Alluvial history of the Porcupine River, : Role of glacial-lake overflow from northwest

ROBERT M. THORSON University of Alaska Museum and Geology/Geophysics Program, 907 Drive, Fairbanks. Alaska 99701 E. JAMES DIXON, JR. University of Alaska Museum, 907 Yukon Drive, Fairbanks, Alaska 99701

ABSTRACT ing, 1978; Matthews, 1974, 1976; Morlan, westward through the lowland basins of 1978, 1979, 1980; Jopling and others, 1981). northern Yukon Territory prior to its entry The stratigraphy and morphology of The results of these studies suggest the fol- into Alaska across the uplands at the alluvial terraces in the lower Porcupine Val- lowing simplified regional model for late Ramparts (Fig. 1). In Alaska, the Porcupine ley permit the definition of twelve river Quaternary events (Fig. 1): River can be divided into five well-defined stages, each marked by distinctive surface local areas, each with distinctive physio- characteristics, sediment composition, and Prior to late Wisconsinan time, the Old Crow, graphy, bedrock geology, and surficial sed- regional gradient. Terraces that exhibit Bluefish, and Bell Basins were gradually filling iments (Fig. 1). In the Upper Ramparts, the characteristics suggestive of extremely high with fine alluvium as they drained eastward to the river flows through a narrow, V-shaped . In late Wisconsinan time, discharge, such as coarse, bouldery, braided canyon that cuts deeply into Precambrian Laurentide ice advanced westward to the east gravel surfaces and intense scouring, formed- flank of the Richardson Mountains and diverted and Paleozoic dolomites, quartzites, and at times when the Porcupine River at the pre-existing and meltwater drainage into the shales (Brabb, 1970; Brosge and others, Ramparts acted as an overflow outlet for three large basins. Eventual blockage of eastward 1966). The valley walls are generally irregu- glacial lakes in northern Yukon Territory drainage at McDougall Pass by ice near its max- lar, often with large talus cones and alluvial imum limit caused the rapid growth of a large fans near their bases. The average depth and which had been impounded by the Lauren- turbid glacial lake in each basin. Lake levels rose tide ice sheet. Terraces capped by sediment until the lowest west-draining stream divide, width of the valley are 100-150 m, and 1-2 suggestive of relatively low discharge mean- located at the Ramparts of the Porcupine River km, respectively. In Howling Dog Canyon, dering streams, and which were strongly near the present Alaska-Canada border, was the Porcupine River traverses a broad area affected by drainage, proba- breached. As downcutting of the outlet pro- of near-horizontal Tertiary basalt flows that ceeded, the lake shorelines were gradually low- bly formed when glacial-lake overflow did ered until the lakes disappeared completely. As overlie irregularly eroded Paleozoic carbo- not occur. Ten radiocarbon dates on allu- later incision continued below the floor of the nate rocks. Valley walls are typically high vial sediments from the lower Porcupine large basins, much of the glaciolacustrine and (75-125 m) and very steep, owing to the River range from greater than 35,000 to preglacial alluvial deposits were flushed westward erosion-resistant lava. Howling Dog Can- via the enlarged Porcupine River, which by this 2,350 ± 55 yr B.P. When combined with yon is relatively narrow (1 km), giving the time had permanently captured the drainage geomorphologic interpretations on terraces from much of northern Yukon Territory. valley in this vicinity a "notch-like" appear- in Alaska and with radiocarbon dates from ance. Fishhook Bend is a broad structural northern Yukon Territory, these dates sug- basin in which poorly consolidated and According to this model, the Porcupine gest repeated glaciolacustrine innundations unconsolidated Tertiary alluvium and la- River in Alaska acted as the major spillway of the Old Crow and Bluefish Basins during custrine sediment dip shallowly basinward. for impounded meltwater; hence, features Wisconsinan time. Above the Tertiary sediments, there is a uni- along this part of the river should provide formly thick mantle of Quaternary allu- important constraints on the interpretation vium. The river bluffs average about 35 m in INTRODUCTION of and timing of glacial and lacustrine height here, and the valley widens to several events in northern Yukon Territory. This kilometres. In the Lower Ramparts, the The discovery and dating of "early" report presents part of the preliminary river is incised steeply below a broad 5- to human artifacts from eastern Beringia fo- results of stratigraphic and geomorphologic 10-km-wide flat erosional valley floor that is cused widespread attention on the archeo- investigations carried out in 1979-1980 developed into Paleozoic carbonates and logic and paleoecologic potential of north- along the Porcupine River in Alaska. Com- carbonaceous rocks. Vertical cliffs 15-25 m ern Yukon Territory (Irving and Harring- parison of data generated during this study high commonly border the river in this area, ton, 1973). Following the earlier work of with published interpretations of the history but steep rock faces can be as high as 100 m Hughes (1969), more detailed studies of the of the Old Crow and Bluefish Basins results where older ridges intersect the valley. The stratigraphy and geologic history of north- in several unresolved paradoxes, which Yukon Flats form a broad expanse of flat- ern Yukon Territory have continued during should provide foci for future research. lying Holocene and late Pleistocene flood- the past decade (Hughes, 1972; Hughes and The Porcupine River originates in central plain sediments beyond the westward limit others, 1981; Lichti-Federovich, 1973; Irv- Yukon Territory and drains northward and

Geological Society of America, v. 94, p. 576-589, 9 figs., 3 tables, May 1983.

576

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thus, prior to lake overflow, it was the main 148" 132* valley. Consequently, non-overflow terraces and sediments in the present Porcupine Val- ley downstream from the Coleen River were created largely by its drainage. Non-over- flow features upstream from the Coleen River formed when the Porcupine River was a small west-draining tributary to the trunk stream. During times of strong glacial lake overflow, however, discharge from the Yukon Territory greatly exceeded that of the Coleen River, causing its influence to be greatly diminished. Owing to these relative discharge relationships, differences in ter- race gradient, valley morphology, and gravel lithology above and below the Coleen River can be used to determine the relative strength of glacial-lake overflow.

148° PORCUPINE RIVER STAGES

Extensive alluvial terraces and good stra- tigraphic exposures along the Lower Por- cupine River provide an excellent opportu- nity to reconstruct the evolution of the valley. Terraces range from broad high-level strath and alluvial surfaces to coarse, boul- der-covered, and irregularly scoured bed- rock channels. In order to correlate strati- graphic exposures and to determine terrace gradients, the height of each exposure was carefully measured above the best available datum with a spirit-level. Neither the high or low 1980 water lines were acceptable as a datum. An intermediate and clearly defined level, referred to as "normal high water" was marked by the upper limit of driftwood and the lower limit of perennial vegetation. High water during spring floods was varia- ble, ranging as much as 3 m above datum, depending on the narrowness of the valley. The normal summer minimum was about 4 m below normal high water, resulting in an Figure 1. Location maps showing regional setting and subdivisions of the lower Porcu- average annual range of about 7 m. pine Valley. Areas above 1,000 ft (305 m) altitude shaded on lower map. Note width and Because of the low gradient of the Lower dissection of the lower Porcupine Valley within and downstream from the Coleen River, in Porcupine River, a large contour interval, contrast to the narrow undissected valley in the Upper Ramparts and Howling Dog and the dearth of bench marks, the true alti- Canyon. Maximum extent of Laurentide Ice Sheet and glacial lakes (shaded) on upper map tude of datum was difficult to determine (from Hughes, 1972). accurately. Only two 100-ft contour lines cross the Porcupine River between the of bedrock (Williams, 1962). Muskeg, dis- Quaternary evolution of the valley. Prior to Canadian Border and the Porcupine Flats continuous spruce forest, abandoned river glacial lake overflow, the Porcupine River on the 1:63,360 scale U.S. Geological Sur- channels, and thaw lakes characterize this upstream from the Coleen River probably vey maps used as a base. The position and large area which extends down-valley to the carried a discharge comparable to that of extent of gravel bars on the maps suggest mouth of the Porcupine River at Fort the present Salmon Trout River (Fig. 1). that they were made from photographs Yukon. The Coleen River, which extends north- taken at a time of low summer discharge Within the study area (hereafter referred ward to the Brooks Range and is now the (about 4 m below datum). Only two bench to as the "Lower Porcupine"), the relative largest tributary of the Lower Porcupine in marks are present on terraces near enough amount of discharge, both water and sedi- the study area, drains approximately five to the river to be used as altitude reference ment, has changed greatly during the late times the area of the ancestral Porcupine; points. Altitudes of datum points deter-

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TABLE I. PORCUPINE RIVER STAGES

Stage Age Name Description Inferred rate of incision Inferred discharge XI0 or aggradation between this upstream from l4C yr and previous stage Coleen River

Lower Ramparts- Upper Ramparts Fishhook Bend

I Very High terrace Isolated high terrace in Lower Very slow Very slow Very low (Alaska old Ramparts capped with fine cherty incision incision Porcupine). gravel. Strong Coleen influence.

