From direct sampling, the deeper Rockall Trough and Faeroe-Shetland Channel are known to have a Tertiary-Quaternary sedimentary sequence up to 3000 m thick, which is in places, particularly in the north, underlain by early Tertiary basaltic volcanic rocks. The seamounts in the Rockall Trough are of basic volcanics of probable Upper Cretaceous age. The eastern shelf areas have a rifted basement of Precambrian-Devonian (-?Carboniferous) age, overlain by Permian + Mesozoic sedimentary rocks that reach 5000 m in thickness in rift basins. Tertiary sediments thicken rapidly from the shelf into deep water. The western shelf areas have extensive early Tertiary basalts from the Faeroe Islands to the southern part of Rockall Bank. A thin Tertiary—Quaternary cover exists and Precambrian basement lies beneath.
The pre-Tertiary geology of the deep water areas and the overall crustal structure have been inferred from geophysical investigations. In the Rockall Trough the crust is of oceanic thickness, about 6 km, but it is probably slightly thicker beneath the Faeroe-Shetland Channel. This fact, coupled with the size of the channel compared with other small ocean basins and the knowledge that fully developed oceanic crust exists just outside the mouth of the Rockall Trough, strongly suggests that at least parts of the deep water areas are floored by oceanic crust. However, seismic reflection and magnetic anomaly profiles do not yield observations characteristic of normal oceanic crust.
The age of any oceanic crust in the Rockall Trough and Faeroe-Shetland Channel is equivocal. Between 54° and 59° N a succession of largely sedimentary rocks up to 3000 m in thickness occurs between the Tertiary and the acoustic basement. To the north this succession is masked on seismic profiles by early Tertiary basalts but it is probably present; to the south it is interrupted by a series of acoustically opaque basement ridges. With slow sedimentation rates, this succession could extend back to the late Palaeozoic, but with rapid rates, only to the mid-Upper Cretaceous. An age of mid-Lower to mid-Upper Cretaceous for oceanic crust, equal to that of the ocean crust outside the mouth of the Rockall Trough, is accepted here. Although rapid subsidence and infill in Upper Cretaceous time is not characteristic of major shelf basins around Britain, it may be acceptable for the Rockall Trough and Faeroe-Shetland Channel if they are underlain by oceanic crust rather than continental crust.
A likely model for the formation of the Rockall Trough and Faeroe-Shetland Channel is of continental rifting and subsidence from late Palaeozoic or earliest Mesozoic to mid-Cretaceous time, then sea-floor spreading in Albian (c.105My)–Santonian (c.85 My) time, accompanied and immediately followed by rapid subsidence and deposition. The Tertiary was heralded by widespread basaltic igneous activity which briefly arrested subsidence, but was largely a period of subsidence without sedimentation keeping pace.