Abstract:

The study describes a large (kilometre-scale) composite mud volcano system within the North Sea Chalk Group, containing both intrusion and extrusion features. The upper part of the system consists of three stacked constructive components. At the top, a Late Palaeocene mound-shaped extrusion with subtle onlaps is observed. Below, a Danian near-surface intrusion caused forced-folding of the c. 100 m thick overburden. This is underlain by a shallow intrusion. The lower part of the mud volcano system is defined by a depletion-related depression and the fluid conduit. Seismic data suggest that the source unit for the linked intrusion–extrusion is to be found either within a Lower Cretaceous diapiric siliciclastic structure or within the fine-grained carbonates of the Chalk Group. The latter is more probable, implying that chalk may generate hitherto undocumented intrusions and extrusions comparable with siliciclastic mud volcano systems. Remobilization is inferred to have been driven by fluidization caused by the influx of mainly pore water expelled from the Lower Cretaceous succession. A distinct depositional variation within the Chalk Group caused early diagenetic differences in porosity preservation and mechanical properties of the chalk units, allowing the remobilization of certain chalk units even at burial depths as great as 200 m.

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