- Email: [email protected]
Field report: Exploring the Doonerak fenster of the central Brooks Range, Alaska, USA
Accepted Manuscript Field Report: Exploring the Doonerak fenster of the central Brooks Range, Alaska, USA Justin V. Strauss, Carl W. Hoiland, Lyle L. Nelson PII:
S1674-9871(17)30033-6
DOI:
10.1016/j.gsf.2017.02.004
Reference:
GSF 540
To appear in:
Geoscience Frontiers
Received Date: 9 February 2017 Revised Date:
20 February 2017
Accepted Date: 22 February 2017
Please cite this article as: Strauss, J.V., Hoiland, C.W., Nelson, L.L., Field Report: Exploring the Doonerak fenster of the central Brooks Range, Alaska, USA, Geoscience Frontiers (2017), doi: 10.1016/ j.gsf.2017.02.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
1
Field Report: Exploring the Doonerak fenster of the central Brooks
2
Range, Alaska, USA
4
Justin V. Straussa,*, Carl W. Hoilandb, Lyle L. Nelsonc
5 6
a
7
03755, USA
8
b
9
USA
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3
SC
Department of Earth Sciences, Dartmouth College, HB6105 Fairchild Hall, Hanover, NH
M AN U
Department of Geological Sciences, Stanford University, 450 Serra Mall, Stanford, CA 94305,
10
c
11
Cambridge, MA 02138, USA
12
* Corresponding author e-mail address: [email protected]
13
ABSTRACT
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14
Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street,
Arctic Alaska is a ‘suspect’ terrane that encompasses approximately 20% of Alaska, stretching from the southern Brooks Range all the way to the continental shelves of the Chukchi
16
and Beaufort Seas. Although the origin and subsequent travels of this large crustal fragment are
17
debated among geologists, most researchers agree upon its composite nature and exotic origin.
18
To constrain the early geological history of this terrane, we describe a recent expedition to the
19
Doonerak fenster of the central Brooks Range. This area has long been regarded as a key locality
20
for understanding the structural evolution of the Mesozoic–Cenozoic Brooks Range orogen;
21
however, our target was different: a unique sequence of volcanic and siliciclastic rocks (Apoon
22
assemblage) exposed beneath a profound pre-Mississippian unconformity, which we argue is of
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15
ACCEPTED MANUSCRIPT
23
key importance to understanding the early Paleozoic tectonic history of northern Alaska and the
24
greater Arctic.
25
Keywords: Arctic Alaska; Doonerak fenster; Brooks Range; Apoon assemblage
27 28
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26 Report from the field
The E–W-trending Brooks Range of northern Alaska borders the Arctic Ocean and
represents a north-vergent fold-thrust belt of Middle Jurassic to Tertiary age (Fig. 1A; Moore et
30
al., 1994). This ~1000 km long orogen is characterized by a southern hinterland of polydeformed
31
metamorphic rocks and a northern foreland belt dominated by allochthonous thrust sheets and a
32
mildly deformed foreland basin (Colville Basin; Moore et al., 1994, and references therein). The
33
Doonerak fenster (Brosgé and Reiser, 1971) is a NE–SW-trending doubly plunging antiform in
34
the central Brooks Range (Fig. 1) that exposes the structurally lowest level of the Brookian
35
orogen (Dutro et al., 1976; Mull et al., 1987; Oldow et al., 1987). The fenster is capped by the
36
Amawk thrust (Fig. 1B), which is the basal detachment of the Endicott Mountains allochthon, a
37
foreland-dipping imbricate stack of thrust sheets comprised of upper Paleozoic siliciclastic and
38
carbonate rocks (e.g., Dutro et al., 1976; Mull et al., 1987; Moore et al., 1997).
