Sandstone architecture and development of the Tunanui slope basin-fill, Hikurangi forearc, New Zealand

Abstract

This thesis describes the facies, architecture and development of the Tunanui Formation, a deep-marine, sand-rich, slope basin-fill of Middle Miocene age (NZ stages Clifdenian to Lillburnian). The study area is located in the northern Hawke Bay region of the East Coast Basin, North Island, New Zealand, and is within the forearc domain of the Hikurangi active margin. It is part of a structurally complex and now largely emergent accretionary wedge with sediments dating from basal Miocene to present. Due to renewed plate subduction and compressive movements along the margin in the Neogene, highly restricted intra-slope basins developed between rising thrust ridges of the inner-forearc. The Tunanui deepwater-clastics were originally deposited as flat-lying sediments within one such elongate slope-basin, atop the deformed sedimentary prism. These rocks are now present within the subsurface of offshore Hawke Bay and extend N/NW to the onshore areas. Structural inversion has produced outcrops of Tunanui sediments in the core of two major anticlines, the Mangaone and the Morere Highs. In the Morere Anticline along the Paritu Coast south of Gisborne, spectacular sea-cliff exposures provide a unique opportunity to investigate the nature and development of over 1000 m thickness of sandy, deepwater, slope basin-fill representing almost the entire stratigraphic section of the Tunanui Formation. Rocks in this remote region have not been previously described in any detail. A database of thousands of digital field photographs, together with 71 stratigraphic logs and some deep borehole information has enabled the production of a series of detailed correlations for the Tunanui sections. In turn, the number of logs available has made possible the construction of a large-scale (over c.25 km in length) stratigraphic cross-section, slightly oblique to the basin axis, through the deepwater clastic succession. Interpretation has allowed a series of deductions to be made concerning the nature of the 'Tunanui Basin' fill and its development, the types of gravity flow elements present and in most field areas, the production of a detailed sandstone architecture for the Tunanui deposits. Five phases of basin fill are recognised. These range from highly restricted, over-thickened sandstone packages, deposited under conditions of high slope gradient and complex basin-floor topography, within the lower parts of the sequence, to laterally continuous 'fan-like' deposits, and thinner-bedded sandstones within the upper part of the Tunanui section. A c.250 m thick sequence that is slightly younger than the upper Tunanui Formation, containing numerous channel-forms (the Tangawa Formation), is present on the East Coast of the Mahia Peninsula c.17 km to the south of the Paritu Coast outcrops. The architecture of the Tangawa Formation, and its regional structural position, indicates that it was the 'spill' of the Tunanui basin-fill into an adjacent down-slope sediment trap within the forearc terrane. This depocentre was also controlled by deep-seated imbricate thrust faulting in common with the Tunanui Basin. Several different types of deepwater channel-forms are present within what is a limited stratigraphic range. The vertical sequence reflects an overall progression within a muddy slope from deeply incised, erosive systems, to laterally offset-stacked channels of a mixed erosional depositional type.