Structural Evolution of the T-Block Brae Fields, South Viking Graben

Abstract

Abstract The structural evolution of the T-Block (U.K. 16/17) Brae Formation fields in the southern part of the South Viking Graben reflects a history of Late Jurassic rifting and Early Cretaceous inversion. Triassic rifting follows an inherited Caledonian trend, with Permian and Triassic depocenters to the northwest and southeast of a ridge trending north–northeast through the South Viking Graben from the area of the Thelma field. In the northern part of the area, in Trees Block (U.K. 16 /12), halokinesis has created accommodation space for Middle Jurassic deposition. Further south, in T-Block, Middle Jurassic deposition does not appear to have been influenced by Caledonide structures. Rifting commenced in Trees Block in the early part of the Late Jurassic, with development of a north–south striking northern fault segment. The faulting propagated southward from the northern segment and northward from a segment to the south of T-Block, to create a relay zone opposite Thelma and Toni. At the segment centers, the fault throws are large, and the Middle Jurassic sequence dips to the west, toward the footwall. In comparison, at the Thelma relay zone, the fault displacements are much smaller, and the Middle Jurassic dips to the east. Flexural uplift and back-tilting have affected the footwall sediments and normal faults. The fault segment evolution is likely to have been a significant control on Brae sedimentation, the back-tilting of the footwalls at the segment centers funneling sediment supply into the Thelma relay zone, and footwall uplift providing emergent source areas adjacent to the developing graben. The basin morphology has been modified by postrifting thermal subsidence, increasing the eastward dip of the fault terraces. Inversion in the Early Cretaceous caused uplift of the hanging wall, creating a bulge over Thelma and Toni, and uplift on the fault adjacent to Trees Block. This inversion event is likely to be the result of oblique northwesterly compression, causing shortening and left-lateral strike-slip on the marginal faults. This event can be related to an unconformity between the Valhall and the Carrack formations, which constrains timing to the late Barremian–Aptian.