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TitleSurficial Geology and Late Cenozoic History of the Stewart River and Northern Stevenson Ridge Map Areas, West-Central Yukon Territory
AuthorJackson, L E, Jr; Froese, D G; Huscroft, C A; Nelson, F E; Westgate, J A; Telka, A M; Shimamura, K; Rotheisler, P N
SourceGeological Survey of Canada, Open File 6059, 2009, 414 pages; 1 CD-ROM, (Open Access)
PublisherNatural Resources Canada
Documentopen file
MapsPublication contains 28 maps
Map Info.surficial geology, glacial deposits and landforms, 1:50,000
Map Info.compilation, 1:100,000
MediaCD-ROM; on-line; digital
RelatedThis publication contains the following publications
File formatreadme
File formatpdf; txt; gbd
NTS115I/13; 115I/14; 115J/16; 115N/01; 115N/02; 115N/07; 115N/08; 115N/09; 115N/10; 115N/15; 115N/16; 115O
AreaStewart River; 17
Lat/Long WENS-140.5000 -138.5000 64.0000 62.7500
Subjects14; surficial geology/geomorphology; stratigraphy; paleontology; Nature and Environment; organic deposits; alluvial deposits; colluvial deposits; glacial deposits; gravels; sands; silts; clays; glacial features; moraines; glacial landforms; Holocene; Pleistocene; Pliocene; Miocene; paleoecology; fossil descriptions; fossil plants; Cenozoic; Quaternary
Illustrationsphotographs; location maps; sections; tables
ProgramGEM: Geo-mapping for Energy and Minerals, Geomapping for Energy and Minerals (GEM) - Minerals Component
Released2009 12 10
The study area (NTS 115 O, east ½ of 115 N, and northern halves of 115 J/14, 15, 16) predominantly lies within the Klondike Plateau, a deeply incised upland of at least early Tertiary age. Surficial geology mapping was carried out at a scale of 1: 50 000. This was generalized to a 1: 100 000 scale summary map with accompanying stratigraphic sections that appears in this report. Although this study constitutes the first systematic surficial geologic mapping of the area, investigations of aspects of the surficial geology of the area go back to the Klondike goldrush (1896-1898) that exploited rich unconsolidated placer gravels of late Tertiary and Quaternary age. Unconsolidated sediments and volcanic rocks within the map area span much of the Pliocene Epoch (ca 5.3-1.8 Ma) and all of the Pleistocene and Holocene epochs (ca. 1.8 Ma to present). Drainage systems are incised into the Klondike Plateau. They contain numerous drainage anomalies such as linear trends in major drainage courses and dramatic changes in valley widths. These infer underlying structural or lithologic discontinuities within the underlying quartz-rich metaclastic, amphibolite and orthogneiss that forms the bedrock and glacially induced drainage changes in some cases. Much of the area has never been glaciated or only was glaciated by valley glaciers that were extensions of a regional ice sheet that covered much of Yukon to the east during the late Pliocene. The only minor exceptions were the highest uplands areas where local alpine glaciers developed. These alpine glaciers apparently existed only during the late Pliocene or early Pleistocene. Glacial sediments in all areas have been extensively eroded from slopes and are only preserved in valley and lowland settings. Consequently, colluvium is the most extensive surficial sediment in the study area. It is typically a stony diamicton resulting from the physical and chemical breakdown of bedrock and its subsequent reworking and transportation largely by seasonal creep and solifluction, deformation of interstital ice and land sliding, including such processes as debris flow and rock fall. It continues to form under present conditions although some deposits may date from the early Pleistocene. Alluvial stream and fan deposits and organic sediments are also actively forming at present as are small dunes along the cliff-tops adjoining stream courses. Organic sediments (predominantly peat) accumulate where waterlogged conditions occur during the summer thaw such as along the lower portions of north-facing slopes, along the floors of deep narrow valleys, and permanently saturated sites such as abandoned stream courses in flood plains. Accumulations of peat up to 3 m in thickness are common. Sediments not forming at present include glacial, non-glacial, and eolian sediments other than cliff-top dunes. Areas underlain by glacial sediments are typically low-lying and heavily vegetated. Consequently, they are lumped together under the heading of undifferentiated drift. These sediments are predominantly early Quaternary or late Tertiary age and include glaciofluvial gravel, till, and glaciolacustrine deposits. They are most extensive within the Tintina Trench or along the Stewart River near the eastern margin of the study area. The glaciofluvial sediment is commonly gravel and forms terraces up to tens to more than 100 m above contemporary valley floors. These sediments commonly have the rubified early Quaternary or late Tertiary Wounded Moose paleosol developed on their surface. Particularly well-developed glaciofluvial terraces occur along the Yukon River above Kirkman Creek, Stewart River and Indian River confluences. The terraces along parts of Indian River are dominated by sand and likely represent either a great glacial damming event in the paleo-Yukon River basin or a series of local non-glacial damming events. If the former is the case, these sediments represent the most widespread glaciolacustrine sediments in the study area. In Tintina Trench, some lower terraces are underlain by glaciofluvial gravel deposited by meltwaters from the middle Pleistocene Reid Glaciation or the late Pleistocene McConnell Glaciation (the penultimate and last glaciations respectively in this region). Older fluvial deposits include the