30 research outputs found
Towards an understanding of thermodynamic and kinetic controls on the formation of clay minerals from volcanic glass under various environmental conditions
No full textlmogolite is the kinetically and thermodynamically favoured weathering product from rhyolitic volcanic glass in the soil-forming environment. However, on thermodynamic grounds imogolite would also appear to be the favoured alteration product of rhyolitic glass deposited in the nearshore marine environment. On the basis that the rate of conversion of glass to clay minerals is a function of the solubility of the clay mineral, smectite is expected to be formed under mildly diagenetic conditions, and formed more rapidly than imogolite in soil. The derived activation energies for formation of imogolite from glass in soils are appropriate for a diffusion controlled reaction, and appear consistent with the diffusion of the tetrahedrally co-ordinated species Al[iv](OH)₂(H2Q)⁺. In the marine environment, however the mechanism for all reactions appear to be surface reaction control
Seismic hazard in the Taupo - Reporoa Region
No full textNew Zealand lies astride a convergent plate margin, as a result it experiences hundreds of earthquakes a year. This study was commissioned in response to Environment Waikato's contingency planning for such natural disasters.
The Taupo - Reporoa region was identified in a report to Environment Waikato (Waikato Regional Council) as needing to be studied, in order to quantify the level of earthquake hazard. This region lies in the centre of the Taupo Volcanic Zone, and has, over thousands of years built up considerable thicknesses of unconsolidated mainly volcanogenic materials. It is recognised that towns and cities that are sited on these materials are at risk from amplification of seismic waves and will experience greater damage, than if they were on hard rock.
Soils were analysed by the following geotechnical tests: hand penetrometer, Bush penetrometer, shear vane, the standard compression test, and dynamic cyclical loading. Samples were also taken for soil moisture and bulk density. The results from these tests have been used to evaluate the region for seismic hazard, and to semi-quantitatively correlate with reports and claims of damage from earthquakes. A number of parameters were able to be derived from the field tests and the cyclic loading which were used in the final analysis.
The results showed that some soils were substantially weaker than others, which correlated with historical accounts of damage. A seismic microzoning map was constructed on the basis of the difference in soil strengths
Geology of the northern Mamaku Plateau
No full textThe geology of the northern Mamaku Plateau is dominated by the rhyolitic Mamaku and Waimakariri lgnimbrites, although exposures of the dacitic Waiteariki Ignimbrite are common in river gorges.
The Mamaku lgnimbrite comprises 4 flow units (sheets 1 to 4). The base of sheet 2 (the bulk of the ignimbrite) is an uncompacted pyroclastic breccia to lapilli tuff grading into a moderate to strongly welded zone, often lenticulitic. The upper zone is incipiently welded, often light pinkish grey, with poor to moderate crystal contents. Fossil fumeroles are common in the upper middle of the flow, devitrification and vapour phase alteration is extensive. The upper zone of the Waimakariri Ignimbrite is usually light grey and incipiently welded. Towards the lower middle of the flow this grades down into a welded lenticulite, occasionally eutaxitic. The base is usually an incipiently welded pyroclastic breccia enriched in lithics. This ignimbrite has a poor to moderate crystal content, is composed of at least three flow units and often has an underlying Plinian airfall. Gas escape structures appear in distal outcrops.
The unwelded top of the Waiteariki Ignimbrite is often eroded. Usually only the middle of the flow, a densely welded lenticulitic with eutaxitic texture is present. The base of the flow is a moderately devitrified, incipiently welded, pyroclastic breccia to fine tuff. Evidence suggests the Waiteariki Ignimbrite comprises 2 cooling units both of which are crystal rich.
With increasing distance from source plagioclase crystal percentages, pumice size and pumice percentages decrease in the Mamaku Ignimbrite (sheet 2). For the Waimakariri Ignimbrite, there is a decrease in crystal lengths with distance from source (assumed to be the Taupo Volcanic Zone). There is an increase in crystal percentages from a medial to distal position, and a moderate increase in pumice percentages. This is explained by elutruation of vitric material from the Waimakariri as a co-ignimbrite air fall ash concentrating pumice and crystals.
