A side-scan sonar mosaic of pockmarks on the Scotian Shelf

Abstract

Pockmarks, cone-shaped depressions that occur in unconsolidated fine sediments at the seabed, are generally thought to be formed by gas ascending from underlying sediments. A mosaic of pockmarks in Emerald Basin on the Scotian Shelf was constructed and the problems of mosaic construction are discussed. Within the study area pockmarks have an average depth of 6 m, an average diameter of 85 m and cover up to 18% of the seabed. Pockmarks that occur in thin clay (LaHave clay) are generally < 5 m deep and those in thick clay are up to 15 m deep. Many pockmarks are elongate in shape and show a preferred orientation, both of which may be due to the influence of bottom currents. Within the study area, the process of pockmark formation has displaced 2.5 × 10 5 m 3 km −2 of sediment. Pockmarks on the Scotian Shelf appear to be inactive (relict) and in cases where they are buried they are referred to as ancient.