NEW INSIGHTS INTO NEOGENE LANDSCAPE AND TOPOGRAPHY WITHIN THE INTERMONTANE BELT OF THE CANADIAN CORDILLERA: MAPPING THE BASE OF THE CHILCOTIN GROUP BASALTS

The Cordilleran Intermontane Belt of British Columbia is characterized by extensive plateaux underlain by apparently undeformed basalt lavas of the Neogene (24 – 1.5 Ma) Chilcotin Group (CG) and overlying glacial till. CG represents a medium-sized igneous province (ca. 36,500 km2 in area) whose distribution constrains estimates of Coast Mountain Belt uplift (Mathews, 1989), and the history of evolution of the Fraser Basin (Tribe, 2003), and Early Pleistocene glaciations (Mathews and Rouse, 1986). The CG was emplaced across highly-variable relief of Eocene volcanic and Mesozoic basement rocks. Poor exposure of the CG hinders mapping of its distribution, thickness, stratigraphy, and possible sources. Mapping of lava emplacement directions is an important proxy for local and regional landscape architecture and provides critical evidence for reconstructing the Neogene tectonic history of the Intermontane Belt.

Our re-examination of the CG, with emphasis on understanding the volcanic facies architecture, is revealing several new and important aspects of the pre-CG topography: 1) the CG is typically very thin (< 25 m) across much of mapped distribution; 2) the CG is only thick where is has infilled paleo-drainages, producing volcanic lithofacies characteristic of lava-water interaction; 3) mapped paleo-drainages coincide with present-day rivers and valleys, suggesting that the regional drainage pattern was unchanged for several m.y.. Further research will investigate and better constrain the interplay between regional tectonic tilting, the ancestral Fraser River basin, and sustained (and, episodic) volcanism. Geoscience BC publication number GBC046.

Mathews, W.H. (1989): Neogene Chilcotin basalts in south-central BC. Canadian Journal of Earth Sciences, 26, pages 969-982.

Mathews, W.H., Rouse, G.E. (1986): An Early Pleistocene pro-glacial succession in south-central BC. Canadian Journal of Earth Sciences, 23, pages 1796-1803. Tribe, S. (2003): Physiography and Tertiary base levels in the southern Interior Plateau and adjacent areas, southwestern British Columbia. Geological Survey of Canada, Current Research, 2003-A2, 11p.