The effect of volcanism on cool-water carbonate facies during maximum inundation of Zealandia in the Waitaki–Oamaru region

Abstract

The paleogeography of the Waitaki–Oamaru region during the Oligocene–Miocene maximum inundation was defined by a volcanic paleohigh producing a rimmed cool-water carbonate shelf geometry. Basaltic surtseyan style cones became the setting for a productive cool-water carbonate factory isolated from terrigenous input. To the west, impure wackestones and calcareous siltstones contain terrigenous material derived from low relief landmasses farther to the west. A lowstand following the cessation of volcanism caused the paleohigh to become subaerially exposed, forming an extensive dissolution surface in the east correlative with submarine firmgrounds to the west. Stronger currents from the south and significant storm events sweeping over the high reworked carbonate and glauconitic sediment and deposited it in channels to the north. Carbonate deposition west of the paleohigh filled in the deeper part of the basin eventually resulting in a wider, shallower and more regular shelf environment. Depositional environments of the Waitaki–Oamaru region during the Waitakian (Late Oligocene–Early Miocene) were overall shallower than during the earlier Whaingaroan (Early Oligocene) Stage when a more complex paleotopography existed. This is unlike elsewhere in New Zealand where maximum depth was reached during the Waitakian Stage. Thus, the existence of an isolated, submerged paleohigh in a cool-water carbonate basin can have a significant effect on the evolution of that basin, stimulating carbonate factories to develop where they might otherwise not.