UNIVERSITY OF CINCINNATI
Date:______
I, ______, hereby submit this work as part of the requirements for the degree of: in:
It is entitled:
This work and its defense approved by:
Chair: ______
Defining the timing of glaciation in the central Alaska Range
A thesis submitted to the
Graduate School
Of the University of Cincinnati
In partial fulfillment of the
Requirements for the degree of
MASTER OF SCIENCE
In the Department of Geology
of the McMicken College of Arts and Science
November 2006
By
JASON MICHAEL DORTCH
B.S., Geology
University of California Riverside, California, December 2004
Committee Chair: Dr. Lewis A. Owen
Abstract
A succession of moraines and terraces in the Nenana River valley, Reindeer Hills, and the
Monahan Flat in the central Alaska Range were dated using Terrestrial Cosmogenic
Nuclides (TCN) and optically stimulated luminescence (OSL). Moraines date at >125 ka
(Lignite Creek glacial stage), ~60 ka (Healy glacial stage), 22-30 ka (Riley Creek 1 glacial stage), ~22 ka (Riley Creek 2 glacial stage), and ~19 ka (Carlo glacial stage).
These are minimum ages of deglaciation.
Comparative examination of boulders shows that the position of boulders and condition/type of landforms are more influential of TCN ages than the condition of boulders. TCN ages on boulders from drumlins are more tightly clustered than morainal boulders. No correlation of boulder condition and accuracy of TCN ages was found.
iii
iv Table of Contents
1 Introduction 1
2. Study Area 3 2.1. Regional Setting 3 2.2 Nenana River Valley 6
3. Methods 12 3.1. Field Methods 12 3.1.1. Surface Exposure Dating (SED) 12 3.1.2. Optically Stimulated Luminescence Dating 14 3.2 Laboratory Methods 15
4. Sample Locations 16 4.1. Moraine succession 16 4.2. Eight Mile Lake Region 26 4.3. Other glacial landforms 27 4.4. Terraces 28 4.5 Loess deposits on outwash terraces 36
5. Results 38 5.1. TCN Results 38 5.2 Evaluation of TCN Data 43 5.2.1. Uncertainty 43 5.2.2. Oldest Boulder Method 44 5.2.3. Evaluating the Data Set 45 5.2.4. Geologic Processes 46 5.3. OSL Results 59 5.4. Evaluation of OSL Data 62
6. Timing of Glaciation 65
7. Correlation with other regions of Alaska 72
8. Conclusions 73
References 75
v 1. Introduction
Reconstructions of paleoclimatic changes through proxy data, such as glacial landforms and
sediments, enable the development and testing of models that may be used to predict future
environmental change. The most severe environmental changes are predicted to occur in high
latitude regions, such as Alaska (Watson et al., 1997). Fortunately, these regions contain
abundant glacial geologic evidence that may be used to help reconstruct paleoclimatic conditions
and that can help in determining the nature of past and future environmental change. Given the
detailed geologic record in these regions it is surprising that few quantitative glacial chronologies
have been developed in key areas of Alaska, such as the Alaska Range (Fig. 1). Consequently,
the forcing mechanisms, spatial, and temporal variations of glaciation and climate throughout
Alaska are not well defined.
The Nenana River valley on the north side of the Alaska Range contains an impressive
succession of glacial and non-glacial landforms that can be used to reconstruct the nature of
Quaternary glaciation. These landforms are particularly well-preserved here because they were
not overrun by the extensive Cordillera Ice Sheet during the Last Glacial Maximum. The glacial
and associated landforms comprise a succession of at least six moraines with associated outwash
terraces. These potentially record the nature of climate change over multiple glacial cycles.
The sequence of glacial events that created these landforms was initially proposed by Wahrhaftig
(1958) and modified by Thorson (1986). However, these researchers could not date many of the
moraines and associated landforms because many were beyond the age range of radiocarbon
methods, the standard dating methods available at the time of their studies. Modern dating techniques, such as terrestrial cosmogenic surface exposure and luminescence dating, now
1 provide an opportunity to date these landforms to help elucidate the nature of Quaternary climate and associated environmental changes in Alaska.
The research presented here focuses on developing a quantitative glacial chronostratigraphy using terrestrial cosmogenic radionuclide (TCN) surface exposure dating (SED) and optically stimulated luminescence (OSL) dating of moraines and outwash terraces in the Nenana River valley to define the timing of glaciation. The objectives of this study are: (1) developing a quantitative chronology of the timing of glaciation in the central Alaska Range, (2) provide an assessment of the applicability of SED on both moraines and terraces for defining glacial chronologies in central Alaska through the comparison of multiple samples collected from each surface, and (3) assess whether glaciation was synchronous throughout the region by comparing the timing of glaciation with other regions of Alaska.
2
Figure - 1: DEM of the central Alaska Range. The topographic divide separates drainage that flows south via Broad Pass and the Monahan Flat to the Cook Inlet and the Gulf of Alaska from drainage that flows north to the Bering Sea via the Tanana River. There are two notable exceptions, the Nenana River and the Delta River, that head south of the divide and flow north across the Alaska Range to the Tanana River. Note the location of Fairbanks and Mt. McKinley for scale. (DEM altered from USGS.Seamless data; the inset is from http:// adgc.usgs.gov)
2. Study Area
2.1. Regional Setting
The Alaska Range is a crescentic, convex north, belt of mountains located in south central
Alaska that stretch ~950 km east/west and 80-200 km north/south (Fig. 1) (Capps, 1940;
Wahrhaftig, 1958). Contemporary deformation is caused by movement of the Denali Fault system, which consists of two major fault strands, the Hines Creek and the McKinley (Fig. 1)
(Hickman, 1977). Cenozoic offset along the Hines Creek strand is limited to local dip-slip
3 movement, while the McKinley strand is responsible for at least 30 km of right lateral slip
(Hickman, 1977). Holocene deformation rates are estimated to range between 1-2 cm/year
(Hickman, 1977). The average elevation of the range crest is between 1,500 m to 2,750 m asl,
containing ~40 mountains higher than 3,000 m asl, including the highest mountain in North
America, Mt. McKinley (6,194 m) (Capps, 1940; Wahrhaftig, 1958). The bedrock of the Alaska
Range consist of two major groups that trend east west (Wahrhaftig, 1958). The older group
comprises of schist, gneiss, phyllite, chert, argillite, limestone, conglomerate, slate, shale, coal,
and greenstone (both intrusive and extrusive) and ranges in age from Precambrian to Cretaceous
(Wahrhaftig, 1958). These rocks are well consolidated or metamorphosed and are resistant to
erosion. The younger group of rocks formed during the Tertiary. They are easily eroded, and
consists mainly of the Coal-Bearing Formation and the Nenana Gravel Formation (Wahrhaftig,
1958). These rocks are poorly consolidated and are restricted to the foothills and lowlands north
of the range crest (Wahrhaftig, 1958).
The crest line of the Alaska Range is both the north-south topographic divide and a climatic boundary. The climatic boundary represents the separation of the moderately warm and moist
(annual precipitation of ~76 cm) climate of southern Alaska from the colder dry (annual precipitation of ~28-35 cm) climate of the interior of Alaska (Capps, 1940). Glaciers on the southern slopes of the Alaska Range are larger than those on the northern slopes due to the higher precipitation to the south.
Two major glacial systems, the Cordilleran Ice Sheet and isolated alpine glaciers, influenced the
Alaska Range during the Quaternary. The Alaska and Aleutian Ranges blocked the expansion of the Cordilleran Ice Sheet to the north and west, while the Kenai and Chugach Mountains blocked its expansion to the southeast (Hamilton et al, 1983). The Cordilleran Ice Sheet reworked
4 sediments and landforms of previous glaciations across southern Alaska (Hamilton et al, 1983).
This has resulted in a complex glacial record with the loss of much of the evidence for earlier glaciation. In addition, much of the margin of the Cordilleran Ice Sheet terminated in proglacial lakes, marine environments, or submerged parts of the continental shelf obscuring the morphological and stratagraphic records (Hamilton et al, 1983). Water-well logs in the
Anchorage lowland contain evidence of five to seven glacial advances separated by intervals of soil formation that probably date from Miocene to early Pleistocene (Schmoll et al, 1999).
However, these sequences cannot be correlated with glacial landforms due to the poor dating control (Schmoll et al, 1999).
During the Late Wisconsin glaciers advanced in south-central Alaska and coalesced to form the
Cordilleran Ice Sheet. High relief and contrasting moisture regimes created differing rates of snow accumulation in individual source areas enabling regions of the Cordilleran Ice Sheet to advance and retreat independently of each other (Hamilton et al., 1983; Schmoll et al, 1986;
Thorson, 1986). These regions include the Copper River Basin, the Cook Inlet-Susitna lowland, and the Alaska Range. The Copper River Basin was influenced by a continental climate, whereas the Cook Inlet-Susitna lowland and the Alaska Range were more maritime and largely influenced by moisture moving northward up the Cook Inlet (Hamilton et al., 1983). However, moisture reaching the Alaska Range, which fed the northwestern region of the Cordilleran Ice
Sheet, was limited by mountains and ice fields to the south (Hamilton et al., 1983).
The crest line of the Alaska Range limited the extent of the Cordilleran Ice Sheet and the moisture supply to the north (Hamilton et al., 1983). The moisture limitation restricted glaciation to individual alpine glaciers in most of the northern valleys of the Alaska Range
5 (Hamilton et al., 1983). The restricted glaciation led to the preservation of successions of glacial and non-glacial landforms in the north trending valleys of the Alaska Range.
Glaciers from the Cordilleran Ice Sheet flowed across the Alaska Range through two north trending valleys, the Delta River and Nenana River valleys (Fig. 1) (Hamilton et al., 1983).
These outlet glaciers coalesced with alpine glaciers such as the Black Rapids, Canwell, Castner, and Eel Glaciers in the Delta River valley and the Yanert Glacier in the Nenana River valley.
The Delta River valley’s glacial record is not as well preserved as in the Nenana River valley.
This is possibly due to a larger outlet glacier entering the Delta River valley, which coalesced with several large alpine glaciers and reworked sediments that were previously deposited (Beget et al., 2002). In contrast, in the Nenana River valley, a smaller ice stream and only one major alpine glacier probably enabled preservation of a more complete glacial record.
2.2 Nenana River Valley
The Nenana River valley is located on the north side of the central Alaska Range near Healy. It is in a transition zone between the cold continental climate of interior Alaska and the milder maritime climate of south central Alaska (Fig. 2) (Thorson, 1986). Contemporary mean annual temperature ranges from –2o to 0o C. The annual precipitation averages 30-50 cm, which is dominantly in the form of snow (Thorson, 1986). Precipitation decreases toward the north due to orographic effects. The current equilibrium line altitude (ELA) of the Yanert Glacier, located on the north side of the Alaska Range, is 1,310 m asl based on toe to head wall ratio and change in glacier form (Meierding, 1982).
The morphology of the Nenana River valley is significantly influenced by rock type. Bedrock within and between the Nenana Corridor to the Nenana Canyon consists mainly of Birch Creek
Schist (Wahrhaftig, 1958; Thorson, 1986). The head of the Nenana River valley (Southern end)
6 is ~15 km long, 3.5 km wide, 1.5 km deep and forms a u-shaped corridor that enabled north flowing ice to coalesce with the Yanert Glacier (Fig. 3) (Hamilton et al., 1983; Thorson et al.,
1985; Thorson, 1986). The Nenana Canyon is 2-3 km wide, ~1.5 km deep, and contains numerous well developed glacial trimlines (Fig. 4) (Thorson, 1986). North of the Nenana
Canyon the valley widens abruptly to 7-10 km because the more easily eroded Nenana Gavel has enabled glaciers to more effectively erode this region (Warhaftig, 1958; Thorson, 1986). From
Healy northward, the Nenana Gravel comprises both ridges and lowlands and is exposed almost continuously along the valley (Warhaftig, 1958; Thorson, 1986).
The setting of the Nenana River valley influences this study due to the affects of geologic processes on TCN dating. Vegetation in the Nenana River valley is dense and can cause significant bioturbation of sediments. Well-drained areas such as moraines and outwash terraces are typically covered with white spruce that is spaced 2-4 m apart. Peat and moss is typically
0.1-0.5 m thick, and dominates the terrain covering some of the largest of boulders in areas where drainage is poor, such as the Eight Mile Lake Region. Significant cryoturbation occurs in these areas and in lowlands where drunken forests (areas of leaning black spruce, which thrive in frozen and/or poorly drained soil) and deformed roads are present. Periglacial processes result in angular shattered rubble, mainly consisting of the Birch Creek Schist, that covers morainal ridges above 800 m asl in the Nenana Canyon. In addition, formerly glaciated benches in the Nenana
Canyon such as those on Sugarloaf Mountain and Mount Healy have abundant pattern ground and gelifluction lobes. The abundant frost shattered rock, exhumed boulders, weathered and disintegrated granitic erratics throughout the Nenana River valley suggest high rates of weathering in this region.
7 The Nenana River valley contains abundant glacial and non-glacial landforms, which include: morainal ridges, ground moraines with hummocky kettle topography; glacial outwash terraces, kame terraces, strath terraces, loess, and large gravel deposits. The Nenana valley is cut by the east/west trending Denali Fault zone. This fault zone is very active as recently demonstrated by the 2002 M7.9 Denali Fault Earthquake, and is responsible for much of the Cenozoic and
Quaternary uplift of the Alaskan Range (Fig. 3) (Thorson, 1986; Lincoln, 2003). In at least three areas (the foothills, Eight Mile Lake region, and the Nenana Canyon) faulting deforms the landforms (Wahrhaftig, 1958; Hickman, 1977; Thorson, 1986; Thoms, 2000). Wahrhaftig,
(1958) and Thorson (1986) used reconstructed ice gradients, topographic position, overall morphological expression, vertical position in the valley, and their relative extents trace and correlate landforms across these faults.
Figure-2: DEM of the central Alaska Range. Red boxes show the location of Figure 7 H, E, & F (from left to right). Fault locations taken from Warhaftig (1958), Thorson (1986), and Hickman (1977). DEM from USGS.seamless data.
8
Figure - 3: DEM showing views of the corridor where ice from the Cordilleran Ice Sheet flowed north into the Nenana River valley near the town of Windy. LGM ice south of the Alaska Range crest flowed through Broad Pass creating lineated landforms such as scour lakes, drumlins, and eroded Reindeer Hills. Image altered from USGS.seamless data and Landsat-7 ETM+ data from the University of Maryland.
Figure – 4: Oblique view of the west side of the Nenana Canyon showing possible trimlines (Thorson, 1986). These indicate a series of at least five nested benches scoured by separate glacial advances (Thorson, 1986). Note the flat bench between the 1st (highest) and 2nd trimlines that was suggested to represent the paleo-valley of glacial advances during the Browne and Bear Creek glacial stages (Thorson, 1986; Wahrhaftig, 1958).
In the Nenana valley, Warhaftig (1958) and Thorson (1986) identified a succession of at least six sets of moraines which they assigned to glacial stages calling them: the Taklanika (the oldest and
9 most extensive), the Browne, the Bear Creek, the Lignite Creek (Warhaftig’s (1958) original Dry
Creek advance), the Healy, the Riley Creek, and the Carlo (Fig. 5, Table 1). No previous numerical dating has been undertaken on the Teklanika, Browne, Bear Creek, and Healy glacial advances or the Eight Mile Lake moraines (Fig. 5). Wahrhaftig (1958) identified and correlated outwash terraces with each glacial stage (Fig. 5). The terraces are named after the glacial stage that preceded them. These will be described in detail in section 4.1 on the moraine successions and terraces.