2 >35 Old valley Broad gravel-capped bedrock Very slow Very slow Very old (Alaska terrace rising up all major incision incision Porcupine), tributaries. Strong concave upward long profile to divide at Ramparts. Gradual erosion in carbonate rocks; gradual deposition in Fishhook Basin. Gravel shows strong Coleen and local influence.

3 >30.6 Braided surface Broadly braided coarse gravel Slow incision Rapid initial Very high (Alaska terrace extending across valley. incision Porcupine and Yukon Gravel occurs either as thin sheet drainage and meltwater). over scoured surface or as thick deposit in deep scour channels. Gravel compositionally immature, showing strong local influence. No Coleen influence.

4 >30.6 High strath Flat unscoured gravel-capped Slow incision Slow incision Low? terrace bedrock terrace in Lower (terrace marks Ramparts. Gravel compositionally pause) similar to modern river. Coleen influence decreases downstream.

5 >30.6 Low strath Common terraces at this level Slow incision? Slow incision? Low? terrace must have formed prior to Stage (terrace marks 6. Gravel stripped by subsequent pause). scour. Slope similar to Stage 4.

6 >30.6 Valley-fill Scoured irregular surface with Rapid incision Rapid incision Very high (Alaska scour common boulder lag below Porcupine and Yukon deposits of Stage 7. Abrupt slope drainage and meltwater). increase upstream from Fishhook Bend suggests soft rocks easily excavated. Narrow, deep, bifurcate channels.

mined from the bench marks were equiva- lithologies (Table I, Figs. 2, 3, 4, and 5). are here considered to include the transition lent to those established from river contours Each stage is marked by an erosional or from the previous stage as well. within the measurement error. The altitudes depositional surface inferred from paired Prior to formation of the oldest terrace, of all terraces and stratigraphic contacts terraces that mark the culmination of a spe- the landscape probably consisted of low were then determined by measuring their cific fluvial event. Isolated rare non-paired subdued mountains drained by well-de- heights above or below datum. terraces at various heights along the Porcu- fined, partly structurally controlled, den- Deposits and fluvial features along the pine were also examined but were not dritic networks similar to those adjacent to Porcupine River can be assigned to 12 grouped into stages. Because a stage bound- the present Porcupine Valley. This land- stages on the basis of altitude, surface mor- ary marked by a fluvial terrace represents scape was developed on Tertiary lava and phology, radiocarbon dating, and clast only an instant in geologic time, river stages alluvial sediments which lie unconformably

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TABLE I. (Continued)

Stage Age Name Description Inferred rate of incision Inferred discharge > IO'1 or aggradation between this upstream from l4C vr and previous stage Coleen River

Lower Ramparts- Upper Ramparts Fishhook Bend

7 30.6- Valley-fill Flat alluvial terrace. Fine sand Gradual No incision and Very low (Alaska 26.6 and sandy gravel commonly with aggradation limited Porcupine). cross-beds of reworked Tertiary aggradation. wood and old Pleistocene wood. Highly variable lithology increases in compositional maturity downstream, showing great local variability and strong Coleen influence. Variability in slope reflects valley width and discharge.

8 26.6 Post-valley- Removal of valley-fill and strong Rapid excavation Slow incision? Very high (Alaska fill scour scouring of Stage 5 surface. of valley-fill. Porcupine and Yukon drainage and meltwater).

9 <26.6 Fishhook Concordant heights of terraces Very slow Very slow Very high (Alaska Terrace showing coarse gravel. Height incision? incision? Porcupine and Yukon possibly controlled by depth of scour drainage and meltwater). during Stage 6 in Lower Ramparts.

10 <• 26.6 Half-way Concordant height of highly Rapid Rapid incision Very high (Alaska pillar scour scoured surfaces in Upper incision (terrace marks Porcupine and Yukon Ramparts. No gravel. pause) drainage and meltwater).

11 <26.6 Caribou bar Variable, but nearly concordant. Beneath Rapid Very high (Alaska Highly scoured surfaces in modern incision Porcupine and Yukon Upper Ramparts. Commonly flood plain drainage and meltwater). overlain by coarse bouldery gray gravel of variable thickness.

12 0-5.2 Modern scour Modern channel cut into Very slow incision High (Alaska Porcupine bedrock. Incision may have been flood-plain and Yukon drainage), rapid at first and minimal later. development. Holocene flood plain is nearly uniform. Some areas are slightly higher than modern flooding and have large thaw lakes.

Figure 2. Map showing localities mentioned in text. Triangles indicate location of radiocarbon dated exposures described in text. Circles indicate location of features described in text.

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KILOMETERS DOWNSTREAM FROM CANADA BORDER

Figure 3. Longitudinal profiles of terraces along the lower Porcupine Valley shown in Figure 2. Data points represent the altitude of terrace segments above datum projected normal to the main axis of the valley. Arrows show maximum and minimum altitudes for terrace segments. Line labeled bedrock ridges shows generalized maximum altitudes of hills within 2 km of the valley axis. Circles, squares, and triangles are used only to differentiate stage profiles. Maximum and minimum ages for culminations of Stages 2, 6, 7, and 12 are shown.

Figure 4. Generalized composite cross section of the Porcu- pine Valley between Fishhook Bend and the Lower Ramparts (localities 3 through 12), showing approximate altitudes of ter- — races related to river stages. Refer to Table 1 for stage descrip- ~ STAGE 8- tions. Compare with Figure 3. § STRIPPED H SURFACE b<

STAGE 12- MODERN FLOODPLAIN

NO SCALE

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Figure 5. Generalized composite cross section of the Porcupine Valley in the Upper Ramparts between locali- ties 18 and 20, showing approximate altitudes of terraces related to river stages. Refer to Table 1 for stage descrip- tions. Compare with Figure 3.

£ tion with the broad erosional terraces in the UJ a D tributary valleys, indicate that the Old Val- H ley Stage marks a prolonged interval of near b< stability in the river's history. At this time, the Porcupine River, upstream from the Coleen River, was a nearly stable, meander- ing stream tributary to the trunk stream.

