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

Abstract

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.

Impact of faulting in depocentres development, facies assemblages, drainage patterns, and provenance in continental half‐graben basins: An example from the Fanja Basin of Oman

“…The localised occurrences of deposits of the Green Qahlah member in fault-bounded basins (i.e. Fanja and Sa'al basins; Figure 1), combined with axially trending drainage (Figure 10a), verify that faulting had initiated when this unit was deposited (Prosser, 1993;Würtzen et al, 2022) (Figure 15). Faulting ascribes to orogenic collapse by extension in the Maastrichtian-Palaeocene, when faults above detachments exerted control on local topography and localised vertical movements.…”

“…The lesser amount of gravity‐flow deposits (FA1) relative to hyperconcentrated‐ and stream‐flow deposits (FA2) in the Al Batah member compared with the Wadi al Theepa member (Figure 7) is consistent with different, possibly interplaying, scenarios. Firstly, a higher accommodation rate relative to the rate of sedimentation prevailed during deposition, as is necessary for the preservation of such thick mudstone intervals in continental basins (Würtzen et al, 2022). Secondly, several trends (e.g.…”

“…Besides, if sediment input exceeded the accommodation creation, the formation of deep and long‐lasting standing‐water bodies may have been prevented (Huerta et al, 2011; Rogers et al, 2016), confirmed by our data in the western section (Figure 7). On a different note, fluctuations in climate with drier conditions during deposition of the Wadi al Theepa member could also result in a more sand‐rich succession due to increased mechanical weathering (Nystuen et al, 2014; Würtzen et al, 2022), minor formation of fine‐grained deposits due to dry‐out of the system, and a possible prevalence of transverse over axial sedimentation (Fordham et al, 2010). Climatic studies suggest two global warming events in the Maastrichtian (Abramovich et al, 2010) overall humid seasonal to perennial but with one dry seasonal period recorded in the Late Maastrichtian (Abramovich et al, 2003; Adatte et al, 2005; Keller, 2004).…”

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/geology/lithostratigraphy/).…”