Table 1: Suggested ages of glacial stages in the Nenana River valley by previous workers. Glacial Stage Suggested Ages (morphostratigraphic order) Wahrhaftig (1958) Thorson (1986) Beget et al. (1991) Taklanika Not recognized Late Tertiary No comment Browne Late Tertiary Mid Pleistocene No comment Bear Creek Recessional of Browne Stage Separate advance No comment Dry Creek Mid Pleistocene Inconclusive No comment Lignite Creek Coupled with Healy Stage Separate advance ~140 ka (Tephra) Healy no comment >Late Wisconsin ~60ka (Suggested) Riley Creek Late Wisconsin Late Wisconsin Late Wisconsin Carlo * Late Wisconsin No comment No comment *Referred to by Thorson (1986) as the Carlo Readvance.
10
Figure 5: DEM with the location of moraines and outwash terraces in the Nenana River Valley (See white box in Fig. 2 for location). Moraines and outwash terraces for each glacial stage is listed in chronostratigraphic order in the key except for the Eight Mile Lake glacial stage moraines, whose morphostratigraphic relationship with the other landforms is unknown (Thorson, 1986).
11
3. Methods
3.1. Field Methods
Geomorphology and Sedimentology
The Teklankia, Browne, Bear Creek, Lignite Creek, Healy, Riley Creek, Carlo, and Eight-Mile
Lake moraines and outwash terraces mapped by Wahrhaftig (1958) and Thorson (1986) were
reexamined and remapped in the field, aided by aerial photography, 2o National Elevation
Dataset DEM (obtained from USGS-Seamless), and Landsat-7 satellite image analysis (courtesy
of the University of Maryland website http://glcf.umiacs.umd.edu/data). Terraces and terrace
capping loess were examined in gravel pits and natural stream cuts. Graphic sedimentary logs
were constructed using the sedimentary techniques of Evans and Benn (2004). Sedimentary
units were described on using particle size, sorting, and internal structures using the
nomenclature of Evans and Benn (2004).
3.1.1. Surface Exposure Dating (SED)
The timing of moraine formation and outwash terrace deposition in the Nenana River valley was
established using (SED), using the concentration of 10Be in boulders and sediments. Significant sources of uncertainty with SED include: production rate and geologic processes including stability, inheritance, erosion, and shielding. These factors are accessed in section 5.2. Loess deposits were observed only on the Lignite Creek moraine. Samples were collected from terrace deposits that appeared to retain original forms such as imbrication of clasts and stratification to help avoid the problems of cryoturbation.
Samples for boulder SED were collected by chiseling off ~500 g of rock from the upper 5 cm of granitic erratics on moraine crests and slopes. Sampling in areas where obvious evidence of
12 exhumation or slope failures (i.e. slopes that are visibly scared, slumped, or cryoturbated) were
avoided when possible. Locations of samples are in Table 2. The characteristics of each boulder
was recorded, which includes: joints, exfoliation, weathering pits, weathering rinds, surfaces that
are bounded by joint planes, and position on the moraines (crest or slope) (Fig. 6 & Table 3).
Boulders with a height >1 m were chosen for sampling when possible to reduce the possibility of shielding by snow for significant periods and to reduce the likelihood of any loess cover in the
past.
To determine the reproducibility of SED dating and to check for the possibility of inheritance of
TCNs, several (4-15) boulders were sampled from moraine ridges, and in some cases two
samples were collected from the same boulder to test for differential weathering on individual
boulders (Fig. 6D). The degree of weathering on boulders, their position on the moraine (crest or side slopes), and slope was recorded (Table 3). Topographic shielding was determined by measuring the inclination from the boulder to the top of the surrounding horizon and recorded if
>20o (Table 2).
Pits ranging from 2-5 m deep were excavated in outwash terraces. Six bulk sediment samples were collected from the pits for depth profiles at 20 cm intervals. Sediment was collected from
5-7 cm above and below the 20 cm intervals. The depth, vertical range of sediment collection, and thickness of terrace capping loess was recorded for each sample. These data are described and plotted onto graphic sedimentary logs and in section 4.4.
13
Figure 6: Views of selected boulders sampled in the Nenana River valley. (A) a large (> 10m diameter) granitic boulder with little to no weathering rind located on the side of the Browne glacial stage moraine (note person for scale). The gap between the base of the boulder and the vegetation indicates movement. (B) shows another large (~8m diameter) boulder on the side of the Browne glacial stage moraine with a slope of approximately 20o. This boulder displays 3 horizontal joints that will eventually weather and cause the dispersion of the boulder into a cluster (C). (C) three moss covered boulders from a cluster of approximately seven, each 3-5m in diameter. (D) shows differential weathering on a boulder surface. (E) a boulder that has a thick weathering rind and an extensively eroded joint. (F) a boulder with a surface bounded by a joint plane. The remnant core of the removed part of the boulder is only a few centimeters away. The boulder in (D) is from the Carlo glacial stage deposit (Fig 7D for location); all other boulders are from the Browne glacial stage moraine (C, E, & F see Fig 7G for location). (A & B) were not sampled.
3.1.2. Optically Stimulated Luminescence Dating
Optically stimulated luminescence (OSL) samples were collected from terrace capping loess to determine the ages of loess and to test the radiocarbon ages proposed by Thorson et al. (1976), which will help estimate the duration of TCN shielding. One OSL sample was collected from a sand lens within the Riley Creek 1 terrace. Several decimeters of sediment were scraped away from the face of the exposure of OSL sampling sites to remove possible slump material and any bleached sediment before sampling. Using a 20 cm black plastic tube, samples were collected perpendicular to the vertical surface that was newly exposed.
14 3.2 Laboratory Methods
TCN
Following crushing, TCN samples were sieved to the 250-500 µm size fraction for each boulder
sample. Depth profile samples were first sieved to < 2mm and then crushed and resieved to
obtain the 250-500 µm size fraction. Sieving was followed by a minimum of four acid leaches:
aqua regia for >9 hours, two 5% HF/HNO3 leaches for approximately 24 hours, and one 1%
HF/HNO3 leach for 24 hours. To remove acid resistant and mafic minerals such as zircon and magnetite, heavy liquid (density of 2.7 g/ml) separations with lithium heteropolytungstates
(LST) were used after the first 5% HF/HNO3 leach. Pure quartz was dissolved in concentrated
HF and then fumed with perchloric acid to remove fluoride atoms. Next, the sample was passed through anion and cation exchange columns to remove Fe and Ti and to separate the 10Be and
26Al fractions. Amonioum Hydroxide was added to the 10Be fractions to precipitate beryllium
hydroxide gel. The beryllium hydroxide was oxidized by ignition at 750oC in quartz crucibles.
Beryllium oxide was mixed with Nb powder and loaded in steel targets for the measurement of
the 10Be/9Be ratios by accelerator mass spectrometry at PRIME Laboratory in Purdue University.
OSL
Five cm of sediment was removed from both ends of the OSL tube in restricted light at the
Luminescence Laboratory at the University of Cincinnati. The ends and the middle portion of
the sample were put into an oven <50oC and weighed every 12 hours until dry. The dry ends were crushed and homogenized then sent to the USGS Nuclear Reactor facility in Denver for
neutron activation analysis. The middle portion was sieved to the 125µm-90µm grain size fraction. Then the sample was leached in 10% HCl for 24 hours, 30% H2O2 for 24 hours, and
10% HF for 20 minuets to eliminate impurities. Next, heavy liquid (sodium polytungstate) was
15 used to separate the quartz fraction by density separation. This was followed by a 49% HF leach for 40 minuets to etch the outer 10µm and remove the alpha dose. Steel target discs were spray with silica spray and a single grain thick layer of quartz grains was dispensed onto steel discs.
The discs were measured in the Geochronology Laboratories at the University of Cincinnati using the single aliquot regenerative method (SARS) on a RisǾ TL/OSL reader (model DA-20)
(Murry et al., 2000). 4. Sample Locations
4.1. Moraine succession
The moraine succession is described in morphostratigraphic order (oldest to youngest). We use
Wahrhafitgs’ (1958) and Thorsons’ (1986) nomenclature and refer to these as glacial stages. In addition, two sets of moraines (Eight Mile Lake Moraines) that could not be morphostratigraphically correlated with other moraines are described in section 4.3 (Figs. 5 & 7).
16
Figure-7: Glacial landforms and sample locations in the Nenana River valley (See Figs. 2 & 5 for location of areas A-H and samples Ala-125, 301-306, 600-605, and 701-706). Dotted lines indicate outer limits of moraine flanks. (A) The Eight Mile Lake region contains both Eight Mile Lake glacial stage moraines, the Lignite Creek glacial stage push moraine complex, and is bounded to the south by Gravels Ridge. (B) The area north of the Nenana Canyon contains landforms from the Healy glaciation including moraines, drumlins (samples 23-25) scoured bedrock (samples 107-108) and Otto Lake (interpreted as a possible kettle by Wahrhaftig (1958). (C) The area east of the McKinley Park entrance contains deposits from the Riley Creek 1 & 2 glacial stages. Riley Creek 1 glacial stage deposits (samples 15-20) are farther north than Riley Creek 2 glacial stage deposits (119-123). (D) Area north of Carlo contains deposits from the Carlo glacial stage. Box (E) is Monahan Flat west and (F) is Monahan Flat east. Samples in both areas were collected from lateral moraine deposits created by the coalescence of the Nenana, West Fork, and Sustina glaciers. (G) The northern end of the Browne glacial stage moraine on the west side of the Nenana River. Note, the scale bar is 0.5km. (H) Shows south flank of Reindeer hills. A total of 13 samples were collected in groups of three from 5 glaciated benches. DEM from USGS Seamless data. Taklanika Glacial Stage
17 Thorson (1986) recognized the Teklanika lateral-frontal moraine and suggested the Teklanika advance was responsible for erratics outside the limits of the Browne glacial stage. Furthermore,
Thorson (1986) suggested that the Teklanika glacial stage represents two separate glacial advances and that it should be assigned to the Late Tertiary (Figs. 5 & 7; Table 1).
The Taklanika lateral-frontal moraine, located ~13 km to the west of the Browne glacial stage moraine, is 12 km long and covered by ~ 14 m of loess (Thorson et al. 1985). Erosion has exposed >20 m of subangular to angular till that contains striated and faceted boulders with a poorly sorted matrix (Thorson, 1986). This moraine suggests that the main ice lobe extended to with in 20-30 km of the town of Nenana (Thorson, 1986).
It was not possible to investigate or sample this moraine due to its remote location. However, sample Ala-125 was collected from a high glacial bench (856 m) on Sugarloaf Mountain that
Thorson (1986) correlated with the Taklankia advance by (Fig. 5).
Browne Glacial Stage
Wahrhaftig (1958) suggested that the Browne glacial stage is also Late Tertiary. He noted that boulders on the moraine crest commonly occur in clusters. Wahrhaftig (1958) and Thorson
(1986) believed that the most extensive glacial stage was Late Tertiary. The discovery of the
Taklanika glacial stage deposits prompted Thorson (1986) reassign ages to younger glacial stages accordingly. Therefore, Thorson (1986) suggested that the Browne glacial stage is Mid
Pleistocene (Table 1).
The Browne glacial stage is characterized by ridges that are continuous for 8 km and 15 km on the west and east side of the valley, respectively (Figs. 5 & 7) (Thorson, 1986). The terminus of the Browne glacial stage moraine is located on the south side of Birch Creek, 115 km from the contemporary snout of the Yanert Glacier. It comprises a 250-500 m wide ridge with a crest that
18 rises <50 m above the present valley floor (Wahrhaftig, 1958; Thorson, 1986). The flanks of the
moraine slope from 15-20o and are currently undergoing deflation (Thorson, 1986).
Surface boulders ranging in size from < 1 m to >10 m in diameter consist mainly of granite.
Boulders on this moraine occur in two populations: clustered and isolated. Boulders found on
the moraine crest in clusters commonly show little sign of weathering and are bounded by joint
planes. Isolated boulders on the crest are extensively jointed, rounded, disintegrating, and have
weathering pits (Fig. 6 C&E). These well-weathered boulders are found isolated from other
boulders and clusters. TCN samples were collected from isolated (Ala 1-5, 112, 117, & 118) and
clustered (Ala 111, 113, 114, 115, & 116) boulders on the crest of the moraine. Two TCN
samples (Ala 109 & 110) were collected from the flank of the moraine.
Bear Creek Glacial Stage
Wahrhaftig (1958) suggested that the Bear Creek glacial stage moraine represents a recessional
or small readvance of the Browne Stage. However, Thorson (1986) suggested that the Bear
Creek glacial stages was separated by a significant time period based on the height of moraines,
terrace development, and weathering criteria (Fig. 5; Table 1). Furthermore, Thorson (1986)
suggested that the glacier that created the Bear Creek glacial stage moraine advanced down the
Nenana River valley and incised about 50-100 m below the Browne drift (Thorson, 1986).
The Bear Creek glacial stage moraine is a prominent ridge with surface erratics (Thorson, 1986).
This moraine is located 110 km from the current toe of the Yanert Glacier south of Bear Creek
(Figs. 5). Thorson (1986) interpreted Bear Creek as an ice marginal melt water channel. TCN samples were not processed for the Bear Creek glacial stage.
Lignite Creek Glacial Stage
19 Wahrhaftig (1958) suggested that the Lignite Creek glacial stage was a recessional or small readvance of the Healy glacial stage. However, Thorson (1986) argued that the Lignite Creek and Healy glacial stages were separated by a significant time gap. Thorson (1986) based this on the fact that the Lignite Creek glacial stage moraine was a significant distance from the Healy glacial stage and the more subdued morphology of Lignite Creek glacial stage deposits.
Irregular stratified gravel deposits are present along Stampede Trail inside the limits of the
Lignite Creek glacial stage moraine. These were described by Thorson (1986) and Beget et al.
(1991). Beget et al. (1991) described deformed sediments in a newly cut gravel pit (63.88811N/
149.10228W) in the main Lignite Creek glacial stage moraine located ~3 km west of the Parks
Highway on Stampede Trail. Based on these deformed sediments and the irregular stratified gravel deposits, Beget et al. (1991) suggested that Lignite Creek glacial stage deposits are the result of glaciotectonism, as advancing ice pushed and deformed sediment into a complex set of ridges creating a series of push moraines (Beget et al., 1991). Lignite Creek glacial stage deposits occur north of Gravels Ridge, which is composed of Nenana Gravel. The push moraines were interpreted by Thorson (1986) to be composed of reworked Nenana Gravel from
Gravels Ridge. The deformed sediments also contained a tephra, which Beget et al., (1991) called the Stampede Tephra. This tephra did not correlate chemically with any previously known tephra. Beget et al. (1991) suggested that the Lignite advance is occurred during the mid-
Quaternary (~140,000 ka) or older because it must be older than the Healy glacial stage, which was tentatively assigned to MIS 4 or 6.
