Late syn-rift to early post-rift basin fill dynamics of a mixed siliciclastic-carbonate succession banked to a basement high, Hornsund, southwestern Spitsbergen, Arctic Norway

The transition from syn-rift to post-rift sedimentation in rift basins is difficult to characterize in terms of stratigraphic architecture and dominating control on sedimentation, due to decreasing tectonic activity interplaying with regional subsidence, eustatic sea level changes, and differential compaction of underlying syn-rift sediments. Our case study of the Late Palaeozoic Inner Hornsund Fault Zone targets late syn-rift strata recorded in the (?Pennsylvanian – ?lower Permian) Treskelodden Formation in Hornsund, southern Spitsbergen, representing a mixed siliciclastic-carbonate succession, with siliciclastics primarily sourced from the adjacent Sørkapp-Hornsund High. We document local scale (<10 km) facies variability, sequence stratigraphy, and evolution of a succession deposited along a flank of the structural high during the late syn-rift stage. We observe that during the transition towards rift termination (glacio-)eustatic sea level changes and overall regional flooding became a more prominent forcing factor controlling sedimentation. Our dataset includes sedimentary logs, microfacies analysis, and high-resolution digital outcrop models. We identify four progressively backstepping stratigraphic sequences, reflecting an evolution from (1) terrestrial siliciclastics through (2–3) nearshore mixed siliciclastic–carbonates, to (4) carbonate ramp deposits. On the small scale (<5 m) the internal sediment cyclicity of the succession was formed by autogenic processes, particularly the changing rate of sediment input from the southwestern source area (the uplifted Sørkapp-Hornsund basement high). On the larger scale (10s of m), the importance of glacio-eustatic sea-level changes, driven by waxing and waning of ice caps in the southern hemisphere (Gondwana), increased as the rift-related tectonics decreased. The interdisciplinary methods used in this study provide new knowledge of the Middle Pennsylvanian to Permian depositional evolution in southern Spitsbergen, besides a novel framework for comparison to adjacent basins in the region and similar basins elsewhere.