Stage 3: Braided Surface

Gravel of Stage 3 is coarse, gray, compo- NO SCALE sitionally immature, and inorganic; it oc- curs as part of a broad paired terrace on Paleozoic and Precambrian rocks. The races present nearly everywhere along the extending intermittently from the Lower relief on the Paleozoic-Tertiary unconfor- Porcupine and Coleen Rivers and their Ramparts to the Canadian Border (Fig. 6). mity suggests that the Porcupine Valley major tributaries. Upstream from the Co- The terrace is covered with a highly braided was already partly defined in lava-covered leen River at the mouths of the Salmon network of gravel bars and channels that areas by Tertiary time and in some areas Trout and Rapid Rivers and David Creek are best defined where the valley is broadest was deeper than at present. The location of (Fig. 2), the terraces of Stage 2 are as broad (3 to 5 km). The bars commonly are longi- the ancestral Porcupine and Coleen River or broader in the tributaries than in the tudinal in shape, 1 to 1.5 km long, 0.5 to 1 Valleys during latest Tertiary and early main valley, suggesting that they carried km wide, and their surfaces are little wea- Pleistocene time apparently were similar to discharges comparable to the Porcupine thered. Present relief on the braided surface their modern counterparts. River at that time. The Stage 2 terrace rises is only about 1 m owing to filling of the steeply toward the Canadian Border, with a abandoned channels with a variable thick- Stage 1: High Terrace progressively concave-upward longitudinal ness of eolian sediments and peat; examina- profile, similar to those of small valleys in tion of bluff exposures that intersect the The oldest terrace along the Porcupine this area (Figs. 4 and 5). Sediments of Stage bars indicates that they originally stood sev- Valley that can be clearly attributed to a 2 show great local lithologic variability, eral metres above the flat channel floors. river stage is a high strath terrace, cut into including a pronounced increase in siliceous The Stage 3 terrace widens in the more limestone in the Lower Ramparts at 210 m components downstream from the Coleen open parts of the valley and is narrow or altitude about 10 km east of John Herbert's River. Terraces at this level can be traced absent in constricted areas. In constricted Village (locality 3; Figs. 2, 3, and 4). The nearly continuously as much as 20 km up areas such as locality 16, Stage 3 consists of terrace extends for several kilometres along the Coleen River. highly scoured and stripped bedrock sur- the north valley wall as a highly dissected In the Fishhook Bend area, the Old Val- faces, that are mantled with cobble gravel bench which rises up several presently ley Stage is represented by alluvial terraces, and isolated boulders measuring up to 1 m underfit small tributary valleys and which is where as much as 15 m of oxidized brown diameter. At the mouths of the Salmon capped by about 3 m of brown, fine, highly fine gravel form part of a capping deposit Trout River and David Creek (localities 17 siliceous rounded pebble gravel. This ter- over the Tertiary sediments (locality 14 in and 15), scour features are particularly well race, which also occurs on the southern val- Fig. 2). Woody layers, peat beds, organic defined, showing streamlined and elongate ley wall, is interpreted as marking the height silt lenses, overbank sediments, and cryo- (whaleback) bedrock ridges up to 4 m high of the Porcupine Valley at some very early turbation horizons marked by prominent that are oriented parallel to the paleocur- time in its history. ice-wedge pseudomorphs are all interbed- rent direction and transverse to the local ded with thin beds of fine alluvial gravel, bedrock structural grain (similar to those Stage 2: Old Valley suggesting episodic aggradation of the basin shown in Fig. 7). alternating with periods of stability charac- In Fishhook Bend near Canyon Village The Old Valley Stage is dominated by terized by frost-deformation on the paleo- (locality 14, Fig. 2), a thin (0.5 to I m) broad, gravel-capped erosional bedrock ter- surfaces. These alluvial terraces, in conjunc- horizontal sheet of imbricate gray gravel

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Stage 3 generally modify and lie just below those of Stage 2, indicating that major inci- sion over much of the valley did not occur prior to the culmination of stages. In this sense, braiding and scouring through much of the valley modified a surface already in existence rather than creating a new level.

Figure 6. Top of 12-m-thick Stage 4: High Strath Terrace deposit of boulder gravel, fill- ing channel scoured into Ter- Terrace remnants of Stage 4 form a fairly tiary basalt on north side of broad and extremely planar surface at the river near locality 19. Upper top of river cliffs throughout most the surface formed part of a Lower Ramparts that shows none of the braided terrace associated with irregular and streamlined scour features Stage 3 (Table 1). Note coarse characteristic of Stage 3. This terrace steep- heterogenous nature of sub- ens upward toward, and is absent above, the rounded gravel which was de- Coleen River. Capping gravel is often thin rived from local bedrock. or absent but, where present, is similar to the modern river gravels and very unlike the bouldery gravels characteristic of the braided terrace of stage 3. Stage 4 is clearly cut below the braided gravel surface but possibly no lower than the bedrock which underlies it. This terrace is interpreted as having been last modified by lateral erosion of the Porcupine River after Stage 3, yet prior to subsequent stages mantles deposits of Stage 2 over an un- filled during one or more closely related characterized by rapid downcutting. The scoured area at least 0.5 km2. These gravels high-discharge events. planar nature of the terrace, its location and are interpreted to have been emplaced as a The broadly braided terrace remnants, gradient in the valley, and the composition broad sheet of coarse gravel that was pronounced streamlined scour features, the of the capping gravel suggests that drainage rapidly deposited over older sediments coarse and compositionally immature char- of the Coleen River was largely responsible without significant erosion. acter of the sediments, and the evidence for for its formation. If enhanced discharge In contrast, at locality 19 near Rapid deep scour and fill and for sheet-like gravel from the Porcupine River during overflow River at the upstream end of a large braided deposition all indicate that Stage 3 was was involved in its formation, spillway dis- zone, 29 m of coarse, boulder gravel lies caused by an extremely high discharge of charge must have been considerably less within a deep scour channel cut into the water that must have resulted from glacio- than it was during Stage 3. underlying Tertiary volcanics. The abun- lacustrine overflow. Stage 3 terraces at the dance of local boulders in the coarse sedi- mouth of the Coleen River are capped by Stage 5: Lower Strath Terrace ments, the unoriented clast-supported char- elongate gravel bars that show orientations acter of the gravels, and the lack of any transverse to the Coleen, indicating that the The Lower Strath Terrace is a uniform significant stratigraphic discontinuities sug- Porcupine River during Stage 3 was little discontinuous level of bedrock terraces 4 to gest that the scour channel was quickly affected by the Coleen drainage. Features of 5 m below those of Stage 4 (Fig. 3). Stage 5 terraces are also restricted to the Lower Ramparts, and slope upstream toward the Coleen River. Almost no gravel is present on its surface, and it is frequently irregularly scoured with occasional streamlined ero- sional remnants. The concordant paired strath terraces of Stage 5, which lie above the level of later incision (Stage 6), yet below the level of subsequent alluvial depos- its (Stage 7), could not have formed after alluviation because the river would have entrenched itself into deep channels filled with easily erodable sediments. Although its Figure 7. Irregularly scoured strath terrace (Stage 8) cut into Paleozoic limestones in the origin remains problematic. Stage 5 may Lower Ramparts on the north side of the river near locality 4. Note streamlined whale-back represent a minor pause in downcutting fol- features on broad terrace which lies below a higher older terrace (Stage 1). lowing Stage 4, which was mantled with

