Late Quaternary Sediments and Geomorphic History ...

Late Quaternary Sediments and Geomorphic History of the Southern Rocky Mountain Trench, British Columbia

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JOHNJ. CLAGUE Department of Geological Sciences, University ofBritish Columbia, Vancouver, British Columbia V6T 1 W5' Received October 1, 1974 Revision accepted for publication January 22,1975

The southern Rocky Mountain Trench was a major outlet valley of the Cordilleran Ice Sheet. Quaternary sediments underlying the floor of the trench in southeastern British Columbia consist mainly of glacial, glaciofluvial, and glaciolacustrine materials deposited during the Fraser (Pinedale) Glaciation, and fluvial and lacustrine sediments deposited during the preceding interglaciation. Deposits of three stades and two intervening nonglacial intervals are recognized. Interglacial sediments which contain wood dated at 26 800 2 1000 y B.P. underlie drift of the early stade. During the interval between the early and middle stades, the Rocky Mountain Trench in southeastern British Columbia probably was completely deglaciated, and sediments were deposited in one or more lakes on the floor of the trench. In contrast, glacier recession between the middle and late stades was of short duration and extent; glaciolacustrine sediments were deposited only along the margins of the Rocky Mountain Trench, and apparently residual ice remained in the center of the valley. Final recession of the trunk glacier occurred prior to 10 000 y B.P. with no major halts and without significant stagnation of the terminus. La Tranchee des Montagnes Rocheuses, dans le sud-est de la Colombie Britannique a semi d'exutoire majeur B la calotte glaciaire de la Cordilliere. Les sediments quaternaires, sur le fond de la tranchee, consistent en materiaux glaciaires glacio-fluviaux et glacio-lacustres abandonnes lors de 1aGlaciationde Fraser (Pinedale), et en materiaux fluviaux et lacustres accumules lors des interglaciaires precedents. On reconnait les dep8ts de trois stades et deux intervalles interposes non-glaciaires. Des sediments interglaciaires sous le drift du stade inferieur contiennent des fragments de bois dates B 26 800 2 1000 ans B.P. I1 est probable que lors de l'intervalle entre les stades inferieur et moyen, cette partie de la Tranchee des Montagnes Rocheuses fiit completement dkglacee et des sediments s'accumulerent dans un ou plusieurs lacs sur le fond de la tranchte. Par contre, la recession du glacier entre les stades moyen et superieur fiit de courte duree et peu Btendue; des dCp6ts glacio-lacustres furent sedimentes uniquement en bordure de la tranchte, alors que le glacier residue1 occupait le centre de la vallee. La recession final du glacier principal s'est produit avant 10 000 ans B.P. sans interruptions majeures et sans stagnation importante du terminus. [Traduit par le journal]

Introduction The Rocky Mountain Trench is a long narrow intermontane valley that extends northwest from Montana through British Columbia (Fig. I). The southern trench consists of tectonic depressions formed by Cenozoic half-graben block faulting superposed on late Cretaceous to early Tertiary allochthonous fold and thrust structures (Leech 1966; Clague 1974). The present configuration of the trench results from sediment infilling of these tectonic depressions, glaciation, and Holocene erosion. During each Pleistocene glaciation, the southern Rocky Mountain Trench served as an outlet

valley of the eastern Cordilleran Ice Sheet which formed through the coalescence of large valley and piedmont glaciers in the Rocky and Columbia Mountains. The trunk glacier in the trench was augmented by tributary valley glaciers in southeastern British Columbia and northwestern Montana, and reached south of 47'30'N at the peak of glaciation. The succession of glacial and nonglacial sediments beneath the floor of the southern Rocky Mountain Trench was studied in order to reconstruct the late Quaternary geomorphic history of the area.

Distribution and Character of Sediments Thick deposits of unconsolidated sediments are 'Present address: Geological Survey of Canada, 100 West Pender Street, Vancouver, British Columbia V6B limited to the major valley bottoms; the drift 1R8. cover is thin or absent on the uplands (Fig. 2). Can. J. Earth Sci., 12,595-605 (1975)


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FIG.1. Locality index map. Location of Rocky Mountain Trench is shown on inset. Unpatterned area is lower than 1220 m (4000 ft) in elevation.