Thorson (1986) defined the Lignite Creek glacial stage limits by a nearly continuous morainal ridge north of Otto Lake between Panguingue and Dry creeks. This is 80 km from the contemporary snout of the Yanert Glacier. The Lignite Creek glacial stage moraine has slopes of
20 ~20o with a crest that rises ~30 m above Panguingue Creek (Thorson, 1986). A gravel pit,
(stampede gravel pit) located in the main Lignite Creek glacial stage moraine, comprises stratified gravel toped by reworked loess and overlain by disturbed sediment (Fig. 7). Deformed stringers of reworked coal are present in the loess (Fig. 8A). South of Otto Lake and in the
Nenana Canyon the Lignite Creek glacial stage moraine is discontinuous and covered in frost- shattered rubble with occasional small angular boulders. The boulders comprise mainly Birch
Creek Schist and granitic erratics. West of Otto Lake, granitic erratics are sparse and the moraine crest is heavily vegetated with shrubs and trees.
Boulders (Ala 137 A/B, & 156) on the moraine are extensively weathered and jointed. Two granitic erratics (Ala 135 & 136) were found south of Panguingue Creek on the north slope of the moraine. A depression up slope of the boulders and the gap between vegetation and the boulders suggests that the boulders recently moved down slope. These boulders have a hard fresh surface with no weathering rind. The lack of weathering rind and recent movement suggest that these boulders have either been exhumed or have moved from the moraine crests. Only five boulders were found on Lignite Creek glacial stage deposits.
A 2 m deep pit was dug to investigate and collect depth profile samples (Ala 201-205) from the
Stampede gravel pit (Fig. 7A). The top of the section contains sediment disturbed by human activity. Organic material is present below the disturbed sediments. Stratification of gravels beneath the organic layer suggests the gravels are undisturbed (Fig. 9). Unit-1 (0-36 cm) consists of 1-5 cm thick layers of fine sand to clay with occasional 1-2 cm pebbles. This unit varies in thickness from 0.0-100 cm. Unit-2 (36-64 cm) is 28 cm thick and matrix supported.
The massive matrix consists of silty medium sand. Clasts range from 1-13 cm in diameter and lack striations. Unit-3 (64-111 cm) is 47 cm thick and clast supported. The matrix is composed
21 of medium to very coarse sand. Clasts range in size from 1-18 cm. Unit-4 (111-141 cm) is 30
cm thick and matrix supported. The massive matrix is composed of silty fine sand. Clasts range
in size from 1-5cm. Unit-5 (141->200 cm) is >59 cm thick and clast supported. The matrix is
primarily very coarse sand with clasts ranging from 1-18 cm.
Figure 8: Views of selected sampling sites. (A) contains the Stampede Tephra and the location of sample Aosl-2 (same location as Fig. 6. A in Beget et al. (1991) where the Stampede Tephra was found). The dark layer folded inside the tephra is composed of reworked lignite. (B) & (C) are bedrock and an erratic on a drumlin sampled on Antenna Hill. (D) & (E) are boulders sampled on the Browne Moraine. (F) is of a large erratic on the Lignite Creek push moraine. The weathering pit to the left of Ala-137A is ~ 30 cm deep. Notice the fracture between the two sample locations that will eventually separate the samples.
Healy Glacial Stage
Thorson et al. (1976), Thorson (1986), and Beget et al. (1991) suggested that the Healy glacial
stage advance occurred during MIS 4 or 6. However, there is no chronological control on this
glacial advance.
The Healy glacial stage advance produced a nearly continuous latero-frontal moraine. This is 70
km from the contemporary snout of the Yanert Glacier (Figs. 5 & 7). The moraine consists of ten
22 individual ridges. The main (outer most) ridge has a crest 10 to 20 meters wide and slopes ~30o
(Thorson, 1986). The Healy advance represents the last time ice advanced out of the Nenana
Canyon into the foothills.
Only four erratics (Ala 11-13, & 157) were found on the Healy glacial stage moraine. Therefore, two samples (Ala 107 & 108) were taken from the bedrock and three samples (Ala 23-25) from drumlins on Antenna Hill to determine the last time ice advanced out of the Nenana Canyon.
Unfortunately erratics were scarce therefore sampling was confined to available boulders on the moraine and bedrock on Antenna Hill. The erratics sampled on the drumlins have weathering pits, weathering rinds, and are undergoing angular disintegration.
Riley Creek Advance
Thorson et al., (1976) recognized three advances during the Riley Creek Advance that are termed
Riley Creek 1 (most extensive), Riley Creek 2, and the Carlo glacial stages.
Riley Creek 1 glacial stage
Based on radiocarbon dating of wood found in loess deposited on Riley age terraces, Thorson et
al. (1976), Hamilton (1982), Thorson (1986), and Beget et al. (1991) suggested that the Riley
Creek 1 glacial stage occurred between 25,000 to 12,000 radiocarbon years B.P.
The Riley Creek1 glacial stage terminal moraine is located ~2 km north of the entrance to Denali
National Park, which is ~50 km from the contemporary snout of the Yanert Glacier (Figs. 5 & 7)
(Thorson et al., 1976). Hamilton (1982) suggested that this stage correlates with the Donnelly
advance in the Delta River Valley, located ~100 km to the east. Thorson et al. (1976) noted that
most of the end moraine has been destroyed but discontinuous lateral moraine remnants; ground
moraine, erratics, and ice marginal drainages are still present.
23 Based on Wahrhaftig,s (1958) map, five erratics (Ala 15-20) were sampled from the ground
moraine surface north of the Denali Park entrance. The moraine surface slope ~10o-15o north toward the Nenana River. These boulders are not buried well and have likely moved.
Unfortunately, this denudated ground moraine is the only surface identified as Riley Creek 1 glacial stage. Therefore, sample collection was limited to this area.
Riley Creek 2 glacial stage
Previous chronological work by Wahrhaftig (1958) on the Riley Creek 2 glacial stage consists of radiocarbon dating on peat. This peat is located in till that is covered by lake sediments. The peat was deposited sometime between the maximum extent of the Riley Creek 2 glacial stage composite moraine and the melting of till covered stagnant ice (Wahrhaftig 1958). This sample had a radiocarbon age of 10,560 ± 200 years B.P (12384 ± 287 cal years through CalPal) (dated by H.E. Suess). Based on this radiocarbon date, in a personal communication to Wahrhaftig
(1958), H.E. Suess stated that the Riley Creek glacial stage 2 advance correlated with the Two
Creeks advance and the Younger Dryas.
Wahrhaftig (1958) and Thorson et al. (1976) noted that a composite moraine defines the limit of the Riley Creek 2 glacial stage advance south of the Denali Park entrance. The core of the moraine is composed of an initial moraine that was overrun and deformed into a recumbent syncline by a readvance. Wahrhaftig (1958), Thorson et al. (1976), and Thorson (1986) noted that the Riley Creek 2 moraine morphology is almost a continuous ridge of sediment that ranges from 20-55 m above the valley floor with slopes at 30-35o that extends over 1 km. Ground moraine deposits on both sides of the Nenana River valley have hummocky topography with numerous kettles that have no interconnected drainage system (Wahrhaftig, 1958; Thorson et al.,
1976). This hummock-and-hollow topography contains depressions that are seasonally wet,
24 containing marshes, or are occupied by deep lakes (Wahrhaftig, 1958). Vegetation is dense with
spruce and white birch 2-5 m spacing on the terminal moraine and 0.25-0.75 m thick peat on the
ground moraine.
Five samples (Ala 119-123) were collected from boulders on the Riley Creek 2 glacial stage
ground moraine (Fig. 7). These boulders were generally hard with no planer sides.
Carlo Glacial Stage
The Carlo glacial stage deposits have only been recognized in the Nenana River valley (Figs. 5
& 7) (Wahrhaftig, 1958). Wahrjaftig (1958) had two scenarios for the formation of the Carlo
glacial stage deposits. The first is a retreat of ice up the Yanert Glacier valley near the current
ice position of the Yanert Glacier and coeval retreat of ice in the Nenana Corridor to the town of
Windy. This was followed by an advance of ~17 km by the Yanert glacier and ~12 km by north
flowing ice in the Nenana Corridor. However, due to the lack of correlative stage deposits in
neighboring valleys, Warhafitg (1958) favored the second scenario. The second scenario is that
during the end of the Riley Creek glacial stage 2, melt water created a proglacial lake and delta.
The Nenana glacier then readvanced and reworked the delta gravel into a mass of bouldery till
(Wahrhaftig, 1958).
Two sets of deposits can be correlated with the Carlo glacial stage deposits in the main Nenana
River valley. These are (1) near the mouth of a small valley to the east of Carlo where a small
cirque glacier existed, and (2) up valley toward the Yanert Glacier (20 km from current ice
margin) (Fig. 5) (Wahrhaftig, 1958). It was not possible to examine the deposits in the Yanert
Glacier valley and the cirque valley because of their inaccessibility. The mapped limits and
convex north shape of the Carlo glacial stage ground moraine in the main Nenana River valley
suggest that it was formed by ice that flowed north into the valley via the Nenana Corridor. The
25 ground moraine is located ~5 km north of Carlo Creek and Wahrhaftig (1958) described this as
comprising hummock topography with numerous hollows. When investigated in the field no
distinct latero-frontal morainal ridge was observed. There is no interconnected drainage between the hollows, some of which are almost 30 m deep (Wahrhaftig, 1958).
Various rock types were sampled from Carlo age deposits, including granite (Ala-130), conglomerate (Ala 126 A/B, 127, 132-134), and schist (Ala-128). The different rock types were susceptible to different denduational forces. Granites were hard and fresh looking, conglomerates were exfoliating, and schist were heavily fractured. Conglomerates dominate the lithology of boulders on the Carlo glacial stage ground moraine whereas all other moraines are dominated by granite. This is likely due to the fact that the only outcrop of granite is located in the Yanert Glacier cirque basin, which did not advance out of its tributary valley and contribute this deposit.
4.2. Eight Mile Lake Region
The two Eight Mile Lake moraines are beyond the limit of the Lignite Creek Stage. However,
they are morphostratigraphically separated from older glacial landforms in the Nenana River
valley. Thorson (1986) suggests the Eight Mile Lake area is likely faulted and therefore its
relationship to the Bear Creek, Browne, and Teklanika Stages can not be determined.
Furthermore, Thorson (1986) suggested that coevil deposits to the Eight Mile Lake glacial stages
are present below a break in slope at 930 m asl on Gravels Ridge. These deposits are large
granitic boulders not found in the Nenana Gravel.
The two moraines are located to the east and west of Eight Mile Lake (Figs. 5 & 7). The west
moraine (outer) was formed by more extensive ice than the East moraine (inner). The east
moraine retains more of its depositional topography and its crest is <100 m wide (Thorson,
26 1986). It is more continuous than the western moraine and is not dissected by drainage. Several
less defined moraines that are likely recessional are present to the east of the inner moraine
(Thorson, 1986). The Eight Mile Lake moraines are located north of Gravels Ridge ~4 km
(west) of the Lignite moraine. Alluvial fans north of Gravels Ridge bury the southern end of the
Eight Mile Lake moraines. The moraines have a parabolic shape that is convex west. Moraine
flanks gently rise in elevation ~20 m from the edge of Eight Mile Lake on 5-60 slopes.
Erratics on these moraines range from hard and fresh to soft with thick weathering rinds.
Erratics are more common on the northern ends of both moraines. Clusters of 6-7 boulders with congruent planer sides can be found on the northern end of the West Eight Mile moraine. Sample
Ala-106 was collected from these clusters. Samples Ala 6, 7, & 104 A/B were collected from the crest of the western moraine. Samples Ala 26-29, 101, & 102 were collected from the crest of the eastern moraine. No samples were taken from the small recessional moraines due to the absence of boulders. Two samples (Ala 138 & 139) were collected from Gravels Ridge to check
Thorsons (1986) association with the Eight Mile Lake moraines.
4.3. Other glacial landforms
South of the Alaska Range crest, the margin of retreating ice moved east from Broad Pass and
the Nenana Corridor, past Reindeer Hills, to the southwestern edge of Monahan Flat. TCN
samples were collected from glacial landforms on Reindeer Hills and in the Monahan Flat to test
the synchroneity of glaciation by determining if there is a significant time difference between the
northern and southern sides of the Alaska Range or if one area was glaciated while the other was
not. Ages from Reindeer Hills and the Monahan Flat are also used to investigate ice retreat
history on the southern side of the Alaska Range.
27 Reindeer Hills
Reindeer Hills (~ 700 m in elevation and ~8 km long) is located south of the Alaska Range Crest near the town of Windy (Fig. 3). Five major glaciated benches and numerous smaller benches wrap almost continuously around. In a reconstruction of LGM ice extent of the Riley glacial stage Wahrhaftig (1958) suggested that glaciers covered almost all of Reindeer Hills leaving only the uppermost peaks as nunataks.
A total of 13 TCN samples (Ala- 151-155, 158-162, &164-166) were collected from granitic erratics on the five glaciated benches to test the timing and style of deglaciation. The erratics were hard with no weathering rind.
Monahan Flat
Monahan Flat is a large glaciated basin (~24 km by ~40 km) that was covered by ice from the
Nenana, West Fork, and Susitna glaciers during the LGM (Wahrhaftig, 1958). Ice from the
Monahan Flat flowed west, passing Reindeer Hills, and joined cirque fed glaciers in Broad Pass
(Fig. 2) (Wahrhaftig, 1958).
A series of roche moutonnees composed of diorite are located in the southern edge of the
Monahan Flat near the Denali Highway. Granitic erratics on top of the roche moutonnee were sampled to help determine the timing of deglaciation. Seven samples (Ala- 40 43 & 145-148) were collected on the southwest edge of the flat (Fig. 2). An additional set of three samples (Ala
140, 141, & 143) was collected from the south central section of the Monahan Flats also to help determine the timing of deglaciation.
4.4. Terraces
After moraine formation glaciers retreated from their maximum extent generating large volumes of meltwater (Wahrhaftig, 1958). The meltwater produced extensive glacial outwash terraces
28 along both sides of the Nenana River valley. The Lignite Creek, Healy, Riley Creek, and Carlo
terraces are currently covered by loess. The history and age of the loess deposits are discussed in section 4.5. Wahrhaftig (1958) and Thorson (1986) identified and correlated these terraces to their glacial stages. Based on their correlations and interpretations, depth profiles were collected in outwash terraces of Browne, Healy, Riley Creek, and Carlo age (Figs 9, 10, & 11). These were collected to compare with the surface samples (bedrock, drumlins, and moraines). Terraces and sampling sites are discussed in morphostratigraphic order.
Browne Terrace
The Browne terrace is located on the eastside of the Nenana River. It is ~3 km wide and curves around the Browne glacial stage moraine (Figs. 5 & 7) (Thorson, 1986). A 100 m-high scarp is present at its northern end where it intersects the Tenana Lowland. This is likely the result of differential uplift of the foothills relative to the Tanana Lowland (Thorson, 1986). Loess deposits are not present on this terrace.
A 2.2 m-deep pit was dug in the terrace east of the town of Brown (64.17771N/ 149.24805W/ altitude 336 m) to examine the sediment and to collect depth profile samples Ala 301-306 (Figs.
5 & 9). Where the pit was dug the flank of the terrace slopes at 30o. Beds comprise stratified gravel but there are no sand lenses. Soil processes have bioturbated the top 10 cm of the terrace to homogeneity. These roots persist but attenuate through the terrace to a depth of 180 cm. The profile can be divided into two units. Unit-1 (0-20 cm depth) consists of soil (fine sand and silt) with abundant roots. Unit-2 (20-220 cm depth) is >200 cm thick consists of homogenous stratified clasts supported sediment with a coarse sand matrix that is dark red/brown in color.