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alluvium during Stage 7, and later swept along the valley. In the Upper Ramparts at Stage 7 terrace usually is flat and feature- clear of sediments. locality 20, the alluvium is only about 4 m less. as is the modern Holocene flood plain thick and consists almost entirely of local, in broad areas between abandoned chan- Stage 6: Valley-Fill Scour subangular clasts. Farther downstream in nels, but faint channeling was observed at a Howling Dog Canyon locality 16, the grav- few localities. Stage 6 corresponds to, not a recogniza- els are 6 m thick and more rounded and Deposits of Stage 7 indicate a prolonged ble surface terrace, but the lowest level of mixed. In Fishhook Bend, the sandy gravel episode of alluviation in the Porcupine Val- valley erosion that existed prior to later val- appears to have been derived largely from ley from local sources. No evidence for sim- ley aggradation. The surface of Stage 6, local Tertiary exposures. The alluvium ilar alluviation was preserved in the Coleen which can be observed only at exposures attains a maximum thickness at Tusk Bluff Valley. The texturally fine character and where younger alluvial sediments overlie it, (locality 12) where 23.5 m of fine, sandy local lithologic variability of the sediments is commonly expressed as thin sheets of alluvium shows a nearly continuous record in conjunction with the over-all wedge shape coarse gravel with rare or isolated large of valley aggradation above a lag of boul- of the deposit preclude strong glaciolacus- boulders that lie along the base of scour ders and coarse gravel that floor a scoured trine overflow of meltwater through the channels cut into bedrock. At localities 16 channel (Fig. 8, Table 2). The lower 8 m of Ramparts during this time. and 20. streamlined erosional features sim- alluvium is a continuous sequence of silt If the lowest level of scour during Stage 6 ilar to those of Stage 3 project above the and interbedded fine sand, rich in delicate and the surface of Stage 7 alluvial terrace level of subsequent alluvial fill, yet can be detrital organics that grade continuously are projected up-valley, they converge at a traced downward below the fill to buried upward first into 10 m of gravelly cross- point about 15 km east of the Canadian bouldery channel bottoms. This buried bedded sand and farther upward into 6 m of Border at 302 ± 10 m altitude. This point scoured surface can be traced intermittently fine sandy, silty, alluvium. Although the may have been the location of the divide downstream from the Upper Ramparts but basal contact of Stage 7 sediments with between Yukon and Alaskan drainages dur- shows an abrupt change in slope at the Stage 6 boulder gravel shows no pro- ing this time. upstream end of Fishhook Bend, possibly nounced change immediately upstream or caused by the relatively greater erodability downstream from the Coleen River, the Stage 8: Post-Valley-Fill Scour of the unconsolidated Tertiary sediments upper surface is greatly oversteepened above and greatly reduced below the Co- there. The depth of Stage 6 scour does not The Post—Valley-fill scour Stage repre- leen. respectively. The form of the upper change appreciably above or below the sents the inferred exhumation and modifica- Coleen River, suggesting that the major dis- surface suggests an alluvial wedge built out tion of an earlier terrace (Stage 5) by charge responsible for its formation was into Fishhook Bend Basin and greatly stripping of the Stage 7 sediment, which derived from the Yukon Territory. reworked below the Coleen River. The must have covered it. and by scouring of its The surfaces associated with Stage 6 lie in channels well below the broad surfaces of earlier stages. Rather than braiding during overflow at this time, the Porcupine River apparently became entrenched in a series of steep-walled bifurcating channels suggestive of very high discharge. At localities 5 and 9 in the Lower Ramparts, large ellipsoid- shaped streamlined bedrock prominences up to 1 km long and 200 m wide occur between scour channels in what must have been the main path of overflow discharge.

Stage 7: Valley Fill

Stage 7 represents a prolonged episode of valley aggradation to a height well above that of some earlier scour levels. One of the most characteristic features of Stage 7 allu- vium is its tendency to be highly locally var- iable in lithology, showing a general trend toward increasing compositional maturity downstream. Regardless of its lithology. the alluvium nearly always is finer in texture than older gravels and rich in cross-bedded reworked peat and wood from older Ter- tiary and Quaternary exposures. The surface slope and thickness of Stage Figure 8. Generalized stratigraphie cross section of Tusk Bluff, showing location of 7 sediments show pronounced changes measured stratigraphie Sections I and II described in Table 2.

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TABLE 2. MEASURED STRATIGRAPH1C SECTION AT TUSK BLUFF stricted occurrence of the terraces, their origin is uncertain. They may represent an Section 1. Old valley terrace interval of high-discharge scour-and-fill in

Unit Thickness Generalized Description the Fishhook Bend Basin that was not pre- (m) served in more confined portions of the val- ley. They may also represent the height of 1 2.0 Eotian Cap. Light brownish-gray silty sand and sandy silt with cryo- the river following re-excavation of fine turbated lenses of peat and finely divided organic matter. Dense peat valley-fill sediment from the Lower Ram- cap at surface. parts. In this sense, Stage 9 may have been 2 10.5 Brown Gravel. Well-bedded clasts of medium to fine gravel in loose, controlled more by pre-existing conditions sandy, oxide-stained matrix, with minor lenses of sand and silty sand. than by the contemporaneous stream Includes several lenses of organically stained silty sand with alloch- thonous and autochthonus peat. Large wood fragments are especially regimen. common in lower half of unit and occur in both organic lenses and sandy gravel. Stage 10: Halfway Pillar Scour 3 31.0 Pre-Laie Pleistocene Deposits. Light gray unconsolidated sandy alluvium with minor gravel, clay, and pebbly, clayey silt with common The widespread presence of concordant ironstone and vivianite concretions and nodules, and very common scoured gravel-free surfaces in the Upper large wood fragments, organic layers, and plant fragments which in- Ramparts and Howling Dog Canyon and clude plant and invertebrate fossils and pollen of Miocene age (Brosge which extend eastward into Canada indi- and others, 1966). Beds tilted as much as 9° and show local normal cate a pause or change in the style of inci- faults. sion that continued after Stage 9. These Section II. Valley-fill terrace surfaces are typified by the scoured bedrock terraces near Halfway Pillar (locality 18). Unit Thickness Generalized Description (m) Above this surface, defined as Stage 10, but below the channels of Stage 6, gravel- 1 1.5 Eolian Cap. (Same as Section 1.) capped nonpaired terraces are rare. The general absence of terraces above those of 2 6.0 Fine Alluvium. Gray thinly interbedded fine sand and silty fine sand with occasional thin autochthonous peat beds and isolated organical- Stage 10 and its scoured nature strongly ly stained zones. Wood fragments from lowest autochthonous peat suggest that this stage represents rapid yielded radiocarbon date of 26,570 ( + 680 to-750) yr B.P. (DIC-1571). and/or continuous incision during high- Grades upward into overlying unit. Abrupt lower contact. discharge drainage from Yukon Territory 3 9.3 Gravelly Sand. Brownish-gray, cleanly washed, interbedded sand and (Fig. 5). Stage 10 terraces are absent below rounded sandy fine gravel in beds 20-40 cm thick. Contains very Fishhook Bend, suggesting that the Porcu- common cross-beds 10-30 cm thick that include large amounts of pine River became incised at or below pres- reworked Tertiary compressed peat and flattened wood fragments. Mammoth tusk near upper contact yielded radiocarbon date of 29,440 ent levels in the Lower Ramparts at that + 670 -750 yr B.P. (D1C-I570). Gradational lower contact. time. 4 8.2 Fine Alluvium. Gray thinly interbedded fine sand, silty fine sand, and medium sand with common small-scale trough cross-beds, ripple drift Stage 11: Caribou Bar Scour cross lamination, and possible dessication crack molds. Contains com- mon thin allochthonous peat beds of delicate plant and wood frag- The scoured gravel-capped bedrock ter- ments. Large fragments of reworked Tertiary organics common race remnants grouped as Stage 11 form a throughout upper part of unit, which becomes progressively sandier distinct surface lower than that of Stage 10 upward and is interbedded with fine gravel near gradational upper contact. Abrupt lower contact. (Fig. 5). These lower terraces, typified by the one at Caribou bar (locality 21), are 5 0.5 Coarse Gravel. Gray, well-washed, rounded to subrounded, medium cobble gravel with occasional boulders. Unit discontinuous and varia- mantled with varying thicknesses (0.5-14 ble in thickness, and commonly present as sparse lag of large cobbles m) of coarse, bouldery, inorganic, locally and boulders to I40-cm diameter. Abrupt channeled lower contact. variable gray gravel, similar to that of Stage 6 13.1 Pre-Late Pleistocene Deposits. (Similar to Section I.) 3. Although the scoured bedrock surfaces form a uniform terrace, the upper gravel surfaces are not concordant, possibly re- surface into irregular forms (Fig. 7). The two of the terraces are well exposed. One flecting differential removal of a once- rationale for this interpretation is discussed shows Tertiary alluvium very near a surface continuous sheet of coarse gravel or possi- under Stage 5. This modification probably that is thinly capped by coarse gravel. The bly reflecting the presence of giant bed occurred during a time of high-discharge other shows about 10 m of gray cobble- forms. overflow from the Yukon Territory. sized, inorganic gravel deposited within a Stages 10 and 11 both indicate high- channel cut into an older surface of Stage 2. discharge drainage from the Yukon Terri- Stage 9: Fishhook Terrace Although other nonpaired terraces extend tory. They may both be interpreted as up-valley from Fishhook Bend at compara- marking a pause in the rate of downcutting, Four concordant terrace remnants in ble relative heights, they cannot be corre- or they may simply reflect a threshold- Fishhook Bend between localities 12 and 14 lated with certainty to the Fishhook ter- dependent or hydrologic change that is not that are well below the terraces of Stage 7 races. presently understood. The absence of gravel are grouped within Stage 9 (Fig. 3). Only Owing to the poor exposure and re- from Stage 10 may suggest extremely strong

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TABLE 3. RADIOCARBON DATES: PORCUPINE VALLEY

Lab no. Date Material Locality Stratigraphic significance C14 yr B.P.

