Late Quaternary sea levels and crustal movements, coastal British Columbia

Abstract

Late Quaternary sea-level fluctuations on the British Columbia coast have been established from studies of terrestrial and marine sediments and landforms. These studies indicate that the sea-level history of mainland British Columbia and eastern Vancouver Island is very different from that of the Queen Charlotte Islands and western Vancouver Island. Specifically, in the former areas, there was a rapid rise of submerged coastal lowlands between about 13 000 and 10 000 years ago. Emergence culminated about 6000–9000 years ago, depending on the locality, when the sea, relative to the land, was 12 m or more lower than at present in some areas. During middle and late Holocene time, relative sea level rose on the mainland coast and at least locally on eastern Vancouver Island, resulting in inundation of coastal archaeological sites and low-lying terrestrial vegetation. Tidal records and precise levelling suggest ongoing submergence of at least part of this region.In contrast, shorelines on the Queen Charlotte Islands were below present from before 13 700 years ago until approximately 9500–10 000 years ago. A transgression at the close of the Pleistocene climaxed about 7500–8500 years ago when relative sea level probably was about 15 m above present in most areas. Most of the emergence that followed apparently occurred in the last 5000–6000 years. There has been a similar pattern of emergence on the west coast of Vancouver Island during late Holocene time.The above patterns of late Quaternary sea-level change are attributed to complex isostatic response to downwasting and retreat of the late Wisconsin Cordilleran Ice Sheet, to transfers of water from melting ice sheets to oceans, and to plate interactions on the British Columbia continental margin. Late Pleistocene and early Holocene crustal movements were dominantly isostatic. Although the recent regression on the outer coast likely is due, at least in part, to tectonic uplift, some late Holocene sea-level change in this area and elsewhere on the British Columbia coast may be either eustatic in nature or a residual isostatic response to deglaciation, which occurred thousands of years earlier.