ABSTRACT: Early Diagenesis in the Fraser River Delta, British Columbia: Implications for the Origin of Early Calcite Cement in Ancient Deltas

SIMPSON, G., and I. HUTCHEON, University of Calgary, Calgary, Alberta, Canada, M. ROBERTS, Simon Fraser University, Burnaby, British Columbia, Canada, and J. LUTERNAUER, Geological Survey of Canada, Vancouver, British Columbia, Canada

Many ancient deltas have early diagenetic calcite cements; the source of this cement is ambiguous. One possible source may be the mixing of sea water (high in Ca) with fresh water (high in HCO3). To investigate this and other early diagenetic reactions taking place in a siliciclastic, mixed water environment, pore water and sediment samples have been obtained from drill core from the Fraser River Delta, a modern, temperate-climate, river dominated delta in British Columbia.

The pore waters have been chemically and isotopically analyzed and the mineralogical composition of the sediments has been established by petrographic, SEM, and XRD methods. The sand-sized sediments are all unconsolidated litharenites; the clay fractions are predominantly smectite, illite, chlorite mixtures. A few high Mg calcite cemented concretions were found in a buried distributary channel, similar to concretions forming in the present-day distributary channels. In order to model the formation of the concretions, the pore water data were processed using an aqueous speciation program (EQ3). The formation of the concretions requires the degradation of organic matter, producing high levels of bicarbonate as well as repeated tidal influxes of Ca-rich sea water into the distributary ch nnel. However, the amount of calcite produced in this way is insufficient to account for the amount found in ancient deltas. The shell fragments in the sediment samples show little if any signs of dissolution. Quantitative shell carbonate content of the sediments suggests that the early calcite cement in ancient deltas is more likely due to the redistribution of shell carbonate than to water mixing processes.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)