DIC-1856 2350 ± 55 Woody Peat Canada Border Holocene overbank sediments below large alluvial fan (Stage 12) (22)

DIC-1855 3690 + 60 Woody Peat Fishhook Bend Base of Holocene overbanks on old portion of flood plain (Stage 12) (13)

BETA-1826 5220 ± 85 Wood Coleen River Dates base of major muck deposition sequence (Stage 12) (10)

BETA-1825 15,940 ± 135 Peat Rat Creek Dates closing phase of cryoturbation and paleosol development (Stage 9?) (7)

DIC-1571 26,570 +680 Wood Tusk Bluff Dates autochthonous peat from overbank silts and sands from near top of valley-fill -750 fragments (12) (Stage 7)

BETA-1827 28,930 ± 425 Wood (spruce) Mouth of Lower part of valley-fill sequence along Porcupine River (Stage 7) Coleen River (ID

DIC-1570 29,440+670 Collagen from Tusk Bluff Sandy gravel from near top of valley-fill (Stage 7) -740 mammoth tusk (12)

DIC-1573 30,490 +730 Wood John Herberts Sandy gravel from near base of valley-fill (Stage 7) -810 Village (1)

BETA-1824 31,840 ± 855 Wood (spruce) Rat Creek Fluvial overbank sediments deposited just prior to valley incision and subsequent (7) loess deposition (Stages 3-6?)

BETA-1823 >35,000 Wood Canyon Peat layer above deposits of Stage 2 (old valley) but just below deposits of Stage 3 fragments Village Bluff (braided surface) (14)

discharge that included the flushing away of organic layers. Gravel indistinguishable and is covered by gray sheet-deposited even the largest clasts. Stage 11 may repre- from that of the modern river is occasion- gravel associated with Stage 3. The date sent a time when discharge was either ally visible below the overbank sediments. indicates that the Old Valley Stage, which reduced or when the river was overloaded Portions of the Holocene flood plain that records gradual alluviation in the Fishhook with coarse gravel, derived from erosion of are slightly elevated above the active flood Bend Basin, ended earlier than about 35,000 unconsolidated sediments in northern Yu- plain commonly show large thaw lakes in yr B.P. kon Territory. varying stages of development and, less The date of 31.840 ± 855 yr B.P. (BETA- commonly, ice-wedge polygon microtopog- 1824) was obtained from fluvial overbank Stage 12: Holocene Flood Plain raphy. sediments in Rat Creek, a tributary to the Porcupine River in the Lower Ramparts RADIOCARBON DATING The Holocene flood plain is the most con- (locality 7 in Fig. 2). The dated sediments tinuous terrace in the lower Porcupine val- Ten radiocarbon dates are presently are interpreted to have been deposited just ley, extending its full length as a prominent available for alluvial and eolian sediments prior to incision of Rat Creek, which forested bench about 2 to 4 m above datum. corresponding to various Porcupine River occurred in response to rapid downcutting The Holocene flood plain is best developed stages (Table 3). The oldest date of greater along the Porcupine River. Because Stage 6 where the valley is widest, reaching its great- than 35,000 i4C yr B.P. (BETA-1823) was represents the most major incision in the est expression in the Yukon Flats. Over- obtained from wood fragments from an Lower Ramparts, the date is thought to bank sediments, which make up the bulk of autochthonous peat lens at Canyon Village provide a closely limiting minimum age for the flood-plain deposits, generally consist of Bluff (locality 14) in the Fishhook Bend this stage. about 5 to 6 m of horizontally interbedded area. The peat lies stratigraphically above The next four youngest dates form an sands and silts with common redeposited the youngest sediments related to Stage 2 internally consistent series of ages on or-

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ganic materials deposited during the Valley- ly higher than the active flood plain, exhib- Ramparts, overflow carved a series of bifur- Fill Stage. The oldest date of 30,590 + 730 to ited large thaw lakes and a well-developed cate, coulee-like, steep-walled, bedrock -810 l4C yr B.P. (DIC-1573), from locality organic frost-heave microrelief on its sur- channels 15 to 20 m deep at localities 6 and 1. was obtained from fresh-appearing face. The youngest date of 2350 ± 55 14C yr 8. Near the inferred Yukon drainage divide, branched wood near the base of valley-fill B.P. (DIC-1850) was from Porcupine River the river apparently incised itself into a sin- sediments. The next youngest date of 29.440 overbank alluvium 200 to 300 m east of the gle narrow notch about 30 m deep with a + 670 to -740 yr B.P. (DIC-1570) was Canadian Border (locality 22). The over- basal altitude at about 300 to 310 m. Why obtained from the collagen fraction of a bank deposits there were overlain by as glacial lake overflow caused braiding during large tusk found in situ in sandy gravel 6 much as 14 m of coarse alluvial fan sedi- Stage 3 and fluvial incision during Stage 6 is to 7 m below the top of valley-fill sediments ments, indicating the recency and rate of fan difficult to determine. During the earliest at Tusk Bluff (locality 12). The complete aggradation in this vicinity. stages of overflow, discharge may have eas- tusk (probably Mammuthus primigenius) ily removed a zone of weathered rock and weighed about 30 kg, was 170 cm long, and INFERRED GEOLOGIC HISTORY soil that mantled the river valley and low was only slightly abraded or eroded, sug- The oldest well-defined stage in the evolu- slopes at this time. After removal of all gesting that it was not transported far prior tion of the Porcupine River was the Old unconsolidated materials, the river may to burial (Table 2). About 120 cm above the Valley Stage. At that time, the Porcupine have incised itself because its energy was no tusk, yet still 5 m below the surface of the River, up-valley from the Coleen, was a longer dissipated by sediment transport. valley-fill deposits, an autochthonous peat, medium-sized west-draining tributary to the The initial overflow of Stage 3 and the which contained small wood fragments, Coleen River. Eastward projection of the intermittent downcutting following braid- l4 yielded a date of 26,570 + 680 to -750 C yr concave-upward longitudinal profile of the ing that culminated in Stage 6 occurred B.P. (D1C-I571). The date of 28.930 ± 425 Porcupine to the drainage divide inferred prior to 30.600 yr B.P. The lack of deep l4 C yr B.P. (BETA-1827) was obtained from Stages 7 and 6 suggests that the divide weathering on these terraces and their rela- from a delicately branched spruce log that between Yukon and Alaskan streams lay tively fresh surface expression suggest that lay 5 to 6 m below the surface of the valley- about 15 km east of the Canadian Border at they are no older than Early Wisconsinan in fill sediments at locality 11 near the mouth about 370 to 385 m altitude. The broad pla- age. The dates from Rat Creek and John of the Coleen River. These four consistent nation surface in the tributaries and the Herbert's Village suggest that braiding and dates, obtained from three separate locali- exposed alluvial facies in sediments in the subsequent reduced(?) overflow (Stages 4 ties spanning 60 km of the river, suggest Fishhook Bend area, indicate that the Por- and 5) occurred prior to 31,000 yr B.P. and that Stage 7 began sometime before about cupine River at this time was a gently that the rapid incision associated with Stage 30,600 yr B.P. and ended sometime after meandering stream of limited discharge. 6 occurred within the next 500 to 1,000 yr about 26.500 yr B.P. In addition, the dates The Old Valley Stage ended prior to 35,000 following that date. obtained from unreworked wood strongly yr B.P. and probably spanned much of Late During Stage 7, a large wedge of rela- support the tusk collagen date by bracket- Pleistocene time. No evidence for glacial- tively fine sediment partially filled the deep ing it within a closely limiting range. lake overflow within or before the Old Val- channels carved during earlier high-dis- l4 ley Stage is recognized. The date of 15.940 ± 135 C yr B.P. charge stages. The wedge, which reaches a (BETA-1825) was obtained from peat which The change from Stage 2 to Stage 3 was maximum thickness in Fishhook Bend, formed part of a cryoturbated paleosol that apparently abrupt. During Stage 3, sheets thins gradually eastward and apparently occurred within eolian sediments at Rat of coarse gray gravel were being spread the pinched out at the inferred drainage divide Creek. Although not directly providing the full width of the valley by a very high dis- at 300 to 310 m altitude. The Valley-Fill age for any Porcupine River stage, the date charge braided river. The required high dis- Stage began sometime shortly before 30,600 does provide an indirect maximum age for a charge could have been provided only by yr B.P. and ended sometime well after significant eolian event in the Porcupine strong overflow of glacial-lake meltwater 26,600 yr B.P. During this interval, over- Valley that deposited 4 m of fine loess. This from the northern Yukon Territory. Al- flow from Yukon Territory almost certainly eolian event may correspond to a time when though valley incision during Stage 3 was did not occur, and erosion of the drainage the Porcupine River was more gravelly or probably initially rapid, it never reached divide was negligible. silt laden than at the present time. depths greater than 5 to 10 m through most Sometime after 26,000 yr B.P., strong The three youngest dates, which were of the valley, and 20 to 25 m in the Upper glacial-lake overflow resumed, causing the obtained from wood samples from Holo- Ramparts at the head of the lake outlet scouring and stripping of earlier terraces cene sediments, indicate the young age for (Fig. 5). Stages 4 and 5 are more difficult to (Stage 8) and the planation and gray-gravel much of the Holocene flood plain. The old- interpret. They suggest that incision follow- deposition associated with Stage 9. The est date of 5,220 t 84 l4C yr B.P. (BETA- ing Stage 3 was halted at least once, possi- amount and rate of incision in the Upper 1826). is from Coleen River alluvium 10 km bly during conditions of reduced or limited Ramparts at this time cannot be determined upstream from the river mouth (locality 10). overflow. from available data. Stage 10 marks a much These dated alluvial sediments are overlain During Stage 6, the response of the Por- lower position of the Porcupine River, 55 m by 10 m of frozen organic muck that con- cupine River to glacial-lake overflow was lower than that of Stage 7 and clearly below tains large ice wedges and segregated ice very different from that of Stage 3. Rather the surfaces of Bluefish, Old Crow, and Bell lenses. The date of 3690 ± 60 l4C yr B.P. than forming a broad, slightly incised gravel Basins in Yukon Territory. Incision in the (D1C-1855) was obtained from basal over- flood plain, the Porcupine River incised Upper Ramparts during this interval may bank sediments at locality 13 in the Fish- itself deeply, similar to the description given have been rapid, and the multiple channels hook Bend area. The Holocene terrace by Bretz (1924) for glacial-lake outburst in the Lower Ramparts were eroded at or above the sample locality, which was slight- flooding in Washington State. In the Lower below present levels. Glaciolacustrine lakes

