Holocene sea-level change and delta growth: Fraser River delta, British Columbia

Abstract

The Fraser River has been building its delta into the Strait of Georgia in southwestern British Columbia for about the last 9000 years. This period encompassed a relative sea-level rise of some 13 m. This study concerns the effects of the rise in sea level on the depositional evolution of the delta.The lithostratigraphy of the delta was established by a series of drill cores. Four major lithostratigraphic units were defined: peat, organic-rich silt, interbedded silts and sands, and massive sands. These units were interpreted in terms of the delta's contemporary depositional environments, including peat bog, floodplain, and intertidal zone.Analysis of the delta's depositional architecture and chronology indicates that the delta continued to prograde during the rise in sea level. A marine transgression, accompanied by the landward migration of marine and intertidal facies, did not occur. Growth of the delta during the rise in sea level was accomplished by both vertical accretion and lateral progradation.Vertical-accretion rates during the rise in sea-level averaged 2.4 mm/year and ranged up to about 5.3 mm/year. Lateral progradation of the delta was most rapid in the early stages of growth (9000 – 8000 years ago), when the average rate was 6.5 m/year, and slowest during the period of most rapid sea-level rise, when the rate declined to less than 1 m/year. The progradation rate of 2.4 m/year calculated for the most recent period (2250–present) compares well with estimates based on bathymetric surveys.A revised sea-level curve for the Fraser Lowland region is proposed on the basis of the identification of former sea-level positions in core at the lithologic transitions from tidal-marsh to intertidal deposits. The curve contains a hitherto unknown stillstand that occurred ca 6000 years ago and shows that the mid-Holocene sea-level rise continued until about 2250 years ago.