Abstract
The Marshall Paraconformity is a widely recognised surface in the mid-Oligocene sedimentary record of New Zealand and the south Pacific. Its type section has been defined at Squire's Farm in South Canterbury, South Island, New Zealand where it forms the burrowed top of the Early Oligocene (Whaingaroan) Amuri Limestone and is overlain by a thin deposit of the Late Oligocene (Duntroonian) Kokoamu Greensand. In the stratigraphic framework of the Canterbury Basin it marks the turning point between the extensional tectonics of rifting from Gondwana and the transpressional tectonic regime associated with the present day Australian-Pacific plate boundary. The development of the unconformity has been attributed to the initiation of the Antarctic Circumpolar Current and the eustatic sea level changes associated with the development of continental ice sheets on Antarctica in the early Oligocene. Controversy has surrounded the Marshall Paraconformity, particularly regarding correlation between exposures and the regional significance of the surface.
A new record for the Marshall Paraconformity has been recovered in a 273 m drillcore (TNW- 1) from near the settlement of Cave in South Canterbury, 30 km to the north of the type section. Rock and environmental magnetic studies reveal weakly magnetised sediments with remanence carried by fine-grained magnetite with a minor contribution from iron sulphides (likely greigite) and iron oxyhydroxides (goethite and hematite). The clearest paleomagnetic signal is held by the Kokoamu Greensand, with 0.2-1 [mu]m grains of magnetite, a very small component of goethite and no evidence of iron sulphides or hematite. The Kokoamu Greensand has a reversed magnetic polarity, and is correlated with Chron C7r (25.183-25.496 Ma) using biostratigraphic constraints. This is the most precise age constraint available for the resumption of deposition on the Marshall Paraconformity. Glaucony deposition at TNW-1 predates that at Squire's Farm and accumulation rates were relatively high (~5 cm/kyr) indicating that glaucony did not develop in situ.
A 3 km seismic profile across the drill site was correlated with the core by a synthetic seismogram. The Marshall Paraconformity was identified as a high-amplitude reflection showing slight angular discordance between discontinuous reflections below the unconformity and continuous, parallel reflections above. Erosion on the Marshall Paraconformity was also inferred from the local absence of the Amuri Limestone at TNW-1. Reflections underlying the Marshall Paraconformity in the seismic profile show evidence of further angular discordance, indicating tectonic activity in the Bortonian, possibly related to the development of the Fairlie Basin to the north. The difference in timing of the unconformity at TNW-1 from Squire's Farm to the south is attributed to these local tectonic variations.
The TNW-1 record has been compared with other records from the Canterbury Basin, in a north-south transect including Otaio Gorge (near the Marshall Paraconformity type section) and the Kakanui River Mouth, a site of Eocene volcanic activity. Throughout these different local tectonic environments, the Marshall Paraconformity is a recognisable surface, indicating a regional cause of the unconformity. Comparing the timing of the initiation of the hiatus and resumption of deposition, the shallow-water Marshall Paraconformity correlates well with the proposed eastward shifting of the Proto-Ross Sea Gyre after the opening of the Tasman Gateway. Shallow currents were redirected across the New Zealand continent in the early Oligocene and may have been responsible for non-deposition and erosion on the unconformity. These shallow currents were diverted by the development of the Antarctic Circumpolar Current, allowing the reworking and deposition of glaucony, possibly from the Campbell Plateau.
Age estimates for the deep-water occurrences of the Marshall Paraconformity (ODP Sites 1123 and 1124) indicate an earlier initiation of the hiatus (~33 Ma). These sites were likely beneath the influence of shallow-water currents, and it is suggested that the development of the unconformity here was due to corrosive bottom waters associated with the development of sea ice around Antarctica, rather than the initiation of circumpolar flow. A record of the Marshall Paraconformity from intermediate depths (IODP Site 1352) most closely resembles the shallow-water Marshall Paraconformity. The resumption of deposition on the shallow water Marshall Paraconformity indicates the initiation of the Antarctic Circumpolar Current while the deep water Marshall Paraconformity formed earlier in response to increased glaciation on Antarctica.