Syn- to post-rift alluvial basin fill: Seismic stratigraphic analysis of Permian-Triassic deposition in the Horda Platform, Norway

The Permian-Triassic alluvial rift succession in the Horda Platform area is analysed to construct a refined depositional model. The study demonstrates how subsurface continental rift successions may be stratigraphically subdivided and correlated by integrating seismic and well-log data in concert with conceptual models. Regional unconformities mark the top and base of the Permian-Triassic succession, which is sub-divided into six seismic stratigraphic sequences, delineated by erosional- and non-depositional surfaces. The definition of seismic stratigraphic sequences is based on seismic facies trends and Gamma Ray log signatures. Time-thickness maps combined with geometries in cross-section display the depocentre development. During the Permian-Triassic, the Horda Platform experienced faulting, shaping the Caledonian pre-rift landscape into a series of N-S trending half-graben basins, contemporaneous to a gradual climate change from arid in the Permian to humid in the latest Triassic. The basin underwent three phases of rifting during the Late Permian-Middle Triassic: (1) disconnected heterogeneous depocentres with strain concentrated in the west; (2) depocentres expanded northward; and (3) mature half graben development with widely distributed strain. Vertical lithological changes in mudstone/sandstone dominance reflect varying rates of accommodation and sediment supply (A/S). Systematic A/S variation reflects strong climatic fluctuations, which controlled facies patterns during both tectonic quiescence in the Middle-Late Triassic, and during syn-rift sedimentation. The Permian-Triassic tectono-sedimentary development in the Horda Platform area provides valuable lessons on the influence of faulting on depocentre development, how the interplay between tectonic and climatic forcing is expressed in subsurface continental deposits, and aid the characterisation of reservoirs.