Variations in Depocentre Style under Mid-Late Jurassic Salt-Influenced Rifting: Norwegian Central Graben, North Sea
Ge, Zhiyuan; Gawthorpe, Robert; Rotevatn, Atle; Wonham, Jonathan
Recent studies suggest fault growth process and fault array evolution control many of the first order features of the sedimentology and stratigraphy in rift basins. Nevertheless, where weak mobile evaporite strata are present within the cover stratigraphy, the structural style and evolution of normal faults and their associated depocentres is more variable than in salt-free settings. For example, in polyphasal rift, fault evolution beneath and above salt layers may be decoupled, thus the location, geometry and evolution of supra-salt depocentres are not directly controlled by sub-salt ('basement') faults. Existing models for fault controlled depocentre evolution in rifts are mainly based on salt-free rifts and therefore do not address the full spectrum of basin settings.
Using three-dimensional seismic and well data in the Norwegian Central Graben, we investigate the interaction between normal faulting, salt tectonics and depocentre evolution during the Jurassic rift phase. During Middle to Late Jurassic rifting mobility of evaporites within the Zechstein Supergroup makes a significant contribution to accommodation creation/destruction and to the overall geometry and evolution of depocentres. Three main types of rift-related depocentre can be recognised based on the dominant factor(s) controlling accommodation space. Type I depocentres are mainly driven by salt mobilization (often, evacuation of salt), and any sub-salt, basement normal faults are effectively decoupled from the cover deformation. In the study area, these depocentres are tens of kilometers long and several kilometers wide and mainly located on the Sorvestland Arch above salt walls and salt ridges. Type II depocentres are mainly fault-controlled and have no significant salt involvement. These depocentres represent typical fault-bounded half-graben depocentres and are found along the axial zone of the Central Graben. Type III depocentres are hybrid-type depocentres in that both the basement normal faulting and evaporite mobility have worked in concert to generate accommodation space. Here, the evaporites are not fully decoupled basement and cover faulting, but still has affected the stratigraphic architecture. These depocentres are asymmetric in the hanging wall of basement normal fault and typically extend from a few kilometers to tens of kilometers depending on the fault geometry and salt availability. They are usually located in areas between the Type I and Type II depocentres.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013