Beneath the floor of the southern Rocky Mountain Trench, Quaternary sediments overlie either bedrock, mainly Precambrian and Paleozoic in age, or semiconsolidated sediments of Tertiary age (Leech 1954, 1958; Clague 1974). The thickness of Quaternary deposits has been estimated from deep stratigraphic exposures along the major river valleys in the trench and from seismic records where low-velocity Quaternary sediments are distinguishable from intermediate-velocity Tertiary deposits. In general, Quaternary deposits are about 100 m or less in thickness along the sides of the trench, but are locally much thicker along the trench axis.

The known Quaternary deposits in the southern Rocky Mountain Trench are included in the following informal units listed in order of decreasing age: interglacial sediments, older drift, inter-drift sediments, younger drift, and postglacial sediments. Stratigraphic sections are summarized in Figs. 3 and 4, and a generalized composite section is shown in Fig. 5. Interglacial Sediments Interglacial sediments (Fig. 3, section 6; Fig. 6A and B) crop out along Sand Creek where they are overlain by till; the base of the unit is not exposed. The sediments consist of parallel-


CLAGUE: QUATERNARY SEDIMENTS

FIG. 2. Thickness of Quaternary sediments. Areas where bedrock is at or near the surface inside the dashed line are stippled; areas of thick Quaternary sediments inside the dashed line are unpatterned. The drillhole data were provided by the Water Resources Branch of the British Columbia Department of Lands, Forests, and Water Resources. Seismic records were supplied by Shell Canada Limited.

and cross-bedded clay, silt, sand, and gravel ranging in color from very light gray to yellowish gray (N 8 to 5 Y 8/1).' Many gravel clasts are weathered in contrast to those in the overlying drift which are fresh. Fragments of wood are abundant; a wood sample from sand directly below till yielded a radiocarbon age of 26 800 f 1000 y B.P. (GX-2032). The unit thus was deposited during the major nonglacial interval ZColorsof wet sediment samples are from Rock-color Chart (Goddard et al. 1963).

referred to as the Olympia Interglaciation in the Pacific Northwest (Armstrong et al. 1965;Fulton 1971). No other Rocky Mountain Trench sediments which definitely correlate with the Sand Creek interglacial sediments have been found. Some silt, sand, and gravel exposed in the walls of the major river valleys may be interglacial in origin. These sediments, however, differ from those at Sand Creek in that they lack visible organic detritus and commonly contain dropstones; the gravel fraction is coarser, more poorly sorted,


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Radiocarbon dote - 26.800* 1 0 m Glaciolocurtrine sediments present in tributary volleys.

FIG.4. Major exposures of Quaternary sediments. ( A ) Inter-drift sediments (unit 1 ) unconformably overlain by younger drift (unit 2), Elk River (see Fig. 3, section 2). Younger drift here consists mainly of till; lenses of ice-contact sand and gravel are common in the upper part of the unit. Glaciofluvial gravel underlying a late glacial meltwater channel is present at the top of the exposure (arrow). (B) Older drift (unit 1 ) unconformably overlain by younger drift (units 2 and 3), St. Mary River (see Fig. 3 , section 1 I). Older drift consists of deformed glaciofluvial sand and gravel. It is overlain across an angular unconformity by younger till and outwash gravel (unit 2). The dotted line within unit 2 is glaciolacustrine silt. Till is overlain by outwash sand (unit 3) that has been in part redeposited as dune sand.

FIG.5. Composite columnar section of late Quaternary sediments exposed in the southern Rocky Mountain Trench. Column heights are proportional to the maximum exposed thickness of each unit.