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39
SC
29
The Doonerak fenster exposes a unique succession of early Paleozoic rocks that are not preserved elsewhere in the Arctic Alaska terrane. These rocks are called the Apoon assemblage,
41
a series of volcanic and siliciclastic rocks proposed to represent the ancient back-arc basin of an
42
exotic Ordovician–Devonian(?) volcanic island arc (Dillon et al., 1986; Mull et al., 1987; Julian
43
and Oldow, 1998). The Apoon assemblage is divided into four, fault-bounded lithological units
44
whose primary depositional/intrusive relationships remain unknown (Julian, 1989; Julian and
45
Oldow, 1998). From north to south, these include an ~500 m thick mafic pyroclastic volcanic
AC C
40
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unit (Pza), an ~1.5 km thick siliciclastic succession (Pzc), an ~400 m thick volcaniclastic
47
package with minor intrusive rocks (Pzv), and an ~3 km thick phyllitic unit with minor limestone
48
and volcaniclastic strata (Pzp). Previous age constraints on the Apoon assemblage (Fig. 1B) were
49
limited to five ~520–380 Ma K-Ar and 40Ar-39Ar ages on hornblende from mafic dikes (Dutro et
50
al., 1976), as well as some sparse paleontological collections that yielded Silurian and
51
Ordovician graptolites (Repetski et al., 1987) and Middle Cambrian trilobites (Dutro et al.,
52
1984).
SC
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46
The main aim of our fieldwork was to collect a new suite of datable lithologies to better
54
constrain the age of the Apoon assemblage. We were also interested in examining the structural
55
fabrics within the Doonerak fenster; based on the continuous nature of the tectonic cleavage
56
between the Apoon assemblage and overlying Carboniferous Kekiktuk Conglomerate, Julian and
57
Oldow (1998) proposed that deformation of the Apoon assemblage is only related to the
58
Mesozoic Brookian orogeny. This has important implications because previous workers (e.g.,
59
Mull et al., 1987) had correlated the Apoon assemblage with strata of the North Slope subterrane,
60
which records a significant Middle–Late Devonian deformational event (Romanzof/Ellesmerian
61
orogeny) that is linked to similar tectonism in Arctic Canada (e.g., Lane, 2007). Therefore,
62
proper assessment of the age of deformation of the Apoon assemblage is another important
63
constraint in understanding the evolution of the Brooks Range and circum-Arctic region.
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64
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53
Due to the location of Mount Doonerak within the confines of the Gates of the Arctic
65
National Park and Preserve, we were only able to secure permits to work in this region in early
66
June. This almost guaranteed that we would be met by late season snow in the high peaks of the
67
central Brooks Range, and sure enough, there was next to no snowmelt when we deployed for
68
our fieldwork. Strauss and Nelson travelled to Fairbanks, Alaska, directly from Yukon, Canada,
ACCEPTED MANUSCRIPT
69
and Hoiland came from Sweden, where he had been studying as a visiting researcher at
70
Stockholm University. Our fourth field member became ill with fever en route to Fairbanks and
71
ultimately needed to return home. After assembling food, gear, and bear protection, the three of us drove up the Dalton
RI PT
72
Highway past Coldfoot, Alaska, to an agreed upon rendezvous point with a helicopter that was
74
based out of Toolik Research Station. Upon flying in with a small Robinson R44 helicopter (Fig.
75
2A), we found snow cover to be worse than expected (Fig. 2B) – we decided to scrap our
76
original plan to camp on the high ridges near Mount Doonerak and opted for a lower elevation
77
camp on the western flank of Amawk Mountain (Fig. 2C). This camp was directly across the
78
valley from the locality where Gil Mull and colleagues described the Amawk thrust (Fig. 2D;
79
Mull 1982; Mull et al., 1987), a large folded thrust fault that marks the base of the Endicott
80
Mountains allochthon (Fig. 1B). This 4–5 km thick imbricate thrust stack travelled at least 90 km
81
northwards during the Brookian orogeny and defines the northern boundary of the Doonerak
82
fenster today (Mull et al., 1987).
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We initially experienced poor weather during our stay near Amawk Mountain, including
84
significant blizzard conditions that left us unable to leave our tents for more than 24 hours (Fig.
