Abstract

The Nass Valley of northwestern British Columbia is a glacial fiord containing extensive glaciomarine and glaciofluvial sediments. Two parallel braidplains, separated by a bedrock ridge, were deposited within the fiord. Mapping of these deposits led to the hypothesis that the braidplains must have terminated at deltas. However, a lack of surface exposures meant that ground-penetrating radar was needed to investigate these deposits. Radar facies analysis aided in the identification of braidplain, braid delta and glaciomarine depositional environments, as well as underlying bedrock. Several deltas graded to different sea levels were discovered, allowing inferences to be made about the relationship of falling sea level to sediment architecture. The upper section of the western braidplain is graded to a sea level of 185 m above sea level (asl), indicating that the proto-Nass River flowed on the western side of the bedrock ridge when the sea was at that level. However, the river moved to the east side of the ridge as sea level fell, depositing the extensive Aiyansh Braidplain – Braid Delta, which is graded to a 152 m sea-level stand. Several other deltas also formed at this sea-level stand. Avulsion occurred and the river flowed on the west side of the ridge again when sea level fell to 134 m asl. The river remained in this position throughout late glacial time and eventually evolved into the modern Nass River. The coarse-grained deposits are indicative of forced regression, with both stepped-top attached and detached stratal architecture present.

You do not currently have access to this article.