Temperate carbonate debrites and short-lived earliest Miocene yo-yo tectonics, eastern Taranaki Basin margin, New Zealand

Abstract

This study examines two unconformity-bound earliest Miocene temperate shallow-marine limestones at remote Gibson Beach, western North Island, on the eastern margin of Taranaki Basin, New Zealand's only producing hydrocarbon province. The local geology comprises an upper limestone of up to 6m of spectacular conglomeratic limestone (rudstone; Papakura Limestone) whose pebble- to boulder-sized clasts were derived from cannibalisation of the lower skeletal limestone (bryomol grainstone; Otorohanga Limestone). Multiple lines of evidence require the Otorohanga Limestone to have been well indurated before erosion and clast generation. Cementation occurred from pressure dissolution of calcitic skeletons at burial depths of probably 400m or more. Subsequent uplift led to local subaerial exposure and karstification of a fault-block cliffed coastline where physical erosion sourced talus limestone debris onto adjacent high-energy pocket beaches. Storm and/or seismic events periodically triggered mass flows of the limestone clasts offshore onto the contemporary mixed siliciclastic-carbonate shelf as channelised carbonate debrites. Lithification of these Papakura Limestone debrites occurred via pressure dissolution, and required a second burial episode. Karst pinnacles preserved on the eroded upper surface of the Papakura Limestone demand uplift again and erosion in a subaerial setting, before subsequent deep burial by Early Miocene siliciclastic shelf sand and turbidite deposits. Strontium isotope dating of brachiopod fossils suggests the two postulated burial-uplift cycles, involving movements up to 400±100m, occurred very rapidly within several 100kyr. Speculative drivers of the yo-yo tectonic events are earliest Miocene movements on the nearby major Taranaki Fault thrust in association with regional changes in subduction tectonics at the Australian-Pacific plate boundary, along with pulsed emplacement of obducted gravity slide deposits (Northland Allochthon) onto northern North Island. The two limestones record a previously undocumented degree of eastern Taranaki Basin margin tectonic mobility relevant to a fuller appreciation of petroleum system development in that basin.