Mass transport deposit (MTD) relief as a control on post-MTD sedimentation: Insights from the Taranaki Basin, offshore New Zealand

Mass transport deposits (MTDs) can affect the routing of turbidite systems and their resultant sedimentary architecture. With the aid of a high-quality 3D-seismic reflection dataset a buried Pleistocene MTD has been identified in the deep-water Taranaki Basin. Three MTD seismic facies have been identified and fall within three morphological domains - extensional, translational and contractional areas. Thickness relationships between the MTD and the overburden deposits were investigated quantitatively and matched using TWT-thickness maps. The results reveal that in proximal and distal areas with significant MTD relief, occupied by megaclasts and folded facies respectively, post-MTD accommodation is dominated by small 'micro-basins' with tortuous sediment pathways. In contrast the translational domain, containing homogeneous MTD facies, hosts larger low-relief basins. These characteristics can be used to define two main types of depositional topography: Type I, High-relief depositional topography, is observed prominently in the headwall and toe regions of MTDs, resulting in loosely-connected and isolated ponds of sediment; Type II, Low-relief depositional topography, is observed in the translational domain where the effects of sediment loading are clearly observed. Turbidite sandstone distribution affected by Type I topography, is characterised by irregular pinch-outs which present a significant risk for stratigraphic trapping whereas Type II topography may form overlying sedimentary bodies which are more extensive. Turbidite deposits in both settings are prone to syn-depositional remobilisation on top of the MTD, which may introduce considerable heterogeneity, particularly at reservoir margins.