Mount Doonerak, Alaska with Justin Strauss

Exploring Mount Doonerak of the Brooks Range, Alaska
Justin is a PhD student at Harvard University working with Prof. Francis Macdonald.


            Arctic Alaska is a ‘suspect’ Cordilleran terrane that encompasses approximately 20% of Alaska, stretching from the southern Brooks Range foothills all the way through the North Slope and onto the continental shelves of the Chukchi and Beaufort Seas. The origin and subsequent travels of this large crustal fragment are hotly debated among geologists; however, many workers agree upon its composite nature (i.e., it is composed of multiple smaller fragments) and origin site in the Arctic Realm of Baltica and Laurentia as opposed to its current location. In order to constrain the early geological history of this exotic terrane, I have conducted fieldwork in the Brooks Range for the past five years. For this entry, I will discuss one of my most recent adventures in the central Brooks Range near Mount Doonerak, the so-called “Matterhorn of Alaska.” This fieldwork was conducted during the summer of 2014 with an undergraduate field assistant from Harvard University named Lyle Nelson and a fourth-year PhD candidate from Stanford University named Carl Hoiland.

Mount Doonerak basking in 2AM alpenglow.
            Why Mount Doonerak? The Doonerak region of the central Brooks Range is a structural high within the Mesozoic Brookian Orogen (the mountain-building event that constructed the Brooks Range) that exposes a unique sequence of Paleozoic rocks that are not preserved anywhere else in the Arctic Alaska terrane. These rocks are called the Apoon assemblage, a series of volcanic and siliciclastic rocks proposed to represent the back arc basin of an ancient Paleozoic volcanic arc (Dillon, 1986; Mull et al., 1987; Julian and Oldow, 1998). Age constraints on the Apoon assemblage are limited to five K/Ar and Ar/Ar ages ranging from 380 to 520 Ma that have uncertainties that span tens of millions of years (Dutro et al., 1976). Other age constraints provided by paleontological collections include a single collection of Silurian and Ordovician graptolites (Repetski et al., 1987) and another collection of Middle Cambrian trilobites (Dutro et al., 1984). The main aim of our work in this region was to collect a new suite of datable lithologies (both igneous and sedimentary rocks) and search for new paleontological specimens in order to better constrain the age of the Apoon Assemblage. We were also interested in examining the structural fabrics within the Doonerak Window, as based on the continuous nature of the tectonic cleavage between the Apoon assemblage and overlying Mississippian (ca. 370 Ma) Kekiktuk conglomerate, Julian and Oldow (1998) proposed that deformation of the Apoon assemblage is only related to the Brookian Orogeny. This has important implications because previous workers (e.g., Mull et al., 1987) have correlated the Apoon Assemblage with strata of the North Slope, which records a very prolific Devonian deformational event (Romanzof/Ellesmerian Orogeny) that is linked to similar events in Arctic Canada. Therefore, proper assessment of the age of deformation of the Apoon assemblage is another important constraint in understanding the history of the Doonerak Window and the entire evolution of the Brooks Range.
A Robinson R44 helicopter in front of Mount Doonerak. This was our means of transportation into the Doonerak Window.
            Due to the location of Mount Doonerak within the confines of the Gates of the Arctic National Park and Preserve, we were only able to secure permits to work in this region in early June to avoid backpacking tourist traffic. This almost guaranteed that we would be met by late season snow in the high peaks of the central Brooks Range; however, we were doubly unlucky, as this season proved to be exceptionally cold and there was next to no melt on the high peaks by mid-June. Therefore, upon flying into our proposed camp with a small Robinson R44 helicopter, we had to scrap our original plan to camp on the high ridges near Mount Doonerak and settle into a new camp on the western flank of Amawk Mountain. Fortunately, this enabled us to camp directly across the valley from the famous locality where Gil Mull and others recognized the Amawk Thrust, a large folded thrust that marks the base of the Endicott Mountains Allochthon. This 4-5 km thick thrust sheet travelled at least 90 km northwards during the Brookian Orogen and essentially defines the northern boundary of the Doonerak Window. Despite some poor weather during our stay near Mount Amawk, we still managed to collect a beautiful sample suite through the Apoon Assemblage and some of the younger Ellesmerian sequence rocks (Late Paleozoic).
 
Our cook tent partially buried in snow after a small blizzard engulfed our camp for approximately 24 hours.
Carl Hoiland searching for coarse-grained siliciclastic lithologies for dating the Apoon Assemblage with U-Pb on zircon.
The Amawk Thrust sits between the light grey carbonate and siliciclastic rocks of the footwall-based Ellesmerian Sequence and the darker brown units of the Hunt Fork Shale of the hanging wall.
            After finishing up our work near Amawk Mountain, we backpacked through deep snow ~4-5 miles down the Karillyukpuk River to establish a new basecamp in the southern part of the Doonerak Window. Here, we were able to spend the next segment of our trip exploring different portions of the Apoon Assemblage, including a very thick gabbroic intrusion that towered above our camp that provided dreams of possible U-Pb dates on zircon and baddleyite. After a few very long days traversing the majority of the southern Doonerak Window, we were picked up by another R44 helicopter and shuttled out of the beautiful wilderness and back to the civilized wonders (and hamburgers) of Coldfoot, Alaska with at least two hundred pounds of samples in tow.
 
Carl Hoiland and Lyle Nelson basking in the afternoon sunshine on the hanging wall of the Amawk Thrust with Mount Doonerak and the Koyukuk River valley in the background.

 

Left: A putative tuff (light green) interbedded with tectonized black phyllite and slate of the Apoon Assemblage.
Right: Subtle graded bedding in phyllites and metasandstone of the Apoon Assemblage.
Gabbroic intrusion making up the majority of the mountain on Karillyukpuk Creek. Our second camp was located directly below this massive wall.
References Cited
Dutro, J.T., Jr., Brosge, W.P., Lanphere, M.A., and Resier, H.N., 1976, Geologic significance of Doonerak structural high, central Brooks Range, Alaska: American Association of Petroleum Geologlists Bulletin: v. 60, p. 952-961.
Dutro, J.T., Jr., Palmer A.R., Repetski, J.E. and Brosge, W.P., 1984, Middle Cambrian fossils from the Doonerak Anticlinorium, central Brooks Range, Alaska: Journal of Paleontology, v. 58, p. 1364-1371.
Julian, F.E., and Oldow, J.S., 1998, Structure and lithology of the lower Paleozoic Apoon
assemblage, eastern Doonerak window, central Brooks Range, Alaska. Special Paper – Geological Society of America, 324, 65-80.
           
Mull, C.G., Dillon, J.T., and Adams, K.E., 1987, The Doonerak fenester, central Brooks
Range, Alaska, Geological Society of America Centennial Field Guide- Cordilleran Section, p. 469-473.
Repetski, J.E., Carter, C., Harris, A.G., and Dutro, J.T. Jr., 1987, Ordovician and Silurian fossils from the Doonerak anticlinorium, central Brooks Range, Alaska, in Hamilton, T.D., and Galloway, J.P., eds., Geologic studies in Alaska by the U.S. Geological Survey during 1986: U.S. Geological Survey Circular 998, p. 40-42.

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