Recognising triggers for soft-sediment deformation: Current understanding and future directions ☆ Geraint Owen a , Massimo Moretti b, ⁎, Pedro Alfaro c a Department of Geography, Swansea University, Singleton Park, Swansea SA2 8PP, Wales, UK b Dipartimento di Geologia e Geofisica, Università di Bari, Via Orabona 4, 70125 Bari, Italy c Departamento Ciencias de la Tierra y del Medio Ambiente, Facultad de Ciencias, Universidad de Alicante, 03080 Alicante, Spain abstract article info Article history: Received 5 November 2010 Accepted 23 December 2010 Available online 6 January 2011 Keywords: Soft-sediment deformation Trigger mechanisms Seismites Most of the 16 papers in this special issue were presented at a session entitled “The recognition of trigger mechanisms for soft-sediment deformation” at the 27th IAS Meeting of Sedimentology in Alghero, Sardinia, Italy, which took place from 20th–23rd September 2009. They describe soft-sediment deformation structures that range widely in morphology, age, depositional environment and tectonic setting. In their interpretations, the authors have been asked to focus on identifying the agent that triggered deformation. Our aims in this introductory overview are to: (1) review the definition and scope of soft-sediment deformation; (2) clarify the significance and role of the trigger; (3) set the contributions in context and summarise their findings; and (4) discuss strategies for reliably identifying triggers and make recommendations for future study of this widespread and significant category of sedimentary structures. We recommend a three-stage approach to trigger recognition, combining the assessment of facies, potential triggers, and available criteria. This focus on the trigger for deformation distinguishes this collection of papers on soft-sediment deformation from other important collections, notably those edited by Jones and Preston (1987), Maltman (1994), Maltman et al. (2000), Shiki et al. (2000), Ettensohn et al. (2002b), Van Rensbergen et al. (2003) and Storti and Vannucchi (2007). © 2011 Elsevier B.V. All rights reserved. 1. Soft-sediment deformation structures: definition and classification 1.1. Definition and scope Soft-sediment deformation is deformation that occurs in unconsoli- dated sediment. Deformation usually occurs rapidly, close to the surface, during or shortly after deposition, and before significant diagenesis. The sediment may be cohesive or cohesionless, but its physical properties are distinct from those of rock. Many alternative names, tied to more rigid definitions, have been proposed for this category of sedimentary structures, including soft-rock deformation, sedimentary deformational structures, penecontemporaneous deformation, synsedimentary defor- mation, early-diagenetic deformation, pre-lithification deformation or contorted bedding (see discussions by Maltman, 1984, 1994; van Loon, 2009). However, “soft-sediment deformation” persists and we consider it to be a useful term that most sedimentologists associate with a particular range of structures. There is a continuum between soft-sediment deformation as defined here and other areas of deformation or sediment mobility that do not constitute soft-sediment deformation. They include the following. 1.1.1. Tectonic deformation Stresses of tectonic origin may deform both near-surface unconsol- idated sediments and more deeply buried sedimentary rock (see Jones and Preston, 1987; Storti and Vannucchi, 2007). The deformation of sediments represents soft-sediment deformation, although there may be a continuum of processes with deformation of lithified rock. 1.1.2. Glacigenic deformation A wide variety of deformation structures occurs in glacigenic deposits (see Maltman et al., 2000); those that affect unconsolidated sediment are soft-sediment deformation structures. 1.1.3. Mass movements Slumps involve the deformation of unconsolidated sediment and are therefore soft-sediment deformation structures. However, there is a continuum of gravity-driven mass-movement and sediment gravity flow processes, many of which cannot be considered soft-sediment deformation, such as debris flows, turbidity currents, or slope failures in lithified rock (Martinsen, 1994). Sedimentary Geology 235 (2011) 133–140 ☆ Papers presented at the 27th IAS Meeting of Sedimentology, Alghero, Sardinia, Italy, 20th–23rd September 2009. ⁎ Corresponding author. E-mail addresses: g.owen@swansea.ac.uk (G. Owen), m.moretti@geo.uniba.it (M. Moretti), pedro.alfaro@ua.es (P. Alfaro). 0037-0738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.12.010 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo