Aspects of an early Miocene unconformity in North Otago and South Canterbury

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Batt, Geoffrey Ernest.

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A distinctive unconformity is present in many of the shallow water earliest Miocene sequences of North Otago and South Canterbury, between the Otekaike Limestone and the overlying sediments. However, this feature is not universally developed throughout this region. Sequences in eastern Canterbury and western Otago are commonly continuous throughout this period. Possibly correlative unconformities are seen in comparable late Oligocene to early Miocene sequences in South Otago, North Canterbury, Westland, and Nelson, suggesting a regional scale of development for this feature.
Where it is well developed, particularly in coastal areas of North Otago and around Blands Bluff in Canterbury, this unconformity is an irregular, extensively solution-affected surface showing relief of up to 1 m or more at an outcrop scale. Its character is inferred to suggest formation by exposure of the Otekaike Limestone to subaerial erosion. Possible terrestrial exposure is also supported by the character of the sediments directly overlying this unconformity. These commonly include well rounded reworked local and exotically derived clasts up to 30 cm in diameter, which in places (most notably at Hutchinsons Quarry in Oamaru) reach high concentrations, forming distinctive basal conglomerates.
Detailed microfaunal dating constrains the development of this unconformity to the mid to upper Waitakian - the Globigerina woodi woodi and Globigerina woodi connecta zones of Jenkins (1971). This interval also coincides with major sedimentological changes throughout the North Otago and South Canterbury regions- even in sections where no intra-Waitakian unconformity is developed. The dominantly bioclastic sediments of the Otekaike Limestone are replaced at this time by more glaucony and/or terrigenous rich formations such as the Mount Harris Formation and Gee Greens and.
These changes in the sedimentary regime of the region are attributed primarily to an initiation of tectonism in the provenance region of the sediments at this time. Evidence for active faulting contemporaneous with the development of this intra-Waitakian unconformity is seen at Gees Bay on the North Otago coast, and is also tentatively inferred for the Otiake region. However, tectonism probably had only a peripheral influence on the development of this unconformity in most areas.
Rather, the extensive geographic distribution of this erosion surface, combined with the apparent simultaneity of its development everywhere it is exposed suggests that this unconformity is primarily the result of a eustatic fall in sea level. This suggestion is further supported by an apparent correlation between the mid to upper Waitakian age inferred for this unconformity and a global sea level drop of approximately 40 m identified by Haq et al. (1987).

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85 leaves (some folded) : ill. (some col.), maps ; 30 cm.

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1993Batt

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http://download.otagogeology.org.nz/temp/Abstracts/1993Batt.pdf

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Batt, Geoffrey Ernest., “Aspects of an early Miocene unconformity in North Otago and South Canterbury ,” Otago Geology Theses, accessed March 9, 2021, http://theses.otagogeology.org.nz/items/show/273.

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