Nelson, Campbell S.Smith, Abigail M.2026-08-062026-08-061992https://hdl.handle.net/10289/18511Carbonate sediments are forming on modern shelves in both tropical and temperate latitudes. Temperate shelf carbonate sediments have a distinctive character and, unlike their tropical counterparts, have many poorly understood aspects. Bryozoans are major skeletal components of many temperate carbonate deposits, but very little is known about their sedimentology. Aspects of the sedimentology of New Zealand bryozoans are investigated here. Many temperate carbonates are also closely associated with terrigenous siliciclastic sediments, forming a mixed deposit. A New Zealand example is presented here from the Hauraki Gulf, a semi-enclosed sea in northeastern North Island. The results of these studies are applied to the temperate shelf carbonate model. Bryozoans are important sedimentologically as producers, stabilizers, encrusters, bioeroders, subjects of diagenesis, and eventually as fossils. Most bryozoans precipitate carbonate at rates of less than 1 g CaCO₃/y (mean = 0.07 g CaCO₃/y). In the Hauraki Gulf the maximum determined is 0.8 g CaCO₃/y and overall production by one bryozoan population is about 24 g CaCO₃/m²/y, generally much less than the few rates published from elsewhere. Bryozoans in New Zealand fall into five mineralogical classes. Calcite may be low-Mg (<3 wt% MgCO₃), moderate-Mg (3-5 wt% MgCO₃), or high-Mg (5-10 wt% MgCO₃). Bimineralic skeletons may be mainly calcite, or mainly aragonite, reflecting two different modes of origin: developmental bimineralism, with discrete age-related layers, or diagenetic bimineralism, where metastable aragonite alters over time. Carbonate mineralogy is most closely linked to higher taxonomic order, while bryozoan colonial growth form is weakly correlated. Isotopes (δ¹⁸O and δ¹³C) and co-precipitates (Ca, Mg, Sr, Fe, Mn) in bryozoan carbonate are related to mineralogy, taxon, water depth, and degree of terrigenous mixing. Bryozoans precipitate carbonate close to isotopic equilibrium with sea water, and thus reflect their temperate climate. After death, bryozoan carbonate is subject to abrasion, bioerosion, and dissolution. Different bryozoan colonial growth forms are more or less resistant to abrasion, and the less resistant forms may produce large quantities of carbonate mud. Bryozoans are active in biological diagenesis, as borers, encrusters, and as substrate. Dissolution of bryozoans depends on mineralogy and on morphology. Depth, hydraulic energy, and substrate are all important controls on bryozoan distribution and on diagenesis. Bryozoan assemblages, therefore, reflect these environmental factors. Destructive diagenesis reduces and limits this relationship. Sediments in the Hauraki Gulf are highly variable spatially, and are a good example of in situ mixing of terrigenous and biogenic material. Four sediment facies are described: high-carbonate gravel (64% CaCO₃), muddy sand (38% CaCO₃), sand (31% CaCO₃), and mud (15% CaCO₃). The patchy distribution of these facies is controlled by water depth, water energy, source of sediment, and by substrate availability. Bivalves are the dominant carbonate-producers, with bryozoans, gastropods, and barnacles subdominant. Holocene sea-level rise has reworked sediments, and much is palimpsest. The degree of carbonate-elastic mixing appears to influence temperate assemblage in New Zealand carbonates, such that foramol and mollusc assemblages predominate when terrigenous material is abundant, while the bryomol assemblage is more common in high-carbonate settings. Early marine diagenesis of temperate carbonates includes constructive and destructive processes, but destruction predominates. Abrasion is controlled by hydraulic energy, and indirectly by climate. Biological diagenesis is evident in Hauraki Gulf shells, and operates on a time scale of 100s to 1000s of years. Bioerosion is controlled by biota, thus by environment and climate, and by substrate. Carbonate mud may indicate its origin by mineralogy or by particle morphology. Dissolution is controlled by sea-water chemistry, and the mineralogy and morphology of grains. Constructive processes such as encrustation and local cementation do occur in temperate shelf sediments. Nevertheless, most temperate carbonates, including bryozoan-dominated ones, are subject mainly to destructive processes, often over a long time due to low sediment accumulation rates. Temperate carbonates thus may have a relatively low preservation potential, and could be under-represented in the fossil record.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Thesis