Cosmogenic Isotope Chronology, Seward Peninsula and NC Brooks Range


SEWARD PENINSULA GLACIAL SEQUENCE

The glacial history of the Kigluaik Mountains of Seward Peninsula has been well described by Kaufman and Hopkins (1986), Kaufman and Calkin (1988) and Peck et al., (1990). Because of the wealth of relative-age data collected on the glacial sequence in the past and the fact that the glacial deposits interfinger with well-known marine sequences at the coast, we felt that this region was probably most relevant for comparison with the glacial sequences on Chukotka. Field work in this area was carried out specifically with two goals: (1) to demonstrate the style and character of the Pleistocene glacial sequence for this part of Alaska to Dr. Glushkova, and (2) to sample the entire glacial sequence in several valleys for cosmogenic isotope dating. This isotopic work is intended to provide for the first time numerical age estimates of the nested moraine sequence for comparison with published relative age estimates and our cosmogenic work in Russia.

Demonstration of the glacial sequence in the Kigulaik Mountains turned out to be crucial to forging a new understanding of the differences between Russian geomorphological mapping and American surficial mapping. In the field we discussed at length the purpose and rationale of our surficial maps as well as the use of relative-dating and the morphometric analysis of landforms. This exercise was extremely important because many Russians like Dr. Glushkova were trained to analyze landscapes in a different way. We determined for ourselves that many differences stem from the Russian preference to assess landscape assemblages and create geomorphology maps. On the other hand, USGS policy and academic training emphasizes the mapping of surficial deposits and their relative age, a tact that produces a fundamentally different style of map. Glushkova's glacial maps of northeast Russia contain a great deal of information but the mapping units consist of landform assemblages different from what we are accustomed. In the field we argued, for example, over the fundamental mapping of cirques vs. the mapping of nivation hollows. The discussions were extremely constructive in generating an open dialog toward respecting each others point of view and training without the language and cultural barriers. We sought out the common ground and discovered a great deal that all we agreed upon. All of this was facilitated by having excellent translators, Ramon Azatov (Univ. AK -Anchorage) and Sonja Benson (USGS-Fairbanks). In Russia in 1995, we worked much of the time without a translator and found that my terrible Russian skills and Olga's weak English kept us from having substantive science discussions.

We sampled moraines mapped by Darrell Kaufman et al (1989) in the Kigluaik Mountains including sites at David Creek (long Nome River), Salmon Lake, Crater Creek and Homestake Creek along the south side using the road as easy access. We were then set out to camp along the Cobblestone Creek by local helicopter hire and worked for six days in two unnamed valleys directly west of Mt. Osborn (the highest peak) and along the lower Cobblestone and Grand Union tributaries along the northcentral part of the range. We obtained good to excellent coverage of the Steward River, Salmon Lake, Mt. Osborn- age deposits as well as a few of the inner cirque sequences; we were unable to collect viable material from middle Pleistocene Nome River-age surfaces due to the lack of erratics at the surface. Because Kirk Yuengling ( MSc at UAF) is studying tor formation inside and outside the Nome River limit in the Kigluaik Mountains using cosmogenic isotopes for this purpose, we did not attempt to duplicate his efforts.

One of the surprising observations we made along the north side of the Kigluaik Mountains was the amount of offset expressed by the northern range-front fault. This fault offsets moraines of different ages along the front, including some Holocene deposits (Kaufman et al., 1989). We observed clear fault scarps in the Salmon Lake moraines on either side of Cobblestone Creek and younger (?) moraines at the mouth of Grand Union Creek are offset as much as ~6 m. Kaufman et al. (1989) report measuring as much as 2- 6 m of offset in moraines at the eastern end of the range. These observations clearly indicate that this large fault system continues to be active at the present time and could be the source of notable earthquakes in the future.

Of special note to regional land managers, we wish to report that we discovered archeological remains likely to be more than 100 years old in the Cobblestone River valley. The outermost terminal moraine segment at the mouth of the unnamed valley directly west of Mt. Osborn Peak is mantled by several circular tent rings, a campfire ring, and a possible meat pit. The tent rings are 3.7 m and 3.8 m in diameter and undisturbed. The stone hearth is 1.4 m in diameter with inner stones that show some fire reddening. The meat pit measures about 90 cm in diameter and is 25 cm deep. The site is marked by an upright slab of rock some 50 cm in height surrounded by 6 boulders. Elsewhere on a high moraine at the mouth of Grand Union valley, we discovered a man-made pile of rocks holding up a large cottonwood branch, perhaps marking the position of a cache or animal kill. The age of this feature is unknown.


CENTRAL BROOKS RANGE GLACIAL SEQUENCE

In order to fully assess and compare the glacial sequences of Chukotka with those familiar to Hamilton in Alaska (Hamilton, 1986, 1994 and references therein), we traveled the Dalton Highway out of Fairbanks and accessed the northern Brooks Range sequences via the University of Alaska Toolik Lake Field Station. Access to the complete moraine sequence was made possible by using the NSF-funded helicopter at Toolik Lake and on alternate days working on foot from the Dalton Highway using a Suburban rented from the UAF Toolik office. The NSF logistics supplement we received for the helicopter work proved to be invaluable for insuring the success of the sampling effort. While we were there, we also interacted with scientists on other NSF projects (esp. Skip Walker) concerning the age of the surrounding landscapes and regional vegetation mapping. Using Hamiltonšs excellent field notes and original air photo sketches and maps, we reoccupied field stations where Hamilton collected relative-age data over 20 years ago so that we could sample glaciated boulders for cosmogenic age dating. Sampling of the Kanayut Conglomerate (chert pebble conglomerate with a quartz cement) proved to be difficult even for a diamond-bit drill, however we persisted using an 8 LB sledge. We successfully sampled the entire Pleistocene glacial sequence of Hamilton in several valleys - including the Sagavanirktok, Itkillik, Toolik, and Kuparuk rivers. At the same time, fruitful discussions with Glushkova on the regional quadrangle mapping strategy and airphoto interpretation provided the detail and rationale behind Hamiltonšs published surficial maps of the area. In total we collected about 120 samples for cosmogenic isotope dating from the Kigluaik Mountains and the Brooks Range.


Seward Peninsula Pictures

Brooks Range Pictures


Beringian Paleoenvironments Workshop (September 1997)

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