ABSTRACT: Quaternary sedimentology and stratigraphy of the Mayo region, Yukon Territory

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Abstract

Sedimentological analysis of Quaternary sediments along the Stewart River near Mayo, central Yukon Territory, is used to determine the environmental history of the region. Thirteen sedimentary facies are defined and interpreted as: lodgement till (Facies 1 ), meltout till (Facies 2), mass-movement diamictons (Facies 3), longitudinal bar deposits (Facies 4, 6 and 8), transverse bar deposits (Facies 5, 7 and 8), planar and ripple laminated sand deposits (Facies 9, 10 and 11 ), organic deposits (Facies 12), and loess (Facies 13). Seven stratigraphic units (Units A to G) are interpreted from these facies and they form five informal chronostratigraphic intervals: interglacial, proglacial, glacial, immediate postglacial and Holocene. Results of this study indicate that during the Mid-Wisconsinan a wandering gravel bed river occupied the Mayo River valley and the Stewart River valley downstream of Mayo. Interglacial river sediments consist of coarse gravels and sands (Unit A) with abundant wood fragments which have yielded several radiocarbon dates (38.1 to 35.4 ka). Environmental reconstructions from pollen and macrofossil data indicate a willow dominated, low arctic tundra around 29.6 ka. Deposits of only one glaciation, the Late Wisconsinan McConnell advance, are recognized. A poorly sorted and highly disrupted sand, gravel and diamicton sequence (Unit B) was deposited proglacially in the Mayo Indian Village Section. As topographically controlled ice advanced from the east along the Stewart valley, the Mayo River was deflected to the northwest. Glacial deposits (Unit C) include mass-movement diamictons overlain by a thin layer of lodgement till, a thicker and more laterally extensive bed of meltout till, and a discontinuous cap of mass-movement diamicton. Postglacially, the region was dominated by an ice-contact lake dammed by stagnating ice in the Stewart valley and a second, higher lake formed behind a lateral moraine across the mouth of the Mayo valley. When the upper lake breached the moraine it deposited a sequence of deltaic sediments (Unit D) in the lower ice-dammed lake. Melting of the ice dam caused instability and debris flows deposited lobes of diamicton (Unit E) as the lake drained. After the lake level dropped gravels of the incipient Mayo River were deposited in channels on top of the delta (Unit F). During the Holocene, loess with multiple paleosols (Unit G) developed on top of the sections as the Stewart and Mayo Rivers incised into the unconsolidated proglacial and deltaic sediments. Prior to the McConnnell glaciation, the Stewart River flowed to the southwest from the Fraser Falls area, rather than following its present course to the west past Mayo. During deglaciation morainal sediment southwest of Fraser Fails deflected the Stewart River into its present channel. This is evidenced by the absence of interglacial gravel in the Stewart River valley between Fraser Falls and Mayo. In addition, pre-McConnell glaciation fluvial deposits in the Mayo area are characterized by clasts of northern provenance and are distinct from the modern Stewart River gravels.

Bibliography of Canadian Geomorphology