No relationship exists between crystal length and vertical position in the Mamaku ignimbrite. Pumice percentages often increase towards the top of the flow, while pumice numbers increase towards the middle of the flow. Welding is at a minimum at the base, at a maximum in the lower middle of the outcrop, reducing towards the top of the flow. Phenocryst increase towards the base of sheet 2 is attributed to welding/compaction. No relation exists between crystal length and vertical position in the Waimakariri Ignimbrite; pumice percentages and size increase towards the base of the flow, while pumice numbers often increase towards the base and top of the flow.
Horizontal sections over distances of 194 and 350 m for the Mamaku and Waimakariri Ignimbrites respectively, demonstrated that they are relatively homogeneous over short distances.
Both the Mamaku and Waimakariri Ignimbrites traveled as moderately fluidized laminar flows, but the surge deposits separating Mamaku flow units were turbulent as were parts of distal Waimakariri. The surges (probably foward jetting from the front of the flow) imply the Waimakariri flowed faster than the Mamaku Ignimbrite. A new flow unit is presented, perhaps more applicable to distal large ignimbrite eruptions, than the standard ignimbrite flow unit. Most field evidence indicated the Mamaku and Waimakariri Ignimbrites were emplaced en masse. The diversity of facies in both Mamaku and Waimakariri Ignimbrites implies a single large pyroclastic flow can operate under several flow regimes, dependent to some extent on local paleo-surface conditions
Morphology and geochemistry of glauconite from the Te Kuiti Group, South Auckland region, New Zealand
No full textThe late Eocene - Oligocene Te Kuiti Group comprises calcareous mudstone, calcareous sandstone and skeletal or sandy limestone formed in marginal marine to fully marine shelf settings. Glauconite is ubiquitous in all sedimentary units in low concentration (5-10%), but at unconformities or near formation boundaries can attain concentrations > 75%.
Binocular microscope studies show· the following morphological form abundances: ovoidal(40-55%) > fragmentary(20-40%) > lobate(5-15%) > casts (0-5%) with trace quantities of tabular, capsule, and vermicular grains. The abundance of ovoidal and fragmentary glauconite suggests the glauconite is dominantly allogenic. Only the highly fragile cast and lobate grains are considered reliable indicators of authigenic glauconite.
Because distinction of morphological types using the binocular microscope is subjective, an alternative classification based on measured variables using image analysis was attempted. Seven measurements (area, formfactor, convexicity, length, fractal, aspect, and fibre) were made for each grain image. Seven glauconite morphological types were investigated: capsule, cauliflower, fragmentary, lobate, ovoidal, tabular, and vermicular. Measurement data were analysed using canonical variate analysis. The calculated discriminating vectors were then used to distinguish the seven morpological forms. Using two canonical variates 81.5% of the variability within the database was accounted for.
X-ray diffraction studies shows a predominance of poorly crystalline glauconite. Many difficulties were encountered when applying previous literature techniques for calculating the expandable content of glauconite. An alternative technique based on the relationship between %K₂0 and % expandables is favoured for the calculation of % expandables. A relationship between host sediment lithology and glauconite expandable content was found; siltstone = 20-32%. expandables in the glauconite, sandstone = 8-15% expandables in the glauconite.
Major element data were obtained with the electron microprobe for 327 individual glauconite grain centres of 70 samples from 45 sites and 7 formations in the Te Kuiti. Group. The. chemical data were analysed using canonical variate analysis and the seven formations were clearly distinguished. Because only a small glauconite sample size is required (4-5 grains) the technique has potential for determining the stratigraphic correlation of drill core samples and small hand specimens of Te Kuiti Group material containing glauconite.
