Seabed pockmarks in the southern Bulgarian Black Sea zone

Abstract

ockmarks were discovered for the first time in the SE of the Bulgarian Black Sea zone in 1988. Echo sounder and shallow seismic data analyses of investigations conducted before and after 1988, allow the main pockmark area to be enlarged and given more details. A total of 305 pockmarks have been located within an area of more than 100 km2 (2-5 km wide and 41 km long). The bottom of the area is covered with Holocene soft silty clay. The following trends are defined when moving from the northern part of the pockmark area to the southern part: 1) increase in seabed sediments thickness from 2.5 m to more than 3.2 m and decrease in mean sediment grain size; 2) increase in general water depth from 160-300 m to 230-340 m in areas where pockmarks occur; 3) increase in average individual pockmark size from 86 to 132 m in diameter and from 2 to 4 m in depth; 4) decrease of pockmark density from 8-9 p.m./km2 to 3-4 p.m./km2 Similar trends are observed seaward where the pockmark profile shape changes from a smooth U-shaped to a sharp V-shaped form, with pockmark wall slope angle reaching more than 10°. A variety of acoustic anomalies such as acoustic turbidity, bright spots, enhanced reflections etc., are recorded in many places within the pockmark area, indicating the presence of gas in the sediments. Water column anomalies have also been recorded. The paper discusses the origin of the gas (most likely to be a mixture of thermogenic and biogenic gas) and the time of pockmark formation.