Sediment gravity-flow deposits in an exhumed, strike-slip, deep-water basin, the Paleogene Forlandsundet Graben, western Svalbard margin

The Forlandsundet Graben, located between the island of Prins Karls Forland and the west coast of Spitsbergen is a narrow, fault-bounded basin of proposed transtensional origin. It has been formed due to Paleogene dextral strike-slip movements between Svalbard and Greenland, which ultimately resulted in the establishment of the western Svalbard transform margin. Despite its geological significance, the evolution of this basin and its several-kilometre-thick fill, reportedly containing sediments of continental to deep-water origin, remains poorly understood. This study investigates the exhumed western basin margin, which exposes >3.5 km of sedimentary strata, to clarify the processes responsible for sediment deposition, focusing on the upper, marine portion of the basin fill. Our investigations identify a variety of conglomerate- to sandstone-dominated beds, representing deposits from diverse sediment gravity flows. These include cohesive to non-cohesive debris flows, gravel- and sand-rich high-density turbidity currents, low-density turbidity currents, and hybrid flows. The beds are organised into rhythmically alternating couplets: thin-bedded, sandstone-rich heterolithic units, and thick-bedded, conglomeratic sandstone units, representing submarine lobe fringe and lobe axis to off-axis deposits, respectively. Coarsening- and thickening-upward cycles, ranging from a few metres to several hundred metres, indicate a hierarchy of progradationally stacked lobe elements, lobes, and lobe complexes. This stacking pattern, and the degree of rounding observed in the conglomerates, suggest that the submarine lobes were fed by high-gradient streams. Overall, the succession indicates a deep-water basin that experienced substantial sediment influx through sediment gravity flows. A basin-wide deepening event marks the onset of rapid fault-induced subsidence likely driven by the regional transtensional regime, which established marine conditions from the middle/late Eocene. The basin served as a deep-water depocentre into the early Oligocene and persisted until exhumation later in the Cenozoic.