The range composition of Te Kuiti Group glauconite calculated from EMP major element data using the "Clayform" program is:
(Si ₃.₄₀₋₃.₆₈ Al ₀.₃₂₋₀.₆₀) (Al ₀.₀₈₋₀.₄₁ Fe ₁.₀₀³⁺₋₁.₃₆ Fe ₀.₁₉²⁺₋₀.₂₆ Mg ₀.₄₀₋₀.₆₀) (Ca ₀.₀₂₋₀.₁₆ Na ₀.₀₀₋₀.₀₄ K₀.₄₉₋₀.₆₇) O₁₀(OH₂)
The Te Kuiti Group passes stratigraphically upward from terrigenous and mixed terrigenous-skeletal carbonate deposits to very pure limestones. Early post-depositional diagenetic processes probably released Mg from the carbonate lattice of accumulating skeletal grains. Some of the released Mg was incorporated into the glauconite lattice as evidenced by the increase in Mg content of glauconite stratigraphically upward, parallel with the increased carbonate content of the rocks.
Major and trace element data were obtained using X-ray fluorescence for 29 bulk glauconite concentrates from 21 sites and 5 formations. Notably there were anomalous Mg, Pb and Ni concentrations in glauconite from the Otorohanga - Waitomo -Te Kuiti area. This suggests the area existed as a small semi-enclosed embayment, with a significantly higher terrigenous input compared with adjacent depocentres.
A strong correlation between the La/Ce ratio of glauconite and paleontological and oxygen isotope-derived sea-water temperatures suggests glauconite La/Ce ratio may be a useful paleotemperature indicator. The data collected in this study show no single previously suggested process adequately decribes the development of glauconite. Rather, glauconite should be considered polygenetic
The use of physical and chemical techniques in the identification of Tephera (volcanic ash) in the North Island, New Zealand
No full textField evidence is no longer adequate to resolve the problems of tephrostratigraphy, particularly in areas far from the likely volcanic source. In such areas the deposits are thin, and consequently more strongly weathered, as they are more or less continually involved in soil development processes. Under such conditions lithologic resemblance of a given ash to the same material closer to source may be lacking. The identification and correlation of tephra in the Waikato Basin of the North Island of New Zealand is one such example. These tephra form the parent materials of the agricultural soils of the region, and are at considerable distances from the likely sources, these being the Egmont volcanic centre in Taranaki, the Okataina volcanic centre in the Bay of Plenty, and the Tongariro and Taupo volcanic centres in the Central North Island. In particular, this thesis sought to resolve the problem of the identity of the Tirau and Mairoa Ash beds; whether they merely represent the products of pedogenesis of the same parent materials under differing weathering conditions.
In the resolution of this problem, many laboratory techniques were exhaustively and systematically investigated, the methods being reviewed and extended in their applicability and their sensitivity being improved where possible. Physical properties of the rhyolitic glass associated with the samples that were found to provide useful data were refractive index, density, and magnetic properties.
(a) Refractive index: This classical method of determination of a parameter indicative of bulk composition was improved an order of magnitude over that typical for petrologic investigation by the development of a thermal variation method. The increased sensitivity so obtained enabled the use of this parameter in the correlation of glass shards. The use of refractive index for this purpose had previously been abandoned because of its lack of sensitivity.
(b) Specific gravity: The determination of density was investigated by gradient techniques using solutions of acetone in bromoform. The natural variation in density exceeded the precision of the technique, but in spite of this the method was shown to have some application in the solution of tephrostratigraphic problems.
(c) Magnetic properties: Simple methods based on the Guoy magneto-balance were found to provide useful information on the magnetic susceptibility of glass, and were found to be particularly useful in the attempted correlation of the Aokautere and Teviotdale Ashes. More generally, the magnetic susceptibility was found to be a ready method of assessing the ferrous contents of glasses.
