Cretaceous to Paleogene Palinspastic Reconstruction of the East Coast Basin, New Zealand
Authors:   , Hines Benjamin 
Published: August 2022

<p><b>The East Coast Basin is a Cretaceous-Cenozoic sedimentary basin that largely formed within a passive margin setting, and has subsequently evolved through the Neogene in a convergent margin setting. The position of the basin prior to the inception of the modern boundary between the Pacific and Australian plates has, for many decades, been a contentious issue in paleogeographic reconstructions of Zealandia and the southwest Pacific region. The sedimentary, tectonic and paleoenvironmental development of the basin have substantial importance in understanding the evolution of Zealandia, and resource exploration in the region. However, the pre-Neogene position, orientation and internal deformation of the basin is poorly constrained. To date, paleogeographic reconstructions have utilised the East Coast Basin to accommodate an unknown volume of deformation, with substantial variability between reconstructions. This thesis combines a number of methods and approaches to produce a rigorously tested, palinspastic model for the Cretaceous to Paleogene East Coast Basin that is integrated within the wider Zealandia paleogeographic and tectonic framework.</b></p>
<p>Late Cretaceous–Eocene marine successions in the basin are dominated by thick, often lithologically monotonous sedimentary units. Moderate- to high-resolution geochemical analysis applied to 1100 samples from late Cretaceous to Eocene stratigraphic sections across Hawke’s Bay, Wairarapa and Marlborough, provide a framework for chemostratigraphic correlations across the basin to supplement bio- and lithostratigraphic studies. The Waipawa Formation holds particular interest, as it forms an important isochronous unit, making it possible to establish facies relationships and correlations between multiple sections in various environmental settings and lithostratigraphic associations across the basin. Trace metal indices and organic carbon relationships reveal evidence of shifting paleo-redox conditions during the Late Paleocene, associated with enhanced preservation of terrestrial organic matter. The increased preservation occurs in tandem with increased sedimentation rates, suggesting that sediment supply and burial rate are controlling factors in the source and preservation of organic matter.</p>
<p>Recently developed rigid-plate reconstructions for Zealandia are integrated with geological and geophysical datasets to emplace constraints on the Neogene deformation of the basin. To facilitate reconstruction, the basin was subdivided along major basement-penetrating faults to produce a micro-plate model. Lineations in basement terranes provide piercing points for the base reconstruction, which are utilised in conjunction with paleomagnetic constraints on the rotation and fault movements of, and between, individual structural blocks to retroactively remove Neogene deformation. Removal of shortening, rotation and lateral displacements establishes a foundation upon which a series of Cretaceous-Paleogene palinspastic reconstructions can be built. These palinspastic reconstructions are then translated into a suite of paleoenvironmental maps through the integration of various datasets, including litho-, bio- and chemostratigraphic and sedimentological measurements, and heavy mineral and detrital grain studies. These data provide insight into environmental and sediment provenance characteristics that are incorporated into paleogeographic reconstructions, in addition to enabling compilation of a composite section used for the identification and interpretation of second- and third-order (1–10 Myr) sequence stratigraphic cycles from the Motuan to Bortonian (c. 105–40 Ma). The joint assessment of multiple provenance indicators informs of sediment sources, supply routes and hinterland tectonics, and basement unroofing and exhumation, contributing to geodynamic models of basin development. A series of paleogeographic maps have been developed for important time-slices through the late Cretaceous to late Eocene, namely the Ngaterian, Piripauan, Haumurian, K-Pg boundary, Teurian, Mangaorapan and Bortonian stages, providing basin-wide temporal and spatial perspectives of ancient depositional environments in a consistent and reproducible format. Information derived from these palinspastic maps has implications for the petroleum prospectivity of the East Coast region, and broader applicability to paleotectonic, paleoclimate, and paleoceanographic research.</p>