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in Northern Yukon Territory must have With the exception of the oldest finite date completely drained for the last time during of 41,100 + 1,650 (GSC-1650), all other pre- Stage 10. Stage 11 marks a younger stage of transgression dates are infinite. overflow in which conditions were some- The collagen dates on redeposited bones what different. Rather than intense scouring from the Old Crow region, which are inter- and complete stripping of the surface (Stage preted to predate the glaciolacustrine phase, 10), coarse gravel was deposited prior to can be interpreted as falling into three final incision. The tremendous bulk of fine poorly defined groups (bone dates are given Holocene overbank sediments in the Por- with a 2 s.d. error). The oldest group cupine Flats, the well-developed Holocene includes, with the exception of S1-2816 flood plain throughout the lower Porcupine (36,650 t 2,600), all dates reported as infi- Valley, and the lack of prominent riffles in nite, ranging from greater than 38,000 (SE- the present channel suggest that the Porcu- 2817) to greater than 42,000 (SE-2814). A pine River is not rapidly cutting its channel younger, more poorly defined group of today and that the Holocene has been a dates ranging from 32,800 ± 1,600 (SI-2820) period of relative stability. to 34,000 ± 2,000 yr B.P. (SI-2822), shows a peak at about 33,000 yr B.P. No dates on COMPARISON WITH THE OLD organic matter correspond to this interval. CROW AND BLUEFISH The third group of collagen dates which ranges from 29,300 t 2,400 (I-I105) to BASIN STRATIGRAPHY SCALE 22,600 ± 1,200 (1-3573), and which peaks at The stratigraphy of sediments in the Old about 28,000 B.P., is also unmatched by Crow and Bluefish Basins has recently been dates on organic matter. summarized by Morlan (1980). The general Figure 9. Comparison of Porcupine River Samples from one locality on a terrace sequence consists of ancient primary and Stages with radiocarbon dates in the Old inset into the upper lake clays form a cluster reworked lacustrine sediments (Units 1 and Crow and Bluefish Basins (Morlan, 1980). of bone collagen dates that range from 2) overlain by thick, predominantly alluvial Bone collagen dates are shown with two 12,660 ± 560 (QU-782) to 11,450 + 400 yr silt and sand deposits with abundant or- standard deviation errors (indicated by tri- B.P. (QU-780), peaking at about 12,000 yr ganic material and clearly defined uncon- angles). Dates on Anodonta beringiana B.P. This interval is also unmatched by formities (Units 3 through 5). Above the shells shown by squares. Dates on plant dates on organic matter. Two bone collagen thick sequence of alluvial sediments and fragments and wood shown by circles. dates of 4,570 t 200 (1-4225) and 6,450 t 270 unconformities, there is a uniform mantle of Question marks indicate dates on wood (1-4221) form the youngest group of bone dates. gray inorganic glaciolacustrine clay (Unit 6) stratigraphically out of place. Shading indi- Dates on organic material stratigraphi- which lies on an erosional contact. Cut into cates inferred intervals of high-discharge cally above the upper lake clays form a con- the upper glaciolacustrine unit is a silt- and overflow in the lower Porcupine and pre- tinuous series from 10,850 ± 320 (1-4224) to sand-filled alluvial terrace (Unit 7) that sumed glaciolacustrine conditions in the 6,430 + 140 (GSC-372). One date on re- marks the early stages of downcutting of the Yukon basins. Cross-hachuring indicates worked wood fragments from Holocene ter- . A low silty terrace (Unit 8) intervals of high discharge and rapid ero- race deposits yielded a date of 14,390 + 160 near the modern river level and capping sion. Boundaries of Stage 6 are well defined; (GSC-730-2), possibly suggesting that the peat sediments throughout the region (Unit all others are speculative. floor of the Old Crow Basin was freely 7B) complete the generalized stratigraphic drained at this time. Alternatively, this date sequence. may reflect either organic material beyond Interpretation of the stratigraphy in the shells, 31,400 ± 660 (GSC-2739) on autoch- the edge of the basin that was reworked into Old Crow Basin has been difficult owing to thonous peat, and 31,300 ± 640 (GSC- the basin sediments at a later time or a mix- the pervasive influence of old organic mate- 1191) on organic detritus. All samples were ture of wood fragments of different ages. rial reworked into younger deposits and to obtained from organic layers beneath bio- Comparison of the alluvial features and the widely disparate dates on bone and logically sterile clays associated with the late radiocarbon dates from the lower Porcu- wood or peat; hence, no clear picture has Wisconsinan glacial lake. Other pretrans- pine River with the stratigraphy and radio- yet emerged which accounts for all of the gression finite dates are significantly older. carbon dates in the Old Crow Basin suggest dates and stratigraphic interpretations. Ma- The two youngest dates of 35,500 * 1,050 a significant reinterpretation of the glacio- jor stratigraphic questions center on when (GSC-2507) and 38,800 ± 2,000 (GSC-2756) lacustrine events which affected both areas, and how the Old Crow Basin was inundated were obtained from samples that were and it presents several unresolved para- by glacial meltwater in Late Wisconsinan mixed with older (greater than 36,000; doxes (Fig. 9). The Alaska data indicate time and when the glacial lake finally disap- GSC-2775) organics and were stratigraphi- that at least one and possibly two episodes peared (Hughes and others, 1981). cally above younger (31,400 ± 660; GSC- of strong overflow from glaciolacustrine Three key dates have been interpreted as 2739) autochthonous peat. The large errors lakes in the Yukon Territory definitely providing close minimum ages for the associated with these samples (GSC-2507 occurred prior to about 30,500 yr B.P. and transgression of the glacial lake in the Old and 2756), their chronological position near that a younger significant lake overflow Crow Basin (Fig. 9; dates taken from sum- the limit of radiocarbon dating, and their event occurred sometime after about 26,500 mary tables from Morlan, 1980, p. 262- discordant position in sequence (Morlan, yr B.P. This interpretation contrasts mark- 264). These are 32,400 ± 770 (GSC-952) on 1980) raise questions about their validity. edly with those of Morlan (1980) and

