Abstract:
The technique of seismic sequence analysis, and the associated global sea-level curve, have revolutionised stratigraphic thought and caused a re-assessment of hydrocarbon exploration techniques. Tests of the global sea-level concept were identified as a high priority for future Ocean Drilling by the COSOD II planning meeting. In the first instance, such tests must be targeted on well-developed sets of shelf sequences, selected worldwide. The database from which the modern global sea-level concepts originated is dominated by northern hemisphere examples. It is therefore particularly important to identify high-quality sites for drilling in the southern hemisphere. During the Plio-Pleistocene, the 200 x 200 km Wanganui Basin developed in a backarc position relative to the North Island, New Zealand plate boundary. Because of the proximity of the plate boundary, the inner edge of the basin was subjected to progressive uplift at the same time that major glacio-eustatic sea-level changes were occuring. The inner edge (TST and HST) of the cylothemic sedimentary sequences which characterise the basin are therefore readily accessible in onshore outcrops, whilst the outer parts of the basin (LST) are preserved as shelf-edge clinoforms offshore. Onshore, cyclothems comprise rhythmic alterations of coarser grained (TST) sand-gravel-coquina and finer grained terrigenous siltstone-mudstone. This facies pattern is precisely that predicted by systems tract stratigraphic analysis. The offshore LST clinoforms therefore form an outstanding target for ocean drilling. A drill transect would establish the lithology and stratigraphy of a mid-Pleistocene lowstand systems tract. Together with the onshore data, this would allow the documentation of the complete 3-dimensional facies pattern associated with a sequence known to have been deposited during a major eustatic sea-level cycle. A cyclothemic sedimentary pattern has not been observed in Pre-Pliocene sequences in the NZ region. Caution should therefore be exercised in interpreting pre-Pliocene seismic sequences as controlled by glacio-eustatism.