Terrestrial cosmogenic nuclide surface exposure dating of moraines and terraces in
the Nenana Valley, northern slopes of the Alaskan Range: a model for dating
dynamic surfaces for neotectonic studies [*J M Dortch*] (Department of Geology,
University of Cincinnati, Cincinnati, OH 45221-0013; ph. 513-252-4705; e-mail:
dortchjm@uc.edu); L A Owen (Department of Geology, University of Cincinnati,
Cincinnati, OH 45221-0013; ph. 513-556-4203; e-mail: lewis.owen@uc.edu); M W
Caffee (Department of Physics, Purdue University, West Lafayette, IN 47907-2036; ph. 765-4945381; e-mail: mcaffee@physics.purdue.edu)
Impressive moraines and terraces are present in the Nenana Valley on the northern slopes
of the Alaskan range. These are tectonically deformed; faulted and tilted. However,
dating these deposits for neotectonic and paloenvironmental studies is challenging. This
is mainly because organic matter required for radiocarbon dating is sparse in these
deposits and where present usually only provides minimum ages for the landforms.
Furthermore, the method does not allow landforms to be dated beyond the normal
radiocarbon dating range of ~30 ka. In addition, the mobility of the landform surfaces,
due to dynamic periglacial processes in the region, erosion and sedimentation by aeolian
(mainly loess) and mass movement processes makes terrestrial cosmogenic nuclide
(TCN) surface exposure dating difficult. However, TCN surface exposure dating when
successful allows landforms to be dated from several hundred years old to hundreds of
thousands of years old. In an attempt to assess the applicability and to improve
methodologies for TCN surface exposure dating methods in Alaskan taiga and tundra
environments, we undertook an extensive program of TCN dating on the landforms in the
Nenana valley. This included processing >130 10Be dates on glacially eroded bedrock,
boulders on moraines, and sediments within depth profiles in terraces. We focused the
assessing variability of dates determined on individual landforms and on determining the
upper limit of the TCN chronology in the Nenana Valley to assess the factors that limits
the techniques. The comparison of terraces (mainly depth profile dates) and moraines
(mainly boulder and ice-polished bedrock dates) provides important insights into the
stability of the surfaces, suggesting that terraces rather than moraines are more stable and
more appropriate for TCN surface exposure dating in these environments. Optically
stimulated luminescences dating currently being undertaken will test the TCN dates and
will provide more insights into the history of the Nenana Valley landforms. Using this
study, we provide a protocol for determining ages using TCN methods on similar surface
surfaces in Alaskan and other Arctic environments that are subjects to periglacial
processes, erosion and sedimentation. Providing reliable ages on landforms is a major
step forward to help define rates of surface deformation in Alaska on geomorphic (100s
to 100,000s years) timescales.