Camilla Louise Würtzen

The Triassic continental succession west of the Øygarden Fault Complex in the northern North Sea is studied using 2D and 3D seismic lines integrated with well pattern analysis to discern the basin fill history and structural development of the area. The Øygarden Fault Complex is located within the N-S trending Horda Platform east of the deeply faulted Viking Graben. A series of large west-dipping faults bounding east-dipping half grabens characterize the Horda Platform area. The focus area is within a rotated fault block between the basement-bounding Øygarden Fault in the east and the Vette Fault in the west. The Triassic fill forms a 1300 m eastward thickening unit, commonly regarded as part of the Late Triassic post-rift stage. A wedging of some intervals is however indicative of active faulting during deposition. Previous studies have subdivided the strata in the Tampen Spur area into a late Permian-early Scythian syn-rift basin fill and three post-rift phases; 1: Late Scythian to Ladinian, 2: Carnian to Norian, 3: Norian to Rhaetian, defined by bounding unconformities and depositional patterns. This study subdivides the Øygarden area succession into seismic facies where reflection patterns indicate depositional characteristics. Reflection pattern analyses reveal depositional features such as channels, hangingwall fans, and footwall fans. A shift in facies trend between mud and sand rich intervals indicates variance in subsidence rate and sedimentation supply related to tectonic displacement rate and climate. The climate changed from arid to semi-humid in the Latest Triassic according to studies from Tampen Spur: (1) a shift in palaeosol types, (2) increase in carbonate material, (3) seasonality indicated by isotope analysis, (4) absence of anhydrite, (5) an increase in abundance and thickness of the mud-dominated intervals related to an increase in pluvial lacustrine conditions, (6) a change in palaeolatitude, and (7) increased fluvial channel stability indicating increasing sinuosity. Humid climate results in denser vegetation, which strengthens levees and their channels. During increased runoff, the sediment yield increased, with correspondingly coarser grained material delivered to the basin. Models for sedimentation fill and stratal package geometries in rift systems can be applied to the Øygarden area. The analysis suggests a depositional system with N-S trending highs bounding narrow elongated basins. Alluvial fans prograded from the ranges into the basins. Changing subsidence rate and climatic variations controlled the magnitude and style of axial draining channels along with the dispersal and preservation of surrounding finer grained alluvial plain style deposits. Keywords. Triassic, Øygarden Fault Complex, northern north Sea, basin fill history, seismic facies, alluvial