In addition to these physical methods the glasses were analysed chemically. As well as these techniques, use was made of mineral assemblages, particularly of felsic minerals; and of the relative proportions of rhyolitic and andesitic glass. The former of these two glasses is easily extracted physically, but the existence and quantitative estimation of andesitic glass, being difficult to determine directly, was determined indirectly by infra-red spectroscopic techniques. The relative proportions of these two glasses was itself found to be a parameter of some importance in correlation and has been used in conjunction with a first-order kinetics model for the weathering of glass to allophane to provide approximate dates for the samples.
While the object of this thesis was to review and develop physical, chemical, and mineralogical techniques of use in tephrostratigraphy, the techniques were used to solve some specific tephrostratigraphic problems, in particular, problems involving thin beds in the Waikato region, and some problems in long-range tephra correlation. These specific problems investigated were:
(a) The Tirau-Mairoa Problem: There has been a long-standing debate over whether the ash beds that make up the Tirau Ash and Mairoa Ash sequences are the same or whether the parent materials differ. From the information derived by the use of the techniques described above, the two ash sequences were found to differ: the Mairoa Ash sequence being both older and more andesitic in character than the Tirau Ash sequence. The Mairoa Ash sequence was considered, as a result of this work to underlie and possibly interdigitate with the Tirau Ash. Samples from sites between the type localities of the two sequences shoved a composite character. Chemical analysis of the rhyolitic glass and refractive index data suggested, that at least in some localities, the Mairoa Ash was contaminated with material from the underlying Oruanui Formation.
(b) Oruanui-Aokautere Correlation: The Oruanui Formation is a widely distributed bed or sequence of beds in the central North Island and had been previously tentatively correlated with the Aokautere Ash in the southern part of the North Island. Application of the techniques developed in this thesis confirm this correlation specifically between the Oruanui Ash and the Aokautere Ash.
(c) Oruanui-Teviotdale Correlation: The Teviotdale Ash of North Canterbury is believed by some to be correlative with the Oruanui and Aokautere Ashes. The magnetic and chemical properties, however, suggested that the Teviotdale Ash represents a separate eruption.
The soils developed on Kaharoa and Taupo Ashes show elemental deficiencies when exploited for agriculture. Soils developed on similar rhyolitic tephra of greater age show no such deficiency. Accordingly, the relative merits of such deficiencies being caused either by pedogenetic factors or by initially low amounts of the microelements in the parent materials was investigated. In the case of selenium, most of this element potentially available is to be found in the glass. Analysis of the glasses from the various ashes showed little variation in selenium content, and the results obtained bore no relationship to the agricultural selenium status of the derived soils. From this it was inferred that the selenium deficiencies are pedogenetic in origin, being caused principally by the short length of time available for the leaching of selenium from the glass. In that cobalt tends to be enriched in the more mafic minerals rather than in the glass, deficiencies of this element in the soils derived from the Kaharoa and Taupo Ashes may be rather more inherent, since the parent tephra concerned contain but small amounts of these minerals
Origin and occurrence of Antarctic lacustrine carbonates, with special reference to Lake Fryxell, Taylor Valley
No full textLake Fryxell is a permanently ice-covered 19 m deep lake located 15m a.s.l. in the lower Taylor Valley, Antarctica (75° 35'S, 163° 35'E). The lake occupies the lowest part of the Fryxell drainage basin and is surrounded by glacial tills with associated lacustrine carbonate and algal material of about Ross Sea I age. Lake Fryxell is a mesotherrnal, stratified, amictic lake with a euphotic (aerobic) zone above 10 m depth and an anaerobic (anoxic) zone below. The lake waters were derived from glacial meltwater into which upward diffusion of salts deposited in the basin or remnant brines occurred creating a gradient in density, which results in a physically stable lake. The so formed diffusion cell gave an estimated age of about 1000 years. Analysis of both lake water and lake bottom sediment cores show precipitation of CaCO₃ to have occurred both in the past and the present. Current carbonate precipitation is a biologically induced process occurring in the euphotic zone lake waters. over deep waters, precipitated carbonates, in this case calcite, fall from suspension through the water column and form flakes on the lake bed. Where the lake bed is within the euphotic zone stromatolites occur.