85
2E). Fortunately, the weather improved and we eventually covered a fair amount of ground, such
86
that we still managed to collect a nearly complete sample suite of siliciclastic and volcanic rocks
87
through from the Apoon assemblage and unconformably overlying Carboniferous rocks of the
88
Kekiktuk Conglomerate (Fig. 1B). After finishing up our work near Amawk Mountain, we
89
backpacked through deep snow ~6 km down Karillyukpuk Creek to establish a new basecamp
90
near the southern part of the fenster (Fig. 1B). This was an arduous traverse, as we needed
91
multiple loads to shuttle our rock samples and backpacking equipment down to the new
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basecamp. Here, we spent the next segment of our trip exploring the southern exposures of the
93
Apoon assemblage (map units Pzv and Pzp), including finding a very extensive, and previously
94
unrecognized, gabbroic body towering above our camp (Fig. 2G). From this southern camp, we
95
also sampled several tuffaceous horizons (Fig. 2F) in the Apoon assemblage and even accessed
96
thrust slivers of Hammond subterrane rocks at the southern edge of the fenster (Fig. 2). After
97
several very long days traversing and sampling units throughout the southern Doonerak fenster,
98
and just as another Arctic front was developing, we were picked up by a R44 helicopter and
99
shuttled out of the wilderness and into the civilized wonders (and hamburgers) of Coldfoot,
SC
Alaska, with more than 100 kg of samples.
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After returning from the field in 2014 and with the support of colleagues Bill McClelland
102
and William Ward at the University of Iowa and Benjamin Johnson at West Virginia University,
103
six samples from the Apoon assemblage, two samples from the overlying Kekiktuk
104
Conglomerate, and one sample from a thrust sliver of the fenster hanging wall (Trembley Creek
105
phyllite of Moore et al., 1997) were processed and analyzed for igneous and detrital zircon U-Pb
106
geochronology and Lu-Hf isotopic geochemistry. Isotopic analyses were performed at the U.S.
107
Geological Survey–Stanford University Ion Probe Laboratory and the University of Arizona
108
LaserChron Center. The results of this work are being published by the Geological Society of
109
America Bulletin (Strauss et al., in press). Additional analytical work is planned for the
110
remaining samples and more fieldwork is anticipated for the summer of 2018.
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ACKNOWLEDGEMENTS Strauss was supported by a Geological Society of America graduate student research grant, Harvard University, Dartmouth College, and a National Science Foundation (NSF)
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Tectonics grant (EAR-1624131). Hoiland was supported by a Stanford McGee Grant and a NSF
116
Graduate Research Fellowship. Nelson was supported by Harvard University. We thank Loic
117
Labrousse, Nicolas Lemmonier, Sarah Roeske, and Trent Hubbard for helicopter support; Nancy
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Brandt at Toolik Research Station, Dana Truffer-Moudra at Polar Field Services, and Åsa
119
Lindgren at Swedish Polar Research Secretariat for logistical support; Will Ward, Ben Johnson,
120
and Bill McClelland for help with analytical work; and Jobe Chakuchin for providing access to
121
Gates of the Arctic National Park. We thank reviewers Tim O’Brien, Francis Macdonald, and
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Kellen Gunderson for providing helpful feedback and Associate Editor Christopher Spencer for
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encouraging us to submit this account.
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Editor’s note
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This article is part of a series of invited field reports, edited by Christopher Spencer, intended to highlight recent or forthcoming publications that are based off a noteworthy degree
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of time and/or effort in remote or challenging field areas.
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129 References
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Brosgé, W.P. and Reiser, H.N., 1971, Preliminary bedrock geologic map, Wiseman and eastern Survey
133 134 135 136 137 138 139 140 141
Pass Quadrangles, Alaska: U.S. Geological Survey Open-File Map 71-56, scale
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1:250,000.
Dillon, J.T., Brosge, W.P., and Dutro, J.T., Jr., 1986, Generalized geologic map of the Wiseman quadrangle, Alaska: U.S. Geological Survey Open-File Report 86-219, scale 1:250,000. Dutro, J.T., Jr., Brosgé W.P., Lanphere, M.A., and Reiser, H.N., 1976, Geologic significance of Doonerak structural high, central Brooks Range, Alaska: American Association of Petroleum Geologists Bulletin, v. 60, p. 952-961.
