REGOLITH-STRIPPING AT THE PLIO-PLEISTOCENE TRANSITION: EVOLUTION OF THE KLONDIKE PLACER

We have performed an experiment removed from complications of direct glacierization, high relief, and rapid rock uplift, and with temporal and paleoclimate control to test the hypothesis that hillslope erosion rates increased at the onset of late Pliocene-Pleistocene glaciation. The Klondike Placer District, Yukon encompasses the auriferous variably altered and quartz-veined Paleozoic chlorite-muscovite-feldspar Klondike Schist. Erosion of regolith in this schist through the late Miocene and Pliocene produced extensive placer deposits associated with the White Channel Gravel, which is exposed within modern drainages. The Gravel is composed of light grey to white leached and unleached braided stream sediments with a high abundance of angular milky-white quartz cobbles and pebbles, sourced from quartz veins within the Klondike Schist. Toward the upper part of the White Channel gravel, syn-depositional ice-wedge casts record cooling and appearance of permafrost during deposition of the Upper White Channel (UWC) Gravel. The uppermost UWC Gravel interbeds with the earliest Cordilleran outwash (Klondike Gravel) in the lowermost valley reaches. The ²⁶Al/¹⁰Be burial age of the UWC Gravel (2.64 +/- 0.20 Ma, 1sigma) is consistent with late Gauss Chron normal polarity and associated glass fission-track ages on volcanic glass. Concentrations of ¹⁰Be and ²⁶Al (150-850 um sand fraction) are corrected for decay, and post-deposition erosion and production, to calculate the depositional concentrations and the up-section variation in paleo-erosion rate. The ²⁶Al/¹⁰Be are relatively constant (2.0 to 2.6 n=8) for all except the deepest sample at the base of the Lower White Channel (LWC) Gravel (ratio=1.4). This suggests that the top of the LWC and the UWC gravel were buried over a relatively short period without significant variations in sediment storage. The mean paleo-erosion rate during the deposition of the UWC gravel is 3.7 +/- 0.1 cm/ka. If the depositional age of the LWC gravel is early Pliocene (based on available data), the catchment paleo-erosion rate during deposition of the top of the LWC was relatively slower, indicating an increase in erosion to strip the mid-Pliocene regolith and deposit the gold-bearing quartz-rich UWC Gravel during the Plio-Pleistocene transition.