GSA Connects 2021 in Portland, Oregon

Paper No. 221-5
Presentation Time: 9:00 AM-1:00 PM


FRANZ, Kenya, WARD, Brent and CLAGUE, John J., Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada

In the temperate rainforest of SW BC thick vegetation can obscure glacial features, making geomorphic reconstructions difficult. Recently acquired lidar data in the Fraser Lowland, between Vancouver to Hope, provide insight into deglaciation of the Cordilleran Ice Sheet (CIS). High resolution lidar DEMs were converted to hillshade imagery, allowing us to map subtle landforms such as streamlined till and bedrock, moraines, eskers, meltwater channels, deltas, terrace complexes, landslides and shorelines in Arc Map and to provide a reconstruction of the deglacial history of this part of British Columbia in more detail than was possible in the past.

Sequential retreat of the CIS and interaction with local valley glaciers were inferred based on the orientation and position of glacial landforms, combined with fieldwork. We discovered many previously undocumented features, notably the unexpected dominant southward flow of ice originating in the valleys of the Coast Mountains to the north; previous reconstructions indicated a dominant westward flow throughout the Fraser Lowland. Additionally, we identified and mapped multiple recessional moraine complexes left by both the CIS and local valley glaciers, eskers and ice contact deposits. Evidence of the late-glacial Sumas Stade includes prominent multiridged, composite moraines and numerous smaller moraines. The locations and size of these recessional moraines have implications for the duration and dynamics of the Cordillera ice sheet during its Termination Pleistocene retreat.

Latest Pleistocene paleo-shorelines and marine terraces are present at numerous sites in the western Fraser Lowland. The highest terrace, at 174 m above sea level (asl), is located in lowermost Capilano Valley near Cleveland Dam. The marine limit decreases in elevation to the east, with preserved shorelines at 152 m asl at Whonnock Lake. The shorelines are subtle; they are evident only in lidar, likely due to rapid uplift of the area, with insufficient time to develop more prominent shorelines. Emergence of the western Fraser Lowland, with the passage of the lowland surface through the intertidal zone, may have destroyed some subtle glacial features.