Outwash occurs as lenses in older till3 as horizontal and cross-stratified beds, and as broadly folded beds locally cemented by calcium carbonate. The folded sediments are of particular interest because they are widely distributed (Fig. 3, sections 7, 11, and 16) and probably represent a thick supraglacial outwash mantle which deformed competently during melting of underlying stagnant ice. Strata commonly dip 5 to 30' with dip components perpendicular to the axis of the valley in which the unit is exposed (Fig. 4B). High-angle normal and reverse faults and fractures cut the outwash and are oriented and consists of less weathered clasts. Thus, it is parallel to the walls of what must have been thought that these sediments were deposited in major meltwater discharge routes corresponding a proglacial environment after deposition of the in position to modern valleys crossing the trench interglacial unit at Sand Creek. floor. Similar faults in glaciofluvial sediments have been shown to originate by adjustments in Older Drift Older drift crops out along major valleys in the sediment pile as masses of associated ice the study area where it overlies bedrock or sedi- melt (McDonald and Shilts 1975). ments of Tertiary age. More commonly, how- Inter-drift Sediments ever, the base of the unit is not exposed. Inter-drift sediments crop out throughout the The glacial deposits consist of till (Fig. 6C) study area along the valley walls of Kootenay and outwash (Fig. 6D). The former is a massive River and its tributaries. They overlie older diamicton, light gray to very light gray (N 7 to N 8) in color. The latter is mainly sandy gravel, 3Till of the older drift is referred to as the older till; till its color differing with lithologic composition. of the younger drift is called the younger till.


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glacial deposits and are overlain by younger till and outwash (Fig. 3). The sediments are very light gray to white (N 8 to N 9) silt, sand, and minor gravel (Fig. 6E). No organic matter has been found in these sediments. Rhythmic bedding is uncommon, and many exposures appear massive. Soft-sediment deformation, although uncommon, has locally produced contorted beds overlain by undisturbed strata, load casts, flamelike projections of fine-grained sediment into overlying silt and sand, and coherent blocks of sediment that slumped toward the center of the depositional basin. Also present are high-angle faults and fractures, clastic dikes intruded from below, and channel scour-and-fill structures. Inter-drift silt and sand were deposited in one or more lakes which formed on the floor of the Rocky Mountain Trench during a period of glacier recession. The transition between these sediments and underlying older outwash is gradational, with dropstones, gravel lenses, and sediment deformation increasing downward; thus deposition of inter-drift silt and sand began in a proglacial environment. Near the top of the unit there is an upward increase in dropstones related to the readvance of the Cordilleran Ice Sheet indicating that the upper part of the unit was also deposited in a proglacial environment. Dropstones and other indicators of nearby glaciers are absent in the middle part of the unit, so it is not known whether these sediments are glaciolacustrine or simply lacustrine in origin. The absence of visible organic matter throughout the unit may indicate that vegetation was never firmly established during this nonglacial interval. Younger Drift Younger glacial deposits crop out over large portions of the floor of the Rocky Mountain Trench and its tributary valleys. They include

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till, glaciofluvial sand and gravel, and glaciolacustrine clay, silt, and sand. The till (Fig. 6F) is similar in color and texture to older till. It is underlain by either advance outwash gravel or inter-drift silt and sand (Fig. 4A). Glaciofluvial sediments, consisting mainly of sandy gravel, occur locally beneath the younger till (advance outwash), as lenses within the till (englacial and subglacial outwash), and as channel and valley train deposits overlying the till sheet (recessional outwash, Fig. 6H). Stone imbrication in recessional outwash shows that meltwater drainage was in a southerly direction, much as the present drainage. Glaciolacustrine clay, silt, and sand occur along the trench margins and in tributary valleys at two stratigraphic levels within the younger drift: (1) intra-till glaciolacustrine sediments, and (2) supra-till glaciolacustrine sediments. Thus, locally the younger drift is divisible into two glacial subunits bounded by stratified sediments. Intra-till glaciolacustrine sediments are massive and rhythmically bedded clay, silt, and sand occurring along the margins of the Rocky Mountain Trench stratigraphically above inter-drift sediments, but within the younger drift. The glaciolacustrine sedimentscrop out near St. Mary Valley along the west side of the trench (Fig. 3, section 11) and near Elk Valley along the east side (Fig. 3, sections 4 and 5), but are absent as a distinct unit in central trench exposures. Instead, at the same stratigraphic level are discontinuous intra-till lenses of ice-contact sand and gravel, and coarse channel fill. Although the glaciolacustrine unit along the trench margins cannot be continuously traced into the coarse outwash pockets at the center of the trench, the two are probably genetically related, the former representing deposits in water ponded behind

