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Amerada Hess Ltd, 33 Grosvenor Place, London SW1X 7HY, UK
3D seismic mapping in the southern part of Quadrant 15 in the Outer Moray Firth Basin reveals an array of circular to oval downthrown blocks bound by steep ring faults. These lows affect Triassic and older strata, are approximately 1.5–2 km in diameter and 0.5–1 km deep. The footwall crests of the structures now lie at a depth of c. 2.5 km. Several alternative causal mechanisms for these circular fault blocks are examined including salt dissolution, strike-slip pull-apart and impact cratering, but in terms of geological and geometrical criteria, the most likely mechanism appears to be small calderas or diatreme-conduit collapse. Seismic data indicates that the circular structures cut Triassic, Permian and Devono-Carboniferous strata and are infilled with pre-Upper Oxfordian sediments, therefore they are broadly synchronous with Mid-Jurassic volcanism–local well control demonstrates the presence of several thousand feet of basalt tuffs and lavas. The model proposed here to account for the circular structures consists of an initial phase of diatreme/maar cratering as magmas associated with the Rattray Volcanics Member intruded shallow aquifers in the Pentland Formation fluviodeltaic rocks. These maars enlarged and deepened via conduit wall collapse, supplemented by downsag-faulting due to magma withdrawal. The post-volcanic evolution of the structures included (1) differential erosion creating accommodation space which may have been infilled with Oxfordian reservoir sands and (2) local reactivation by underlying basement faults in the Kimmeridgian.
Keywords: Jurassic, Moray Firth, Rattray Volcanics Member, caldera, maar.
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