Smectite-Mudstones: Origin & Geochemistry

Analysis of insoluble clay fractions reveals a dominance of well-crystallized smectite and illite, with accessory quartz (chert). Crucially, these clay minerals exist as discrete phases, lacking interstratification. This absence of interstratification is significant because it indicates that there is no post-sedimentary transformation from smectite to illite. Detailed phase analysis identifies the smectite as primarily montmorillonitic, although interstratification with other dioctahedral smectite species and variations in layer charge are present. Importantly, no discernible stratigraphic trends are observed in either the overall clay fraction mineralogy or the smectite mineralogy. This consistent mineralogical composition across different stratigraphic layers is a key finding of this study.

The sand fractions of the mudstones show a different composition than the clay fraction. These are overwhelmingly dominated by authigenic or non-volcanic detrital minerals. The presence of authigenic minerals, formed in place, contrasts with the detrital minerals, which are transported from other sources. This observation provides further insight into the depositional environment and the origin of the sediments. The specific types of authigenic and detrital minerals present, and their relative proportions, can provide valuable information for understanding the sedimentary processes that led to the formation of these mudstones. The identification of non-volcanic detrital components is further evidence supporting the hypothesis that the smectite mudstones did not form from volcanic processes. Further studies could investigate the specific mineral types in more detail to refine the understanding of the source material.

These marine smectite-mudstones, dating back to the Lower Tertiary (Teurian to Runangan), are widespread throughout New Zealand's East Coast Deformed Belt. Specific formations studied include the Amuri Limestone's Marl lithofacies in Marlborough (characterized by calcareous, siliceous smectite-mudstone alternating with biomicrite), and the Kandahar Formation in Wairarapa (composed of calcareous smectite-mudstone, micritic limestone beds, and mass-flow greensand beds). Calcareous smectite-mudstone also occurs as a minor interbedded lithology in the Mungaroa Limestone. In Marlborough, the Arouri Limestone contains two marl units, originally called the Lower Bentonite and Upper Bentonite. The study notes the use of 'Lower Marl' and 'Upper Marl' as more accurate descriptors of their calcareous mudstone lithology. These marl units have gradational contacts with surrounding rock formations and range in thickness from 50-100 meters. In the Middle Clarence Valley, there is only one mapped marl unit within the Arouri Limestone.

The study presents compelling evidence refuting a volcanic origin for the smectite-mudstones. Combined sedimentological, mineralogical, and geochemical data point towards a different genesis. The smectite-mudstones did not form through in-situ alteration of ash falls, nor are they likely derived from transported or reworked ash. This conclusion directly contradicts the previous assumption of a bentonitic origin, a term often associated with volcanic processes. The absence of volcanic glass in the sand fraction, and the lack of evidence for significant amounts of free silica in the clay fraction (as might be expected in a volcanic origin) strongly supports this non-volcanic hypothesis. The study instead explores alternative explanations for the formation of these mudstones.

The study explicitly challenges the historical classification of these mudstones as 'bentonites'. The term 'bentonite' implicitly suggests a volcanic origin, a notion that the research directly refutes. This misclassification highlights the importance of thorough investigation into the formation mechanisms of sedimentary rocks. The authors emphasize that using the term 'bentonite' for these deposits is misleading because it implies a volcanic genesis that is not supported by their findings. The study's detailed mineralogical and geochemical analysis forms the foundation for rejecting the 'bentonite' label and suggests that a more appropriate classification is needed, reflecting the non-volcanic nature of these smectite-mudstones.

Given the rejection of a volcanic origin, the study explores alternative formation mechanisms for these smectite-mudstones. The possibility of a detrital origin, with the smectite derived from older sediments and/or weathered soils, is considered. The presence of abundant detrital illite in the mudstones supports the notion of sedimentary input from preexisting formations. This detrital model aligns with the observed characteristics of the smectite mudstones; however, two points suggest that further investigation is needed. Firstly, the smectite appears well-crystallized, which is atypical of typical detrital clays. Secondly, there is a lack of typical detrital clay minerals. The study proposes that the smectite could potentially originate from well-developed soils under warm, subtropical to tropical climates, which were likely prevalent during the Lower Tertiary in New Zealand. The study advocates for a more extensive investigation to confirm this hypothesis. The study points out that a significant quantity of smectite needs to be explained, and further work is necessary to identify possible smectite-bearing source rocks.

X-ray fluorescence (XRF) spectrometry was employed to analyze trace elements (Sr, Rb, Zr, Y, Nb, Pb, Th, and Ga) in whole-rock samples of the smectite-mudstones. The aim was to identify any consistent variations, particularly stratigraphic trends, in trace element geochemistry. The geochemical data obtained through XRF analysis was compared to the average composition of mudstones as reported in the literature. The majority of the analyzed smectite-mudstones were calcareous, so the mean values of trace element concentrations in an average shale and an average carbonate rock were used for comparison and normalization. The resulting trace element data contributes significantly to the assessment of the origin of the smectite-mudstones, providing insight into the depositional environment and potential source materials.

The trace element concentrations in the smectite-mudstones were normalized against mean shale-carbonate concentrations for comparison. The resulting compositional field encompasses the mean shale-carbonate composition, with all elements exhibiting variation above and below mean concentration levels. This finding contrasts sharply with studies on bentonites, like that by Pacey (1984) which showed a strongly differentiated trace element pattern for bentonites in the Chalk of England. The contrast between the observed trace element pattern in these New Zealand smectite-mudstones and that of known bentonites strengthens the evidence against a volcanic origin for these mudstones. The study used the average composition data of mudstone from Turekian and Wedepohl (1961) as a baseline for comparison. This comparison helps determine if the mudstones possess any geochemical characteristics consistent with a volcanic origin.