Clasts consist mainly of pebbles and cobbles.
29 Dry Creek and Lignite Creek Terraces
Thorson (1986) suggested that the Lignite Stage replace Wahrhaftig’s (1958) Dry Creek Stage
based the identification of the Lignite latero-frontal moraine. Thorson (1986) also suggested that
the Dry Creek terrace is an erosional landform. The Dry Creek terrace mapped by Wahrhaftig
(1958) is located discontinuously on both sides of the Nenana River north of the town of Healy.
The Lignite Creek terrace is only located on the West side of the Nenana River, north of the
Lignite Moraine (Fig. 5). Due to the lack of suitable quarries or road cuts in terrace deposits of
Dry Creek and Lignite Creek age, depth profiles were not collected from Dry Creek or Lignite
Creek Terraces. Instead a depth profile was collected in the Lignite push moraine complex
(discussed above in section 4.1.) (Figs. 5, 7, & 9).
Healy Terraces
The Healy terrace is located on the east and west sides of the Nenana River (Figs. 5 & 7)
(Thorson, 1986). Two depth profiles were examined in the Healy terrace (Healy 1 & 2) (Fig. 9
& 10).
Healy 1 depth profile, samples Ala 701-706, was collected from large gravel pit has been quarried into this terrace near the town of Browne (64.16035N/149.28510W/ altitude 280 m)
(Figs. 5 & 9). Several decimeters of sediment were removed from a vertical flank along with
>1m of slump to expose a section 340 cm deep. The profile can be divided into two units. Unit-
1 (0-110 cm) is 110 cm thick loess that comprises silt, clay, and fine sand. The upper 50 cm of loess has been bioturbated by roots to homogeneity. The base of the homoginized loess is defined by a gray soil horizon. The lower 60 cm retains its depositional stratification. However, roots persist throughout the section to a depth of 1.3 m. Unit-2 (110-400 cm) is 290 cm thick
30 clasts supported gravel. Stratification and imbrecation of clasts were unable to be determined due to the loose unconsolidated matrix, which led to slumping several times during sampling.
Healy 2 depth profile samples (Ala 801-806) were collected from the Dry Creek riverbank where subsequent erosion by Dry Creek has cut a near vertical south facing section in this terrace (Figs.
7B & 10). A 430 cm deep pit was dug into this terrace (63.88043N/149.03963W/ altitude 481 m) to examine the sediments for cryoturbation, bioturbation, and to collect samples. This terrace is more indurated than Healy 1 depth profile. The profile can be divided into four units. Unit-1
(0-170 cm) is 170 cm thick loess. Roots penetrate to a depth of 210 cm. Unit-2 (170-240 cm) is
70 cm thick stratified clasts supported gravels. In the top 27 cm of Unit-2 there is carbonate present on bottom of some clasts and the matrix is red, which suggest paleosol development.
Unit-3 (240-290 cm) is 50 cm thick. It comprises a dark gray coarse sand matrix with several pebble stringers. Unit-4 (290-430 cm) is 140 cm thick stratified clasts supported gravels that range in size from 1-55 cm. The matrix is dark gray coarse sand.
31
Figure 9: Browne, Lignite Creek, and Healy 1 depth profiles showing the sample positions. The graphic sedimentary logs and age graphs are plotted next to an image of each profile. Age graphs are log/normal plots of 10Be concentrations with R2 values displayed. The corrected line represents modeled 10Be concentrations after accounting for shielding by loess. The model recalculates 10Be concentrations after every 10 cm of loess deposition. The large difference is due to the exponential decrease in 10Be production with depth.
32 Riley Creek 1 Terrace
The outwash terrace from the Riley Creek glacial Stage is located on both sides of the Nenana
River from the confluence of the Nenana and Yanert Rivers to beyond town of Browne
(Wahrhaftig, 1958). Loess deposits ~1 m thick are present on the terrace.
The Riley Creek 1 depth profile samples (Ala 801-806) were collected from the Dry Creek river bank northeast of the town of Healy (Fig. 7B). Since deposition, Dry Creek has cut a near vertical south facing section in the Riley age tributary terrace. A 385 cm deep pit was dug into the face (63.88531N/149.02424W/ altitude 452 m) (Figs. 7 & 10). Abundant roots are present to a depth of ~80 cm. This profile can be divided into two units. Unit-1 (0-121 cm) is composed of loess. The upper 30 cm have been bioturbated by roots while the lower 91 cm maintains its stratification. Unit-2 (121-385 cm) is composed of imbricated stratified gravel dominated by pebbles and cobbles. There are 2-3 small boulders and sand lenses ~5 cm thick that pinch out laterally over 0.5 m.
Carlo Terrace
The outwash terrace from the Carlo glacial stage is located on both sides of the Nenana River from the Nenana Corridor north, beyond the town of Browne. A 270 cm deep pit was dug into the Carlo terrace north of the town of Browne to collect samples Ala 600-605
(64.19183N/149.29774W/ altitude 241 m) (Figs. 5, 7, & 10). This profile can be divided into three units. Unit-1 (0~65 cm) is 65 cm thick loess with a convoluted base. It comprises fine sand and silt. Diapirs ~25 cm thick are present laterally that are likely the result of reworking.
Unit-2 (~65-110 cm) is ~45 cm thick stratified clasts supported gravel comprised of pebbles and cobbles. The matrix is well-sorted coarse sand. Unit-3 (110-270 cm) is 160 cm thick clasts supported stratified gravel composed of pebbles, cobbles, and boulders. The matrix comprises
33 well-sorted coarse sand. All contacts between units and within each unit in this terrace are gradational. Roots are present throughout the profile. No vertical standing clasts or sand lenses
(to collect OSL samples) were noted.
34
Figure 10: Healy 2, Riley Creek, and Carlo Readvance depth profiles showing the sample positions. The graphic sedimentary logs and depth profiles for the 10Be concentrations are plotted next to each profile. The corrected line represents modeled 10Be concentrations after accounting for shielding by loess. The model recalculates 10Be concentrations after every 10 cm of loess deposition. The large difference is due to the exponential decrease in 10Be production with depth.
35
Figure 11: Summary of outwash terraces with the location of their respective moraines. Relative height of terraces is not to scale.
4.5 Loess deposits on outwash terraces
Loess deposits in the Nenana River valley are abundant on terraces (Figs. 9 & 10). Thorson et
al. (1976) conclude that loess deposition began ~12,000 radiocarbon years ago (~13000 cal years through CalPal) on Healy age terraces and ~6,000 radiocarbon years ago (~6900 cal years through CalPal) on Riley Creek 1 Stage terrace from radiocarbon dating. Older loess units that reach a thickness of 14 m have been identified by Thorson et al. (1985) between the Teklanika and Nenana River valleys 20 km west of the town of Browne. A radiocarbon date on wood 5 m below the surface of the loess West of Browne yielded a radiocarbon age of 40,000 ± 800 yr B.P.
(43884 ± 706 Cal years through CalPal). Thorson et al. (1985) suggested that this older loess unit was also deposited in the Nenana River Valley but has been stripped away by katabatic winds during subsequent glacial times. Modeling by Thorson et al. (1985) showed that the strength of katabatic winds during glacial times was strong enough to completely strip away any loess deposits in the Nenana River Valley. The thickness of the paleoloess in the Nenana River valley has not been estimated.
Thorson et al. (1976), Hamilton (1982), Thorson et al (1985), and Bigelow et al. (1989) suggests that the initiation of aeolian deposition closely followed the final deglaciation of the Riley Creek
36 Glaciation around 12,300 radiocarbon yr B.P. (~14,500 cal year through CalPal). This is confirmed by radiocarbon dates in the basal units of loess on the Healy Outwash Terrace around
13,000 cal yr. If the same processes of loess deposition operated in the past then deposition of the older loess found in the Teklankia River Valley likely began shortly after deglaciation of the
Healy advance. Carbonate deposition and surface weathering of the Healy terrace including fracturing of cobbles by frost action, polishing, and minor ventifacting by wind erosion was interpreted by Thorson et al. (1976) as evidence for a previous loess. Thorson et al. (1976) suggested that this loess was striped away to expose the terrace surface for some time before the deposition of the loess currently found in the Nenana River Valley. The older loess unit was most likely removed during the Riley Creek Glaciation.
Bigelow et al. (1989) suggested that loess deposition and changes in grain size in the loess profile in the Nenana River Valley correlates with climatic change. Bigelow et al. (1989) suggested that at the base of the loess, a coarse grained sand layer (S1) bracketed by radiocarbon dates of 11,120 ± 85 - 10,690 ± 250 yr B.P. (13025 ± 140 - 12508 ± 344 cal years through
CalPal), correlates with the Younger Dryas Stade. In a technical comment Waythomas et al.
(1991) disagrees with this interpretation and suggested that the changes in particle grain size do not represent climatic change but are the result of short-term weather events or more likely changes to proximity of source areas.
OSL samples were collected from sandy loess at the TCN depth profile on the Lignite Creek push moraine (Aosl 1 & 2) and Healy 1 & 2 (Aosl 7 & 8B, respectively), Riley 1 (Aosl 9), and
Carlo (Aosl 6) terraces (Figs. 8A, 9, & 10) to define the age of loess and to help quantify its role in shielding TCN production.
37 5. Results
5.1. TCN Results
Ages were calculated from 10Be concentrations using the Stone (2000) scaling factor on the
PRIME Laboratory Age Calculator. The PRIME Laboratory Age Calculator calculates
production 10Be rates and makes geomagnetic corrections. The 10Be production rate used for surface samples is ~9 10Be atoms/gram of quartz/year. The 10Be production rate for depth profile samples was calculated using the PRIME Laboratory Age Calculator based on latitude, longitude, and depth. The 10Be concentrations and resulting ages are displayed in Table-2 and
Fig. 12 & 13.
38 Table 2: Contains location and details of each sample. Location Boulder size Sample Length/Width/ Topographic Minimum Number Altitude Height shielding factor Depth 10Be Error Be Error Latitude Longitude (Horizon @ below (Atoms/g (Atoms/g exposure (Ala-) (N) (W) (m asl) (m/m/m) Angle >20o) surface quartz) quartz) age (ka) (ka) 1 64.1708 149.3016 423 3.5/1.8/1 none N/A 320425 43.9 3.5 2 64.1706 149.3041 397 1.9/1.8/0.7 none N/A 196357 26.7 2.5 3 64.1709 149.3036 390 2.4/1.6/1) none N/A 551314 38278 76.1 7 4 64.1712 149.3025 401 3/2.7/1 none N/A 421553 18780 57.3 4.4 5 64.1713 149.3019 403 3.5/2.5/1.2 none N/A 261315 36.1 2.6 6 63.8901 149.278 715 3.5/2/0.5 none N/A 451903 46.2 3.3 7 63.8904 149.2799 706 2/1/2001 none N/A 249076 25.6 2.4 11 63.8446 149.0335 579 1.1/.6/0.15 none N/A 419476 16904 48.4 3.7 12 63.8475 149.0248 568 2.1/1.4/1 none N/A 474872 55.4 5.2 13 63.8475 149.0246 568 1.8/1.2/0.4 none N/A 469637 54.8 4.8 15 63.7355 148.893 529 0.9/0.9/0.45 none N/A 72716 8.8 0.8 16 63.7357 148.8932 509 3.3/2.9/1 none N/A 98407 5528 12 1.2 18 63.7352 148.8937 524 0.6/0.45/0.3 none N/A 69007 6619 8.3 1.1 19 63.735 148.8946 526 0.8/0.7/0.45 none N/A 105554 19680 12.7 3 20 63.7349 148.8943 530 0.5/0.45/.3 none N/A 84690 10.3 1 23 63.8335 148.9995 579 1.5/1.2/0.6 none N/A 513098 59.3 4.8 24 63.8332 148.9996 574 2.9/1.6/0.5 none N/A 487703 56.6 4.8 25 63.8341 149.0011 576 1.3/1/0.7 none N/A 521122 15602 60.4 4.2 26 63.8802 149.2301 710 2.6/.9/0.45 none N/A 612998 63.2 4.7 27 63.881 149.2312 709 2.2/1.9/0.5 none N/A 239902 24.5 1.9 28 63.8881 149.2387 692 2.8/2.4/1.2 none N/A 124681 12.9 1.2 29 63.878 149.2255 716 2/0.7/0.25 none N/A 79385 8 0.9 40 63.3062 148.2098 863 4.5/3/1.4 none N/A 174430 15.6 2.1 Depth profile ages are extrapolated from an exponential line fit to the 10Be concentrations of the profiles samples (Figs. 9 & 10). Ages displayed are not corrected for erosion or loess cover. *Latitude recorded from a Garmin hand held GPS is not accurate due to poor satellite coverage.
39 Table 2 con’t: Contains location and details of each sample. Location Boulder size Sample Length/Width/ Topographic 10 Minimum Number Altitude Height shielding factor Depth Be Error Be Error Latitude Longitude (Horizon @ Below (Atoms/g (Atoms/g Exposure (Ala-) (N) (W) (m asl) (m/m/m) Angle >20o) surface quartz) quartz) age (ka) (ka) 141 63.23832 147.77692 936 1.8/1.6/0.9 none N/A 177150 14.9 1.4 143 63.23773 147.77445 949 1.56/1.5/0.6 none N/A 176295 14.7 1.6 145 63.30612 148.21227 859 1.47/0.94/0.76 none N/A 273532 24.6 2.1 147 63.30518 148.21048 856 1.72/1.44/0.73 none N/A 198585 17.9 1.6 148 63.30487 148.20962 855 1.7/1.07/0.56 none N/A 378381 34.2 2.7 151 63.4037 148.84336 1108 2.74/1.22/0.52 none N/A 203251 14.8 2 152 63.40328 148.84293 1109 1.46/1.20/0.32 none N/A 203851 14.8 1.4
153 63.40121 148.84692 1034 1.4/1.05/0.59 40o horiz @ 15o N/A 215763 16.7 1.5
154 63.40076 148.8402 1032 0.83/0.65/0.37 40o horiz @ 15o N/A 199064 15.5 1.6
155 63.40048 148.84742 1023 0.91/0.91/0.43 40o horiz @ 15o N/A 203207 15.9 1.9 156 63.86848* 149.12951 626 1.89/1.48/1.13 none N/A 337778 37.3 2.9 157 63.85472* 149.04311 558 2.4/1.7/0.7 none N/A 252469 14903 29.6 2.7 158 63.40154 148.8576 972 2.53/0.97/0.78 none N/A 160667 13.1 1.2 159 63.40116 148.85783 965 1.52/1.39/0.67 none N/A 231979 19.1 1.8 160 63.40119 148.85798 964 1.24/0.83/0.31 none N/A 235579 37535 19.4 3.6 161 63.89876* 148.86595 914 2.1/1.91/0.57 none N/A 208608 27453 17.9 2.8 162 63.89850* 148.86589 915 2.04/1.6/0.38 none N/A 177364 16666 15.2 1.9 164 63.89306* 148.85961 875 1.37/0.83/0.23 none N/A 213768 26238 18.9 2.9 165 63.89298* 148.85997 869 1.83/1.57/0.41 none N/A 176820 15.7 2.4 166 63.89290* 148.86027 869 2.81/2.48/0.69 none N/A 210305 31985 18.7 3.3 Depth profile ages are extrapolated from an exponential line fit to the 10Be concentrations of the profiles samples (Figs. 9 & 10). Ages displayed are not corrected for erosion or loess cover. *Latitude recorded from a Garmin hand held GPS is not accurate due to poor satellite coverage.
