ABSTRACT

The Fraser River Delta, with its high organic matter content, mixed water setting and accessibility, provides an ideal site for investigating early diagenetic reactions.

In ancient deltaic rocks calcite is often considered a very early diagenetic cement. Calcite concretions have been reported forming in the present-day tidal channels of the Fraser River Delta, although their mode of formation has not been fully resolved. In order to study the formation of calcite concretions and other diagenetic reactions occurring in the delta, thirteen shallow vibracored holes and five, deeper, sonic and rotary-cored holes have been drilled across the delta. Approximately 90 sediment samples have been obtained from these holes. All of the samples were unlithified, although a number of calcite concretions were found in one of the holes. Analyses of pore waters is a sensitive method of studying diagenetic reactions, therefore the sediment samples were "squeezed" to release their pore waters. The pore waters have been analyzed for pH, alkalinity, cations, anions, oxygen and hydrogen isotopes, carbon isotopes in dissolved carbonate species and sulphur isotopes in dissolved sulphide and sulphate. The sediment samples have been examined by SEM and XRD techniques to characterize the diagenetic minerals. A geochemical thermodynamic computer modelling technique is being used to study mineral reactions within the sediments, as well as to quantify the mode of origin of calcite concretions and its relationship to marine-meteoric water mixing in the shallow subsurface of the Fraser Delta.

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