Mid Tertiary Unconformities in the Waitaki Subdivision, North Otago

Abstract

Abstract Two mid-Tertiary unconformities of wide extent in North Otago are documented and discussed. The older is a previously recognized surface of early to mid Oligocene (late Whaingaroan or early Duntroonian) Age which rests on the early Oligocene (Whaingaroan) McDonald Limestone. Angular discordance across the surface indicates tectonic deformation, probably mild open folding; eustatic fall may account for the bulk of other characteristics, although paleokarst features occur only in modern coastal exposures. The younger surface, not previously recognized as a continuous unconformity in this area, is of late Oligocene-early Miocene (early Waitakian) Age, and rests principally on the lower part of the Otekaike Limestone (Duntroonian-Waitakian). Paleokarst features attest regional emergence; angular discordance and varying thicknesses of sediment below the surface attest tectonic deformation, again probably mild folding. Eustatic fall may or may not have been causally significant. Both unconformities probably extend widely in the New Zealand region. The older appears to correspond with the “Marshall Paraconformity” (Carter and Landis, 1972, 1982), but confusion in the definition and implications of this term renders its use questionable. The younger is considered to be further evidence of a tectonic event represented elsewhere by unconformity and sedimentary diapirism which affect the Tertiary succession at least as far north as the Wairarapa.