Interactions between the prograding Giant Foresets Formation and a subsiding depocentre: Insights from the Parihaka 3D & ES89 2D seismic surveys

This seismic interpretation project provides new insights into the interaction between the Pliocene-aged Giant Foresets Formation and the faults bounding the Northern Graben. A newly named fault-bounded depocentre within the North Taranaki Graben, the Arawa Sub-Basin, has subsided during the Pliocene, attracting volumes of sediment across the Parihaka Fault within large-scale channels. The study images kilometer-scale channels and explores the interplay between the progradation of the Giant Foresets Formation and normal faulting along the Cape Egmont Fault Zone. A focus is placed on imaging the provenance and depositional facies of sedimentary packages throughout the foresetting sequence of the Giant Foresets Formation. Mapping of the Waipipian-Nukumaruan-aged foresetting sequence within the offshore northern Taranaki Basin has previously shown the primary sediment transport direction is primarily NNW. This is contradicted by sediment-transport features mapped within the study area showing the sediment transport direction fluctuates between NE and SE. The primary mechanism of sediment redirection is faulting along the Cape Egmont Fault Zone and subsidence within the North Taranaki Graben, an elongate SW-NE graben within the northern Taranaki Basin. Smaller (˜10s m-scale) channels concentrate into much larger (˜100s m- to km-scale) mega-channels that travel E/NE into the subsiding Arawa Sub-Basin. Volcanic intrusions of the Mohakatino Volcanic Formation have also influenced the evolution of the mega-channels in the study area, via uplift and doming of the seafloor which provided a barrier to the transport of sediment. The Parihaka 3D and ES89 2D seismic surveys are interpreted using the IHS Kingdom software package to create a basic framework of horizons and faults over the Pliocene-Recent interval. Depth grid maps are produced from the grid of horizon picks. Isochore maps are produced which span key intervals between depth grids. A coherency cube of the Parihaka 3D is generated from the 3D seismic volume using OpendTect. Using the framework of faults and horizons within the coherency cube, imaging sediment transport and deposition features in the vicinity of normal faulting is made possible by flattening on a top foresets horizon and horizontally slicing the data at regular intervals. This recreates past conditions by removing the effects of fault-slip and differential compaction. These “time-slices” contain clear images of channels, canyons and fan-deposits allowing sediment provenance and transport direction to be mapped and interpreted. Finally, seismic section images from the Parihaka 3D and ES89 2D seismic surveys are generated along paths intersecting key geological features within the study area.