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Hughes and others (1981) which suggests fish Basins. Their results indicate that Wis- 29,300 yr B.P., are all too young by as much that a single lake episode occurred in the consin-age Laurentide ice had impounded as 9,000 yr. It is also improbable that these Old Crow and Bluefish Basins between meltwater against the Richardson Moun- dates are accurate, and that they all repre- about 31,000 and 12,000 yr B.P. tains and began to recede prior to about sent grazing animals that intermittently fell Porcupine River Stage 6, which repre- 37,000 yr B.P. A subsequent and more through ice in the center of a large lake over sents an interval of deep incision of the river extensive late Wisconsinan advance oc- a 7,000-year time span. The preferred inter- during strong overflow from Yukon Terri- curred sometime prior to about 16,000 yr pretation presented here, is that the Yukon tory, occurred prior to about 30,500 yr B.P. ago. Basins were freely drained through Mc- This event was preceded by a probable The radiocarbon chronology and terrace Dougall Pass at this time and that the dated interval of diminished overflow or nonover- geomorphology along the Alaska Porcu- bones constitute the remains of animals that flow (Stages 4 and 5), which was in turn pine River strongly suggest that Stage 3 is inhabited this surface. preceded by an interval of strong overflow correlative to part of the pre-37,000-yr-old, The absence of organic matter during this associated with Stage 3. Gravel terraces of post-early Wisconsinan Laurentide ad- 7,000-yr interval documented by the bone Stage 3 along the Alaskan Porcupine River vance in the Bonnet Plume Basin. If this dates is puzzling, but perhaps can be exhibit almost no postdepositional modifi- correlation is valid, conditions of reduced explained partially by three factors. As cation of the preserved channel-and-bar overflow or nonoverflow indicated by Por- pointed out by Morlan (1980), this interval morphology and show only limited oxida- cupine River Stages 4 and 5 are probably spans a time during which large woody tion and weathering, suggesting that they correlative to the subsequent retreat of plants probably did not exist in much of are no older than early Wisconsinan time. Laurentide ice prior to the main advance Beringia. Additionally, if the basin floor The absence of glaciolacustrine sediments during the Hungry Creek Glaciation. Close- were exposed, erosion or nondeposition relating to this interval in the Bluefish and ly limiting maximum dates for lake trans- rather than sediment accumulation may Old Crow Basins may possibly be explained gression in the Old Crow Basin about have prevailed. Finally, sediments and or- by the inferred brief innundations or by des- 31,000 yr B.P. and for overflow discharge ganic materials which may have been depos- truction of early lake sediments during sub- along the Porcupine River about 32,000 yr ited could have been oxidized or floated sequent transgressions. The cluster of bone B.P., when combined with the minimum away during the subsequent transgression. collagen dates peaking at about 33,000 yr limiting dates about 30,000 yr B.P. for The closeness in altitude between the in- B.P. may be the result of nonrandom sam- Stage 7 alluvium, suggest that Porcupine ferred Ramparts outlet and the basin floors pling and dating, but it may also correspond River Stage 6 represents a brief episode of suggest that the glacial lakes were very shal- to the inferred episode of diminished over- strong lake overflow that is not recognized low, and probably well oxygenated by flow associated with Stages 4 and 5. During in the Hungry Creek section. Hughes and waves and current circulation. Such condi- the formation of these terraces, the lake others (1981, p. 358) noted that there may tions may have allowed thorough oxidation outlet had not been cut below the surface of be "a significant gap in the record due to of existing plant remains, prior to burial the Old Crow and Bluefish Basins; conse- glacial erosion" in this section during this with silt and clay. The shallowness of the quently, reduced overflow may require that interval. lakes may also have contributed to repeated the basins were freely drained at this time. During the Valley-Fill Stage in Alaska minor transgressions and regressions during The presence of in situ vegetation in basin from about 30,600 to sometime well after their filling and draining, a situation which sediments from this time interval supports 26,600 yr B.P., glacial-lake discharge over would be expected if the lake were dammed this interpretation (Morlan, 1980). the Ramparts almost certainly did not by an ice sheet near its outer limits and therefore sensitive to minor fluctuations. The recent paper of Hughes and others occur. By this time, the floor of the spillway These explanations for the absence of (1981) presented several radiocarbon dates at the Ramparts had been cut to nearly the organic matter between 22,000 and 29,000 and stratigraphic relations which are critical same altitude as the surfaces of the Old yr B.P. are plausible, although not com- to the ages of Porcupine River stages and Crow and Bluefish Basins and the base of pletely satisfactory. The alternate hypothe- their relationship to glacially impounded McDougall Pass, the only alternate drain- sis, which requires westward lake overflow lakes in the northern Yukon. At the Hungry age route. Because the altitude of the Ram- during a time of fine-sediment aggradation Creek Section in the Bonnet Plume Basin, parts outlet cannot be exactly reconstructed in the Porcupine Valley, and which must glaciolacustrine clays (Unit 2a) which "indi- and because the effects of hydraulic pond- invalidate a consistent series of collagen cate the near approach of an advancing ing in the narrow outlet and of tectonism radiocarbon dates, is here suggested to be Laurentide ice sheet" grade upward into and glacioisostasy are presently poorly less plausible. Whatever the cause of the fluvio-deltaic sediments (Unit 2b) that im- understood, the direction of drainage dur- discrepancy between bone dates and dates ply "retreat of the Laurentide ice due to cli- ing this time interval cannot be positively on organic matter, it is not unique to the matic warming" (Hughes and others, 1981, determined. The subsequent reconstitution interval between 22,000 and 31,000 yr B.P.; p. 358). A beaver-chewed spruce stick, of lakes in the basin which required block- a similar discrepancy is apparently present which was obtained from Unit 2b, yielded a age at the Ramparts and the absence of within all other intervals represented by radiocarbon date of 36,900 ± 300 yr B.P. high-discharge features in Alaskan valley- bone dates. (GSC-2422), which was supported by age fill sediments suggest that drainage was estimates based on amino-acid dating. eastward through McDougall Pass during The absence of radiocarbon dates be- Higher in the section, Units 3 and 4 are this interval. One lake or more, possibly of tween about 22,000 and 14,000 yr B.P. in outwash and till, respectively, deposited nonglacial origin, may have existed for part the Old Crow and Bluefish Basins almost during the advance of Laurentide ice to its of this time in the Old Crow and Bluefish certainly corresponds to a time in which maximum extent during the Hungry Creek Basins. large glacial lakes occupied the basins and Glaciation. They correlate the Hungry It is unlikely that the large number of during which strong overflow conditions Creek Glaciation to the single glacial epi- bone collagen dates cited by Morlan (1980), existed in Alaska. This interval is probably sode recognized in the Old Crow and Blue- which range in age from about 22,600 to equivalent to the Hungry Lake Advance of

