Basement-Cover Relationships and Regional Structure in the Transtensional Orcadian Basin

Abstract

Geological evidence suggests that the Orcadian Basin, a thick development of Devonian age continental facies, comprising alluvial, fluvial, lacustrine and aeolian deposits, is analogous to the Clair Group which developed in the smaller, offshore Clair Basin. Studies for the Clair Group to date have focussed on the predominantly fluvial-lacustrine rocks of the central Orcadian Basin in Caithness and Orkney. However, alternatives exist in Shetland and around the margins of the Orcadian Basin which are of a closer affinity to the predominantly fluvial-alluvial rocks of the Clair Group.

Detailed field studies and structural analysis of basement-cover contacts and the overlying Devonian sedimentary sequences were undertaken in Shetland, Orkney and NE Scotland, in order to understand the scale, geometry and distribution of structures, and as an analogue to equivalent features in the subsurface, at sub-seismic to outcrop scales. This was supplemented by the integration of legacy and new multiscale data (topographic, bathymetric, geological and geophysical) and the production of photogrammetric 3D Virtual Outcrop Models.

A diverse range of basement/cover relationships occurs both spatially and temporally in the basin, together with synchronous Middle Devonian syn-sedimentary faulting and growth folding, as observed on Foula. Thus, some folds interpreted as Permo-Carboniferous inversion structures may have instead initiated earlier, as Devonian growth folds. These folds, together with the large-scale structural geometries of the basins in Shetland are consistent with models of regional constrictional strain, related to sinistral transtension along the Walls Boundary and Great Glen Fault Zones during the Middle Devonian. Analysis of high-resolution bathymetric data from offshore Caithness/Orkney has provided new insights into the distribution and scale of structures in the Orcadian Basin that are poorly exposed onshore, with kilometre-scale folds and numerous faults and fracture zones being mapped for the first time. Provenance studies indicate that Foula and Melby in Western Shetland show a stronger affinity with the Clair Group and Orkney successions, suggesting a large, linked depositional system with Foula representing a transitional region located between the Clair and Orcadian basins. These findings suggest that the Devonian basins of Western Shetland and marginal areas of the Orcadian Basin are suitable analogues to the Clair Basin. Models of Devonian basin development used offshore should also consider the role of strike-slip tectonics and sinistral transtension during basin evolution.