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GusCavson, T. C., Ashley, G. M., and Boothrayd, J. C.,
1975, Depositional sequences in glaciolacustrine
deltas, & Jopling, A. V., and McDonald, B. C.,
eds., Glaciofluvial and glaciolacllstrine sedimen-
tation: Society of Economic Paleontologists and
Mineralogists Special Publication 23, p. 264-280.
Hamilton, T. D., 1982, A late Pleistocene glacial
chronology for the southern Bmks Range: Stra-
tigraphic record and regional significance: Geo-
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p. 700-716. .
Hopklns, D. M., 1982, Aspects of the paleageography
of Beringla during the late Pleistocene, &
Hopkins, D. M., MaUlews, J. V,, Jr., Schweger, C.
E., and Young, S. B., eds., Pateoecology of
Beringia: Mew York, Academic Press, p. 3-28.
Jopling, k V., and Walker, R. B., 1968, Morphology
and origin of rippledrlft cross-laminati011 with
examples from the Pleistocene of Massachu-
setts: Journal of Sedimentary Petrology, v. 38,
no. 4, p. 971-984.
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Qlacial4ake deposits in the M m t Barpec area,
Yukon-Tauana Upland
By Florence R Weber and lhomes A. Ages
During the past year, two new 14c ages and
pollen data have thrown light on some events in the
Pleistocene history of the Yukon-Tanma Upland.
Woody debris within a volcanic ash layer previously
described (Weber, 1982) has been dated, as well as
some organic material from farther down in the same
section. The locality with the new ages is an actively
thawing 15-rn-high cutbank In unconsolidated sediment
on the Middle Fork of the Pottymile River about 18 krn
southeast of Mount Harper (loc 12, fig. 23; fig. 42).
Mount Harper, with an elevation of 1,994 m, is the
highest point in the Yukon-Tanana Upland and has
repeatedly supported active glacial systems.
The lower 3 to 6 m of the dated cutbank section
(fig. 43) is made up of stratlftd polymictic gravel and
lenticular coarse sand, including subrounded cobbles
and boulders as large as 25 cm in diameter. The larger
clasts are mainly of granitlc and other igneous rocks
but also include gneiss, augen gneiss, quartzite, and
some schist. Locally at the top of this lower section is
as much as 30 cm of sand. In places, several centimeters
of this sand and gravel is oxidized orange.
The overlying 9 to 10 m in the middle and upper
part of the section (fig. 43) is made up of alternating
thin layers of sand and silt. The sand is coarse and
gravelly, but, in contrast to the lower part of the section,
the gravel is made up of angular bits of schist or
quartzite and contains no igneous rocks. The metamorphic~ock
clasts are mostly abut 2.5 cm In diameter;
the largest schist clast observed wes 20 cm in
diameter, although such large flat angular fragments
are very rare. Much fine dark organic material is
mixed in the silty lagers, particularly near the base.
The top meter of the entire section is loes containing
some organic material, but because of frost
mixing it is unclear whether the contact is sharp or
gradational between the loess and the underlying sand
and silt. Close to the bottom of this meter of loes is
a layer of white volcanic ash 3 to 15 cm thick (here informally
designated as the Mount Harper ash).
Studies in the Mount Harper area of the upland
indicate three major glacial advances, which, for convenience,
are here designated I, 11, and 111, from oldest
to youngest. Figure 42 maps the extent of recognized
glacial advances in the Mount Harper area. The
glacier of maximum extent (I) In valley X (fig. 42)
opposite the mouth of Molly Creek extended down to
and blocked the valley of the Middle Pork of the
Fortymile River in two separate episodes. Episode IB
is recognized by a well-defined inner terminal
moraine. Instead of typical Irregular morainal topography,
however, the upper surface of this feature is
weathered to a terrace of low relief because of its
age; such level morainal surfaces are characteristic of
older glacial advances in the upland. Moralne of the
earlier episode (IA) is no longer extant, but its presence
is defined by erratic boulders at an elevation of
slightly more thnn 900 m, by n concentration of large
lag boulders in the stream valleys, and only locally by
topographic form.
A large lake, here Informally named "Lake
Harper," formed when the Middle Pork was blocked by
glacial ice extending from valley X. The approximate
shoreline of Lake Harper is drawn at the 900-m contour
(fig. 42); at its maximum extent the water level
was probably somewhat higher, Outwash of the advance
I glaciers in valleys Y end Z (fig. 42) discharged
into the lake and formed alluvial fans. A smaller lake
may have formed in the lower valley of MoUy Creek,
which was a h blocked by valley X ice advances.
The flat f#m of the Lake Harper basin, approximately
40 km in area, at present is covered with
unconsolidated sand and gravel. The gravel, mostly of
schist or schistose gneiss, is evidently derived from the
local bedrock. A few thin sand and clay layers occu~
under loess copping the low bluffs that stand slightly
above the flats.
The cutbank is in sediment making up the floor
of the valley. The basal stratified gravel and sand are
thought to represent a distal part of the outwash fan
of valley 2, probably of glacial episode IA. The large
amount of granite in this gravel indicates that the
gravel came from the Mount Harper area to the west.
Metamorphic racks surround and probably underlie
much of the Lake Harper basin. Granite is found only
near Mount Harper at the head of the glacial valleys
and at the far southward reaches of the Middle Fork
drainage (Poster, 1976). This gravel was exposed and
oxidized after glacial episode IA.
A sharp break occurs at the tog of the oxidized
gravel, above which a thick succession of finely
layered organic and finegrained glacial3ilt lakebeds
was deposited. This lake (Lake Wpm) Pormed#ainst
the terminal moraine of glacial eplsode 19. A C age
on fine woody debris collected from near the base of
the lakebed section is older than 50,300 years (USGS
1122).
A pollen sample (80A Wr 332A) from mentially
this dated horizon contains abundnn't fine woody debris
and well-preserved pollen. The fossil assemblage includes
pollen of spruce (Picea), birch (Betub sp.), alder