|Lawless, A. S. (2012). Nature, Distribution, Origin and Economics of Glauconite in Carbonate-Phosphate-Glauconite Surficial Deposits on Central Chatham Rise, Southwest Pacific (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/6502
|Chatham Rise is a broad, elongated submarine platform that extends 1100 km out into the Southwest Pacific off eastern South Island. The surficial sediments on the central portion of the Rise at depths of 200-500 m include significant concentrations of the mineral glauconite (10-80 wt%) associated with skeletal carbonate, fine terrigenous material and locally abundant phosphatic nodules. This is the first study to fully analyse the nature, origin and economic potential of the glauconite component. Sub-bottom 3.5 kHz profiles have established the distribution, geometry and thickness (10-200 cm) of the surficial deposits, which overlie partially indurated Oligocene chalk of probable Whaingaroan age (~34-27 Ma). 137 grab, dredge or piston core samples have been analysed for their texture, mineralogy and geochemistry, which has produced a new surficial sediment map for the central Chatham Rise.
Glauconite most commonly occurs as dark green to black, very fine to fine sand-sized (0.2 mm), polished ovoidal and lobate pellets, and less commonly as infills within foraminiferal tests or as variably replaced rock fragments and phosphatic clasts. Full physical analysis reveals seven morphological types of glauconite including, in order of decreasing abundance, ovoidal, lobate, composite, internal molds, fossil casts, pigmentary, tabular and pellets within rock fragments. Their internal fabrics are dominated by random microcrystalline varieties with some oriented fibroradiating rim and skeletal infill types. Other notable features of some glauconite pellets include their expansion cracks, opaque inclusions and variable degrees of limonitisation. XRF/XRD analyses reveal mainly diagenetically mature varieties having elevated K2O (7-9 wt%) and Fe2O3 (19-23 wt%) values, and a dominant 10Å glauconitic mica structure involving from 10-20% expandable smectite layers.
K-Ar dates on pellets indicate a Late Miocene age (av. 5.75 Ma). This “old” age, as well as the clear evidence of reworking shown by the dominance of smooth polished ovoidal grains, mean that the central Chatham Rise glauconites are predominantly allogenic (i.e. derived/reworked), and neither strictly authigenic nor in situ, despite occurring in “modern” seafloor deposits. The formation of glauconite is linked to prominent upwelling within the Subtropical Front (STF) zone over the Rise in the Late Miocene, and the attendant heightened primary productivity, bacterial activity and nutrients associated with widespread biogenic blooms at that time. The source of the smectite required for glauconite formation could have been from the devitrification of volcanic ash from diverse eruptive centres, or through “neoformation”/authigenic precipitation within pore water solutions of carbonate-rich waters in the Oligocene/Miocene chalk substrate. Glauconite formation largely ceased after the Late Miocene due to an increase in terrigenous input, a reduced smectite source, and a major period of lowered sea level associated with Antarctic ice sheet growth. The “modern” Chatham glauconite pellets were sourced from submarine erosion of the former Late Miocene sediments and have been dispersed by intensified bottom current circulation at times of lowered sea level within the complex STF over Chatham Rise, and possibly ice scouring. Effectively there exists a widespread “Chatham unconformity” (mid-Oligocene to Recent) which cuts out most of the last 30 myr of the sedimentary record on central Chatham Rise, except for the highly condensed surficial deposits whose glauconite grains formed 5-6 Ma, in the Late Miocene. This long-lasting “Chatham unconformity” is a greatly extended version of the well documented Oligocene Marshall Paraconformity.
The thickness of the surficial sediment cover on central Chatham Rise ranges from 0.06-1.35 m. By using a conservative estimate of 0.5 m thick, and the wt% of glauconite within the sediment cover, a resource estimate of the glauconite has been made for various areas. In the most glauconite-rich area (>50 wt% glauconite) in the vicinity of Reserve Bank in 200-300 m water depths, which has an aerial extend of 4,500 km2, the glauconite resource amounts to about 2 Bt. The Chatham Rise glauconites have substantial economic potential as a potash fertiliser in support of the strong agricultural-based economy of New Zealand.
Glauconite is widespread in many Late Cretaceous-Cenozoic sedimentary deposits in the New Zealand rock record and the Chatham Rise occurrences provide a useful uniformitarian analogue for at least some of these.