late Tertiary White Channel gravel and pediment and bajada gravels that were deposited prior to the onset of regional glaciation. Streams flowed tens of metres above contemporary valley bottoms at that time and much of the drainage in the area flowed into a southward draining stream system that predated the modern Yukon River system. The sand and gravel deposited by the preglacial streams were commonly quartzose due to their apparent source in the erosion of a thick colluvial mantle that formed during the Tertiary. Quartz clasts were derived from quartzose veins and lensoidal segregations or 'sweats' in the schistose bedrock that was otherwise reduced to silt and clay. In contrast, sediments underlying intermediate and lower terraces in the area were deposited by streams integrated into the modern Yukon River system. Their sediments are commonly different in composition from White Channel gravel: they are polymictic because they had their source in terrain that lost its mantle of deeply weathered bedrock and subsequently underwent predominantly physical weathering during the largely periglacial climate of the late Tertiary and Quaternary. These intermediate terrace sediments commonly have the Diversion Creek paleosol developed at their surfaces. This paleosol is less well developed than the Wounded Moose soil but is thicker and better developed than the post glacial Stewart soil which developed upon latest glacial and Holocene sediments. Eolian sediments in the form of loess were predominantly deposited during the last glacial maximum although some date back to the middle Pleistocene. A thin veneer of loess is common in many areas but the thickest accumulations occur on the north-facing margins and floors of narrow valleys. There, loess has been reworked from slopes by mass flow and has formed accumulations of up to 10 m and overlie alluvial terraces and valley floors. The reworked loess is commonly interbedded and capped by organic sediments. It is commonly referred to as muck. Its silty texture and location in deep valleys that receive low levels of direct sunlight and trap bitterly cold winter air makes them host to extensive bodies of segregated ground ice and continuous permafrost. Volcanic rocks were erupted locally in the study area during the late Pliocene: columnar alkaline olivine basalt and flow breccia that occur along Rosebud Creek date to 3.23±0.05 Ma. Vesicular basalt is intersected by a placer mining access trail along the north side of Frisco Creek near its confluence with Yukon River. It is undated but was apparently erupted into the extant valley of Frisco Creek. The oldest and most extensive glaciations of the area occurred during the late Pliocene and early Pleistocene. They are referred to the pre-Reid. During the most extensive of these regional glaciations, a valley glacier extended down the Stewart River valley and terminated between the confluences of Rosebud and Black Hills creeks. This advance also reached the divide between Stewart River and Indian River in the area of Wounded Moose Dome. Meltwaters spilled into the Indian River basin depositing an extensive valley train. The extent of ice in the Stewart River basin makes it certain that this glacial ice also extended into Tintina Trench in the northeastern corner of the study area. It likely deposited part of the drift composing the Flat Creek beds and may have diverted the ancestral Klondike River into its present course (confluent with Bonanza and Hunker creeks and Yukon River near the present site of Dawson immediately north of the map area). Regional evidence also suggests that this glaciation was responsible for reversing the flow of the ancestral Yukon River into its present north and west course below the present confluence with White River. Deposits of the most extensive pre-Reid advance overlie the 3.23 Ma basalt flows along Rosebud Creek. This is in accord with the estimated age of the oldest and most extensive glaciation in Tintina Trench north of the study area. Following this maximum glaciation, streams in the area progressively incised preglacial and glacial fill with intermittent periods of channel aggradation and terrace formation. This aggradation can be attributed to increased sediment loads due to glacial advances that terminated north, east and south of the study area and the deposition of thick loess and eolian sand carried by katabatic winds. These events are known from investigation of the stratigraphy of an intermediate-level terrace exposed in a placer mine along the north side of the Klondike River valley below Midnight Dome, immediately north of the study area. The top of the terrace fill is about 100 m above the contemporary Klondike River flood plain. At the Midnight Dome site, fluvial and glaciofluvial gravel is overlain by up to 14 m of loess, retransported loess and colluvium. Paleomagnetic, paleoecological and paleopedological evidence suggest that the sediments span approximately 1.5 Ma: this encompassed four glaciations and three interglaciations and spanned the early Pleistocene and the earlier portion of the middle Pleistocene. All of these predate the penultimate Reid Glaciation and consequently are grouped within the pre-Reid glaciations. Areas higher than 1000 m a.s.l. in the terrain centered on Cragg Mountain, west of Yukon River valley, supported scattered cirque and valley glaciers during at least some of the pre-Reid glaciations. In addition to eroded cirques, evidence includes 'U' shaped valley cross-sections and, in one instance, a moraine. The most extensive areas of local glaciation include the Cragg Mountain upland and Mt. Tyrrell areas. Degraded cirque-like features also occur around the highest uplands east of Yukon River