Stable Isotope Geochemistry of Paleocene to Early Eocene Strata around Southern New Zealand
The Late Teurian (Paleocene) Tartan Formation is an organic-rich mudstone that has been identified in five of the eight exploration wells drilled in the Great South Basin, and three of four exploration wells drilled in the Canterbury Basin. In this study, the geochemistry of two wells from the Great South Basin (Pukaki-1 and Rakiura-1) and four wells from the Canterbury Basin in southern New Zealand (Resolution-1, Clipper-1, Galleon-1, and Endeavour-1) have been investigated using elemental analyser isotope ratio mass spectrometric (EA-IRMS) analyses on selected sidewall core and cuttings samples. This study builds on previous geochemical work by the author from five other wells from the Great South Basin (Takapu-1A, Toroa-1, Pakaha-1, Kawau-1A, and Hoiho-1C). All wells except Rakiura-1, Takapu-1A, and Resolution-1 showed geochemical characteristics that allowed recognition of the Tartan Formation. The formation is characterised by enrichments in TOC (typically above 3%) and 13C (generally delta13C ratios are between -21 and -17 per 1000), indicating a significant marine contribution. C/N ratios recorded within the Tartan Formation are all above 20, which suggest that the organic matter contains a significant contribution from terrestrial and/or altered marine material. Geochemical evidence of samples within the Tartan Formation suggests that it contains a mixture of marine bacterial/plant/algal and C3 terrestrial plant source components. This is consistent with the findings of Killops et al. (2000), who reported from biomarker studies that the organic matter of some Great South Basin samples contained organic matter derived from a marine source with varying degrees of terrestrial contribution. The Tartan Formation is distinct from enclosing formations which are characterised by low organic contents (generally below 2%), isotopically light delta13C values (typically around -26 per 1000), which is indicative of terrestrial C3 plant matter, and a wide range of C/N ratios (ranging from 4 to 64). The latter suggests that there were varying degrees of preservation of the deposited organic matter within these formations. Organic matter within enclosing formations appears to be derived from a combination of C3 land plants and marine material. The high TOC content of Tartan Formation sediments compared to the underlying formation suggests that it represents a profound change in depositional conditions. Conditions for the preservation and accumulation of organic matter were more favorable prior to deposition of the Tartan Formation than following it. The enrichment of 13C and the high TOC contents within the Tartan Formation are similar to those for the mid to Late Teurian Waipawa Formation that has been identified throughout many of New Zealand's major sedimentary basins; however, TOC and delta13C values for the Tartan Formation exceed those previously reported for the Waipawa Formation. Geochemical changes characteristic of the Tartan Formation are recognised below the lithological base of the formation in some wells, contemporaneous with the onset of the Paleocene Carbon Isotope Maximum (PCIM), and represent different lithostratigraphic expressions of that event. Termination of the environmental effects associated with the PCIM around New Zealand appears to have been diachronous and differences between the exact ages and stratigraphic positions of the Tartan and Waipawa formations are attributed to local environmental variations during deposition. TOC and delta13C enrichments associated with the Tartan Formation are not ubiquitous, and the formation has variable thickness throughout the Great South and Canterbury basins. It is concluded that the Tartan and Waipawa formations are correlatives.