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

Abstract

Discrepancies in models of continental rift-basin dynamics and stratigraphic response calls on further investigation on the subject. Geometric- and lithological trends between stages of faulting is studied in the Permian- Triassic continental rift succession in the Horda Platform. The Horda Platform occupies the northeastern margin of the North Sea aulacogen where Late Permian-Early Triassic faulting shaped the Caledonian pre-rift landscape into a series of N-S trending half-graben basins, filled by Permian-Triassic strata. A tectonostratigraphic model developed from seismic- and well-data details the Permian-Triassic basin fill and structural basin development. Regional unconformities mark the top and base of the succession, while internally, six depositional sequences are delineated by erosional- and transgressive surfaces. Thickness maps reveal three syn-rift stages, where strain migrated and concentrated in different parts of the developing rift basin, from disconnected faults and scattered depocentres with varying accommodation space hosting deposits of different thicknesses, to fully linked faults bounding half-graben basins with expanded and connected depocentres. The lithology through the syn-rift stages reflect how sedimentation gradually outpace accommodation space creation. The overlying thick post-rift succession shows no evidence of rifting besides a minor fault-displacement along the Øygarden Fault Zone in the Late Triassic, reflected by slight wedging. Meanwhile distinct interchanging sand- and mud-dominated intervals reflect strong climatic fluctuations during the post-rift. The spatio-temporal heterogeneity in fluvial facies makes stratigraphic correlation challenging and is further complicated by discontinuous surfaces caused by basin floor tilting. With this study, classical sequence stratigraphic models are evaluated and revised against the observations from the Horda Platform. The stratigraphic model presented takes into account the dynamics of strain along faults through time and space and the resulting diachronic boundaries between the rift stages.

Rift kinematics preserved in deep‐time erosional landscape below the northern North Sea

“…Here, we cannot measure or estimate bedrock erodibility (or changes thereof), because the surface is buried under 4 km of sedimentary strata and the basement rocks, into which the surface is eroded are not fully characterised (Fazlikhani et al, 2017). Similarly, although the climate appears to have been consistently arid during late Permian-early Triassic times (e.g., Würtzen et al, 2021), there are no reliable estimates of precipitation rates for that time interval. Consequently, the geomorphic analysis of buried landscapes is by necessity more limited than what can be achieved with modern topographies.…”

“…Note that the depth migration has been calibrated with borehole checkshots. (c) West-east chronostratigraphic chart for the Permian-middle Jurassic in the Horda platform (from Würtzen et al, 2021). Modified from NPD (https://npd.no/en/facts/ geolo gy/litho strat igrap hy/).…”