Evolution of a deep-water lobe system in the Neogene trench-slope setting of the East Coast Basin, New Zealand: Lobe stratigraphy and architecture in a weakly confined basin configuration

• We analyze a Miocene deep-water lobe system in a trench-slope basin.
• Six lithofacies were identified from stratigraphic and architectural data.
• System is interpreted to represent deposition in a weakly confined basin.
• A four-phase model of evolution relates lithofacies to the filling of healed slope accommodation.

This study presents a new depositional analysis of the stratigraphic architecture of a deep-water lobe system in a trench-slope basin setting by examining the upper Miocene Hikuwai sandstone and Mapiri Formation of the East Coast Basin (ECB) in the Tolaga Bay area, New Zealand. The Hikuwai sandstone is up to 385 m thick and is enveloped by the mud-rich middle and upper Mapiri Formation. Stratigraphic sections measured at centimeter-scale and high-resolution photo-panoramas were collected from sea-cliff exposures for stratigraphic and architectural analysis and definition of six lithofacies. The Hikuwai sandstone is interpreted to represent a succession of frontal lobe deposits that contain a distributary network of meter-scale erosional channels and scours on their surface. Lobe lithofacies depend on their proximity to the sediment source, and their proximity to a distributary channel and/or scour.The late Miocene basin setting is interpreted to be a weakly to moderately confined trench-slope basin. The basin configuration controlled the development of the depositional system through elongate fault-controlled ridges that directed sediment dispersal pathways through longitudinal troughs. The basin filled from north to south in four phases: 1) lobe aggradation related to healed slope accommodation, 2) retrogradation and lateral migration of the system 3) back-stepping of the system upslope causing reduced confinement and allowing flows to become wider, longer, and thinner, and 4) shut off of sediment supply and mass wasting of the upper Mapiri Formation, representing either renewed fault movement or equilibration of the system to the regional slope profile.This paper provides a detailed description of the internal structure of lobes in a trench-slope setting, and a depositional model that relates lobe lithofacies to the filling of healed slope accommodation. Therefore, this work presents an analog applicable for elongate basins, such as in the trench slope or settings with mud diapirism, where only seismic-scale or limited data is available.