Giant irregular pockmark craters in the Palaeogene of the Outer Moray Firth Basin, UK North Sea

A cluster of irregular pockmark craters of Palaeogene age in block 15/18 in the Outer Moray Firth, UK North Sea have been mapped on 3D seismic data wi th well control. The craters range from 0.5 to 4 km in diameter, between 50 and 200 m in depth, and have been buried by Tertiary subsidence and sedime ntation to present-day depths of between 1000 and 1400 m sub-sea. They are circular to elliptical in plan view, and are located in a delta front and u pper pro-deltaic slope position on the Dornoch delta. The structures are co nfined to the top of the Lower Eocene Balder Formation, and regional mappin g across the Moray Firth Basin shows they are restricted to an area of ca. 15 x 15 km(2). Well data show that the craters are sand-filled, and have ex cavated an argillaceous and tuffaceous unit. The crater-filling sands are s ealed by overlying mudstone units. Planar wing-like features emanate from t he margins of the craters. Well control indicates that these seismically id entified 'wings' consist of sandstone, suggesting that the craters have bee n modified by an episode of sand remobilisation and injection. The proposed mechanism for the formation of these structures is large-scale fluid erupt ion (probably shallow gas) at the Early Eocene seafloor. These craters are significantly larger in size than those typically observed on contemporary sea floors. We argue that the gas is more likely to have been sourced from deeper Jurassic or Carboniferous sediments within the Witch Ground Graben, than from shallow biogenic origins. These structures rank in scale alongsid e the largest known pockmarks and demonstrate that natural fluid and hydroc arbon seeps have a long ancestry in this basin. (C) 2000 Elsevier Science L td. All rights reserved.