Characteristic features of pockmarks on the North Sea Floor and Scotian Shelf

Abstract

This paper presents the results of a comparative study of pockmarks and associated features appearing on both sides of the North Atlantic: on the Scotian Shelf off Nova Scotia and in the northern North Sea. Pockmarks are formed in seabed material consisting of soft silty clay. The seismic, sonar and lithologic characteristics of the sediments on the Scotian Shelf are remarkably similar to those found in the northern North Sea. Sediment clouds suspended in the water column immediately over the seabed have previously been observed on side‐scan records associated with gas‐charged sediments on corresponding shallow‐seismic records. These and similar observations strongly suggest that most pockmarks are caused by gas efflux through the seafloor. However, the detailed mechanism of formation and the origin of the gas in the sediments is still unknown.

Multibeam bathymetric surveys of submarine volcanoes and mega-pockmarks on the Chatham Rise, New Zealand

“…Many such pockmarks seem to be inactive, but rising gas above some of them supports the deduction that they form when gas escapes (e.g. Hovland et al 1984;Hovland & Somerville 1985;Hovland & Judd 1988;Loncke et al 2004). Some believe that they form in short periods, perhaps episodically (Hustoft et al 2009) or even catastrophically (Hovland et al 2005).…”

Pockmarks in the inner Oslofjord, Norway

“…Pockmarks occur in a variety of shapes, with circular and elliptical as the most common types. The orientation of elliptical and elongated pockmarks is often correlated with the dominant bottom current direction (Hovland et al 1984, Gontz et al 2002. They do not exhibit raised rims and range from cone shaped to flat-bottomed with internal slopes of 2P (Cole et al 2000).…”

Timescales for the development of methanogenesis and free gas layers in recently-deposited sediments of Arkona Basin (Baltic Sea)

“…For instance, there are many features indicative of gas expulsion (e.g. pockmarks, Nelson et al, 1979;Hovland et al, 1984), or remnants of methanogenesis where none exists today (e.g. 13 C-enriched carbonate, Malone et al, 2002).…”