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142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186
Dutro, J.T., Jr., Palmer, A.R., Repetski, J.E., and Brosgé W.P., 1984, Middle Cambrian fossils from the Doonerak anticlinorium, central Brooks Range, Alaska: Journal of Paleontology, v. 58, p. 13641371.
187
FIGURE CAPTIONS
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Julian, F.E., 1989, Structure and stratigraphy of lower Paleozoic rocks, Doonerak Window, central Brooks Range, Alaska (Ph.D. Thesis): Houston, Texas, Rice University, 127 p. Julian, F.E., and Oldow, J.S., 1998, Structure and lithology of the lower Paleozoic Apoon Assemblage, eastern Doonerak Window, central Brooks Range, Alaska, in Oldow, J.S., and Ave Lallemant, H.G., eds., Architecture of the central Brooks Range fold and thrust belt, Arctic Alaska: Geological Society of America Special Paper 324, p. 65-80.
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Lane, L.S., 2007, Devonian–Carboniferous paleogeography and orogenesis, northern Yukon and adjacent Arctic Alaska: Canadian Journal of Earth Sciences, v. 44, p. 679-694.
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Moore, T.E., Wallace, W.K., Bird, K.J., Karl, S.M., Mull, C.G., and Dillon, J.T., 1994, Geology of northern Alaska, in Plafker, G., and Berg, H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-1, p. 49-140. Moore, T.E., Wallace, W.K., Mull, C.G., Adams, K.E., Plafker, G., and Nokleberg, W.J., 1997, Crustal implications of bedrock geology along the Trans-Alaska Crustal Transect (TACT) in the Brooks Range, northern Alaska: Journal of Geophysical Research, v. 102, no. B9, p. 20645-20684. Mull, C.G., 1982, The tectonic evolution and structural style of the Brooks Range, Alaska: An illustrated summary, in Powers, R.B., ed., Geological Studies of the Cordilleran Thrust Belt, Volume 1: Denver, Colorado, Rocky Mountain Association of Geologists, p. 1-45.
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Mull, C.G., Adams, K.E., and Dillon, J.T., 1987, Stratigraphy and structure of the Doonerak fenster and Endicott Mountains allochthon, central Brooks Range, Alaska, in Tailleur, I., and Weimer, P., eds., Alaskan North Slope Geology: Bakersfield, California, Society of Economic Paleontologists and Mineralogists (SEPM), Pacific Section, and Alaska Geological Society, Book 50, p. 663-679.
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Oldow, J.S., Avé Lallemant, H.G., Julian, F.E., and Seidensticker, C.M., 1987, Balanced cross sections through the central Brooks Range and North Slope, Arctic Alaska: American Association of Petroleum Geologists Special Publication, v. 19, 19 p., 8 plates.
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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. Strauss, J.V., Hoiland, C.W., Ward, W., Johnson, B.G., Nelson, L., and McClelland, W.C., in press, Orogen transplant: Taconic–Caledonian magmatism in the Brooks Range of Alaska: Geological Society of America Bulletin, v. xx, p. xxx.
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Figure 1. (A) Simplified subterrane map of the Arctic Alaska terrane after Moore et al. (1994)
189
depicting the location of the Doonerak fenster within the central Brooks Range. (B) Simplified
190
geologic map of the Doonerak fenster, central Brooks Range of Alaska, after Dillon et al. (1986),
191
Mull et al. (1987), Oldow et al. (1987), Moore et al. (1997), Julian and Oldow (1998), and
192
Strauss et al. (in press). The yellow stars depict our two camps in the Doonerak fenster.
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193
Figure 2. Selected photographs from 2014 field season to the Doonerak fenster, central Brooks
195
Range, Alaska. (A) Robinson 44 (R44) helicopter provided transportation into and out of the
196
Doonerak fenster, thanks to special permission granted from the National Park Service. (B) 2AM
197
alpenglow on Mount Doonerak after an early June snow storm. (C) Hoiland searching for coarse-
198
grained siliciclastic lithologies to date the Apoon assemblage with zircon U-Pb geochronology.
