A subsurface evacuation model for submarine slope failure Suzanne Bull n , Joe Cartwright n and Mads Huusew n 3D Lab, School of Earth Ocean and Planetary Sciences, Cardiff University, Cardiff, UK wDepartment of Geology and Petroleum Geology, College of Physical Sciences, Kings College, Aberdeen, UK ABSTRACT Analysis of three-dimensional (3D) seismic re£ection data from the Norwegian continental margin provides an insight into an unusual, buried submarine slope failure, which occurred adjacent to the later Holocene-age Storegga Slide.The identi¢ed failure, informally named the ‘SouthVÖring Slide’ (SVS), occurs in ¢ne-grained hemipelagic and contourite sediments on a slope of 0.51, and is characterised by a deformed seismic facies unit consisting of closely spaced pyramidal blocks and ridges bound by small normal faults striking perpendicular to the slope.The SVS contrasts with other previously described submarine slope failures in that it cannot be explained by a retrogressive model. The de¢ning characteristic is the high relative volume loss.The area a¡ected by sliding has thinned by some 40%, seen in combination with very modest extension in the translation direction, with line length balancing yielding an extension value of only 4.5%.The volume loss is explained by the mobilisation of an approximately 40 m thick interval at the lower part of the unit and its removal from beneath a thin overburden, which subsequently underwent extensional fragmentation. Evidence for the mobilisation of a thick ¢ne-grained interval in the development of a submarine slope failure from a continental margin setting may have implications for the origins of other large-scale slope failures on the Norwegian margin and other glacially in£uenced margins worldwide. INTRODUCTION Submarine slope failures are a common occurrence on continental margins (Canals et al., 2004), where they play a signi¢cant role in their evolution, in£uencing both mor- phology and stratigraphy (Pratson, 2001). Once failure initiates, the slope failure may progress by means of a number of mass movement processes, from translational sliding to £uidal £owage (see Martinsen, 1994, and refer- ences therein). Althoughvarious subdivisions and classi¢ - cation schemes for these processes exist (Martinsen,1994; Mulder & Cochonant, 1996), each process represents part of a continuum, whereby one type may evolve into or trig- ger another (Martinsen, 1994). As a result, submarine slope failure events can be highly complex and are likely to have involved a number of processes, possibly active at di¡erent stages of failure. Because of this complexity, sev- eral important aspects of submarine slope failure develop- ment and occurrence remain poorly understood. For example: (1) the physical processes involved in the transi- tion from failure to post-failure stages of development (Locat & Lee, 2002); (2) the mechanisms responsible for generating exceptional mobility and long run out dis- tances (Locat & Lee, 2002); and (3) how to better predict the timing and location of future submarine slope failure events (Pratson, 2001). In recentyears, 3D seismic data have proven to be useful in the continued study of submarine slope failures, and have been used to increase our knowledge of the detailed morphology and 3D architecture of their deposits (Huvenne et al., 2002; Frey Martinez et al., 2005, 2006; Moscardelli et al., 2006). The aim of this paper is to use 3D seismic re£ection data to describe a submarine slope failure unit from the Norwegian continental slope that contrasts markedly with previously described examples in both gross morphology and process of origin. Classical models suggest the formation of slope failure events de- pends critically on the necessary failure conditions being exceeded on a discrete basal shear surface (Bjerrum, 1967; Martel, 2004; Petley et al., 2005).This basal surface evolves morphologically during failure to become the base of the slope failure, over which material is translated downslope. On seismic data the basal surface is invariably represented as a sharply de¢ned seismic facies boundary separating the failed and translated mass from the underlying, unde- formed slope sediments (e.g. Frey Martinez et al., 2005). The distribution and volume of the translated material overlying the basal shear surface is largely a function of its location within the slope failure, as it is generally expected that net depletion in the upslope realm of the slope failure occurs due to mobilisation and translation of the failed mass downslope. This is balanced at some point down- slope in a zone of general accumulation due to arrest and Correspondence: Suzanne Bull, 3D Lab, School of Earth Ocean and Planetary Sciences, Cardi¡ University, Main Building, Park Place, Cardi¡ CF10 3YE, UK. E-mail: sbull@talisman-energy. com Basin Research (2009) 21, 433–443, doi: 10.1111/j.1365-2117.2008.00390.x r 2009 The Authors Journal Compilation r Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 433