Mobile Salt Thickness as a Control on the Structural Style and Evolution of Rift Basins: Danish Central Graben, North Sea
Duffy, Oliver B.; Gawthorpe, Rob; Brocklehurst, Simon H.; Docherty, Matthew
The Danish Central Graben, in the Danish sector of the North Sea is characterised by a lateral variation in the thickness and mobility of pre-rift Zechstein salt, allowing investigation of how cover salt variability influences the structural style and tectono-stratigraphic evolution of the rift. The study area is divided into two structural domains based on the interpreted depositional thickness and mobility of the salt-rich Zechstein Supergroup. The northern domain was influenced by a thin to absent (<100 ms TWT) unit of immobile Zechstein Supergroup, whilst the southern domain was influenced by thicker (ca. 200-500ms TWT) unit of mobile Zechstein salt. Within each domain we examine the overall basin morphology and the structural styles in the sub-salt (basement) and supra-salt (cover) units. We integrate these data with pre- and syn-rift isochore maps allowing fault activity and salt migration maps to be constrained.
The northern domain is not influenced by mobile salt, and has developed in a manner typical of brittle, basement-involved rifts. Syn-rift basins display classical half-graben geometries and are bounded by thick-skinned faults, with rift-related megasequence boundaries easily distinguishable. In contrast, the southern domain is influenced by mobile salt, with salt thickness increasing southward. Within the southern domain there are variable degrees of decoupling between basement and cover fault and fold systems and fault- and salt-related folding is a prominent structural style, including salt pillows, diapirs and walls. The addition of mobile cover salt to the rift influences processes at segment boundaries along-strike of the border fault and results in complex and spatially-variable modes of tectono-stratigraphic evolution, including i) syn-rift stratal geometries that are condensed above salt swells and over-thickened in areas of salt withdrawal, ii) compartmentalised syn-rift depocentres, and iii) masking of rift-related megasequence boundaries. These observations indicate that cover salt may exert a first order control on the distribution and stacking patterns within syn-rift sequences.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013