199
The W-trending ridge in the background was the location of our first camp. (D) The Amawk
200
Thrust (Mull et al., 1987) sits between the light grey carbonate and siliciclastic rocks of the
201
Ellesmerian sequence in the footwall and the darker brown units of the Hunt Fork Shale of the
202
Endicott Mountains allochthon in the hanging wall. (E) Cook tent partially buried in snow after a
203
small blizzard engulfed camp for ~24 hours. In the background is the North Fork of the Koyukuk
204
River. (F) Olive-green massive tuffaceous horizon with calcite veins in penetratively-deformed
205
grey-black phyllite of map unit Pzp of the Apoon assemblage. Hammer for scale is 32 cm in
206
length. (G) Massive gabbro sampled for zircon U-Pb geochronology from map unit Pzv near the
207
confluence of Karillyukpuk Creek and Clear River. This was also the location of our second
208
basecamp.
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S1674-9871(17)30033-6
DOI:
10.1016/j.gsf.2017.02.004
Reference:
GSF 540
To appear in:
Geoscience Frontiers
Received Date: 9 February 2017 Revised Date:
20 February 2017
Accepted Date: 22 February 2017
Please cite this article as: Strauss, J.V., Hoiland, C.W., Nelson, L.L., Field Report: Exploring the Doonerak fenster of the central Brooks Range, Alaska, USA, Geoscience Frontiers (2017), doi: 10.1016/ j.gsf.2017.02.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
1
Field Report: Exploring the Doonerak fenster of the central Brooks
2
Range, Alaska, USA
4
Justin V. Straussa,*, Carl W. Hoilandb, Lyle L. Nelsonc
5 6
a
7
03755, USA
8
b
9
USA
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3
SC
Department of Earth Sciences, Dartmouth College, HB6105 Fairchild Hall, Hanover, NH
M AN U
Department of Geological Sciences, Stanford University, 450 Serra Mall, Stanford, CA 94305,
10
c
11
Cambridge, MA 02138, USA
12
* Corresponding author e-mail address: [email protected]
13
ABSTRACT
TE D
14
Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street,
Arctic Alaska is a ‘suspect’ terrane that encompasses approximately 20% of Alaska, stretching from the southern Brooks Range all the way to the continental shelves of the Chukchi
16
and Beaufort Seas. Although the origin and subsequent travels of this large crustal fragment are
17
debated among geologists, most researchers agree upon its composite nature and exotic origin.
18
To constrain the early geological history of this terrane, we describe a recent expedition to the
19
Doonerak fenster of the central Brooks Range. This area has long been regarded as a key locality
20
for understanding the structural evolution of the Mesozoic–Cenozoic Brooks Range orogen;
21
however, our target was different: a unique sequence of volcanic and siliciclastic rocks (Apoon
22
assemblage) exposed beneath a profound pre-Mississippian unconformity, which we argue is of
AC C
EP
15
ACCEPTED MANUSCRIPT
23
key importance to understanding the early Paleozoic tectonic history of northern Alaska and the
24
greater Arctic.
25
Keywords: Arctic Alaska; Doonerak fenster; Brooks Range; Apoon assemblage
27 28
RI PT
26 Report from the field
The E–W-trending Brooks Range of northern Alaska borders the Arctic Ocean and
represents a north-vergent fold-thrust belt of Middle Jurassic to Tertiary age (Fig. 1A; Moore et
30
al., 1994). This ~1000 km long orogen is characterized by a southern hinterland of polydeformed
31
metamorphic rocks and a northern foreland belt dominated by allochthonous thrust sheets and a
32
mildly deformed foreland basin (Colville Basin; Moore et al., 1994, and references therein). The
33
Doonerak fenster (Brosgé and Reiser, 1971) is a NE–SW-trending doubly plunging antiform in
34
the central Brooks Range (Fig. 1) that exposes the structurally lowest level of the Brookian
35
orogen (Dutro et al., 1976; Mull et al., 1987; Oldow et al., 1987). The fenster is capped by the
36
Amawk thrust (Fig. 1B), which is the basal detachment of the Endicott Mountains allochthon, a
37
foreland-dipping imbricate stack of thrust sheets comprised of upper Paleozoic siliciclastic and
38
carbonate rocks (e.g., Dutro et al., 1976; Mull et al., 1987; Moore et al., 1997).