A further comparison was made between the trace element geochemistry of the smectite-mudstones and that of the Grass Seed Volcanics. The Grass Seed Volcanics are characterized by elevated concentrations of Zr, Nb, and Ga, and depleted concentrations of Rb, Pb, and Th relative to the smectite-mudstones. These differences, evident in scatter plots (e.g., Rb vs. Nb and Ga vs. Th), further support the argument for a non-volcanic origin of the smectite-mudstones. This contrast in trace element geochemistry between the mudstones and a known nearby volcanic source highlights the distinct differences in origin and formation processes. The study specifically points out that the Marl in the Clarence Valley, which is laterally equivalent to the Grass Seed Volcanics, does not exhibit a similar trace element geochemistry, providing more support to the non-volcanic origin hypothesis.

The smectite component in the smectite-mudstones is predominantly montmorillonitic, but with notable variations. Interstratification with other dioctahedral smectite species and differing layer charges are observed. The study uses heat treatment and various saturation tests (e.g., Li+-saturation and K+-saturation) to characterize the smectite's properties. Heat treatment at 300°C causes complete collapse of the smectite basal peak, indicating the absence of significant interlayer Mg2+ or Al3+. These analyses are crucial for understanding the type of smectite present and its potential formation pathways. The detailed characterization helps distinguish it from other smectite types and informs the discussion regarding its origin, particularly differentiating it from volcanically-derived smectites.

Quantitative X-ray diffraction (XRD) analysis was performed to determine the proportions of the major components (smectite, illite, and quartz) in the clay fractions. This involved creating a set of standard mixtures of known compositions to calibrate the XRD measurements. The use of API No. 26 Clay Spur Bentonite and No. 35 Illite as standards allowed for a reasonably reliable estimation of the proportions of the major components in the clay materials. The method is described, including the preparation of glycerol-treated mounts and the selection of specific peaks for analysis (3.34Å for quartz). Reproducibility checks using two oriented mounts were performed. The accuracy of this quantitative XRD analysis is essential for understanding the mineralogical composition and to support conclusions about the origin of the mudstones.

A significant finding is the absence of interstratification between smectite and illite. This observation is crucial because interstratified smectite-illite clays are characteristic of K-bentonite (or meta-bentonite) deposits, generally formed by burial diagenesis of initially pure smectite. The absence of such interstratification in these smectite-mudstones suggests they did not undergo this type of diagenetic transformation. This supports the argument against a volcanic origin and instead points toward a primary deposition of the distinct smectite and illite phases. The study highlights that in highly tectonized units, like the Kandahar and Wanstead Formations, the illite remains a discrete phase, further supporting this conclusion. This detailed mineralogical assessment forms a key part of the larger argument for a non-volcanic origin.

In Marlborough, the study focuses on the Amuri Limestone, specifically its Marl lithofacies. These lithofacies consist of alternating calcareous and siliceous smectite-mudstone layers interbedded with biomicrite. The smectite-mudstones within the Arouri Limestone are described as occurring in two units, originally referred to as the Lower Bentonite and the Upper Bentonite (Hall, 1964; Prebble, 1976). However, this study uses the more descriptive terms 'Lower Marl' and 'Upper Marl', reflecting their lithology. These units are approximately 50-100 meters thick and display conformable, gradational contacts with adjacent formations like the Lower Limestone, Middle Limestone, and Fells Greensand units of the Arouri Limestone. The study notes the detailed stratigraphic relationships within these units.

In Wairarapa, the investigation includes the Kandahar Formation, characterized by calcareous smectite-mudstone, micritic limestone beds, and mass-flow greensand beds. Calcareous smectite-mudstone is also present as a minor interbedded lithology in the Mungaroa Limestone. The Kandahar Formation is described as extremely complexly deformed, presenting challenges in establishing precise stratigraphic control. The age range of the Kandahar Formation is established as Heretaungan to Bortonian (Waterhouse and Bradley, 1957), confirmed by microfossil dating in this study. The study notes a significant shear zone within the formation and the obscure contact with the underlying Pukermuri Siltstone, suggesting possible sedimentary origins. The Mungaroa Limestone is described, noting its rapid thinning and lateral transition into different lithologies northwest of Mungaroa Point.

In Hawkes Bay, the smectite-mudstones are included within the Wanstead Formation, described as “soft beds of clay-like character” overlying the Whangai beds (Lillie, 1953). The Wanstead Formation shows an age range from Teurian to Bortonian. Later workers recognized a wider variety of Lower Tertiary lithologies in Wairarapa, leading to the adoption of a Wanstead Group status (Waterhouse and Bradley, 1957). The study notes that the Lower Tertiary column in Hawkes Bay resembles that of southwest Marlborough, consisting of a smectite-mudstone unit that is estimated to be several hundred meters thick (Pettinga, 1980). The smectite-mudstones in southern Hawkes Bay are found in three structural highs comprised of Cretaceous and Paleogene rocks and several specific sample localities (Waimarama Beach, Waewae Stream, coastal slope near Red Island, and the bentonite quarry near Porangahau) were studied. The study highlights the correlation between these different regional formations and their relative ages and thicknesses.