low accommodation space, this resulted in a coarsening-upwards trend in the Statfjord Group, braided-stream sandstones with wide lateral extent and floodplain mudstones with palaeosols reflecting highly varying degrees of pedogenic alteration. Allostratigraphic units in the Lunde Formation and the Statfjord Group may have formed in response to more high-frequency climatic cycles or base-level changes.