M AN U
TE D
EP
39
SC
29
The Doonerak fenster exposes a unique succession of early Paleozoic rocks that are not preserved elsewhere in the Arctic Alaska terrane. These rocks are called the Apoon assemblage,
41
a series of volcanic and siliciclastic rocks proposed to represent the ancient back-arc basin of an
42
exotic Ordovician–Devonian(?) volcanic island arc (Dillon et al., 1986; Mull et al., 1987; Julian
43
and Oldow, 1998). The Apoon assemblage is divided into four, fault-bounded lithological units
44
whose primary depositional/intrusive relationships remain unknown (Julian, 1989; Julian and
45
Oldow, 1998). From north to south, these include an ~500 m thick mafic pyroclastic volcanic
AC C
40
ACCEPTED MANUSCRIPT
unit (Pza), an ~1.5 km thick siliciclastic succession (Pzc), an ~400 m thick volcaniclastic
47
package with minor intrusive rocks (Pzv), and an ~3 km thick phyllitic unit with minor limestone
48
and volcaniclastic strata (Pzp). Previous age constraints on the Apoon assemblage (Fig. 1B) were
49
limited to five ~520–380 Ma K-Ar and 40Ar-39Ar ages on hornblende from mafic dikes (Dutro et
50
al., 1976), as well as some sparse paleontological collections that yielded Silurian and
51
Ordovician graptolites (Repetski et al., 1987) and Middle Cambrian trilobites (Dutro et al.,
52
1984).
SC
RI PT
46
The main aim of our fieldwork was to collect a new suite of datable lithologies to better
54
constrain the age of the Apoon assemblage. We were also interested in examining the structural
55
fabrics within the Doonerak fenster; based on the continuous nature of the tectonic cleavage
56
between the Apoon assemblage and overlying Carboniferous Kekiktuk Conglomerate, Julian and
57
Oldow (1998) proposed that deformation of the Apoon assemblage is only related to the
58
Mesozoic Brookian orogeny. This has important implications because previous workers (e.g.,
59
Mull et al., 1987) had correlated the Apoon assemblage with strata of the North Slope subterrane,
60
which records a significant Middle–Late Devonian deformational event (Romanzof/Ellesmerian
61
orogeny) that is linked to similar tectonism in Arctic Canada (e.g., Lane, 2007). Therefore,
62
proper assessment of the age of deformation of the Apoon assemblage is another important
63
constraint in understanding the evolution of the Brooks Range and circum-Arctic region.
TE D
EP
AC C
64
M AN U
53
Due to the location of Mount Doonerak within the confines of the Gates of the Arctic
65
National Park and Preserve, we were only able to secure permits to work in this region in early
66
June. This almost guaranteed that we would be met by late season snow in the high peaks of the
67
central Brooks Range, and sure enough, there was next to no snowmelt when we deployed for
68
our fieldwork. Strauss and Nelson travelled to Fairbanks, Alaska, directly from Yukon, Canada,
ACCEPTED MANUSCRIPT
69
and Hoiland came from Sweden, where he had been studying as a visiting researcher at
70
Stockholm University. Our fourth field member became ill with fever en route to Fairbanks and
71
ultimately needed to return home. After assembling food, gear, and bear protection, the three of us drove up the Dalton
RI PT
72
Highway past Coldfoot, Alaska, to an agreed upon rendezvous point with a helicopter that was
74
based out of Toolik Research Station. Upon flying in with a small Robinson R44 helicopter (Fig.
75
2A), we found snow cover to be worse than expected (Fig. 2B) – we decided to scrap our
76
original plan to camp on the high ridges near Mount Doonerak and opted for a lower elevation
77
camp on the western flank of Amawk Mountain (Fig. 2C). This camp was directly across the
78
valley from the locality where Gil Mull and colleagues described the Amawk thrust (Fig. 2D;
79
Mull 1982; Mull et al., 1987), a large folded thrust fault that marks the base of the Endicott
80
Mountains allochthon (Fig. 1B). This 4–5 km thick imbricate thrust stack travelled at least 90 km
81
northwards during the Brookian orogeny and defines the northern boundary of the Doonerak
82
fenster today (Mull et al., 1987).