FIG 6. Quaternary sediments. (A, B). Interglacial sediments. (A) Interbedded sand and gravel overlain by silt and sand, Sand Creek. (B) Interbedded sand and gravel containing fragments of wood, Sand Creek. (C, D ) . Older drift. (C) Till, Elk River. ( D ) Glaciofluvial sand and gravel partially cemented by calcium carbonate, St. Mary River; sediments are cut by high angle faults and fractures. ( E ) . Inter-drift sediments. Thick-bedded silt with a blocky fracture pattern, Elk River. (F, G, H ) . Younger drift. (F) Till, St. Mary River. (G) Supra-till rhythmically bedded silt and sand, St. Mary River. (H). Gravel underlying late glacial melt-water channel, Kootenay River. (I, J ) . Postglacial sediments. (I) Sand overlying outwash gravel, Kootenay River; sand is probably eolian. ( J ) Mudflow gravel of an alluvial fan built out onto the present Kootenay floodplain.


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sediment or ice dams, and the latter deposits beneath or near remnant ice masses in the central part of the trench. Intra-till glaciolacustrine sediments were deposited in ice-marginal lakes during an interval of glacier recession. Trenchmarginal deposition during this interval contrasts with trench-wide deposition of inter-drift sediments during an earlier nonglacial interval when the entire floor of the Rocky Mountain Trench in the study area was temporarily free of active ice. Supra-till glaciolacustrine sediments (Fig. 3, sections 12, 13, and 14; Fig. 6G) are horizontally bedded silt and clay containing dropstones. They were deposited in ice-dammed lakes in the major tributary valleys during final deglaciation of the southern Rocky Mountain Trench. These lakes drained rapidly during deglaciation, and many of the large meltwater channels on the floor of the trench were formed or modified by the resultant floods (Clague 1973, 1975).

position of Miocene sediments of the St. Eugene Formation (Clague 1974) and late Pleistocene sediments. If Pliocene and early Quaternary sediments were deposited on the floor of the trench, they either have been removed by Quaternary glacial and fluvial erosion or are concealed beneath late Quaternary deposits below present local base level. Multiple glaciation in British Columbia during the Pleistocene Epoch is well documented (for example, see Armstrong and Tipper 1948; Armstrong et al. 1965; Fulton 1972; Fulton and Halstead 1972), but the stratigraphic record for pre-Wisconsinan glaciations is poor and is largely known from drill records or by comparisons with the more complete glacial record in Washington, Idaho, and Montana. Wisconsinan glaciers covered nearly all of British Columbia and removed or buried most of the record of earlier glaciations. The oldest recognized Quaternary sediments in the southern Rocky Mountain Trench were Postglacial Sediments deposited during the Olympia Interglaciation. Postglacial deposits (Fig. 61 and J) include However, drift of an early Wisconsinan glacialoess, eolian sand, colluvium, and alluvium. tion (Bull Lake Glaciation) has been identified Loess and eolian sand up to 1 m thick form a in northwestern Montana (Richmond et al. surface mantle over much of the study area. 1965). Fulton (1971), in a paper on the radioSurface sand in some meltwater channels and carbon geochronology of southern British drained ponds has been reworked by wind into Columbia, has concluded that the Olympia Insmall dunes (Fig. 4B). terglaciation began more than 52 000 years ago Talus cones, alluvial fans, and thin colluvial~ and ended about 19 000 years ago. A nonglacial mantles occur along the margins of the trench sequence in the Purcell Trench spans the interval and major river valleys. Degradation by rivers from 43 800 800 to 25 840 f 320 y B.P. Durand streams began during or shortly after de- ing the Olympia Interglaciation, as at present, glaciation. The rivers have since cut down the major sites of sedimentation in the southern through sediments and bedrock alike and have Interior of British Columbia were valleys located left multiple terraces bordering their valleys. between hilly or mountainous upland areas. Beneath the terraces interstratified sand and Channel, overbank, and alluvial fan sediments gravel are inset on older sediments. These terrace were deposited in lake basins. The climate in sediments were deposited during an interval of the Interior during at least part of this period rapid downcutting following deglaciation and was sufficiently warm and moist to support thus are very late Pleistocene or early Holocene forests and large vertebrates (Fulton, 1971, p. 5). in age. The main evidence for this is: (1) the This nonglacial interval terminated with the occurrence of 6600 year old Mazama 0 tephra invasion of the Rocky Mountain Trench by ice (glass analyses by J. A. Westgate) in alluvial at the onset of the late Wisconsinan Glaciation. fans graded to alluvium of the present Kootenay This glaciation is called the Fraser Glaciation in River flood plain, and (2) the presence of kettled the coastal Pacific Northwest (Crandell 1965) terraces at Wasa Lake which are only 5 to 10 m and the Pinedale Glaciation in the Rocky Mounabove the present flood plain. tain region of the United States (Richmond 1965). The trunk glacier in the trench probably Geomorphic History and Discussion was fed from ice sources in the Columbia and There is a hiatus in the geologic record of the Rocky Mountains to the north and was augsouthern Rocky Mountain Trench between de- mented by local alpine glaciers (Fig. 7). Erratics

