Delaciation of the Prince Rupert – Kitimat area, British Columbia

Abstract

Late Wisconsinan sediments in the Prince Rupert – Kitimat area of west-central British Columbia were deposited in an isostatically depressed marine environment affected by grounded tidewater glaciers. The most common sediments are (1) massive, bedded, and laminated muds, and (2) stratified sand and gravel. The former are accumulations of clay- and silt-size detritus transported in suspension from the mouths of meltwater streams and possibly in turbidity flows generated by submarine slope failures. The muds locally contain scattered ice-rafted stones, fossil marine molluscs, and foraminifera. The main sand and gravel deposits are elevated deltas, including both small incised fan-shaped forms at the margins of the main valleys and enormous ice-contact delta–sandur complexes on the floor of Kitsumkalum–Kitimat trough, the largest valley in the study area.Delaciation occurred by downwasting and complex frontal retreat between about 13 000 and 10 000 BP. The open outer coast became ice free first, in response to wholesale destabilization of the western periphery of the Cordilleran ice sheet by eustatically rising seas. Glaciers retreated rapidly by calving and soon were confined to fjords and mountain valleys; thereafter, they receded more slowly. The distribution of glaciomarine mud and raised deltaic deposits in Kitsumkalum–Kitimat trough indicates that retreat there was nonuniform, with intervals of catastrophic retreat separated by periods during which glacier snouts were relatively stable.Delaciation was accompanied and followed by rapid isostatic uplift. In Kitsumkalum–Kitimat trough, shorelines fell from about 200 m elevation at 10 500 BP to present sea level at about 8000 BP; about half of this fall occurred in perhaps as little as 500 years.