1 Department of Earth Sciences, Faculty of Science, Aarhus University, Aarhus University2 Department of Geoscience, Science and Technology, Aarhus University3 University of Aberdeen4 Department of Geoscience, Science and Technology, Aarhus University
This study gives the first description of 33 mid-Oligocene and 646 late Miocene pockmarks mapped in the Danish part of the central North Sea. The pockmarks are all highly elongated, with average long- and short axes of 2.5 km and 700 m, and average internal depth of 30 m. The Miocene pockmarks strike 140-160° while the Oligocene pockmarks strike 100-105° paralleling the strike of the major Miocene and Oligocene clinoforms on which they occur. In cross-sections, the pockmarks appear as U-, V- and W-shaped or tabular depressions. Based on their geometry and degree of symmetry, three distinct pockmark groups have been distinguished: (A) 58% of the pockmarks are symmetrical, ellipsoidal depressions; (B) 40% are asymmetrical, lunate depressions; and (C) 2% comprise asymmetrical ring-shaped depressions. Group A pockmarks are interpreted as representative for the first stage in the pockmark development, only influenced by fluid expulsion and subsequent erosion by bottom currents. Group B pockmarks are influenced by bottom current erosion, current and gas winnowing and possibly authigenic carbonate precipitation. Group C pockmarks represent a further development from Group B pockmarks with further winnowing, carbonate precipitation and removal of seafloor sediment to form the ring-shape. Current erosion has exerted the major influence on the final geometry of 83% of the pockmarks. The pockmarks occur above gas-mature Jurassic source rocks, modelled to be in the generative window during the late Miocene, and thermogenic gas is suggested as the main fluid involved in the pockmark formation. The timing of gas expulsion from the Jurassic source rocks in combination with loading imposed to the basin by the progradational Miocene clinoforms are interpreted as the main factors controlling the timing and location of the pockmarks. The pockmarks thus tell a story of thermogenic gas venting to the surface and paleo-current scour of the seabed in the eastern part of the central North Sea during the mid Oligocene and late Miocene.