The evolution of catchment-depositional system relationships on the dip slopes of intra-rift basement highs: an example from the Frøya High, Mid-Norwegian Rifted Margin

Abstract

Basement highs form one of many potential sediment source areas during the evolution of continental rifts and rifted margins and add to the topographic complexity typical of active rifts. Footwall basement highs acting as a source area to sedimentary systems in the hangingwall of major faults have been documented in many systems worldwide. However, the back-tilted footwall dip slopes of such highs have received comparatively little attention. Here, we investigate a subsurface case study from the Norwegian continental shelf, where catchments and shallow marine syn-rift sedimentary systems on a dip slope are preserved due to early transgression of an intra-rift high. At the onset of Late Jurassic rifting, the Frøya High emerged as a prominent, N-S trending, 25 km-wide basement high tilted towards the east in response to several kilometres of displacement along the Klakk Fault Complex, a major normal fault zone at the Frøya High's western edge. Using well-calibrated 3D seismic reflection data, we observe a series of conspicuous Upper Jurassic wedges along the eastern edge of the Frøya High along the margin of the Froan Basin. Internally, these wedges show sigmoidal geometries marking top and foresets of clinoform packages with a maximum thickness of ca. 200 m with foresets between 30 and 200 m high, dipping ca. 10° towards the east, southeast and northeast. We interpret these wedges to represent a series of eastward prograding deltas positioned along a constructional shoreline, connected to E-W trending valleys and river catchments up-dip. The deltas show strong progradation, interpreted to reflect the impact of the continued uplift of their catchments prior to the abrupt termination of sediment supply from drainage capture by footwall scarp drainages. The presence of a connected, largely constructional shoreline has implications for Late Jurassic sediment distribution around the Frøya High, providing primary sedimentary input for longshore-driven sedimentary systems in the Draugen Ridge to the north. Comparisons with other syn-rift dip slope systems highlight a broadly similar evolution but shows a distinct lack of the protracted backstepping observed in other dip slope systems. We postulate that different structural configurations of dip slope systems, being footwall uplift or hangingwall subsidence driven, may drive the strongly progradational character of the deltaic systems on the Frøya High. The Frøya High example highlights the need to constrain primary sediment input points to aid the interpretation of volumetrically significant, but short-lived and subtle depositional systems, especially within complex, tectonically active settings.