Permo–Triassic sedimentary fills and tectonic phases off Mid Norway: seismic investigation of the Trøndelag Platform

Abstract

Seismic interpretation (2D and new 3D surveys) has been used to investigate sedimentary fills and timing of tectonic activity offshore Mid Norway. This study was focused on upper Permian and Lower Triassic sedimentary basin fills, but a longer stratigraphic interval (Devonian-Upper Triassic) was analysed in order to get a broad understanding of what happened prior to, during and after deposition of the upper Permian-Lower Triassic successions. The ages of the sedimentary fills were partly constrained by well ties. Seismic reflectors and sedimentary successions below the upper Permian interval are of Late Devonianmid Permian age. Six sedimentary fill geometries (fill type A -F) were identified. These are (A) fault-ward thickening packages with internal strata thickening towards bounding faults, (B) wedge-shaped packages whose internal strata have more or less uniform thickness, (C) sedimentary fills containing fill type A overlain by sedimentary strata with more or less uniform thickness, (D) gently dipping packages that thicken towards deeper areas of the basin, and downlap onto pre-existing topography, (E) gently dipping strata filling depressions, and (F) sedimentary wedge with rotated internal strata and folded top. These fill types were used to determine phases of active tectonics and quiescent phases. Based on temporal changes of the fill types, five late Palaeozoic-Triassic unconformities have been mapped: a nonconformity where the Palaeozoic strata onlap onto the basement, a mid-Permian unconformity, two Early Triassic unconformities and a Middle Triassic angular unconformity. Results show that Devonian-Permian, mid-Permian, late-

Cretaceous–Quaternary seismic stratigraphy of the Tanga offshore Basin in Tanzania and its petroleum potential

“…Seismic geometries of sedimentary bodies, palaeodepositional basin shapes, stratal relationships and termination patterns (for example angular erosional truncations, onlap and downlap) were used to identify unconformities which bound different depositional regimes. The geometries of sedimentary fills, stratal termination patterns, internal seismic expressions and basin shapes were interpreted based on the works of Mitchum et al (1977), Vail (1987), Prosser (1993), Nøttvedt et al (1995, Ravnås and Bondevik (1997), Posamentier et al (2000), Posamentieier and Kolla (2003), Armitage et al (2012), Braathen et al (2012), Kyrkjebø et al (2004) and Kiswaka and Felix (2020). The unconformities presented herein were identified based on the original definitions by Mitchum et al (1977) and recent works of Kyrkjebø et al (2004) and Kiswaka and Felix (2020).…”

“…The geometries of sedimentary fills, stratal termination patterns, internal seismic expressions and basin shapes were interpreted based on the works of Mitchum et al (1977), Vail (1987), Prosser (1993), Nøttvedt et al (1995, Ravnås and Bondevik (1997), Posamentier et al (2000), Posamentieier and Kolla (2003), Armitage et al (2012), Braathen et al (2012), Kyrkjebø et al (2004) and Kiswaka and Felix (2020). The unconformities presented herein were identified based on the original definitions by Mitchum et al (1977) and recent works of Kyrkjebø et al (2004) and Kiswaka and Felix (2020). The use of the term unconformity follows previous explanations of the seismic expression of the stratigraphy off the coast of East Africa (Franke et al 2015;Kiswaka 2015;Fonnesu et al 2020).…”

“…The packages are characterized by weak discontinuous internal reflectors. This fill type indicates that deposition took place during tectonic quiescence (see Kiswaka and Felix (2020)); faulting shows a post-fill tectonic deformation. Parts of the Late Cretaceous deposits of the Tanga offshore Basin are characterized by this fill type (Fig.…”

“…The lower pattern indicates that Miocene sedimentation took place during active faulting, i.e. syn-rift deposition (Nøttvedt et al 1995;Ravnås and Bondevik 1997;Ravnås and Steel 1998;Kiswaka and Felix 2020). The constant thickness of this unit implies that deposition took place after the fault movement has stopped, i.e.…”

“…The constant thickness of this unit implies that deposition took place after the fault movement has stopped, i.e. post-rift sedimentation (see Kiswaka and Felix 2020). Continuous reflector blanketing the wedge-shaped sub-basin marks complete infill of a remnant rift topography above which a new depositional configuration is established (Prosser 1993).…”