Depositional environment on eastern Svalbard and central Spitsbergen during Carnian time (Late Triassic): a sedimentological investigation of the De Geerdalen Formation

Sedimentological data were obtained for the De Geerdalen Formation (Upper Triassic) from outcrop studies at Edgeøya, Hopen and central Spitsbergen to reconstruct the palaeo-depositional environment of the formation. Data from fieldwork 2009 in addition to earlier studies makes it possible to assess the formations changing sedimentation pattern. Lateralvariations were mapped by logging several vertical sections in adjacent areas and helicopterbased LIDAR-scans were taken at Edgeøya to be used for later spatial studies of sandstonebodies. Good exposures, especially in the eastern part of the Svalbard archipelago giveimportant information on unit thicknesses, lateral and vertical facies variations and spatial distributions of sand bodies. This investigation is a part of the Longyearbyen CO2 project, aiming to store CO2 in bedrock below Longyearbyen, where sandstones from De Geerdalen Formation are the expected target. Earlier studies, using sedimentological data, have suggested the De Geerdalen Formation to represent a shallow marine to prograding delta deposit (Mørk et al. 1982). Recent studies of the Middle and Late Triassic succession show prograding clinoforms from ESE, likely to represent De Geerdalen Formation east of Svalbard, (Riis et al. 2008). Our data support the later studies indicating a dominating ESE source. On central Spitsbergen the lower part of De Geerdalen Formation contains fine-grained sandstone units with thicknesses about 5 m, showing storm (hummocky) and shoreline structures. Sandstones in the upper part have greater thicknesses up to 25 m, displaying both channel systems and tidally influenced shoreface deposits. The coarsest grain-sizes found in the sections are medium-grained. Measured sections on the eastern part of Svalbard containsand units representing channel, tidal and shoreface environment. Growth faults are evident insome areas on Edgeøya. Sandstone thicknesses up to 30 m are present, and it shifts laterally between fine to medium-grained channel sands and fine-grained shallow marine sandstones.Thin coal layers, some with rootlets, are developed in the eastern sections, and channel sands are present more frequently. The data indicates an earlier development of proximal facies associations on eastern Svalbard than on central Spitsbergen, generally with a regressive pattern in both areas