40 Table 2 con’t: Contains location and details of each sample. Location Boulder size Sample Length/Width/ Topographic Minimum 10 Number Altitude Height shielding factor Depth Be Error Be Error Latitude Longitude (Horizon @ Angle below (Atoms/g (Atoms/g exposure (Ala-) (N) (W) (m asl) (m/m/m) >20o) surface quartz) quartz) age (ka) (ka) 202 63.88811 149.10228 514 250µm-4mm (Fig. 9) 80-90 cm 474086 34626 125.3 7.6 203 63.88811 149.10228 514 250µm-2mm 120-130 cm 135094 10535 204 63.88811 149.10228 514 250µm-2mm 160-170 cm 158550 8487 205 63.88811 149.10228 514 250µm-2mm 185-200 cm 85237 7201 301 64.17771 149.24805 366 250µm-2mm See Browne Profile 30-45 cm 387255 33783 85.5 5.1 302 64.17771 149.24805 366 250µm-25mm (Fig. 9) 65-80 cm 265764 14143 303 64.17771 149.24805 366 250µm-2.5mm 100-115 cm 182130 18124 304 64.17771 149.24805 366 250µm-2mm 135-150 cm 105276 35161 305 64.17771 149.24805 366 250µm-4mm 170-185 cm 66467 33294 306 64.17771 149.24805 366 250µm-2mm 205-220 cm 97334 600 64.19183 149.29774 241 250µm-2mm See Carlo Profile 70-85 cm 17631 7652 3.7 Mean 601 64.19183 149.29774 241 250µm-2mm (Fig. 10) 115-130 cm 36540 7726 602 64.19183 149.29774 241 250µm-2mm 150-165 cm 15972 7267 603 64.19183 149.29774 241 250µm-2mm 185-200 cm 18778 10079 604 64.19183 149.29774 241 250µm-2mm 220-235 cm 31135 6340 605 64.19183 149.29774 241 250µm-2.5mm 255-270 cm 24841 6475 701 64.16035 149.2851 280 250µm-2mm See Healy 1 Profile 110-125 cm 82212 10761 18 0.7 702 64.16035 149.2851 280 250µm-2mm (Fig. 9) 155-170 cm 46013 703 64.16035 149.2851 280 250µm-2mm 200-215 cm 40178 8603 704 64.16035 149.2851 280 250µm-2mm 245-260 cm 30278 10496 705 64.16035 149.2851 280 250µm-4mm 290-305 cm 19508 706 64.16035 149.2851 280 250µm-2mm 335-350 cm 28624 11995 801 63.88043 149.03963 481 250µm-2mm See Healy 2 Profile 235-245 cm 266141 32536 51 1.8 802 63.88043 149.03963 481 250µm-2mm (Fig. 10) 275-290 cm 130948 34414 803 63.88043 149.03963 481 250µm-2mm 320-335 cm 88245 16421 804 63.88043 149.03963 481 250µm-2mm 365-380 cm 43612 5773 805 63.88043 149.03963 481 250µm-2mm 410-425 cm 43048 16242 806 63.88043 149.03963 481 250µm-2mm 462-473 cm 36704 12977 901 63.88531 149.02424 452 250µm-2mm See Riley Profile 170-185 cm 33697 7009 3.3 Mean 902 63.88531 149.02424 452 250µm-2.5mm (Fig. 10) 230-240 cm 38316 6804 903 63.88531 149.02424 452 250µm-2mm 280-290 cm 12007 10204 904 63.88531 149.02424 452 250µm-2mm 350-344 cm 31094 3302 905 63.88531 149.02424 452 250µm-2mm 375-390 cm 17513 6153 906 63.88531 149.02424 452 250µm-2mm 420-435 cm 23107 7692 Depth profile ages are extrapolated from an exponential line fit to the 10Be concentrations of the profiles samples (Figs. 9 & 10). Ages displayed are not corrected for erosion or loess cover. *Latitude recorded from a Garmin hand held GPS is not accurate due to poor satellite coverage.
41
Figure 12: Cosmogenic ages grouped by landforms. Landforms in the figure are arranged in morphostratagraphic order with the oldest (Teklnakia) on the left and the youngest (Monahan Flat) on the right. Erosion is not accounted for in the ages displayed. D 1 is Healy 1 terrace and D 2 is Healy 2 terrace. TCN dates for boulders in each glacial stage are arranged in order of decreasing value.
42
Figure 13: Plot of TCN ages for the Eight Mile Lake Region grouped by landform. This data was plotted separately due to its unknown morphostratigraphic order. TCN dates for boulders in each glacial stage are arranged in order of decreasing value.
5.2 Evaluation of TCN Data
5.2.1. Uncertainty
Two sets of factors that affect TCN ages are production rate uncertainties and geologic processes. The choice of an absolute production rate, scaling technique, and geomagnetic corrections can influence TCN ages. However, these uncertainties have less influence on relative chronologies for events in limited geographic areas, where the data are used to recognize trends and set limits on the timing of features.
The data needs to be evaluated in order to access geologic processes and their influence on TCN ages. Therefore, the oldest boulder method and conditions of use are discussed. Then the data
43 set is evaluated to meet the conditions for use of the oldest boulder method. Last, the geologic
processes that are likely responsible for outliers are identified and evaluated.
5.2.2. Oldest Boulder Method
The oldest boulder method can be a powerful tool for determining the most accurate age of
deglaciation using SED. Briner et al. (2005) showed that using the oldest boulder proved to be a
reliable method in Alaska. However, it is not always appropriate to apply this technique due to
the scatter of SED ages. Based on the data in Fig 12 & 13, geologic processes prevent tight
distributions of boulder TCN ages from landforms older than 100 ka in the central Alaska Range.
Therefore we set limiting conditions for valleys in the central Alaska Range where a sequence of
landforms have been dated to eliminate inaccurate data before the oldest boulder method is
applied. These conditions are: (1) clustering and (2) geomorphic order.
For clustering, the data sets should be examined to identify a minimum of 2 ages within error of
each other. TCN ages within a data set older than the clustered data are unreliable and are
interpreted as outliers. Geologic processes prevent clustering on old landforms (>100 ka).
Therefore, data sets from old landforms that follow geomorphic order but do not cluster should
be interpreted as very minimum or trivial ages. Modeled ages from depth profiles should be
considered separately based on the R2 value of the 10Be concentration trend line. R2 values of less than 0.6 are interpreted too be unreliable. These data should be removed before applying the oldest boulder method.
For geomorphic order we assume that the glacial landforms are created by the same source and that the more distal landforms will be the oldest. Therefore, when SED ages and depth profiles on distal (older) landforms are younger than more proximal (younger) landforms, they are
44 considered inaccurate and significantly underestimated. These data sets should be removed before applying the oldest boulder method.
5.2.3. Evaluating the Data Set
Examining the data set for clustering will remove samples Ala 120, 121, 127, 145, & 148 (Fig.
14). Samples from the Lignite Creek data set are not removed because they follow morphostratigraphic order and are only considered to be very minimum ages. The R2 for the trend on Riley Creek 1 and Carlo depth profiles could not be determined due to the scatter of
10Be concentrations. Therefore, these depth profiles are removed from the data set.
TCN ages from samples on Teklanika, Browne, and Riley Creek 1 glacial stage landforms
(including surface boulders and Browne/ Riley Creek 1 terrace depth profiles) do not meet the condition of being in morphostratigraphic order (Fig. 14). Therefore, TCN ages from these samples are considered trivial/inaccurate. Note that the OSL age on the Riley Creek 1 terrace is determined by a different dating technique, which is discussed in section 5.3 and 5.4.
45
Figure 14: Data set after following the conditions for the oldest boulder method by excluding trivial data sets and outliers. Orange data were excluded from interpretations because they do not follow morphostratigraphic order. Outliers (red data points) were excluded from interpretations because they are older than clustered ages within a data set from a glacial stage. Interpretations are based on ages (black data points) that meet the limiting conditions of the oldest boulder method.
5.2.4. Geologic Processes
Geologic processes that affect TCN ages include: (1) the stability of landforms, (2) prior exposure inheritance, (3) erosion of boulders, and (4) shielding. This is followed by a (5) summary of the impact of the 4 processes. Due to the scarcity of boulders on the Browne moraine, sampling was limited to suboptimum boulders. However, we take advantage of these suboptimun samples to discuss problems that can affect TCN ages. Samples from the Healy,
46 Riley 1 & 2, and Carlo moraines are used to determine deglaciation ages. However, their data
are also used to highlight geologic process problems by accessing scatter of boulder TCN ages.
(1) Stability of Landforms
The stability of boulders is governed by: the type of landform (moraine, drumlin, terrace), the
condition and inherent stability of the landform, the position of the sample on the landform, and
erosion of the landform.
Browne Glacial Stage
Samples on the Browne moraine were collected from boulders on the flank (Slope of 15-20o) and crest of the moraine (Table 3). Most of the erratics sampled were located on the crest (n=12), the remaining three from the flank. The three erratics from the flank (Ala-109, 110, & 118) have
TCN ages (35-13 ka) that are half of the oldest age (78 ka) from an erratic on the crest. Of the
15 samples collected on the Browne moraine the oldest 8 samples (78-44 ka) were all collected from the crest. Erosion of boulders on the crest is thought to occur at a higher rate than on slopes
(Putkonen, 2003). This is supported by observations of boulder clusters on the crest of the
Browne moraine and the presences of large (>4 m diameter) boulders that have not yet disintegrated on its slopes. Therefore, erosion of boulders on the slopes of the Browne moraine is not as extensive because they are undergoing exhumation by slope processes.
Healy Glacial Stage
Samples for the Healy Stage were collected from bedrock, moraines, and drumlins (Table 3).
Samples from bedrock yielded mixed results, one old (Ala-107 at 57 ka) and one young (Ala-108 at 53 ka). Samples collected from the moraines (Ala-11-13 & 157) have TCN ages that range from 55-30 ka. Three samples collected from drumlins on Antenna Hill (Ala 23-25) have TCN
47 ages that cluster between 60-57 ka. The tighter clustering of these ages is interpreted as drumlins being inherently more stable than moraines. Collecting samples from more stable landforms
(drumlins, fans, and terraces) in addition to moraines can help bracket the age of glaciation.
Riley Creek 1 Glacial Stage
Boulders from the Riley Creek glacial stage 1 (samples Ala-15, 16, & 18-20) have TCN ages that range from 8.3-12.7 ka B.P. (Fig. 11 & table 1). Assuming the oldest boulder (Ala-19) as the minimum age of glaciation suggests an age of ~13 ka. However, the Riley Creek 2 and Carlo
Readvance moraines, which are morphostratagraphically younger, have older TCN ages. The
Riley Creek 1 moraine has likely been destroyed, as noted by Thorson et al. (1976). This event likely occurred ~13 ka, and exhumed the boulders currently found on the surface of the Riley
Creek 1 glacial stage moraine.
Eight Mile Lake Region (West/East and Gravels Ridge)
Erratics (Ala-6, 7, 104 A/B, & 106) from the West Eight Mile Lake moraine (W-8 moraine) have
TCN dates from 12-103 ka (Fig. 13). Sample Ala-104 A was collected from the top surface of the boulder while Ala-104B was collected from underneath the boulder. The TCN age of Ala-
104A is 28 ka. The TCN age of Ala-104B can be calculated two ways. First, assuming the boulder has been stable over its entire history; corrections can be made to compensate for the 1.3 m thickness of granite above sample Ala-104B. This results in an age of 103 ka and would require a thickness of ~2 m of granite to be removed from the boulder to expose the surface where Ala-104A was collected. This seems unlikely. The second way to estimate the boulders age is to assume that the boulder has toppled/rolled at some point during its exposure. I.E. the surface where Ala-104B was collected was exposed for some time then the boulder toppled and exposed the surface where Ala-104A was collected. Following this, TCN ages from 10Be
48 concentrations for both samples (Ala-104 A/B) are calculated as surface samples and have ages
of 28 ka and 12 ka, respectively. The exposure age would roughly be equal to the sum of Ala-
104A and Ala-104B (~40 ka). This is the most likely explanation.
The 40 ka age is meaningless, in terms of assigning an age, due to the possibility of inheritance
(when exposing one side there is some TCN production on the opposite side) and toppling occurring more than once. However, this information is useful because it indicates that toppling is one of the reasons for the young ages in the Eight Mile Lake region. Active permafrost
(evident in undulating roads and peat bogs) is likely responsible for toppling of boulders in the
Eight Mile Lake region. The active layer could also be responsible for exhumation of boulders, which would also yield young ages. Age underestimation is also demonstrated by TCN ages from the East Eight Mile Lake moraine (Ala-26-29, 101, & 102) that range from 8-63 ka and
Gravels Ridge (Ala-138 & 139) that range form 23-25 ka. Both of these data sets are morpohstratigraphically older than the Lignite Creek glacial stage (>125 ka) and are therefore significantly underestimated. These TCN ages are interpreted as significant underestimation due to the subdued, deflated, and fluvially dissected nature of the moraine and the further geographic extent of the morphostratigraphically younger Lignite Creek glacial stage (>125 ka). The young ages are likely the result of exhumation due to an active permafrost layer close the surface, deflation and fluvial dissection of the moraine, and chemical and physical weathering processes acting on the boulder surfaces. Further SED dating in this area will not likely yield useable results.
(2) Inheritance
There are 5 inherited boulders from a total of 82 boulders collected. These samples are Ala 120,
121, 127, 145, & 148. All of these boulder samples were collected on younger landforms
49 (LGM). This suggests that TCN ages from younger landforms are more susceptible to
inheritance due to reworked boulders than older landforms (>100 ka). One hypothesis is that
reworked boulders on landforms >100 ka will likely reach a steady state between erosion and
TCN production in wet environments such as the central Alaska Range. Therefore, reworked
and freshly plucked boulders are likely to have indistinguishable TCN ages in scattered datasets
>100 ka in age.
Riley Creek 1 Glacial Stage
The depth profile sampled in outwash correlated by Wahrhaftig (1958) has a mean TCN age of
3.3 ka (Fig. 10). This age was calculated from the mean 10Be concentrations because the scatter of 10Be concentrations prevented the use of a trend line. This age is interpreted as inaccurate
because the homogeneity of 10Be concentrations at all depths. Scatter of the 10Be concentrations
did not permit corrections for loess cover. The stratification of sediment suggests that
bioturbation/cryoturbation is minimal in spite of abundant roots. The most likely cause for the
scatter of 10Be concentration is inheritance. During deposition of the Riley Creek 1 outwash
terrace, the meltwater charged river likely undercut and eroded sediment from previous outwash
terraces and incorporated them.
Riley Creek 2 Glacial Stage
Following the conditions of the oldest boulder method, samples Ala-120 and 121 are considered
outliers and were removed from the data set. The TCN age (61 ka) of Ala-120 and the
morphostratagraphic position of the Riley Creek 2 in the glacial succession leads to the
interpretation that it is most likely a reworked erratic from a previous glacial advance (Healy).