Dissolution processes predominate in Lake Fryxell bottom waters and account for the discontinuous carbonate record in the uppermost lithologic unit (unit E) of Lake Fryxell sediments. At the time of the Ross Sea I advance a calcareous mud (unit B) was deposited on top of fluvialglacial sediments. The mud, a mixed calcite-aragonite deposit of up to 20 wt% CaCO₃ is C ¹⁴ dated at about 20,000 yrs B.P., was deposited in a deep pro-glacial Lake Washburn. At this time the Ross Sea Ice occupied part of the Fryxell basin damming the lower Taylor Valley and supplying the lake with meltwater. In ~1is ancient lake precipitation occurred in a similar manner to present Lake Fryxell except that aragonite was precipitated. Post-depositional change in the form of the aragonite/calcite transformation caused the observed dual mineralogy. Retreat of the ice sheet caused the deposition of transitional unit C and an evaporite deposit (unit D). Unit D is a varve-like aragonite ~10,000 years old, deposited in a lake of much reduced volume. The iv ice retreat from the drainage basin meant that the lake had an increased ablation surface enabling evaporation to exceed lake inputs resulting in brine concentration and subsequent precipitation of aragonite. Comparison of the Lake Fryxell carbonates with those of the Marshall Valley indicates that the Lake Fryxell depositional model, particularly evaporitic unit D, can adequately explain other Dry Valley carbonate deposits
Weathering of basalt: geotechnical & geochemical aspects
No full textGeochemical influences on the geotechnical parameters of weathered Karamu Basalt are determined and the likely processes involved in basalt weathering are investigated. Geochemical and geotechnical parameters of basalt at different stages of weathering are evaluated; possible relationships between these two sets of parameters are determined and quantified; models are formulated based on statistical analysis; and an hypothesis is postulated to explain the weathering processes.
At an abandoned basalt quarry, materials belonging to different weathering grades are observed and described. In situ geotechnical tests such as Schmidt rebound hardness, penetration resistance and vane shear and laboratory tests such as uniaxial compressive strength, point load, direct shear, Shore scleroscope hardness, California Bearing Ratio, water content, density, particle size distribution, Atterberg limits, permeability, X-ray fluorescence and diffraction, abrasion pH, electrical conductivity, petrography, and scanning electron microscopy are carried out. Using statistical methods, comparisons between geochemical and geotechnical parameters are determined, and their relationships shown in graphical form. Model equations, which depict these relationships quantitatively, are evaluated.
Geochemical results show the following major element concentrations in fresh Karamu Basalt: SiO₂ 42.39 ± 1.66%; Al₂O₃ 12.04 ± 0.23%; MgO 12.53 ± 0.59%; CaO 10.82 ± 0.31%; FeO 10.34 ± 0.35%; Fe₂O₃ 3.09 ± 0.10%; TiO₂ 2.41 ± 0.11%; Na₂O 1.30 ± 0.35%; K₂O 0.70 ± 0.26%; MnO 0.18 ± 0.00%; and loss on ignition 3.68 ± 0.99 %. In this study, the loss on ignition was assumed to be the structural water (H₂O+) concentration. As weathering proceeds, there is a reduction in CaO, MgO and FeO and an increase of Al₂O₃, Fe₂O₃ and H₂O+. When the chemical concentrations are recalculated assuming a constant Al concentration, these trends remain the same for CaO, MgO, FeO, Fe₂O₃ and H₂O+. The following trace elements are also identified: V, Cr, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Nd, Pb and Th. At the slight stage of weathering, an increase in concentration is shown by: V, Cr, Ni, Zn, Ga, Y, Zr, Nb, Ba, La, Ce, Nd, Pb and Th. Some trace elements follow the abundance patterns of the major elements containing similar charges and compatible ionic radii (Ba follows Ca while Ga follows Al). Principal component analysis indicates that CaO and MgO values serve as discriminators of fresh basalt.
