Stratigraphic Architecture of the Upper Miocene Hikuwai Sandstone — Upper Mapiri Formation, Raukumara Region, East Coast Basin, North Island, New Zealand

This study represents the first assessment of Deepwater depositional system evolution of the outcropping Hikuwai sandstone and upper Mapiri Formation located in the Tolaga Bay area of the East Coast Basin (ECB), North Island, New Zealand, which is part of an active forearc setting. Deepwater depositional systems in forearc basins present unique challenges for developing predictive stratigraphic and architectural frameworks due to the dynamic tectonic environment and local variability of the basin configuration. In particular, uplift related to the development of the accretionary subduction complex and confinement from inner forearc highs can impact the evolution of depocenters, morphology, and the direction of system development. In the ECB, basin development is highly localized and variable because it is dissected by numerous margin-parallel, ridge-forming thrust faults creating intervening elongate depressions with distinct basin fill histories.

The upper Miocene Hikuwai sandstone and upper Mapiri Formation record the complex evolution of a Deepwater system likely confined by such inner forearc ridges. The Hikuwai sandstone is comprised of fine-grained sheet-like sand deposits and grades upward into the overlying mud-rich deposits of the upper Mapiri Formation, representing a succession of overall decreasing energy. This study examines the depositional system evolution of this ~635 m thick section, which is well-exposed along 9 km of the Tolaga Bay coast in the Raukumara Region, through development of a lithofacies scheme and identification of architectural elements and bounding surfaces. Biostratigraphic analysis and radiometric dating are used to determine sedimentation and subsidence rates throughout the succession, which provides a holistic understanding of system development. This method is particularly relevant in active tectonic settings because it relates the influence of allocyclic factors, such as subsidence of the forearc region and basin uplift, to the architecture and lithofacies successions of the Deepwater system.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California