Mark A. Barry

Gas domes in soft cohesive sediments

Accumulation of gaseous methane can generate seabed domes in soft cohesive sediments. Such structures can, in turn, lead to seafloor instability, act as precursors to pockmark formation, and arguably pose a threat to seafloor drilling if they contain significantly overpressured gas. Future melting of gas hydrates within the seabed, due to global warming, will likely lead to a significant long-term release of methane, which could potentially produce a new and abundant generation of gas domes and associated pockmarks. Despite their geological and practical significance, our understanding of gas-dome formation in marine sediments has been limited to observations and qualitative analyses. To provide a quantitative understanding, we conducted small-scale laboratory doming experiments. We found that thin layers of clayey sediment behave elastically over the range of deformations needed to create seabed domes. The observed behavior is well described by elastic thin-plate mechanics, from which it is possible to predict the gas pressure required to create natural domes. Our results suggest that observed shallow dome geometries require surprisingly small overpressures to form; however, large overpressures can build under increasingly thicker and stiffer layers of sediment.

THE MECHANICS OF SOFT COHESIVE SEDIMENTS DURING EARLY DIAGENESIS

Natural, surficial, cohesive (clay-bearing), aquatic sediments are subject to a variety of phenomena in which physics, rather than say chemistry, plays an essential role; this includes, but is not limited to, bioturbation, self-weight compaction, and phase growth. Scientific monographs (e.g., Berner, 1971, 1980; Boudreau, 1997; DiToro, 2001; Burdige, 2006; Schultz and Zabel, 2006) that focus on early diagenesis, i.e., those changes occurring in the top 1–10 meters (m) of aqueous sediments, make only passing reference to the physics of early diagenetic phenomena. In contrast, civil engineers, soil physicists and geophysicists have afforded great attention to the physics/mechanics of compaction, particularly in soils, anthropogenic sediments and basin-scale studies (e.g., Yong and Warkentin, 1966; Giles, 1997; Wang, 2000; Craig, 2004; Mitchell and Soga, 2005; Das, 2008); yet, this knowledge has not been effectively transferred to obtain a better understanding of early diagenesis.