Defining the Timing of Glaciation in the Central Alaska Range

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).

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