Search

A unique 10–140 cm thick veneer of greensand and glauconite-rich deposits (10–80 wt% glauconite) presently sits atop central Chatham Rise (CR) in water depths of 200-500 m. The glauconite is dominated by chemically mature, polished ovoidal and lobate pellets, with a 10–12 Å phyllosilicate structure and K-Ar ages of 7–5 Ma, indicating that the glauconite pellets are predominantly allogenic (i.e. derived/reworked) grains, and not of in situ authigenic origin. Glauconite possibly evolved from seafloor alteration of detrital and/or volcanogenic smectitic clays, likely concentrated within organic-rich faecal pellets under unique paleoceanographic conditions (nutrient cycling, upwelling, carbon isotope gradients) within the latest Miocene Subtropical Front. The distribution of glauconite abundance suggests a 'glauconite factory' existed at this time about Reserve Bank on western central CR. The pellets were dispersed eastwards along the crest by intensified bottom currents within the dynamic Subtropical Frontal zone, perhaps during the numerous glacial periods of lowered sea level throughout the Plio-Quaternary. The bioturbated greensand veneer, and its mix of other reworked Neogene and Quaternary skeletal, phosphatic and siliciclastic components, rest unconformably on mainly Early Oligocene chalks, forming a highly condensed, relict/palimpsest deposit that alone embodies the last up to 30 My of sedimentation history atop central CR. [ABSTRACT FROM AUTHOR]

Cool-water shelf carbonates differ from tropical carbonates in their sources, modes, and rates of deposition, geochemistry, and diagenesis. Inorganic precipitation, marine cementation, and sediment accumulation rates are absent or slow in cool waters, so that temperate carbonates remain longer at or near the sea bed. Early sea-floor processes, occurring between biogenic calcification and ultimate deposition, thus take on an important role, and there is the potential for considerable taphonomic loss of skeletal information into the fossilised record of cool-water carbonate deposits. The physical breakdown processes of dissociation, breakage, and abrasion are mediated mainly by hydraulic regime, and are always destructive. Impact damage reduces the size of grains, removes structure and therefore information, and ultimately may transform skeletal material into anonymous particles. Abrasion is highly selective amongst and within taxa, their skeletal form and structure strongly influencing resistance to mechanical breakdown. Dissolution and precipitation are the end-members of a two-way chemical equilibrium operating in sea water. In cool waters, inorganic precipitation is rare. There is conflicting opinion about the importance of diagenetic dissolution of carbonate skeletons on the temperate sea floor, but test maceration and early loss of aragonite in particular are reported. Dissolution may relate to undersaturated acidic pore waters generated locally by a combination of microbial metabolisation of organic matter, strong bioturbation, and oxidation of solid phase sulphides immediately beneath the sea floor in otherwise very slowly accumulating skeletal deposits. Laboratory experiments demonstrate that surface-to-volume ratio and skeletal mineralogy are both important in determining skeletal resistance to dissolution. Biological processes on the sea floor include encrustation and bioerosion. Encrustation, a constructive process, may be periodic or seasonal, and can be reversed. It produces both information and material. Bioerosion, in contrast, is destructive and permanent. In temperate areas bioerosion may destroy even very large shells during their long residence at the sea floor, on the order of hundreds to thousands of years. Overall, processes on the temperate sea floor may combine to destroy more carbonate than they produce, and the preservation potential of temperate shelf carbonate into the rock record may be significantly affected. Where preservation does occur in such a destructive regime, the effects of early sea-floor processes will be key determinants of the deposit, resulting in a 'taphofacies' characteristic of temperate shelf carbonate sediments. Keywords: Early diagenesis; Taphonomy; Cool-water carbonates; Abrasion; Breakage; Dissolution; Cementation; Encrustation; Bioerosion; Sediments

The North Island Fault System (NIFS), coincides with North Island's axial ranges, accommodating crustal strain associated with the Hikurangi subduction margin. Detailed geological mapping along c. 80 km of the NIFS constrains uplift/subsidence and displacement patterns since c. 12 Ma. Uplift within the NIFS area and/or vertical displacement on faults occurred at 12–10, 7–6.5, 5–4.5, 3.5–3.0, 2.5–2, and post-1 Ma. Each deformation episode was associated with vertical offsets across the NIFS of >300–500 m (total c. 5 km). Total dextral strike-slip across the NIFS since 5 Ma is <20 km. Little of the relative plate motion is accommodated on the NIFS (<10%) or in the upper plate (<20%), the majority occurring on the subduction interface. The tectonic evolution of North Island is best explained by reconstructions incorporating vertical axis block rotations and <30 km strike-slip distributed across the entire margin on multiple discrete structures. [ABSTRACT FROM AUTHOR]