Analysis of the stratigraphic architecture of the fills of Wanganui, King Country, and eastern Taranaki Basins reveals the occurrence of five 2nd order Late Paleocene and Neogene sequences of tectonic origin. The oldest is the late Eocene-Oligocene Te Kuiti Sequence, followed by the earlyearly Miocene (Otaian) Mahoenui Sequence, followed by the late-early Miocene (Altonian) Mokau Sequence, all three in King Country Basin. The fourth is the middle Miocene to early Pliocene Whangamomona Sequence, and the fifth is the middle Pliocene-Pleistocene Rangitikei Sequence, both represented in the three basins. Higher order sequences (4th, 5th, 6th) with a eustatic origin occur particularly within the Whangamomona and Rangitikei Sequences, particularly those of 6th order with 41 000 yr periodicity.The base of each 2nd order sequence is marked by marine flooding and represents a discrete phase in basin development.

The Mungaroo Formation in the Gorgon Field is a stratigraphically complex fluvial system of Triassic age. It is also a major hydrocarbon reservoir, therefore understanding its internal stratigraphic architecture is of paramount importance to exploitation of its reserves. Here, the technique of chemostratigraphy is used to construct a correlation framework for the Mungaroo Formation of the Gorgon Field. Chemostratigraphy is a tool that employs variations in inorganic whole rock geochemistry to enable the characterisation and subsequent correlation of sediments. For this study, a total of 1,514 cuttings and core samples from eight wells in the Gorgon Field have been analysed. Using data derived from both claystone and sandstone lithologies, the Mungaroo Formation is divided into nine chemostratigraphic packages, 22 geochemical units and 19 sand units. Additionally, three surfaces identified as time lines (T1–T3) are geochemically defined. Changes in values of Ga/Rb and Al2O3/(CaO+ MgO+K2O+Na2O) indicate that during deposition of the Mungaroo Formation, the paleoclimate became warmer and wetter, resulting in increasingly intense hydrolytic weathering. Steps in the values of these ratios allow three surfaces to be identified (T1–T3), at which there is a marked and sustained change in the paleoclimate. These three surfaces represent time lines that provide a quasi-chronostratigraphic framework for the formation. Values of Cr/Al2O3, Cr/Na2O and Nb/Al2O3 are related to changes in sediment provenance and indicate that during deposition of the Mungaroo Formation the provenance became more mafic and less intermediate. It is variations in paleoclimate and provenance modelled from the geochemical data that allows the packages, units and sand units to be characterised and correlated. The chemostratigraphic correlation is more detailed than is available from other stratigraphic techniques. Although in most instances the lithostratigraphic correlation of sand units based on wireline log correlation matches the one defined using chemostratigraphy, there are some significant differences between the two that influence reservoir models and gas production.

An investigation of the taphonomy, palaeoecology and stratigraphy of cool-water skeletal concentrations (shell beds) of the Matemateaonga Formation (Late Miocene-Early Pliocene) of Wanganui Basin, New Zealand, has provided the basis for the classification of taphofacies presented here. Two taphofacies described from transgressive systems tracts include the amalgamated shell bed and sediment starved shell bed taphofacies, representing skeletal concentration dominated by wave and current agitation, and sediment starvation, respectively. A further five taphofacies described from highstand and regressive systems tracts exhibit a gradient of sedimentological, taphonomic and palaeoecological properties that result from variation in storm event and fair-weather wave processes across the palaeoshelf bathymetric gradient. A principal components analysis of semi-quantitative data (53 observations) from sequences in Manutahi-1 well core demonstrates that taphonomic properties may be limited to particular systems tracts in some cases, but can also be repeated in different system tracts where the depositional environments are similar. Taphofacies, which are contained within siliciclastic-dominated portions of sequences (highstand and regressive systems tracts) possess little direct relevance to sequence stratigraphic analyses, but do provide valuable information on environmental conditions, in particular, depth relative to storm and fair-weather wave base, and proximity to shoreline.

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