Mid-Tertiary unconformities in north Otago — A review and assessment

Abstract

Recent papers by Homibrook, Jenkins and Lewis discuss the identity, interpretation and time-evaluation of discontinuities within the markedly calcareous and glauconitic Oligocene-Miocene succession of North Otago, which often have been in dispute in the past. It now seems established that, of numerous physical breaks in the locally extremely condensed sequences, two are widely identifiable in the region but are telescoped in some coastal sections. One of them may well be equivalent to intraOligocene breaks in other South Island regions, but reliable, exact correlation is beyond the present limits of resolution by biostratigraphical means. For this and other reasons the term “Marshall Paraconformity” should be dropped. Regional upwarping was chiefly responsible for particularly thin, discontinuous sequences in coastal North Otago, but actual emergence, as claimed by Lewis and Belliss (1984) to explain so-called “paleokarstic” features, is considered unnecessary and as yet unproven. Tens of metres at the most of discontinuous sedimentation represent from 10 to 15 Ma; time unrepresented by sediment at the lower important break has been estimated, at different places, at between 2 and 8 Ma.