ABSTRACT: Sedimentology and paleomagnetism of Plio-Pleistocene Terraces, Yukon Territory.

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Abstract

The lower Klondike valley and its gold-bearing tributaries, Bonanza and Hunker Creeks, west-central Yukon, contain some of the best preserved and exposed late Pliocene to early Pleistocene deposits in the Canadian Cordillera. In this study, sedimentologic and paleomagnetic evidence is used to present a summary of depositional environments and processes of terrace formation correlated to the geomagnetic polarity timescale. Four alloformation, bounded by discontinuities are defined on the basis of their sedimentologic and paleomagnetic properties: Lower White Channel; Upper White Channel, Klondike Wash and Midnight Dome alloformations. The Lower White Channel Alloformation overlies the topography of an older erosional surface, the White Channel strath. This strath surface represents an extended period of dynamic equilibrium, favouring extensive valley widening, beginning in the early Pliocene, and perhaps late Miocene time. A change in equilibrium conditions by at least middle Pliocene (Gilbert chron - 3.85 Ma) favoured aggradation and the preservation of the Lower White Channel alloformation. Basal facies of the Lower White Channel show a greater frequency of direct hillslope facies, suggesting higher sediment load inputs initially and greater re-working of gravelly sediment in later phases. A later aggradational phase of the White Channel, post-extensive hydrothermal alteration of the gravel at some sites, is named the Upper White Channel alloformation. This aggradation event occurred in response to late Pliocene cooling and is likely derived from the disappearance of boreal forest and changes in hillslope regime brought about by active freeze-thaw cycles. Facies at the base of the Upper White Channel tend to be coarser and less organized, gravelly diamicts and massive gravel are common, and cross-bedded gravel and sandy facies increase in frequency upward. In the lower valleys of Bonanza and Hunker Creeks, the Upper White Channel alloformation is overlain by, and interfingers with, the Klondike Wash alloformation. This unit marks the first direct evidence of glaciation in the area and was deposited as proglacial outwash by the Cordilleran Ice Sheet. Polarity of both the Upper White Channel and Klondike Wash alloformations is normal and assigned to the Gauss chron (>2.58 Ma). Incised into the upper terrace level, the Midnight Dome alloformaiton is characterized by 3 depositional successions. First, a lowermost interglacial wandering gravel bed river association responsible for the extensive strath formation; second, a proximal glaciofluvial association which aggraded the valley by 2-3 metres with coarse massive cobbly gravel; and third, following incision of the terrace, an aeolian-colluvial mantle on the Midnight Dome alloformation gravel preserves an extensive period of glacially derived loess and hillslope colluvial deposition during cold periods and preserves evidence of at least 3 interglacial events, marked by dominance of boreal forest pollen. Deposition of the loess and colluvial sediment occurred between the late Matuyama (~1.4 Ma) and the early Brunhes (~700 ka BP) Chrons. Incision of the Midnight Dome terrace level to the present valley bottom had occurred by at least 39.2 ka BP, indicating late Wisconsinan glaciation had little effect in the Klondike valley.

Bibliography of Canadian Geomorphology