Tectono-sedimentary evolution and source rock distribution of the mid to Late Cretaceous succession in the Great South Basin, New Zealand

Abstract

The Great South Basin, off New Zealand’s southeast coast, has attracted renewed exploration interest from major petroleum companies since 2005. The distribution of the mid Cretaceous to Paleocene source rocks (coals and coaly mudstones) is a critical component in evaluating basin prospectivity. This paper delineates source rock distribution from seismic facies characterisation, and presents a series of updated paleogeographic maps over the initial (Cretaceous) phases of basin evolution. Basin evolution has been analysed from mapped sequence stratigraphic boundaries and isochron maps. Seismic facies were characterised based on the amplitude, continuity, and stacking pattern of the reflection packages. The identified facies were calibrated with well data for age, gross lithology, and gross depositional environment. Areas of source rock deposition were demarcated using seismic attribute interval maps, from which a series of updated paleogeographic maps was prepared. Four second-order sequences have been identified within the Cretaceous succession. The lower two sequences are mainly fault bounded and were deposited in a syn-rift phase. In contrast, the upper two sequences reflect a change in basin character from rifting to a post-rift thermal sag phase. Source facies within both the syn- and post-rift sequences were deposited in mainly non-marine to marginal marine settings, although there is also the possibility of lacustrine source rocks in isolated syn-rift depocentres. The wide geographic spread of source rock intervals within the Cretaceous sequences allows for a variety of petroleum generation and exploration play scenarios.

Terrane Boundary Reactivation, Barriers to Lateral Fault Propagation and Reactivated Fabrics: Rifting Across the Median Batholith Zone, Great South Basin, New Zealand

“…The NE‐SW fabric appears to be related to the NE‐trending faults that characterize the Great South Basin (Figures and ). These faults formed due to NW‐SE‐oriented extension between Zealandia and Western Antarctica (Kula et al, ; Sahoo et al, ; Tulloch et al, ), and faults associated with this extension are proposed to be listric and link to a detachment at midcrustal depths (Uruski et al, ). The NE‐SW fabric may possibly relate to this NW‐SE‐oriented extension.…”

“…The Great South Basin is a NE-trending Late Cretaceous rift basin located offshore of the east coast of the South Island of New Zealand. It contains a maximum sedimentary thickness of 8.6 km, which includes up to 4 km of Upper Cretaceous strata (Beggs, 1993;Morley et al, 2017;Sahoo et al, 2014) (Figures 1 and 3). The Great South Basin, along with the adjacent Canterbury Basin and Bounty Trough, is situated within the Campbell Plateau, a submerged (<1000 m water depth) area of thinned continental crust (22-32 km) that extends~1000 km southeast of the South Island ( Figure 1) (Beggs, 1993;Grobys et al, 2009;Higgs et al, 2019;Mortimer et al, 2002;Uruski, 2010).…”

“…Extensional activity may have begun in the Jurassic-Early Cretaceous in a back-arc setting along the southern margin of Gondwana, with Jurassic strata identified in the deepwater Taranaki Basin and potentially in the Great South Basin (Grobys et al, 2007;Uruski, 2010;Uruski et al, 2007). Initial NE-SWoriented extension occurred between Australia and the contiguous Zealandia and Western Antarctica at 101-89 Ma related to the opening of the Tasman Sea and the northward propagation of the Tasman Ridge (Kula et al, 2007;Kula et al, 2009;Sahoo et al, 2014;Tulloch et al, 2019). A second rift phase, related to the breakup of Zealandia and Western Antarctica and associated with the eventual formation of the Pacific-Antarctic Ridge, occurred from 90-80 Ma (Kula et al, 2007;Tulloch et al, 2019).…”

“…Late Cretaceous extension, forming the Great South and Canterbury Basins and the Bounty Trough, reduced crustal thickness to 22-32 km across the area (Mortimer et al, 2002). In the Great South Basin, faults related to this activity have been proposed to sole out onto a midcrustal detachment (Sahoo et al, 2014;Uruski et al, 2007).…”

“…The NE-to ENE-trending Sisters Shear Zone along the southern coast of Stewart Island is aligned with the northern margin of the Great South Basin and records extensional activity at this time (Kula et al, 2009). The NW-SE extension direction during this rift phase resulted in the formation of NE-trending faults across the Great South Basin (Beggs, 1993;Grobys et al, 2009;Sahoo et al, 2014;Uruski et al, 2007). As the extension direction was oriented at a high angle to the WNW-trending Terrane boundary, this structure does not appear to have been active during this rift phase.…”