Post-breakup stratigraphy of the Kaikoura Synthem (Cretaceous-Cenozoic), continental margin, southeastern New Zealand

Abstract

The stratigraphy and structure of the Kaikoura Synthem (Cretaceous—Cenozoic) is synthesised from available seismic records and well reports on the continental margin of southeastern South Island. Four seismic sequences are distinguished, corresponding to lithostratigraphic groups that can also be recognised nearby on land: Matakea Group (Cretaceous), Onekakara Group (late Cretaceous—Oligocene), Kekenodon Group (Late Oligocene—Miocene), and Otakou Group (Miocene—Recent). These sequences achieve maximum thickness within three sedimentary basins underlying the continental shelf (Canterbury and Foveaux Basins) and slope (Great South Basin). The Great South Basin is separated from the Canterbury and Foveaux Basins across the WaipounamouFaultSystem, anortheasterly oriented zone of faults which controlled rifting and Matakea Group sedimentation along the western edge of a Cretaceous aulacogen now manifest as the Bounty Trough. The Canterbury andFoveaux Basins contain transgressive sequences (Onekakara Group) that were deposited in response to post-rifting thermal subsidence of the margin. By the mid Cenozoic, land areas were greatly reduced, and at peak transgression there was the development of a widespread Oligocene unconformity (Marshall Unconformity at the base of the Kekenodon Group). Subsidence continued until the Miocene, though punctuated by phases of mild faulting and volcanism in thePaleocene, Middle Eocene and Late Eocene–Oligocene. Regression commenced in the Miocene (Otakou Group), consequent upon uplift of the Southern Alps along the Alpine plate boundary in the west. Crustal deformation attained a climax in eastern Otago in the late Middle Miocene, with substantial volcanism and block faulting.