TERTIARY GEOMORPHOLOGY AND CHRONOLOGY OF CANADA'S WESTERN INTERIOR, BASED ON A LATE MIOCENE MAMMALIAN ASSEMBLAGE INCORPORATED INTO HILLTOP GRAVELS OF THE HAND HILLS, SOUTH-CENTRAL ALBERTA

Western Interior's landscape has a peaceful aspect that suggests little has happened for a very long time, but nothing could be further from the truth.

The region experienced nearly 900 metres of deposition in the early Cenozoic, and nearly as much fluvial erosion through the middle and latter Tertiary. The landscape was subsequently only slightly altered by Pleistocene processes. The controls and chronology of Tertiary fluvial processes and events are only broadly understood, but hill-top fluvial gravel deposits on the Hand Hills of south central Alberta have yielded mammalian fossils, including Plesiogulo (NALMA Hemphillian, HH3) that provide good biochronologic bracketing ages for the deposit. The gravel and sand, deposited by the ancestral Bow River, became a remnant upland as the river migrated south and eroded downward nearly 190 m, forming part of the Alberta plain. Hill top gravel found in other locations typically are not fossiliferous, but a few specimens found in the Cypress Hills to the southeast at higher elevations provide more dates, and show that erosion and planation rates likely accelerated through the latter Tertiary. Erosion rates, deduced by using reconstructed surfaces from river profiles, show increases of 2.5 X in the last 4 million years of the Tertiary, compared to the previous 10 million years. Those increases were possibly a positive feedback to isostatic rebound caused by crustal unloading. It seems unlikely that erosion was due to uplift caused by thrust sheet propagation to the west, as ages of overthrust sediment show that thrusting ended much earlier, in the Paleocene, and the tectonic history of British Columbia to the west indicates the middle and latter Tertiary were times of crustal tension.