Sedimentology and Diagenesis of the Triassic Snadd Formation in the Barents Sea
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A palaeodepositional model and an analysis of the diagenetic evolution of the Upper Triassic Snadd Formation in the Barents Sea are here presented. Attention has especially been given to the pedogenic structures dominating the sediments. The interpretations are based on core logging and thin section analyses including three shallow, stratigraphic wells from the south- western Barents Sea area. The data from the Nordkapp Basin represent a sediment succession of only a few meter thicknesses, and the main interpretation is therefore based on data from the Bjarmeland Platform core, which covers the upper 108.9 m of the Snadd Formation of Carnian to Norian age. Sandstones are mineralogical immature, fine- grained lithic arenites. Main carbonate cements include siderite, calcite and ankerite. Iron carbonates formed early in the diagenetic evolution and a progressive decrease in the Fe- content of the system with time has been evidenced. Kaolinite is a dominating, early diagenetic mineral, and indicates an overall humid climate. Regional interpretations of the Upper Triassic palaeoclimate, are that there was a switch from arid to humid on the Triassic- Jurassic border, and this goes well with the observations made in this study. Alternating sand and mud layers dominate the Upper Triassic strata and are suggested to represent deltaic sequences formed as a consequence of autocyclic switching of lobes on the delta plain. Sedimentary structures indicate a tidally influenced environment, while thin coal deposits and abundance of root structures indicate a transitional to continental setting. In the Bjarmeland Platform core there is a switch from coarsening upward units dominated by tidal structures in the lower portions to dominantly fining upward units with rooted horizons and lack of tidal influence in the upper portion, suggesting a transition from lower delta plain to mid/upper delta plain. The Nordkapp Basin cores bear resemblance to the upper portion of the Bjarmeland Platform succession and a similar setting has been interpreted for these deposits. The depostional model correlates well with former interpretations of a large- scale delta system prograding in a westward direction in Late Triassic. Mottled sediments clearly modified by pedogenic processes are found inbetween smaller fining upward or coarsening upward sand units. The interpreted delta plain environment is a favourable setting for pedogenesis and several micro-textures e.g. carbonate-coated grains, alveolar textures, circumgranular cracking and Microcodium have been identified in the polarization microscope. Coal beds are often associated with dark grey to black carbonaceous mudrocks reflecting high water table and reducing conditions during pedogenesis. The alternations between oxidized ferruginous palaeosols with carbonate concretions and carbonaceous palaeosols most likely reflects variation in ground-water level, caused by avulsion of channels on the delta plain. Oxidized palaeosols probably formed during low-water table while carbonaceous palaeosols were formed during high ground-water table perhaps in small pools on the delta plain. The well- preserved soil profiles indicate subaerial exposure for a sufficient period of time, followed by lack of subsequent erosion. Extensive carbonate cementation at an early stage has probably enhanced the preservation of the soils.