Lithostratigraphy and Biostratigraphy of Swinburn; Proxies for Paleogeography from Central Otago Marine Strata

Richards, Marcus

The Maniototo Basin preserves localised Cenozoic marine strata that can be correlated into Waitaki Valley, with formation names as used in the southern Canterbury Basin. The clastic-dominated strata were derived from an Otago Schist source on Zealandia during maximum submergence. Long-recognised marine strata from the Maniototo are the Naseby Greensand and Green Valley Limestone. This study recognises formations including: Kauru, Wharekuri (truncated by the Marshall Unconformity), Kokoamu, and Otekaike. The basin-wide greensand resting on the Marshall Unconformity is Kokoamu Formation with an age of Whaingaroan- Duntroonian. The Naseby Greensand of Kyeburn is now identified as a composite of units including Whaingaroan to Waitakian-aged fossils and is equivalent to Wharekuri, Kokoamu and Otekaike Formation. The use of Naseby Formation should be discontinued. Kokonga - Green Valley Limestone is Otekaike equivalent. We now detail a 200 m section at Swinburn, comprising massive fossiliferous marine rocks with a pervasive micaceous, very fine quartz sand to silt component. The sequence is variably (in places richly) glauconitic, grading up through calcareous sandstones to a packstone unconformably overlain by Wedderburn Formation and capped by basaltic volcanics. Foraminifera and ostracods date these units as upper Runangan - lower Whaingaroan to mid-Waitakian; low planktic foraminiferal percentages are consistent with limited oceanic influence. Uncommon fossil scallops (Pectinidae) in the Janupecten - Lentipecten lineage are consistent with microfossil ages. This sequence is thicker, more clastic - dominated, and nearer to sediment source (probably Otago Schist) than in the Waitaki. The background sedimentation of subangular very fine sand and silt found iii throughout the Oligocene indicates a continuous supply from the terrestrial source. A nearby stratigraphically lower sequence includes quartzose coal measures, inferred estuarine mudstones with rare molluscs, and intertidal shales, consistent with a paralic to estuarine setting, and is a correlative of Kauru Formation. The beds show a pattern of marine transgressions and regressions peculiar to this site. Hedecardium indicates a Bortonian to Kaiatan age. In summary, the Swinburn sequence formed in a shallow, possibly early- to mid-Cenozoic epicontinental sea with a productive source of terrigenous clastics. There is no firm evidence of outer shelf settings.

xv, 127 pages A4

2014Richards

MULTIPOLYGON (((170.326851103521562 -45.144400009853953,170.31041878064002 -45.14458554850502,170.310588624841017 -45.135253224545238,170.32701826354986 -45.135067598840919,170.326851103521562 -45.144400009853953)),((170.386652137944708 -45.161409039434176,170.340630264763689 -45.16072702208109,170.340415834161263 -45.130617162769575,170.388313129141864 -45.130961131131521,170.386652137944708 -45.161409039434176)),((170.277684253347246 -45.012051743114796,170.270162459994538 -45.012062604979292,170.270916171862837 -45.007507655583765,170.27816005784868 -45.00768065677368,170.277684253347246 -45.012051743114796)),((170.304746339913066 -44.979158865655613,170.299651199328224 -44.979038418045057,170.300357342772259 -44.975434835633664,170.304111420823631 -44.975523579779761,170.304746339913066 -44.979158865655613)))