Petrographic studies show that fresh basalt is holocrystalline to hypocrystalline, fine grained, porphyritic with phenocrysts of olivine, titanaugite, plagioclase and rare titanomagnetite set in a groundmass composed of plagioclase laths, titanaugite, olivine, glass and titanomagnetite; approximately olivine 33%, titanaugite 28%, plagioclase 34%, glass 2% and titanomagnetite 2%. Apatite and chromite are accessory minerals. As weathering proceeds the mineral assemblage changes and finally the completely weathered basalt consists of only secondary minerals (clays, hematite and goethite).
The weathering profile was divided into five grades according to the New Zealand Geomechanics Society Standards: fresh, slightly weathered, moderately weathered, highly weathered and completely weathered. Only some geotechnical results could be obtained for all grades of weathering due to the varying nature of the testing materials. They are California Bearing Ratio (CBR), density, porosity and water content. Fresh rock is strong (uniaxial compressive strength ∼ 262 MPa) with near-vertical columnar jointing. Along joint planes, discolouration occurs at slightly weathered stage. From fresh to slightly weathered stage, there is a dramatic drop in strength parameters (average point load strength index 5.59 for fresh rock, 0.41 for slightly weathered basalt; CBR drops from 100 % to 45 %), and horizontal joints develop. From fresh to completely weathered CBR drops from 100 % to 1 %; density drops from 2902 kg m⁻³ to1502 kg m⁻³; and porosity increases from 0.18% to 27.19 %. From slight to completely weathered stage cohesion and water content increase (cohesion 0 to 7 kPa; water content 12 % to 59 %). From the moderately weathered stage, corestones are reduced in size and a clayey matrix develops. The material shows plastic behaviour, but Atterberg limits show no consistent trends with weathering grade.
Based on chemical analyses, an easily calculable “new weathering index” (NWI):
NWI = (MgO+CaO+FeO)/(Al₂O₃+Fe₂O₃+H₂O+)
which may be used to define the degree of weathering of Karamu Basalt, is proposed. It correlates well with compressive strength, water content, density, porosity, and California Bearing Ratio but does not correlate well with shear strength, Atterberg limits, cohesion or angle of internal friction.
Abrasion pH value is indicative of the degree of weathering. It is higher in fresh rock (8.5) and gradually decreases to 4 due to weathering. Abrasion pH has good positive correlations with compressive strength, density, and California Bearing Ratio and negative correlations with water content, cohesion, and porosity. By obtaining the abrasion pH of Karamu Basalt material, its degree of weathering can be determined.
Several chemical predictors of geotechnical parameters are formulated. Predictor “x”, given by:
x = 225 pH + 283 TiO₂ + 7.4 Sr - 80 Fe₂O₃
provides the best prediction of geotechnical properties of Karamu Basalt. Its suitability for other lithologies could not be evaluated due to unavailability of data regarding trace element content and abrasion pH value. Modified versions with fewer components were tested against other lithologies, with some success for specific parameters. Obtaining trace element analysis and abrasion pH values is thus crucial for predicting geotechnical parameters from geochemical data.
A causal relationship between the loss of cations Mg and Ca and initial loss of strength is hypothesised. The early stage of weathering is diffusion controlled (with some hydrolysis) whereby cations are lost from the constituent primary minerals and are replaced by H⁺ and possibly Al³⁺. This process affects the lattice structure due to radius to charge ratio imbalances, thus causing a marked loss of strength, leading to microfractures and later macrofracturing following stress release. Later weathering is controlled by hydrolysis, dissolution, Redox and leaching, leading to the development of clay minerals.
Based on the results of this study, it is suggested that the basalt weathering profile be divided into only 3 categories as follows: fresh rock, weathered rock and saprolitic soil. Fresh rock should remain as it is; slightly weathered, together with moderately weathered material should be called weathered rock; highly and completely weathered material should be included in the saprolitic soil. This is a simpler classification, and better reflects the geotechnical behaviour of the materials. Further investigation may indicate the suitability of this simpler classification for other lithologies