There are deposits from the Healy glacial stage proximal to the ice source from the Riley Creek 2
50 moraine. Advancing ice during the Riley Creek 2 glacial stage likely reworked a boulder from
one of these deposits.
Samples Ala 119, 122, and 123 were removed from the data set because they did not fit in
morphostratigraphic order. This leaves Ala-121, which is likely a reliable age, as an outlier
because it was older than the clustered data that was too young morphostratigraphically (Fig. 14).
Therefore, we make an exception for sample Ala-121 and keep it in the data set. Justification for this is (1) the clustered data used to identify outliers is too young, (2) the TCN age of Ala-121 is in morphostratigraphic order, and (3) the lack of clustering is likely due to a small sample set
(n=5).
Carlo Glacial Stage
The older erratic (Ala-127) with an age of 23 ka is interpreted as an outlier because it is older than samples (Ala 126 A/B, 128, 130, and 132-134,), which cluster at ~ 19 ka. This difference in
TCN age is ~4 ka. However, due to the short time period between recessional or readvance moraines, influence by inheritance may not be obvious (Briner et al. 2005). The older age of
Ala-127 is likely due to inheritance. Due to the close chronological and geographic proximity,
reworking of material from the Riley Creek 1 & 2 glacial stages is likely.
The depth profile sampled in Carlo age outwash correlated by Wahrhaftig (1958) has a mean
TCN age of 3.7 ka (Fig. 10). This age was calculated from the mean 10Be concentrations because the scatter of 10Be concentrations prevented use of a trend line. It is interpreted as inaccurate because the homogeneity of 10Be concentrations between all six sub-samples
regardless of depth. Scatter of the 10Be concentrations did not permit corrections for loess cover.
Samples Ala-126 A/B from a boulder located on the terrace appears stable and clusters well with other samples from the moraine. Also, the stratification of sediment suggests that
51 bioturbation/cryoturbation is minimal in spite of abundant roots. The most likely cause for the
scatter of 10Be concentration is inheritance. During deposition of the Riley Creek 1 outwash
terrace, the meltwater charged river likely undercut and eroded sediment from previous outwash
terraces and incorporated them.
Monahan Flat West
Samples Ala 148 & 145 (25 ka & 34 ka) are removed from the data set and considered outliers
because they are older than the TCN ages that cluster at ~18 ka (Ala 40-43 & 147). Ice nearly
covered Reindeer Hills during the LGM to form Riley Creek glacial stage 1 & 2 and Carlo glacial stage deposits. Therefore, glaciers likely covered and eroded the roche montenees where the boulders were sampled (Fig. 2 – red box labeled Fig 7E). This implies that the oldest two samples (Ala 148 & 145) are inherited. Simplifying the shape of a boulder to a cube there is a
1:6 chance of sampling a surface with prior exposure. Therefore, the odds of sampling two inherited boulders are 1:36, which seems unlikely. However, a total of 4 boulders interpreted as inherited in a dataset of 82 boulders results in <5% of boulder samples with inheritance.
(3) Erosion of Boulders
The production rate of TCN’s is approximately constant for the top 5 cm of rock. Below this 5 cm depth the production rate decreases at an exponential rate of 1/e. If the thickness of the material removed is known, then corrections can be made to account for the difference in TCN production rate. However, the amount of erosion on the surface of a boulder since deposition is hard to quantify. There are several erosional processes that affect TCN age including; fracturing along joint planes, frost shattering, exfoliation, and disintegration.
Browne Glacial Stage
52 No correlation was found between the oldest TCN ages on the Browne moraine and condition of
boulders (Table 3). For example the oldest age (78 ± 6.3 ka) was obtained from boulder Ala-
111, which was located in a boulder cluster on the crest of the Browne moraine. Its surface is
bounded by joint planes that likely match with another boulder (not sampled) and no weathering
rind (Table 2). This boulder is likely one of several remnants of a much larger boulder (likely
>10 m diameter) that has been subjected to significant fracturing along joints caused by frost
action. Very large boulders (>10 m) were exclusively found on slopes not the crest of the
Browne moraine. Furthermore, large boulders usually show evidence of movement and have
likely been exhumed by slope processes. The second oldest age (76 ± 7 ka) was obtained from a
solitaire boulder (Ala-3) well away from any cluster. This boulder has a 3-4 cm thick weathering
rind, is undergoing angular disintegration, and is rounded. Boulder Ala-3 is visibly more
degraded than Ala-111 but has a TCN age only 2 ka younger (within error).
Lignite Creek Glacial Stage
The oldest two SED ages 104 ka and 79 ka (samples Ala-137 A/B) on the Lignite Creek moraine
are from the same boulder (Fig. 12). This boulder is 8 m in diameter and contains significant
joints and weathering pits up to 30 cm deep. It is rounded with a very hard surface. The surface
where the older age sample Ala-137B was taken is flatter and about 5 cm lower than the younger
sample Ala137A (Fig 8F). A difference of ~25 ka demonstrates the significance of differential
weathering on old boulders in the Alaska Range.
The depth profile in the Lignite Creek push moraine (125 ka) has an older TCN age than the
oldest boulder (Ala-137B at 104 ka). The plot of 10Be concentrations for the Lignite Creek depth
profile suggests that inheritance is small (Fig. 9). The R2 of 0.68 of the trend line is acceptable for giving an approximate age. Therefore, it is unlikely that the difference of ~20 ka is due to
53 inheritance or uncertainty in the modeled depth profile age. The 20 ka difference in age is interpreted to be result of the sediment between 40-200 cm depth in the moraine complex being more stable and incurring less erosion than a boulder that is exposed on the surface.
Regardless of boulder condition, the Lignite Creek Stage represents the upper limit of CRN dating in this region. The oldest sample Ala-137B (104 ka) and the depth profile (125ka) have
TCN ages that are reasonably close in age (Fig. 11, Table 2). Landforms older than the Lignite
Creek Stage dated using TCN’s are significantly underestimated. This is supported by morphostratigraphically older stages having younger TCN ages.
Carlo Glacial Stage
A boulder (Ala-126 A/B) located on the Carlo terrace was sampled twice to determine the importance of differential weathering on young erratics. The older age of 18 ka was obtained from a hard intact surface (Ala-126 A) on the erratic while the younger 17.4 ka age was obtained from a detached eroded chip (Ala-126 B) sitting in a weathering pit on top of the boulder (Fig.
6D). The age difference between the two samples on this young Carlo age erratic are within error of each other while the twice dated Lignite Creek age erratic (Ala-137 A, B) have TCN dates with a difference of ~25 ka (See Section 6) (Fig. 8F). This elucidates the importance of the sampling location on boulders older than LGM ages when trying to constrain the age of landforms.
(4) Shielding
Geologic processes that can influence the shielding of a boulder are: exhumation, burial, and cover (loess and snow).
Healy Glacial Stage
54 Shielding of TCN’s by loess cover can have a significant effect due to the nonlinear relationship
between depth and TCN production rate. Two depth profiles, Healy 1 and Healy 2, taken from
Healy age outwash yielded uncorrected ages of 13 ka (R2= 0.78) and 31 ka (R2= 0.90), respectively (Figs. 9 & 10). The thickness of loess on Healy 1 and Healy 2 depth profiles is 110 cm and 170 cm respectively. The base age of loess, determined from extrapolating the age of
Aosl 7 & 8B and assuming a constant deposition rate, is used to account for TCN shielding
(Section 5.3 & 5.4). The age of initial loess deposition based on this extrapolation, which agrees reasonably well with Thorson et al. (1976) radiocarbon dating, is ~14 ka and ~12 ka, Healy 1 &
2 respectively. The loess ages were subtracted from the depth profile ages. Then the 10Be concentrations were recalculated for every 10 cm of loess deposition due to the nonlinear relationship between 10Be production and depth. After correcting for the contemporary loess cover Healy 1 & 2 have ages of 18 ka and 51 ka, respectively (Figs. 9, 10, & 14).
However, history of loess cover is unknown. Evidence for possible stripping of paleoloess is the slightly ventifacted clasts on the terrace surface observed by Thorson et al. (1976) and also by first hand observations of pedogenic carbonate development in the upper 27 cm of Healy 2 depth profile. Shielding from paleoloess deposits that were likely striped away can not be accounted
for. Therefore, the modeled ages from TCN depth profiles are minimum ages. This age
underestimation of Healy 1 and Healy 2 depth profiles is likely due to a paleo-loess cover that
shielded the terrace in the past and/or stripping of some thickness of loess before the modern
vegetation shielded it from deflation. Furthermore, the anomalously low age of the Healy 1
depth profile is likely a result of mixing of sediment due to the unconsolidated nature of the
terrace. Another possibility is a miss association of the terrace to the Healy glacial stage on
Wahrhaftig’s (1958) map.
55 (5) Summary
Denduation of older landforms (>100 ka), which leads to exhumation of boulders, can have a significant impact on TCN ages. Deflation of older moraines and erosional stripping on older terraces can cause TCN ages to significantly underestimate the age of deglaciation.
Younger landforms (LGM and younger) are more sensitive to inheritance than older landforms
(>100 ka) due to their lower 10Be concentrations. Therefore, while inheritance may only have little to no effect on Browne and Lignite Creek boulders and depth profiles, it has a significant impact on the Riley Creek 1 and Carlo boulders and depth profiles. Furthermore, due to other processes that affect stability of the landform, toppling of boulders, and erosion of landforms and boulders; the effects of inheritance on samples older than 100 ka are likely indistinguishable.
Erosion of boulders on very old landforms (Browne) does not appear to affect TCN ages as shown by deeply weathered boulders (thick weathering rinds, weathering pits, and soft surfaces) yielding TCN ages within error of boulders that are hard with no weathering rinds. The TCN ages of very old landforms are dominated by degradation of the landforms and exhumation of boulders. However, erosion of boulders can have a significant impact on landforms ~ 100 ka in age (Lignite Creek) as shown by the differential weathering of samples Ala-137 A/B that have
TCN ages ~20 ka apart. Erosion on boulders from young landforms (Carlo) does not impact
TCN ages as strongly. This is shown by samples Ala-126 A/B, which are within error of each other.
Shielding due to loess cover can have a significant impact on modeled depth profile ages. This is shown by the 20 ka difference in age between uncorrected and corrected ages of the Healy 2 depth profile. Shielding can be accounted for if the age and thickness of paleoloess cover is known. However, if it is not the TCN ages should be interpreted as minimum ages.
56 Table 3: Characteristics of boulders and bedrock sampled in the Nenana River valley. Geologic Processes
Glacial Stage (Morpho strati Exfoliation/ Bounded Weath Weath graphic TCN Age Sample Perma- Disenti - Hard/ Soft Jointed/ by Joint -ering -ering Order) (ka) # (Ala-) Landform Position frost gration Surface Factured Planes Rounded Pits Rind Teklankia 32900 125 Bench Flat Y Y S N N Y N Y 78400 111 Moraine Crest N N H Y Y N N N 76100 3 Moraine Crest N N S N N Y Y Y 63200 113 Moraine Crest N Y H Y Y Y N N 59900 112 Moraine Crest N Y S Y N Y Y Y 57300 4 Moraine Crest N N H N N Y N N 46700 117 Moraine Crest N N H N N Y N N 44000 115 Moraine Crest N Y H N Y Y N Y 43855 1 Moraine Crest N Y H N Y Y N N 36136 5 Moraine Crest N N H Y N Y N N 34700 109 Moraine Flank N N S Y N Y N Y 29200 116 Moraine Crest N Y H N Y Y N N 27500 114 Moraine Crest N N H Y N Y N N 26744 2 Moraine Crest N N H N Y Y N N 20000 118 Moraine Flank N Y H Y Y Y N N Browne 13300 110 Moraine Flank N Y H Y Y N N N 103800 137 B Moraine Flank N Y H Y N Y Y N 79200 137 A Moraine Flank N Y H Y N Y Y N 37300 156 Moraine Flat N Y H Y Y Y Y N 19200 136 Moraine Flank N Y S N Y Y N Y Lignite Creek 15100 135 Moraine Flank N N H Y Y N N N 60400 25 Drumlin Crest N Y S N N Y Y Y 59300 23 Drumlin Crest N Y S N N Y Y Y 57100 107 Bedrock Summit N Y H Y Y N N N 56600 24 Drumlin Flank N Y S Y N Y Y Y 55400 12 Moraine Crest N N H Y N Y N N 54800 13 Moraine Crest N Y H N N Y N N 53300 108 Bedrock Summit N Y H Y Y N Y N 48400 11 Moraine Crest N Y S N N Y N Y Healy 29600 157 Moraine Crest N N H Y Y Y N N 12700 19 G-Moraine Slope N Y H N N Y Y N 12000 16 G-Moraine Slope N Y H N N Y N N 10287 20 G-Moraine Slope N Y H N N Y N N 8805 15 G-Moraine Slope N N H N N Y N N Riley Creek 1 8300 18 G-Moraine Slope N Y H N N Y N N Samples are grouped by glacial stage and arranged in descending age. Primary and secondary characteristics are arranged in alphabetical order not by degree of influence of TCN ages.
57 Table 3 con’t: Characteristics of boulders and bedrock sampled in the Nenana River valley. Geologic Processes
Glacial Stage (Morpho Exfoliation/ Bounded Weath Weath strati graphic TCN Sample # Perma- Disenti - Hard/ Soft Jointed/ by Joint -ering -ering Order) Age (ka) (Ala-) Landform Position frost gration Surface Factured Planes Rounded Pits Rind 60600 120 Moraine Crest N Y H N N Y N Y 22100 121 Moraine Flank N N H N Y Y N N 14100 122 Moraine Crest N Y H N N Y N Y
Riley Creek 13700 119 Moraine Flank N Y H Y N Y N N 2 11000 123 Moraine Crest N N H Y Y Y N N 23000 127 G-Moraine Flat N Y H N N Y N Y 18700 132 G-Moraine Flat N N H N N Y N N 18400 134 G-Moraine Flat N Y H N N Y N N 18000 126 A Terrace Flat N Y S N N Y Y Y 17400 126 B Terrace Flat N Y S N N Y Y Y 17000 128 G-Moraine Flat N Y H Y Y Y N N 15900 130 G-Moraine Flat N N H N Y N N Y Carlo Re- advance 15100 133 G-Moraine Flat N Y H Y N Y N N 19400 160 Bench Slope N Y H N N Y Y N 19100 159 Bench Slope N Y H N Y N N N 18900 164 Bench Flat N N H N Y Y N N 18700 166 Bench Slope N Y H N N Y N N 17900 161 Bench Drainage N N H N N Y N N 16700 153 Bench Slope N Y H Y Y N N N 15900 155 Bench Slope N Y H N Y Y N N 15700 165 Bench Flat N Y H N N Y N N 15500 154 Bench Slope N Y H N Y N N N 15200 162 Bench Flat N Y H N N Y Y N 14800 151 Bench Flat N Y H N Y N N N 14800 152 Bench Flat N Y H N Y N Y N Reindeer Hills 13100 158 Bench Flat N Y H Y Y N N N 34200 148 RM On Top N Y H Y Y Y N N 24600 145 RM Drainage N N H N Y N N N 17900 147 RM On Top N Y H Y N Y Y Y 15600 40 RM On Top N Y S Y N Y Y Y 13300 43 RM On Top N Y H N Y Y N N
Monahan 10364 41 RM On Top N Y S N N Y N Y Flat West 9618 42 RM On Top N Y S Y N Y N Y 15400 140 RM On Top N Y H Y Y Y Y N 14900 141 RM On Top N Y H N Y Y N N Monahan Flat East 14700 143 RM On Top N Y H N N Y Y Y Samples are grouped by glacial stage and arranged in descending age. Primary and secondary characteristics are arranged in alphabetical order not by degree of influence of TCN ages.
