NEOGENE AND QUATERNARY EVOLUTION OF THE FRASER RIVER BASIN, BRITISH COLUMBIA: EVIDENCE FROM THE CHILCOTIN GROUP BASALTS

The Fraser River Basin has experienced dramatic hydrologic and geomorphic changes in the past 3 Myrs; not least it's reversal to flow southward, and eventually into the Pacific Ocean. There is abundant geomorphic evidence in the form of young canyons truncating broad, mature valleys that are now abandoned. The Miocene to Pleistocene Chilcotin Group basalts were erupted into the Fraser Basin. Our mapping and dating of these lavas, in conjunction with published works, is revealing the temporal evolution of the Basin's paleo-geomorphology. Specifically, our data establish stream-flow directions and base-levels, and the presence of and early Pleistocene ice sheets.

Here we present new field observations and Ar/Ar ages from Pliocene and Pleistocene lavas that indicate reversal of the Fraser River after 1.17 Ma. This interpretation is based on identification and dating of a large (~3 km³), lava and pillow-breccia-delta dam indicating northward flow at 3.1 Ma. The dam was approximately 80 m high and several kilometers long where it formed at the confluence of Dog Creek and the Fraser River. The lavas flowed into the Fraser River Valley from a proximal vent, forming a dam across the Fraser and filling the Dog Creek drainage, before forming a southward-propagating lava-fed delta into a developing lake. The delta grew upstream beyond the mouth of Sword Creek (~15 km), to a depth of ~75 m. The 3.1 Ma volcanic sequence was buried by a pro-glacial and glacial succession (preserved in Dog Creek), and a 1.17 Ma lava. The presence of this thin but extensive lava on either side of the present-day Fraser River Canyon provides a maximum age for it. A conservative minimum of age is established by it being part-filled by Fraserian sediments, a base-level drop of 700 m.

We propose that canyon formation and base-level drop were intrinsically linked to the reversal of the Fraser River, most likely resulting from enhanced erosion in its southern Coast Mountain headwaters due to tectonic uplift and repeated development of Pleistocene ice caps, leading to the capture of rivers that flowed south into the Present-day Lower Fraser Valley. This history explains the abandonment of mature tributaries on the adjacent plateaus and the formation of similar canyons along the Thompson and Clearwater Rivers, fundamentally changing the drainage network and producing much of the dramatic relief observed today.