TE D
M AN U
SC
73
We initially experienced poor weather during our stay near Amawk Mountain, including
84
significant blizzard conditions that left us unable to leave our tents for more than 24 hours (Fig.
85
2E). Fortunately, the weather improved and we eventually covered a fair amount of ground, such
86
that we still managed to collect a nearly complete sample suite of siliciclastic and volcanic rocks
87
through from the Apoon assemblage and unconformably overlying Carboniferous rocks of the
88
Kekiktuk Conglomerate (Fig. 1B). After finishing up our work near Amawk Mountain, we
89
backpacked through deep snow ~6 km down Karillyukpuk Creek to establish a new basecamp
90
near the southern part of the fenster (Fig. 1B). This was an arduous traverse, as we needed
91
multiple loads to shuttle our rock samples and backpacking equipment down to the new
AC C
EP
83
ACCEPTED MANUSCRIPT
basecamp. Here, we spent the next segment of our trip exploring the southern exposures of the
93
Apoon assemblage (map units Pzv and Pzp), including finding a very extensive, and previously
94
unrecognized, gabbroic body towering above our camp (Fig. 2G). From this southern camp, we
95
also sampled several tuffaceous horizons (Fig. 2F) in the Apoon assemblage and even accessed
96
thrust slivers of Hammond subterrane rocks at the southern edge of the fenster (Fig. 2). After
97
several very long days traversing and sampling units throughout the southern Doonerak fenster,
98
and just as another Arctic front was developing, we were picked up by a R44 helicopter and
99
shuttled out of the wilderness and into the civilized wonders (and hamburgers) of Coldfoot,
SC
Alaska, with more than 100 kg of samples.
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100
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After returning from the field in 2014 and with the support of colleagues Bill McClelland
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and William Ward at the University of Iowa and Benjamin Johnson at West Virginia University,
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six samples from the Apoon assemblage, two samples from the overlying Kekiktuk
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Conglomerate, and one sample from a thrust sliver of the fenster hanging wall (Trembley Creek
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phyllite of Moore et al., 1997) were processed and analyzed for igneous and detrital zircon U-Pb
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geochronology and Lu-Hf isotopic geochemistry. Isotopic analyses were performed at the U.S.
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Geological Survey–Stanford University Ion Probe Laboratory and the University of Arizona
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LaserChron Center. The results of this work are being published by the Geological Society of
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America Bulletin (Strauss et al., in press). Additional analytical work is planned for the
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remaining samples and more fieldwork is anticipated for the summer of 2018.
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ACKNOWLEDGEMENTS Strauss was supported by a Geological Society of America graduate student research grant, Harvard University, Dartmouth College, and a National Science Foundation (NSF)
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Tectonics grant (EAR-1624131). Hoiland was supported by a Stanford McGee Grant and a NSF
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Graduate Research Fellowship. Nelson was supported by Harvard University. We thank Loic
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Labrousse, Nicolas Lemmonier, Sarah Roeske, and Trent Hubbard for helicopter support; Nancy
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Brandt at Toolik Research Station, Dana Truffer-Moudra at Polar Field Services, and Åsa
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Lindgren at Swedish Polar Research Secretariat for logistical support; Will Ward, Ben Johnson,
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and Bill McClelland for help with analytical work; and Jobe Chakuchin for providing access to
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Gates of the Arctic National Park. We thank reviewers Tim O’Brien, Francis Macdonald, and
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Kellen Gunderson for providing helpful feedback and Associate Editor Christopher Spencer for
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encouraging us to submit this account.
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Editor’s note
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This article is part of a series of invited field reports, edited by Christopher Spencer, intended to highlight recent or forthcoming publications that are based off a noteworthy degree
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of time and/or effort in remote or challenging field areas.
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129 References
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Brosgé, W.P. and Reiser, H.N., 1971, Preliminary bedrock geologic map, Wiseman and eastern Survey
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Pass Quadrangles, Alaska: U.S. Geological Survey Open-File Map 71-56, scale
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1:250,000.
