A giant (5.3  10 7 m 3 ) middle Miocene (c. 15 Ma) sediment mound (M1) above the Siri Canyon, Norwegian–Danish Basin: Origin and significance Katrine Juul Andresen a, * , Ole Rønø Clausen a , Mads Huuse b a Department of Earth Sciences, University of Aarhus, Høegh-Guldbergs Gade 2, 8000 Århus C, Denmark b School of Geosciences, University of Aberdeen, Aberdeen AB24 3UF, Scotland, UK article info Article history: Received 20 March 2008 Received in revised form 12 February 2009 Accepted 13 February 2009 Available online 28 February 2009 Keywords: Norwegian–Danish Basin Cenozoic Siri Canyon Sand volcano Hydrocarbon migration Lateral pressure transfer abstract A large-scale enigmatic mound structure (M1) has been discovered in middle Miocene strata of the Norwegian–Danish Basin, c. 10 km east and updip of the Central Graben. It is located about 1 km beneath the seabed and clearly resolved by a 3D seismic data set focused on the deeper, remobilised, sand-filled Siri Canyon. M1 comprises two culminations, up to 80 m high and up to 1400 m long, constituting a sediment volume of some 5.3  10 7 m 3 . It is characterized by a hard reflection at the top, a soft reflection at the base, differential compaction relative to the surrounding sediments, and 10 ms TWT velocity pull up of underlying reflections, indicating a relatively fast mound fill, attributed to the pres- ence of sand within the mound. Internal seismic reflections are arranged in an asymmetric concentric pattern, suggesting a progressive aggradation to the NW, downstream to a mid-Miocene contour current system. Numerous elongated pockmarks occur in the upper Miocene succession close to the mound and indicate that the study area was influenced by gas expulsion in the mid- and late Miocene. The reflection configuration, velocity, dimensions, regional setting, and isolated location can best be explained by interpreting the mound as a giant sand volcano extruded >1 km upward from the Siri Canyon during the middle Miocene (c. 15 Ma). The likely causes of this remarkable structure include gas charge and lateral pressure transfer from the Central Graben along the Siri Canyon reservoir. While this is the first such structure described from this part of the North Sea, similar-aged sand extrudites have recently been inferred from seismic observations in the North Viking Graben, thus suggesting that the mid-Miocene was a time of widespread and intense sediment remobilization and fluid expulsion in the North Sea. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Mounded structures comprise a wide range of genetically distinct geological features and examples exist from all over the world. Seismic scale (10’s to 100’s m) examples include siliciclastic sedimentary mounds such as deep-sea channels, fans and con- tourites (Timbrell, 1993; Viana et al., 1998; Jennette et al., 2000), carbonate mounds (Hovland et al., 1994; Feary and James, 1995; Huuse, 1999; Nielsen et al., 2003), and structures related to clastic remobilization, intrusion and extrusion (Brooke et al., 1995; Dixon et al., 1995; Graue, 2000; Huuse et al., 2004; Hurst et al., 2006; Frey- Martı ´nez et al., 2007; Hamberg et al., 2007). The latter category is often related to the expulsion of thermogenic gas and formation fluids, which results in the entrainment of sediments and in the formation of mud and sand volcanoes and clastic intrusions, which can reach hundreds of metres to kilometres in extent (Jolly and Lonergan, 2002; Hansen et al., 2005; Huuse et al., 2005). Sand- prone sediment mounds represent high-quality hydrocarbon reservoirs in the Paleogene North Sea area (Timbrell, 1993; Dixon et al., 1995; Jennette et al., 2000; Galloway, 2001; Hamberg et al., 2007). Other mound types include hyaloclastite mounds associated with igneous vents (e.g. Davies et al., 2002). The Central Graben in the North Sea is a well known hydro- carbon province, while areas outside the Central Graben, have been demonstrated to be less prospective. Paleocene sandstones in the Siri Canyon thus form the only economical reservoirs in the Danish area outside the Central Graben (Hamberg et al., 2005, 2007; Ohm et al., 2006). Studies from the Siri Canyon show that fluid migration and sediment remobilization, resulting in both large and small scale sand injection structures, are important factors in location and compartmentalisation of prospective sand bodies in the canyon (Hamberg et al., 2005, 2007). The mound (M1) analysed here is the first middle Miocene mound structure observed in the Norwegian–Danish Basin, and its * Corresponding author. Tel.: þ45 8942 9539; fax: þ45 8942 9406. E-mail addresses: katrine.andresen@geo.au.dk (K.J. Andresen), ole.r.clausen@ geo.au.dk (O.R. Clausen), m.huuse@abdn.ac.uk (M. Huuse). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2009.02.005 Marine and Petroleum Geology 26 (2009) 1640–1655