CENOZOIC TEMPERATE AND SUBTROPICAL CARBONATE SEDIMENTATION on AN OCEANIC VOLCANO - CHATHAM ISLANDS, NEW ZEALAND

The Chatham Islands, at the eastern end of the Chatham Rise in the SW Pacific, are the emergent part of a late Cretaceous - Cenozoic stratovolcano complex that is variably covered with limestones and fossiliferous tuffs. Most deposits accumulated in relatively shallow, high-energy, tide-dominated paleoenvironments with deposition punctuated by periods of deeper-water pelagic accumulation. Carbonate components are biogenic and dominated by molluscs and bryozoans; a heterozoan assemblage. The Middle - Late Eocene Matanginui Limestone contains local photozoan elements such as large benthic foraminifers (especially Asterocyclina) and calcareous green algae, reflecting the general Paleogene subtropical oceanographic setting. More localized Late Eocene - Oligocene deposits (Te One Limestone) as well as Pliocene limestones (Onoua Limestone) are wholly heterozoan and confirm a generally cooler-water oceanographic setting. Early seafloor diagenesis is interpreted to have removed most aragonite components (infaunal bivalves and epifaunal gastropods). Lack of aragonite resulted in the absence of calcite cementation during subaerial exposure such that most carbonates are friable or unlithified. Cementation is present at nodular hardground-firmground caps to m-scale cycles. Such cements are microcrystalline or micrometer-thick isopachous circumgranular rinds with not enough definitive attributes to pinpoint their environment of formation. The overall paleoenvironment is interpreted as mesotrophic, resulting from upwelling about the Chatham volcanic massif and from nutrient element delivery from the adjacent volcanic terrane and coeval volcanism. Biotic diversity in tuffs is 2 to 3 times that in limestones, supporting the notion of especially high nutrient availability during periods of volcanism. These mid-latitude deposits are strikingly different from their low-latitude, tropical, photozoan counterparts in the volcanic island-coral reef ecosystem. Groundwater seepage and fluvial runoff attenuate coral growth and promote microbial carbonate precipitation in these warm-water settings. By contrast, nutrients from the same sources feed the system in the Chatham Islands cool-water setting, promoting active heterozoan carbonate sedimentation.