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A mineralogical and chemical study of the lower Fraser River alluvial sediments Mackintosh, Erven E.

Abstract

A mineralogical and chemical study was conducted on the clay fractions of lower Fraser River alluvial sediments. The major objectives of the study were to characterize the mineralogy of these sediments and to evaluate the influence of a marine environment and sedimentary phases of deposition on their mineralogical content. Twenty-one sampling sites representing the four major soil series developed on these sediments and six sea bottom samples were collected. Surface and subsurface samples were taken for the soil series. X-ray diffraction analyses were conducted on the coarse and fine clay fractions of all samples and the total K, Mg and Ca contents of the clay fractions were also investigated. With the exception of samples from the Pitt Meadows area, there was a marked similarity in the clay mineral suite present in these sediments. The major clay mineral components of the coarse clay fraction were montmorillonoid and chlorite. Lesser amounts of micaceous material and several interstratified clay minerals were also present. The interstratified clay minerals identified included a randomly interstratified chlorite-montmorillonoid and chlorite-mica and in a limited number of cases regularly interstratified chlorite-montmorillonoid. The identification of a regularly interstratified chlorite-mica was quite questionable. Positive identification of kaolin was prevented in most instances by a heat unstable chlorite. However, kaolin was identified in a sample from the Pitt Meadows area and there was strong evidence to suggest its presence in other samples. Quartz, feldspars and amphiboles were the only non-phyllosilicates identified. The fine clay fractions were dominated by montmorillonoid and much lesser amounts of chlorite. Micaceous material, inter-stratified clay minerals and quartz were present in only questionable amounts and in some instances appeared to be absent. The chlorite was identified as an iron rich variety possessing thermally unstable higher order reflections. A progressive decrease in the relative intensity of these reflections was observed on heating from 4OO to 450°C. Further heating to 500°C resulted in the disappearance of the peaks. The montmorillonoid component identified appeared to be of two types: An octahedrally substituted member and a tetrahedrally substituted member. The presence of the latter mineral prevented identification of vermiculite. The results support the findings of other workers that marine deposited sediments are highly detrital in nature, dominantly reflect their source area and are influenced by sea water to only a minor extent. Diagenesis of 14+ A° material of the marine sediments was indicated by X-ray diffraction analyses. Chemical analyses were also indicative of the minor influence of a marine environment. Mineralogical variations within and among soil series were largely quantitative in nature. These variations tended to be minimized within a particular soil series. Mineralogical differences between the two clay fractions were observed, however these were to be expected. The variations noted in the mineralogy of the clay fractions of these sediments were attributed to sedimentary processes, seasonal variations in the detrital components carried by rivers, yearly variations in particular source areas and the local influence of sediments carried by several tributaries of the lower Fraser River that flow out of the Coast Mountains. The X-ray and chemical analyses indicated that there was a valid basis for continued mapping of the Pitt soil series separate from the Monroe and Fairfield series. The two sampling sites from the Pitt Meadows were considerably higher in randomly interstratified chlorite-montmorillonoid and lower in micaceous material than those of the other sediments. Chemical analyses were also indicative of these differences. The variability noted in soils from the Pitt Meadows area may be related to the influence of sediments carried by the Alouette River.

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