+



FIG. 7. Pattern of ice flow in southeastern British Columbia and southwestern Alberta during the last continental glaciation (from Prest et al. 1967). Short lines represent glacial lineations parallel to the ice-flow direction. Arrows represent regional flow patterns. Line with darkened triangles is western limit of Laurentide Ice Sheet. Lines with darkened circles are limits of Cordilleran Ice Sheet. Study area is stippled.

and striae on the east side of the trench near 49'30'N indicate the trunk glacier reached an elevation of at least 2260 m there. The Cordilleran Ice Sheet was about 1500 m thick over the trench at the International Boundary (Daly 1912). It terminated in a lobate front marginal to Glacial Lake Missoula in northwestern Montana approximately 150 km south of the International Boundary. In the southern Rocky Mountain Trench in British Columbia, tills and ice-contact sediments of three late Wisconsinan stades are separated by stratified interstadial sediments (Figs. 3 and 5). At the beginning of the early stade, ice advanced over weathered bedrock and sediments on the trench floor; weathered clasts of various units were incorporated in the basal till of this

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advance. As the early stade ended, portions of the glacier in the trench apparently became stagnant and were buried beneath thick accumulations of glaciofluvial sand and gravel along routes of meltwater discharge and beneath glaciolacustrine silt and sand elsewhere. The glaciolacustrine sediments are in part contemporaneous with and in part younger than the coarse outwash. With melting of the underlying stagnant ice blocks, these sediments were broadly folded and faulted. It is not known whether the fine-grained sediments accumulated in one or a number of lakes occupying the floor of the trench; neither is the cause of impoundage known, although possibilities include ice or sediment dams and differential isostatic depression of the surface of the trench in a northerly to northwesterly direction due to the presence of an ice cover to the north. Ice-rafted clasts were deposited with inter-drift sediments both early and late in the interstade. Glaciolacustrine deposition ended when glaciers of the middle stade advanced down the trench. Recession between the middle and late stades was of short duration and extent; ice may have remained locally in the center of the trench throughout this interval. During final deglaciation of southeastern British Columbia, the glacier in the trench became increasingly confined by the flanking mountains. The shrinking trunk glacier remained active throughout deglaciation and did not stagnate as did glaciers of the early stade. The presence of drumlinized ground moraine and the absence of end moraines and widespread major ice stagnation features such as kettle and kame topography and eskers indicate the trunk glacier receded by downwasting and orderly frontal retreat without major glacial standstills. Tributary glaciers retreated far back into their valleys prior to the disappearance of the trunk glacier (Clague 1973). The latter blocked the side valleys, thus impounding lakes in which clay, silt, sand, and deltaic gravel accumulated. Drainage from these lakes and from the receding trunk glacier was south beneath and along the margins of the glacier. As the ice surface was lowered, ice-marginal channels at lower elevations were progressively occupied and abandoned (Clague 1973). The courses of major streams entering the trench reflect this phase of deglaciation. Streams such as Gold Creek and Elk River swing abruptly to the southeast along the trench

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TABLE1. Summary of Quaternary stratigraphy and geomorphic history of the southern Rocky Mountain Trench


Deposits

Radiocarbon age (YB.P.)

Dominant depositional environments

Postglacial sediments

Eolian, fluvial, colluvial, and mudflow deposition

Younger drift

Inter-drift sediments Older drift

Glaciolacustrine,* glaciofluvial Glacial Glaciolacustrine** Glacial Lacustrine(?), glaciolacustrine Glacial

Interglacial sediments

Lacustrine(?), fluvial

Tentative correlation Holocene

> I 0 0 0 0 f 140 Fraser (Pinedale) Glaciation