and apparently indicate that local cirque glaciation occurred there as well although the evidence is less compelling. The last occurrence of alpine glaciation in the Cragg Mountain upland is undated. A pre-Reid age is based upon the extensive erosion of cirques and the presence of Wounded Moose soil on a single morainal feature. Through most of the middle Pleistocene, with the exception of areas influenced by distal glaciofluvial sedimentation, stream incision continued and stream beds approached their contemporary levels. Sedimentation was largely restricted to alluvial and colluvial fans along valley margins, eolian (loess) deposition in valleys by katabatic winds during late pre-Reid glaciations and resedimentation during warmer intervals and glaciofluvial aggradation in the Yukon, Stewart and Klondike rivers associated with the late middle Pleistocene Reid Glaciation (Marine Isotope Stage (MIS) 8 or 6). The premier paleoenvironmental record for this period comes from the Eddas Bench site along Thistle Creek. This site contains the longest and most significant record of middle Pleistocene muck sedimentation within the map area. These sediments contain independently-dated tephras. They span the period from ca. 740 ka (Gold Run tephra) to ca.140 ka (Old Crow tephra) as well as part of the late Pleistocene (post ca. 130 ka). Work to date on the Eddas Bench site indicates that collapse deformation has occurred within parts of the succession due to periodic thawing of permafrost. The sediments record a middle Pleistocene warm period when the area was forested and one late Pleistocene interval as well as intervening muck beds deposited during periods of glacial climate. Middle Pleistocene sediments also occur along lower Dominion Creek. There, the early Pleistocene (reversely magnetized) Ross Gravel documents fluvial incision to a level below present stream bottoms prior to the last magnetic reversal. This was followed by aggradation in the form of the Dominion Creek gravel which was deposited by fluvial activity spanning the middle Pleistocene (<780 ka) through the Holocene. The oldest deposits within it indicate that Dominion Creek had aggraded to its present flood plain level by early-middle Pleistocene time. The late Pleistocene was characterized by organic and colluvial sedimentation followed by the extensive deposition of eolian sediments carried by katabatic winds during McConnell Glaciation. Most of the megafaunal remains recovered from placer mining in the region dates from this period. Extensive areas of bog developed on these deposits during the early Holocene along with the development of segregated ground ice bodies. The late Pleistocene age of these sediments is directly indicated from radiocarbon dating, inclusion of the ca. 25 ka Dawson tephra or superposition on sediments containing the late middle Pleistocene Old Crow tephra. The Klondike Plateau was created through apparent regional uplift of a formerly low-lying erosional surface. The cutting of valleys approximately 600 m deep since the eruption of Carmacks Group lavas at the end of the Cretaceous Period (ca. 70 Ma) suggests a long term uplift rate of about 0.9 cm/ka during the Tertiary Epoch. It has resulted locally in the superimposition of streams across mountainous uplands such as the course of Yukon River across the Dawson Range/Thistle Mountain upland in the southernmost part of the study area. Tephrochronology, radiometric dating of tephras and lava flows in the region and recognition of surfaces created by the reversal of Yukon River (ca. 2.7 Ma) allow estimates of regional uplift based upon the amount of fluvial incision since streams were graded to former flood plains. The estimates considered most reliable are those based on dating of lava flows or tephras overlying fluvial gravels that now form terraces. Estimates for periods exceeding 1 Ma range yield maximum values from 0.5 to 2.6 cm/ky. Rates of incision calculated from the incision of horizons of known age during the past 1 Ma are inconsistent and contradictory with values ranging from negative values based on the Ross Gravel along Dominion Creek to ~3.4 cm/ka along lower Sixty Mile River. These contradictory rates of incision suggest that uplift has not been uniform, occurred in pulses, occurred non-uniformly within discrete blocks that moved differentially along faults or by differential warping. Climate change was likely also a factor: sediment input into the fluvial system during later pre-Reid, Reid and McConnell glaciations halted incision and caused temporary aggradation in valleys during, between and after these events yielding inconsistent incision rates. Surficial geology and stratigraphy have direct application in recognition and avoidance of geologic hazards. For example, muck deposits are highly prone to thaw collapse and flow slides; drift of the Flat Creek beds in Tintina Trench are extensively affected by land sliding as are areas underlain by the Carmacks group. The Carmacks group can be mapped on the basis of landslide occurrence. Potential auriferous placers likely exist where former channels of late Tertiary age (the age of White Channel gravel) have been buried. Examples are a former course of Indian River near the mouth of Montana Creek that was buried beneath the valley train deposited during the maximum pre-Reid glaciation. Furthermore, recognition of the reversal of Yukon River (which postdates the deposition of White Channel gravel) creates possibilities for tracing White Channel gravel beyond its apparent termination at the mouth of Klondike River. The most likely outliers would occur as high terraces along Yukon River upstream from the mouth of Klondike River rather than downstream from it.