Triassic to Jurassic chemostratigraphy and heavy mineral assemblages on the Sleipner Terrace, South Viking Graben, Norway

Abstract

The Triassic in the Norwegian sector of the South Viking Graben (SVG) has been historically subdivided into the shale-prone Smith Bank Formation overlain by the sand-rich Skagerrak Formation. Depositional environments include fluvial, alluvial sheetflood, playa, floodplain and lacustrine. This terrestrial character usually results in poor biostratigraphic control and uncertain inter-well correlations, which is further hampered by the varying erosional depth of the Aalenian or Cretaceous unconformities. Seismic correlation between wells is also limited by the lack of acoustic impedance contrasts within the Triassic section. This study reports chemostratigraphy and heavy mineral assemblages of selected wells in blocks 15/9 and 16/7 on the Sleipner Terrace and in the Ling Graben. The data presented in this study suggest a three-fold subdivision of the Triassic succession, informally termed Unit 1/1* to 3. Unit 1/1* represents a mudstone interval dominated by high content in Al₂O₃, which is dissected by two channel systems. Unit 2 forms the older channel system with wacke sandstone, sublitharenite and Fe-rich sandstone lithologies, and indicates multi-recycling of Permian or older siliciclastics with some input of freshly exposed metamorphic basement. Unit 3, equivalent to the Skagerrak Formation, represents a channel system derived mainly from erosion of the Western Gneiss Region. Unit 3 sandstones are immature and include wacke, litharenite, subarkose and arkose sandstones. Increased Ga/Rb ratios in the lower part of Unit 3 indicate a switch to warmer humid climate, likely related to the Carnian Pluvial Episode in the Late Triassic. Thus, the underlying units 1/1* and 2 are tentatively interpreted to be of Middle to Late Triassic age. Climate proxies do not indicate the preservation of sediments deposited during the Early Triassic hot house climate. The occurrence of Unit 3 on the Sleipner Terrace and within the Ling Graben suggests that the NE–SW-trending Ling Graben boundary fault did not act as a topographic barrier. However, Unit 3 shows a significant thickness expansion in the Ling Graben, indicating Late Triassic activity of this fault. Observed correlation length of individual chemozones (5-10 km′s), the facies equivalence between unit 1/1* mudstones and unit 2 sandstones, differential preservation of chemozones at the base and at the top of the Triassic, coupled with observational bias related to the final well penetration depth suggest that individual mudstones and/or sandstones do not lend themselves suitable for basin wide correlation.