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3D seismic geomorphology within Chalk strata, North Sea

Three-dimensional (3D) seismic data from the southern Danish North Sea are used to analyse the morphology and spatial distribution of circular/elliptical depression features in the Danian chalk deposit. Previously these depressions were either interpreted as karst structures (subaerial exposure) or submarine pockmarks. This study presents a detailed morphological analysis of 170 of these depression features, which occur in an interval from ca. 25 ms below to 12 ms above the Top Chalk surface. Three types of depressions were differentiated based on their plan view geometry and their degree of symmetry. Type 1, comprising sub-circular and symmetrical depressions, is the dominant group (ca. 70%). Type 2, elongated and symmetrical depressions, represents only a small fraction (ca. 5%). The elongated and asymmetrical depressions of Type 3 compose ca. 25%.  In cross section, each depression type can be either characterized by a V- or a U-shape.  The maximum size of the depressions ranges from 50 m to 580 m, with an average internal depth of 10 m. Based on their morphology and distribution, the depressions are interpreted as pockmarks formed by expulsion of biogenic or thermogenic fluids at the Danian sea floor. Likely, the initial form of the pockmarks has been circular (Type 1) and was subsequently modified for Type 2 and 3 to an elongated form by currents. The majority (74%) of the long axes of the pockmarks (Type 2 and 3) strike 140 -160° (NW-SE) and the minority strike between 90 and 100° (W to E). The long axis of the pockmarks is interpreted to represent the effective current direction. The inferred direction is sub-parallel to the paleobathymetric contours of the Danian Central North Sea, and is therefore interpreted to result from contour currents. The project in general contributes to the understanding of fluid-migration and palaeocirculation patterns during the sedimentation of the terminal Chalk Group in the southern Danish North Sea.

Associated researchers: Samaneh Masoumi, Stefan Back, Lars Reuning