Nature and occurrence of modern and Neogene active margin limestones in New Zealand

Abstract

Modern nontropical skeletal carbonate sediments are presently forming over 20% of New Zealand's continental shelf within or next to the obliquely convergent Australia-Pacific plate boundary. Their distribution is inversely related to terrigenous sedimentation rates, which steadily increase towards the plate boundary. There is a broad separation of major skeletal types into different associations with distance from the plate boundary, bivalve-foraminiferal assemblages dominating on the forearc shelves and bryozoan or bryozoan-bivalve assemblages in backarc regions.

Limestones are a persistent constituent, both geographically and stratigraphically, within the thick terrigenous-dominated onshore Neogene successions. They are more common about the Hikurangi ocean-continent convergent margin than the South Island continent-continent convergent margin. The Neogene limestones in the different tectonic sectors of the plate boundary can be distinguished from each other by a combination of their geometry, thickness, skeletal composition, area, and thickness of associated clastics. In contrast with the modern pattern, the proportion of limestone does not reduce uniformly with increasing proximity to the plate boundary. This is due to two features of the evolution of the modern plate boundary: a delay in tectonically overprinting the Eocene-Oligocene Challenger Rift System, and uplift of the forearc basin through inner shelf depths during the Late Pliocene.

The modern skeletal associations at increasing distances from the plate boundary are broadly similar to the Neogene limestones; this spatial trend has a superimposed time trend, such that, at some localities, the skeletal associations change stratigraphically, thereby monitoring the increased tectonic intensity with decreasing age. During the Late Pliocene and early Pleistocene, glacio-eustatically induced sea-level movements combined with tectonism to form widespread limestones during the glacial episodes in eastern North Island.

Keywords:

• carbonate sediments
• limestone
• tectonics
• Australia-Pacific
• plate boundaries
• Neogene
• regression
• transgression
• terrigenous materials
• sedimentation
• rates
• continental shelf