The DTI 2002 programme of new deep-water seabed multibeam and sample data acquisition in
SEA4 included surveys of a field of seabed mud mounds, collectively named the Pilot Whale
Diapirs. The largest of these occur over a buried anticline and they are set in sediment debris
flows that originated from grounded ice and submarine landslides. Other diapirs and mud
mounds are sited on and adjacent to the north-east plunging Fugloy Ridge and buried transfer
fault zones within a region subject to modern earthquakes. The focus of this study is on the SW
group of the five main groups of large-scale mud diapirs with seabed elevation of 30m or more
above the surrounding seabed and with very complex seabed geometries.
Diapiric sediment has been transferred to seabed from deep sources, in places from more than
500m below modern seabed and from strata more than 24 million years old. Interpretations of
the fossil biota, sediment properties and seismic reflection profiles indicate that there is submetre
scale heterogeneity in the composition and age of sediments cropping at or near seabed on
the large-scale mud diapirs. Interpretations from the seismic reflection profiles and the fossil
data from one site on the large-scale diapirs indicate that the large-scale mud diapirism postdates
approximately 5 million years ago and might have been initiated as late as 1.1 million years
ago. The evidence suggests that rapid large-scale mud diapirism is not occurring at the present
day. In contrast, interpretations of the regional geological setting and the sub-seabed data
indicate that there are large areas with potential for modern, active and small-scale diapirism.
Reconnaissance sample surveys indicate that some of the steepest slopes on the large-scale
diapirs are composed of rock and overlain by thin soft sediments with gravel at seabed. A
numerical static stability model is presented that predicts the general conditions under which the
modern seabed will become unstable on the large-scale diapirs. The model predicts that thin-skin
seabed failures prevent thick accumulations of normally consolidated sediment on the steep
flanks of the large-scale mud diapirs. These failures will contribute to the variability of the
seabed substrates