58 Table 3 con’t: Characteristics of boulders and bedrock sampled in the Nenana River valley. Geologic Processes
Glacial Stage (Morpho strati Exfoliation/ Bounded Weath Weath - graphic TCN Sample # Perma- Disenti - Hard/ Soft Jointed/ by Joint -ering ering Order) Age (ka) (Ala-) Landform Position frost gration Surface Factured Planes Rounded Pits Rind 46200 6 Moraine Crest Y Y S N N Y Y Y 28000 104 A Moraine Crest Y Y H Y N Y N N 25600 7 Moraine Crest Y Y S N N Y Y Y
West Eight 11700 106 Moraine Crest Y Y H N Y Y Y N Mile Lake 11700 104 B Moraine Crest Y Y H Y N Y N N 63200 26 Moraine Crest Y Y S N Y N Y Y 40100 101 Moraine Crest Y Y S N N Y N N 30600 102 Moraine Crest Y N H Y Y N N N 24500 27 Moraine Crest Y Y H N N Y Y N 12900 28 Moraine Crest Y Y S Y Y Y Y N East Eight Mile Lake 8000 29 Moraine Crest Y Y H N N Y N Y 25000 139 Bench Slope N Y H Y Y N N N Gravels Ridge 21300 138 Bench Slope N Y H Y Y Y Y N Samples are grouped by glacial stage and arranged in descending age. Primary and secondary characteristics are arranged in alphabetical order not by degree of influence of TCN ages.
5.3. OSL Results
The equivalent dose on aliquots were averaged for each sample then divided by the dose rate
giving a mean age (Tables 4 & 5). Calculation uncertainties and methods used to calculate dose
rates are explained in the note of Table 5. Probability density graphs and histograms of aliquot
ages were plotted to confirm the mean equivalent dose ages (Fig. 15). Assuming a constant
sedimentation rate, the age of the base of the loess was calculated by dividing the mean age by
sample depth then multiplying by loess thickness (Table 5).
59 Table 4: Instrumental neutron activation analysis, cosmic dose rates, total doses and ages
Sample # Sediment K Rb Th U Water Cosmic Total Number DE Age /Lab ID typea (%)b (ppm)b (ppm)b (ppm)b content dose rate dose rate of aliquots (Gy) f (ka) (%)c (Gy/ka)d (Gy/ka)e Aosl-1 CGQ-B 1.56 82.5 13.10 3.26 17.1 0.20±0.02 3.10±0.12 43 25.97±9.30 8.3±0.8 Aosl-2 CGQ-B 2.00 95.0 13.20 3.18 16.4 0.20±0.02 3.46±0.14 12 180.43±51.08 52.0±5.0 Aosl-6 CGQ-C 1.92 113.0 11.90 3.80 5.3 0.20±0.02 3.46±0.14 52 9.58 ±1.27 2.8±0.2 Aosl-7 CGQ-C 1.93 99.4 15.20 4.60 3.5 0.19±0.02 3.83±0.15 44 39.23±9.69 10.2±0.8 Aosl-8B CGQ-C 2.29 115.0 21.70 4.96 10.6 0.18±0.02 4.63±0.18 21 35.66±5.43 7.7±0.5 Aosl-9 CGQ-C 4.15 209.0 23.00 4.90 0.8 0.19±0.02 6.30±0.26 18 24.71±2.91 3.9±0.3 Aosl-10 CGQ-A 1.47 68.1 12.00 2.57 3.2 0.06±0.01 2.66±0.11 23 49.45±16.55 18.6±1.7 Notes a. Analysis for coarse-grained (90-125 µm) quartz fraction (CGQ) on (A) well sorted sand, (B) pebbly sand and (C) loess. b. Uncertainties on analyses of U, Th, Rb and K are taken to be ± 10%. c. Assumed 10 ± 5% for calculations. d. Cosmic doses and attenuation with depth were calculated using the methods of Prescott and Stephans (1982) and Prescott and Hutton (1994). e. Total doses combine the cosmic dose rate and the beta and gamma doses calculated from the K, Rb, Th and U concentrations in the sediment using the methods of Adamiec and Aitken (1998) and Mejdahl (1979) with corrections for moisture content after Zimmerman (1971). f. Average equivalent dose (DE) and error (1σn-1 of the DE), incorporating the error from beta source estimated at about ± 5%.
60
Figure 15: Probability plots and histograms of aliquots for samples Aosl: 1, 2, 6, 7, 8B, 9, & 10. The sample location and DE age is located in the upper right corner for each sample. All histogram bins are 1 ka in width except for Aosl 8B & 9, which are 0.5 ka in width.
61 Table 5: Aosl 1-9 are mean OSL ages in ka of terrace capping loess.
Sample Loess Extrapolated Age Sample Location Age (ka) Error (ka) Depth (m) Thickness (m) of Loess Base Aosl-1 Lignite Loess 8.3 0.8 0.6 0.0-0.9 Reworked Aosl-2 Lignite by tephra 52.0 5.0 ? ? Reworked Aosl-7 Healy 1 Loess 10.2 0.8 0.79 1.1 14.2 Aosl-8B Healy 2 Loess 7.7 0.5 1.1 1.7 11.9 Aosl-9 Riley Loess 3.9 0.3 0.88 1.21 5.4 Aosl-10 Riley Sand Lens 18.6 1.7 N/A N/A N/A Aosl-6 Carlo Loess 2.8 0.2 0.29 0.65 6.3 Aosl 10 is an OSL age on a sand lens in the Riley Creek outwash terrace. Samples are listed in relative order of associated glacial stage.
5.4. Evaluation of OSL Data
Lignite Creek Glacial Stage
Aosl-1 has an equivalent dose age (DE age) of 8.3±0.8 ka. The Histogram and probability plots in Fig. 15 show some aliquots have DE ages older than 8.3 ka. This is likely due to partial bleaching. This sample was collected in Unit-1 (Fig. 9) above the lignite depth profile. The presence of pebbles suggests reworking of loess after deposition even though the stratified gravels in Unit-2 appear undisturbed. The thickness of Unit-1 varies and pinches out laterally, which also suggests that is has been disturbed. Therefore the age of the base of the loess was not estimated.
Aosl-2 has a DE age of 52±5 ka. The histogram and probability plots show a bi modal distribution of ~50 ka and ~70 ka (Fig. 15). The ~50 ka age is more reliable because there are a higher number of overlapping aliquots. This sample was collected from glacially deformed sediments near the Stampede Tephra to obtain an age of glaciation (Beget, 1991). The Stampede
Tephra (140 ka), TCN depth profile indicate that glacialtectonic deformation occurred >125 ka.
Therefore the DE age of 52 ka is interpreted to be the result of subsequent reworking after deposition. Reworking of the sediment and quartz illuviation can mix younger quartz grains with the older grains or possibly bleach old gains and reset the electron traps. Reworking of
62 quartz and exposure to light will not affect the chemical correlation of the Stampede Tephra,
making the tephra age more reliable.
The age of Aosl-2 indicated that there was loess deposition ~52 ka ago in the Nenana River
Valley. This is the only site in the Nenana River valley where loess >13 ka cal yr has been
described. It likely correlates with Thorson et al (1985) 14 m thick loess (~43 ka at 5 m depth)
in the Teklanika River valley and the eolian erosional event described by Thorson et al (1976)
that created ventifacts on the surface of the Healy outwash terrace.
Healy Glacial Stage
Aosl-7 has a DE age of 10.2±0.8 ka. The histogram and probability plot overlaps with the DE
age. However, there are aliquots with DE ages older than 10.2 ka (Fig 15). These DE ages are
likely the result of partial bleaching. A constant sedimentation rate of 0.0077 cm/yr was
determined by dividing the thickness of loess (79 cm) above Aosl-7 by 10.2 ka. Extrapolating
this sedimentation rate results in the basal age of loess at 14.2 ka (Table 5).
Aosl-8B has a DE age of 7.7±0.5 ka. The DE ages overlaps with the peak of the histogram and
probability plots (Fig 15). The histogram shows little scatter and has a higher statistical
probability of being accurate compared to Aosl-7. This suggests the sediment was well bleached
before deposition. Using the DE age of 7.7 ka and assuming a constant sedimentation rate of
0.0143 cm/yr, the basal age of the loess is 11.9 ka (Table 5).
The aliquots for sample Aosl-7 are more widespread with a skew to older ages compared to
Aosl-8B (Fig. 15). The peaks in both the histogram and the probability plots of Aosl-7 range
from 7.5-10 ka, while Aosl-8B ranges from 7-8 ka. The samples have an overlap from 7.5-8 ka,
which also overlaps with the DE age of Aosl-8B. Therefore the DE age of 7.7 ka is interpreted
to be the most accurate. Loess is not usually formed by continuous sedimentation on >1 ka time
scales. As noted by Biglow et al. there are periods of more rapid deposition and erosional
events. The extrapolated basal age of (11.9 ka) based on Aosl-8B (DE age of 7.7 ka) is in
63 reasonable agreement with Thorson et al (1976) radiocarbon date on paleosol 1 (~13 ka cal yr) located near the base of the loess. Therefore, the deposition of contemporary loess began 12-13 ka on the Healy terrace.
Riley Glacial Stage
Aosl-9 has a DE age of 3.9±0.3 ka. The histogram indicated a bimodal distribution of 4.2 ka and
5.2 ka (Fig. 15). The probability plots show a higher statistical likelihood of the 4.2 ka age being accurate. Also, the 4.2 ka age overlaps with the error of the DE age 3.9±0.3 ka. Using the DE age of 3.9 ka and assuming a constant sedimentation rate of 0.0226 cm/yr, the basal age of the loess is 5.4 ka (Table 5).
The difference between the 6.9 ka cal yr radiocarbon date by Thorson et al. (1976) on a paleosol near the base of the loess unit and the estimated 5.4 ka ages are within reasonable agreement.
The paleosol dated by thorson et al. (1976) is above the base. Therefore, initiation of loess deposition on the Riley terrace likely began ~7 ka.
Aosl-10 has a DE age of 18.6±1.7 ka, which overlaps with the histogram age of ~20 ka (Fig. 15).
This sample was collected from a sand lens within the Riley Creel 1 glacial stage outwash terrace. However, the probability plots indicate an age of ~10 ka. This is largely due to two aliquots with narrow probabilities. Most aliquots have much broader probabilities that create a skew of older ages. The DE age of older aliquots are likely due to partial bleaching. The clarity of the meltwater charged river that deposited the sand lens is unknown. However, the increased discharge due to the meltwater likely incorporated many fine-grained glacial sediments through suspension and saltation. This likely resulted in poor exposure of the quartz grains to light and partial bleaching of some quartz grains. This would account for the aliquots with DE ages >22 ka. The overlap of the DE age (18.6±1.7 ka) and histogram age (~20 ka) suggests that the most accurate age of the sand lens is ~ 20 ka.
Carlo Glacial Stage
64 Aosl-6 has a DE age of 2.8±0.2 k, which overlaps with both the highest bin of the histogram (2.8 ka) and the probability density plots (2.6 ka) (Fig. 15). The correlation between the three ages suggest that 2.8 ka is an accurate age for sample Aosl-6. The younger aliquots in the histogram and likely due to post depositional reworking of sediment as indicated by the incorporation of small pebble in the loess unit. Using the DE age of 2.8 ka and assuming a constant sedimentation rate, the basal age of the loess is 6.3 ka. This age is close to the basal age of loess on the riley Creek 1 terrace. Initiation of loess deposition likely occurred at the same time on both the Riley and Carlo terraces.
6. Timing of Glaciation
Teklankia Glacial Stage
The only sample (Ala-125) collected from the Teklankia Stage (oldest and most extensive glaciation) has age of 33 ka (Fig. 14). Significant erosion on the glaciated bench where the sample was collected is evident through active gellification lobes and the weathering of the granitic eracitic to near ground level. Also, morphostratigraphically younger landforms have significantly older TCN ages (See below). Therefore, The TCN age of 33 ka (Ala-125) is interpreted as a significant underestimate for the age of the Teklankia glacial stage. Furthermore, because only one sample was collected it was not possible to access general effects of geologic processes.
Browne Glacial Stage
The sample set (n=15) for the Browne glacial stage has TCN dates that range from 13-78 ka
(samples Ala- 1-5 & 109-118) (Fig. 14, Table 2). This large range of TCN dates likely results from degradation of the Browne glacial stage moraine. The depth profile from Browne Stage glacial outwash yields an age of 86 ± 5.1 ka and is within error of the oldest boulder78 ± 6.3 ka.
However, this data was removed because it does not meet the limiting criteria for the oldest boulder method. Both terrace and boulder ages of the Browne glacial stage are interpreted to be
65 significantly underestimated based on morphostratigraphy (see discussion of Lignite Creek glacial stage below). The age underestimation of the terrace depth profile age is likely due to one or more paleo-loess covers that have shielded the terrace in the past, such as the loess described by Thorson et al. (1985), and/or significant erosion of the terrace surface.
Considering the age of the Lignite Creek glacial stage and the larger extent of the Browne glacial stage suggests that the Browne glacial stage is > MIS-8. Due to the geologic processes stated above, the age of the Browne Stage is beyond the limits of standard CRN dating. Although no samples were dated from the Bear Creek Stage the close geographic position to the Browne and morphostratigraphic order suggests a similar interpretation of > MIS-8.
Lignite Creek glacial stage
Five boulder samples (Ala- 135-137 & 156) from the Lignite push moraine complex have ages that range from 15-104 ka (Fig. 14 & Table 2). The depth profile sampled in the push moraine complex yields an age of 125 ka (Fig. 9). Since the depth profile is located in the push moraine, it represents the stabilization of glaciotectonically-deformed sediment. The Lignite Creek depth profile age of 125 ka, which occurs within MIS-5e, is interpreted to represent a very minimum age of glaciation following the critera of the oldest age method. Although the push moraine is more stable than the surface of a boulder, it has likely been subjected to erosion and possible surface striping of reworked loess. Due to the reworking at the base of the loess and the uncertainty of the paleo thickness of loess, the depth profile age has not been corrected for shielding. Error on the depth profile age (125 ± 7.6 ka) overlaps with MIS-6. Due to 125 ± 7.6 ka being a very minimum age and using the independently dated Stampeede Tephra (140 ka,
Beget personal communication), the Lignite Creek glacial stage is bracketed to 125-140 ka and is interpreted to correlate with MIS-6.
66 Table 6: Ages of glacial stages. Oldest Boulder w/o Glacial Stage Range of Ages limiting conditions This Study Teklankia 38 ka 38 ka ? Browne 13-78 ka 78 ka ? Bear Creek ? ? ? Lignite Creek 15-104 ka >104 ka 140-125 ka Healy 30-60 ka >60 ka >60 ka Riley Creek 1 8-13 ka 13 ka 22-30 ka Riley Creek 2 11-61 ka 61 ka ~22 ka Carlo Readvance 15-23 ka 23 ka ~19 ka Reindeer Hills 13-19 ka ~19 ka ~19 ka Monahan Flat west 9-34 ka 34 ka ~18 ka Monahan Flat east 14-15 ka 15 ka ~15 ka Eight Mile Lake 8-63 ka ? ? Ages of glacial stages in this study are assigned through examination of clustering and geomorphic order of TCN. Ages assigned by the oldest boulder method are based on the oldest TCN date from a surface erratic.