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Laurentide ice (Hughes and others, 1981). the stratigraphy of the Old Crow and Blue- p. 121-145. Stages 8 and 9 in the lower Porcupine River fish Basins create additional outstanding Hughes, O. L., 1969, Pleistocene stratigraphy, chronology probably date to this interval of paradoxes, rather than solving existing Porcupine and Old Crow Rivers, Yukon Territory: Geological Survey of Canada glaciolacustrine sedimentation in the Yukon problems. During preparation of this re- Paper 69-1, p. 209-212. basins, but specific events are yet undated. port, we attempted to infer the history of 1972, Surficial geology of northern Yukon Radiocarbon dates and terrace relation- the Old Crow and Bluefish Basins solely on Territory and northwestern District of ships in the Old Crow Basin indicate that the basis of the independent radiocarbon- MacKenzie, Northwest Territories: Geologi- cal Survey of Canada Paper 69-36. dated sequence of overflow discharge varia- the glacial lake there had ceased to exist, Hughes, O. L., Harington, C. N., Janssens, J. A., and its deposits were already being incised tions along the Alaskan Porcupine River. Matthews, J. V., Jr., Morlan, R. E., Rutter, by 12,000 yr B.P. and possibly as early as Comparisons with previous interpretations N. W„ and Schweger, C. E„ 1981, Upper 14,000 yr B.P. Formation of the Anadonta of the stratigraphy in the Old Crow and Pleistocene stratigraphy, paleoecology, and terrace in the Old Crow Basin by 10,000 to Bluefish Basins were then made, and appar- archaeology of the northern Yukon interior, eastern Beringia, I Bonnet Plume Basin: 11,000 yr B.P. (Morlan, 1980) indicates that ent inconsistencies were discussed. We hope Arctic, v. 34, p. 329-365. the Porcupine and Old Crow Rivers in that this report will stimulate testing of the Irving, W. N., 1978, Pleistocene archaeology in Yukon Territory rapidly incised the basin alternate hypotheses presented and thus will eastern Beringia, in Bryan, A. L., ed.. Early sediments during the succeeding 1,000 to aid in focusing future research. Man in America from a circum-Pacific per- spective: University of Alberta, Edmonton, 2,000 yr. This interval of extremely rapid Department of Anthropology, Occasional erosion in Yukon Territory probably corre- ACKNOWLEDGMENTS Papers No. 1, p. 96-101. sponds to a similar interval of rapid incision Irving, W. N., and Harrington, C. R., 1973, and deposition of reworked sediments iden- This research was carried out as part of a Upper Pleistocene radiocarbon-dated arte- facts from the northern Yukon: Science, tified by Stages 10 and 11 along the Porcu- larger project that involved the survey for v. 179, p. 335-340. pine River. and testing of early archeologic sites along Jopling, A. V., Irving, W. N., and Beebe, B. F., One major problem regarding erosion of the lower Porcupine River in Alaska. This 1981, Stratigraphic, sedimentological and the outlet at the Ramparts is the apparently project, which was based at the University faunal evidence for the occurrence of Pre- rapid incision during final drainage of the of Alaska Museum, was funded largely by Sangamonian artifacts in Northern Yukon: Arctic, v. 34, p. 3-33. glacial lakes and the comparatively negligi- the National Geographic Society. Addi- Lichti-Federovich, Sigrid, 1973, Palynology ble erosion during much of the glaciolacus- tional financial support was obtained from of six sections of Late Quaternary sedi- trine interval. For a partial answer to this the University of Alaska, the State of ments from the Old Crow River Yukon question, the supply of bedload to the outlet Alaska, and the Bureau of Land Manage- Territory: Canadian Journal of Botany, floor must be considered (Shepherd and ment Outer Continental Shelf Office. v. 51, p. 553-564. Matthews, J. V., Jr., 1975, Insects and plant Schumm, 1974). During much of the time of Other project personnel who generously macrofossils from two Quaternary expo- lake overflow, the supply of clastic material contributed time and advice to this study sures in the Old Crow-Porcupine Region, which might have served as tools for stream include David C. Plaskett (Research Asso- Yukon Territory, Canada: Arctic and Alpine bed scour were limited; hence, outlet ero- ciate), Daniel E. Hunter (Field Assistant), Research, v. 7, p. 249-259. Matthews. J. V., Jr., 1976, Arctic steppe: An sion may have been very slow. Once the and Dixon Sims, Mark Standley, and Kris- extinct biome: Tempe, Arizona, American outlet was incised below the floor of Blue- tine Thorson (General Field Assistants). Quaternary Association Abstracts, p. 73-77. fish Basin, however, a great quantity of This study benefited greatly from a joint Morlan, R. E., 1978, Early Man in northern clastic material would become immediately -Canadian stratigraphic trip Yukon Territory: Perspectives as of 1977, and increasingly available, possibly contri- in July 1980, which examined exposures in Bryan, A. L., ed., Early Man in buting to a rapid and progressive rate in America from a circum-Pacific perspective: along the Porcupine River from its mouth University of Alberta, Edmonton, Depart- outlet incision following drainage of the at Fort Yukon, Alaska, to the Bell River, ment of Anthropology, Occasional Papers glacial lakes. The highly scoured character Yukon Territory. Art Dyke, D. M. Hop- No. I, p. 78-95. and coarse gravel associated with Stages 10 kins, O. L. Hughes, J. V. Matthews, Jr., 1979, A stratigraphic framework for Pleisto- and 11, respectively, strongly suggest that C. E. Schweger, and N. W. Rutter all took cene artifacts from Old Crow River, north- high discharge, probably involving melt- ern Yukon Territory, in Humphrey, R. L., an active and cooperative role in examining and Stanford, Dennis, eds., Pre-Llano cul- water from the retreating Laurentide Ice many exposures critical to this study. Inter- tures in the Americas: Paradoxes and possi- Sheet continued after the glacial lakes were pretations in this report, however, are sole- bilities: Washington, D.C., Anthropological completely drained. Radiocarbon dates ly the responsibility of the authors. Society of Washington, p. 125-145. from the Old Crow Basin indicate that the 1980, Taphonomy and archaeology in the Old Crow River, and presumably the Por- upper Pleistocene of the northern Yukon REFERENCES CITED Territory; A glimpse of the peopling of the cupine, had stabilized near their present New World: Archaeological Survey of Can- position by about 10,000 yr B.P. Similar ada, Paper No. 94, 398 p. conditions may also have existed along the Brabb, E. E., 1970, Preliminary geologic map of Shepherd, R. G.. and Schumm, S. A., 1974, the Black River quadrangle, east-central Porcupine River in Alaska at this time. Experimental study of river incision: Geo- Alaska: U.S. Geological Survey Map 1-601. logical Society of America Bulletin, v. 85, scale 1:250,000. p. 257-268. Bretz, J. H., 1924, The Dalles type of river chan- Williams, J. R., 1962, Geologic reconnaissance CONCLUSIONS nel: Journal of Geology, v. 24, p. 129-149. of the Yukon Flats district, Alaska: U.S. Brosgé, W. P., Reiser, H. N„ Dutro. J. T., Jr.. Geological Survey Bulletin llll-H, and Churkin, Michael, Jr., 1966, Geologic The preceding discussion is by no means p. H289-H331. a solution to the stratigraphie problems sur- map and stratigraphic sections, Porcupine River Canyon: U.S. Geological Survey rounding interpretation of the geologic Open-File Report 263, scale 1:63,360. events in this part of eastern Beringia. MANUSCRIPT RECEIVED BY THE SOCIETY Hopkins, D. M., 1972, The paleogeography APRIL 6, 1982 Instead, comparisons of the alluvial history and climatic history of Beringia during REVISED MANUSCRIPT RECEIVED JUNE 21, 1982 late Cenozoic time: Inter-Nord, v. 12, inferred for the lower Porcupine River with MANUSCRIPT ACCEPTED JUNE 28, 1982

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