SIMPSON, GRAHAM, Department of Geology and Geophysics, Univ. of Calgary, Calgary, Alberta

ABSTRACT: Early Diagenesis in the Fraser River Delta

Sediment mineralogy and associated pore waters from the Fraser River Delta, British Columbia, have been studied to investigate early diagenesis of clastic sediments in a coastal environment.

Mixing between river and sea water as the sediments were deposited is an important control on the concentrations of major ions in the pore waters. Deviations from simple mixing indicate that pore waters are being modified by diagenetic reactions. Weathering reactions such as the breakdown of annite and smectite may explain SiO<2>(aq), Fe and K concentrations. Bacterial degradation of organic matter and concomitant reduction of oxidizing species produce successive diagenetic zones within the sediment. Framboidal pyrite and calcite are the only diagenetic minerals observed. Calcite concretions formed near the sediment/water interface in an ancient distributary channel. Tidal influxes of Ca rich sea water combined with high concentrations of HCO<3> produced by organic matter egradation. Ca concentration may also be influenced by shell fragment dissolution and exchange of Na for Ca in smectite. Concretions which form in the seaward edge of the delta will eventually be surrounded by undersaturated fresh water as the delta progrades. Therefore, calcite formed in this manner is unlikely to be preserved in a prograding delta but may be preserved if the delta is transgressed.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.