Healy Glacial Stage
TCN ages (samples Ala- 11-13, 23-25, 107, 108, & 157) on Healy age landforms range from 30-
60 ka (Fig 14; Table 2). Seven of the nine samples (two from moraines, three from drumlins,
and two from bedrock) cluster between 53-60 ka (Fig. 2; Table 2). Two depth profiles, Healy 1
and Healy 2, taken from Healy age outwash yielded uncorrected ages of 13 ka (R2= 0.78) and 31 ka (R2= 0.90), respectively (Figs. 9 & 10). After correcting for the contemporary loess cover
Healy 1 & 2 have ages of 18 ka and 51 ka, respectively (Fig. 13).
This tight clustering of TCN ages on boulders between 53-60 ka suggests reliability and
reproducibility. The 18 ka age of the Healy 2 depth profile is interpreted to significantly
underestimate the age of the Healy glacial stage. The corrected Healy 2 depth profile age (51 ka)
is ~9 ka younger than erratics sampled on Healy age landforms. Due to the uncertainty of paleo-
loess cover on Healy age outwash terraces, the timing of Healy glaciation is interpreted from
surface boulders. These TCN ages meet the criteria for the oldest boulder method. Using the
oldest boulder (Ala-25), located on a drumlin, the deglaciation is interpreted to have occurred
>60 ka and is correlated with MIS-4.
67 Riley Creek 1 Glacial Stage
All TCN ages (12.7-8.3 ka) from the Riley Creek 1 glacial stage ground moraine were removed from the data set because they do not meet the morphostratigraphic order for the oldest boulder method (Fig. 14). The TCN ages are younger than clustered TCN ages from the younger Carlo glacial stage (~19 ka). However, the morphostratigraphic order can be questioned. Therefore, two possible explanations are considered.
First, the Riley Creek 1 glacial stage is not morphostratographically older than the Riley Creek 2 and Carlo glacial stages. Riley Creek 1 glacial stage could be the result of the Yanert alpine glacier advancing down to the Denali Park entrance during the YD cold period without completely destroying the Riley Creek 2 glacial stage and Carlo glacial stage deposits. This is possible because during the YD the Cordilleran Ice Sheet did not form. Therefore, a north flowing outlet glacier, which would have overridden and destroyed the Riley Creek glacial stage
2 and Carlo glacial stage deposits, did not occupy the Nenana Corridor.
The second, and more likely, explanation is that the Riley Creek 1 glacial stage moraine is an erosional surface. Both a previous report by Thorson et al. (1976) of the Riley Creek 1 glacial stage moraine being heavily eroded and mostly destroyed and first hand field observations of the sloping ground moraine surface made by the authors conclude that erosion has been extensive.
Based on the close geographic proximity of the Riley Creek 1 and 2 stages the authors agree with the previous interpretations of Thorson (1976) that they were both formed during the same glacial cycle. Therefore, the age of the Riley Creek 1 glacial stage is interpreted to be older than the Riley Creek 2 glacial stage (~22 ka) but within the LGM advance, which suggests ages between 22-30 ka. Other supporting evidence for the later interpretation comes from the Aosl-10
(~20 ka), which indicates that terrace formation occurred >20 ka. The outwash terrace formation likely postdates deglaciation and moraine stabilization of the Riley Creek 1 glacial stage moraine.
68 Riley Creek 2 Glacial Stage
Erratics (Ala 119-123) from the Riley Creek 2 glacial stage ground moraine have TCN ages ranging from 11-61 ka (Fig. 14, Table 2). Samples (Ala-120 (61 ka) and Ala-121 (22.1 ka) are considered outliers and have been removed from the data set following the conditions of the oldest boulder method stated above. The other three samples (Ala-119, 122, & 123) range from
11-14.1 ka have also been removed because they do not fit in morphostratigraphic order. A depth profile was not collected in the terrace that correlates with Riley Creek 2.
An exception for sample Ala-121 was made to keep it in the data set because it is likely a reliable age. Justification for this is (1) the clustered data used to identify outliers is too young, (2) the
TCN age of Ala-121 is in morphostratigraphic order, and (3) the lack of clustering is likely due to a small sample set (n=5). Using this TCN age places Riley Creek 2 deglaciation at ~22 ka.
This single TCN age is not interpreted to represent a solid approximation of deglaciation.
However, it is useful because it suggests a time gap between the Riley Creek 2 and Carlo glacial stages. Also, it can be used as a lower bracketing age for the Riley Creek 1 glacial stage.
Carlo Glacial Stage
Erratics (Ala-126-128, 130, & 132-134) from the Carlo glacial stage ground moraine yielded ages that range from 15.1-23 Ka (Fig. 14). Sample Ala-127 (23 ± 2.8 ka) and the depth profile a have been removed from the data set following the conditions of the oldest boulder method. The twice dated erratic (Ala-126 A & B) laying directly on top of the associated outwash terrace has
TCN ages of 17.4 ka and 18 ka respectively.
The tight age clustering of the second two oldest erratics (18.4 ka & 18.7 ka) on the moraine and the twice dated erratic Ala-126 A/B on the associated outwash terrace (TCN ages of 17.4 ka and
18 ka) suggests that the Carlo Readvance occurred ~19 ka and correlates with MIS-2. Based on
Wahrhaftig’s (1958) correlation of the two main Carlo Readvance deposits, in the Nenana
69 Corridor and in the Yanert glacial valley, the area by the Denali Park entrance must have been
deglaciated by ~19 ka.
This is in contrast to Thorson et al. (1976), Hamilton (1982), Thorson et al (1985), and Bigelow
et al. (1989) suggestion that the initiation of aeolian deposition closely followed the final
deglaciation, which implies deglaciation ~14.5 ka cal yr. This 5 ka age difference suggests that
there was a lag between deglaciation and loess deposition or that there was stripping of older
loess. Another possibility is the incorporation of reworked dead carbon from coal bering rocks
outcropped in the Nenana River valley, which would make the radiocarbon ages younger.
Eight Mile Lake Region
All data from the Eight Mile Lake region (TCN ages range from 63-8 ka) is interpreted as
significantly underestimated following the conditions of the oldest boulder method (FIG. 14).
Deglaciation of the Eight Mile Lake region must have occurred before the Lignite Creek glacial
stage (140-125 ka). The Eight Mile Lake moraines are deflated, dissection by drainage, and have
boulder clusters. This morphological expression and the close geographic proximity of the West
and East Eight Mile Lake moraine, suggest a correlation with the Browne Stage and Bear Creek
glacial stages, respectively.
Reindeer Hills
The glacial erratics sampled (Ala-151-155, 158-162, & 164-166) on the five glaciated benches of
Reindeer hills have TCN ages that are within error of each other, which meets the conditions of
the oldest boulder method (Fig. 14; Table 2). These TCN ages range from 13.1-19.4 ka with
four samples clustering between 18.7-19.4 ka. The 19.1 ka and 19.4 ka ages (Ala- 159, 160)
were sampled on the 3rd (middle) bench while 18.4 ka and 18.7 ka ages (Ala-166 & 164) were sampled on the 5th (lowest) bench. Some of the youngest TCN ages are from erratics on the highest benches (Fig. 11; Table 1). This suggests that ice covered all five glacial benches during
70 the last ice advance. Using the oldest boulder (Ala-160), the minimum age of deglaciation and stabilization of Reindeer Hills is 19 ka.
The elevation of the highest bench sampled is 600 m above the valley floor. The trimline used by Wahrhaftig (1958) for LGM ice reconstructions is ~700 m above the valley floor. Therefore, using the oldest erratic (Ala-160), ice 600 -700 m thick deglaciatied from the lowest bench by 19 ka. This also requires the deglaciation of the Nenana Corridor by 19 ka, which agrees with the ~
19 ka deglaciation of the Carlo glacial stage moraine. The preserved terraces in the Nenana
River Valley indicate the Yanert Glacier has not occupied the valley since Riley Creek glacial stage 2 time (~22 ka) and that no ice could have flowed north since the deposition of the Carlo glacial stage (~19 ka). The TCN ages on deglaciation from the Carlo position and Reindeer
Hills, both at ~19 ka, suggests rapid ice retreat (~38 km in <1 ka).
Monahan Flat West
Seven granitic erratics (Ala-40-43, 145, 147, 148) located on top of roche montenees composed of diorite have TCN ages that range from 9.6-34.2 ka (Fig. 14; Table 2). Since deposition of boulders is on thin sediments or bedrock, stabilization of the erratics should be rapid. Samples
Ala 148 and 145 have been removed from the data set following the conditions of the oldest boulder method.
Using the oldest boulder (Ala-147) in the new age range (18-10 ka) suggests deglaciation from the western end of Monahan Flat at ~18 ka. This is consistent with the age of deglaciation of
Reindeer Hills (19 ka) and implies that glaciers retreated ~30 km in ~1 ka (glacial retreat rate of
~30 m/yr).
Monahan Flat East
Three granitic erratics (Ala-140, 141, 143) located on top of roche montenees composed of diorite have TCN ages that range from 14.7-15.4 ka (Fig. 14; Table 2). These three ages are tightly clustered and meet the conditions of the oldest boulder method. Since deposition of
71 boulders is on thin sediments or bedrock, stabilization of the erratics should be rapid. Using the oldest boulder (Ala-140), the minimum age of deglaciation on the southeastern side of Monahan
Flat is 15 ka.
Tight clustering (14.7-15.4 ka) of the three granitic erratics (Ala-140, 141, 143) is interpreted as glaciers retreating from the south-eastern side of Monahan Flat back toward the Nenana and
West Fork sources by ~15 ka. Based on these data, glaciers retreated 20 km from the Monahan
Flat west site (~18 ka) to the Monahan Flat east site (~15 ka), which indicates a glacial retreat rate of ~7 m/yr. The rate of 7 m/yr is ~1/4 of the glacial retreat rate from Reindeer Hills to the western edge of Monahan Flat (~30 m/yr). This is likely due to the fact that Monahan Flat is a large basin that was completely covered by glacial ice during the LGM, whereas glaciers flowing west to reach Reindeer Hills flowed through a comparatively narrow pass (Wahrhaftig, 1958). 7. Correlation with other regions of Alaska
The Delta River valley, located ~100 km to the east of the Nenana River valley, contains a record of at least two glacial advances, the Delta (most extensive) and Donnelly glacial stages. The age of the Delta glacial stage is bracketed between 140±10 ka and190±20 ka using tephra chronology by Beget et al. (2002) and is correlated to MIS 6. This study correlates the Lignite Creek glacial stage with the Delta glacial stage and MIS 6. The younger Donnelly glacial stage likely correlates with the Healy glacial stage and MIS 4. These correlations should be tested using similar TCN techniques in the Delta River valley.
Based on a probability plot of 95 moraine boulder TCN ages, Briner et al. (2005) suggests that late Wisconsin deglaciation occurred in two intervals, 24-26 ka and 17-21 ka. Riley Creek glacial stage 1 (22-30 ka) likely correlates with the first interval 24-26 ka. The Carlo (~19 ka) glacial stage is suggested to correlate with the later interval of 17-21 ka. Due the rough approximation of the Riley Creek 2 glacial stage (~22 ka) a correlation can not be made.
72 8. Conclusions
The oldest age method is most applicable where there is no evidence of inheritance (Briner et al.
2005). This method is also limited where there are few TCN ages or they are too scattered
(Briner et al. 2005). Our study agrees with these observations. The oldest age method proves unreliable on poorly clustered ages by underestimating old landforms (due to exhumation and erosion such as on the Browne moraine) or overestimation young landforms (due to inheritance such as on the Riley 2 and Carlo glacial stages) (Table 5). Therefore limiting conditions are defined to minimize the impact of geologic processes. These conditions are: (1) clustering of at least 2 TCN ages with in error and (2) TCN ages must fit in geomorphic order. Data that did not meet these criteria were removed from the data set before application of the oldest boulder method.
The age of the Teklankia, Browne, and Bear Creek Stages could not be determined through standard TCN dating techniques (Table 7). Also, the association of the Eight Mile Lake moraines could not be quantified. The Lignite Creek glacial stage (>125 ± 7.6ka) represents the upper limit of CRN dating in this region and is interpreted to correlate with MIS-6. The Healy glacial stage has clustered TCN ages between 53-60 ka and is interpreted to have occurred >60 ka and correlate with MIS-4.
Significant erosion has occurred on Riley Creek glacial stage 1 landforms and the TCN ages are significantly underestimated. Riley Creek glacial stage 1 likely occurred in the same glacial cycle as Riley Creek glacial stage 2, which also sets the lower age limit at ~22 ka. Therefore,
Riley Creek glacial stage 1 is interpreted to have occurred 22-30 Ka and correlates with early
MIS-2. The Riley Creek 2 glacial stage is loosely approximated at 22 ka. The Carlo glacial stage sample Ala-127 with an age of 23 ka is interpreted as a reworked boulder from the Riley
Creek 1 & 2 glacial stages. The clustering of the remaining TCN ages from the Carlo glacial
73 stage suggests deglaciation occurred ~19 ka. Also, ice has not occupied the Nenana River valley since Carlo time.
Clustering of TCN ages from glaciated benches on Reindeer Hills suggests deglaciation by 19 ka. This requires rapid glacier retreat rates since the onset of deglaciation of the Carlo
Readvance. Glaciers retreated from Reindeer Hills to the western edge of Monahan Flat by 18 ka at a rate of ~30 m/yr. Deglaciation of the Monahan Flat basin was comparatively slow (~7 m/yr).
Table 7: Summary of ages for glacial landforms in the Nenana River valley. Suggested Ages Oldest Boulder Glacial Stage Beget Range w/o (morphostratigraphic Wahrhaftig Thorson et al. of limiting This order) (1958) (1986) (1991) Ages conditions Study Not Late Taklanika recognized Tertiary --- 38 ka 38 ka --- Mid 13-78 Browne Late Tertiary Pleistocene --- ka 78 ka >MIS 8 Recessional of Separate Bear Creek Browne Stage advance ------>MIS 8 Mid Dry Creek Pleistocene Inconclusive ------Coupled with Separate ~140 15-125 140-125 Lignite Creek Healy Stage advance ka ka >104 ka ka >Late 30-60 Healy no comment Wisconsin ~60 ka ka >60 ka >60 ka Late Late Late Riley Creek 1 Wisconsin Wisconsin Wis 8-13 ka 13 ka 22-30 ka Late 11-61 Riley Creek 2 Wisconsin ------ka 61 ka ~22 ka Late 15-23 Carlo Wisconsin ------ka 23 ka ~19 ka 13-19 Reindeer Hills ------ka ~19 ka ~19 ka Monahan Flat west ------9-34 ka 34 ka ~18 ka 14-15 Monahan Flat east ------ka 15 ka ~15 ka Eight Mile Lake ------8-63 ka 63 ka --- Ages of glacial landforms in the Nenana River valley are from previous work, TCN data range, oldest boulder method with out limiting conditions, and the modified oldest boulder method (this study).
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