Late Miocene pockmarks in the Danish Central Graben: Origin and significance

Late Miocene pockmarks have been identified and mapped in 3D seismic surveys covering large parts of the Danish Central Graben. The upper Miocene succession comprises a thick westward progradational wedge of clay, silt and sand grade siliciclastic sediments. The majority of the pockmarks are located in the clay prone toesets. More than 250 pockmarks were identified by changes in reflection amplitude and continuity on time-slices and amplitude maps. The length, width, depth, and bottom geometry of each of the observed pockmarks was quantified and recorded in order to facilitate a statistical analysis of their relation to depositional and structural elements that may have affected their distribution and morphology. The pockmarks are generally elliptical in plan view with ellipticity between 1 and 11, the majority having ratios between 2 and 6. In cross section the pockmarks have U- V- and box-like shapes, and could be mistaken for channel incisions. The orientation of the elliptical pockmarks is clustered around 150° (NNW-SSE), parallel to the general strike of the Miocene clinoforms. The marked ellipticity suggests an influence of contour-parallel bottom currents whilst the toeset location suggests that pockmarks only formed in the fine-grained (sealing) parts of the depositional system.