Dillon, J.T., Brosge, W.P., and Dutro, J.T., Jr., 1986, Generalized geologic map of the Wiseman quadrangle, Alaska: U.S. Geological Survey Open-File Report 86-219, scale 1:250,000. Dutro, J.T., Jr., Brosgé W.P., Lanphere, M.A., and Reiser, H.N., 1976, Geologic significance of Doonerak structural high, central Brooks Range, Alaska: American Association of Petroleum Geologists Bulletin, v. 60, p. 952-961.
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Dutro, J.T., Jr., Palmer, A.R., Repetski, J.E., and Brosgé W.P., 1984, Middle Cambrian fossils from the Doonerak anticlinorium, central Brooks Range, Alaska: Journal of Paleontology, v. 58, p. 13641371.
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Julian, F.E., 1989, Structure and stratigraphy of lower Paleozoic rocks, Doonerak Window, central Brooks Range, Alaska (Ph.D. Thesis): Houston, Texas, Rice University, 127 p. Julian, F.E., and Oldow, J.S., 1998, Structure and lithology of the lower Paleozoic Apoon Assemblage, eastern Doonerak Window, central Brooks Range, Alaska, in Oldow, J.S., and Ave Lallemant, H.G., eds., Architecture of the central Brooks Range fold and thrust belt, Arctic Alaska: Geological Society of America Special Paper 324, p. 65-80.
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Lane, L.S., 2007, Devonian–Carboniferous paleogeography and orogenesis, northern Yukon and adjacent Arctic Alaska: Canadian Journal of Earth Sciences, v. 44, p. 679-694.
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Moore, T.E., Wallace, W.K., Bird, K.J., Karl, S.M., Mull, C.G., and Dillon, J.T., 1994, Geology of northern Alaska, in Plafker, G., and Berg, H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-1, p. 49-140. Moore, T.E., Wallace, W.K., Mull, C.G., Adams, K.E., Plafker, G., and Nokleberg, W.J., 1997, Crustal implications of bedrock geology along the Trans-Alaska Crustal Transect (TACT) in the Brooks Range, northern Alaska: Journal of Geophysical Research, v. 102, no. B9, p. 20645-20684. Mull, C.G., 1982, The tectonic evolution and structural style of the Brooks Range, Alaska: An illustrated summary, in Powers, R.B., ed., Geological Studies of the Cordilleran Thrust Belt, Volume 1: Denver, Colorado, Rocky Mountain Association of Geologists, p. 1-45.
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Mull, C.G., Adams, K.E., and Dillon, J.T., 1987, Stratigraphy and structure of the Doonerak fenster and Endicott Mountains allochthon, central Brooks Range, Alaska, in Tailleur, I., and Weimer, P., eds., Alaskan North Slope Geology: Bakersfield, California, Society of Economic Paleontologists and Mineralogists (SEPM), Pacific Section, and Alaska Geological Society, Book 50, p. 663-679.
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Oldow, J.S., Avé Lallemant, H.G., Julian, F.E., and Seidensticker, C.M., 1987, Balanced cross sections through the central Brooks Range and North Slope, Arctic Alaska: American Association of Petroleum Geologists Special Publication, v. 19, 19 p., 8 plates.
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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. Strauss, J.V., Hoiland, C.W., Ward, W., Johnson, B.G., Nelson, L., and McClelland, W.C., in press, Orogen transplant: Taconic–Caledonian magmatism in the Brooks Range of Alaska: Geological Society of America Bulletin, v. xx, p. xxx.
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Figure 1. (A) Simplified subterrane map of the Arctic Alaska terrane after Moore et al. (1994)
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depicting the location of the Doonerak fenster within the central Brooks Range. (B) Simplified
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geologic map of the Doonerak fenster, central Brooks Range of Alaska, after Dillon et al. (1986),
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Mull et al. (1987), Oldow et al. (1987), Moore et al. (1997), Julian and Oldow (1998), and
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Strauss et al. (in press). The yellow stars depict our two camps in the Doonerak fenster.
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Figure 2. Selected photographs from 2014 field season to the Doonerak fenster, central Brooks
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Range, Alaska. (A) Robinson 44 (R44) helicopter provided transportation into and out of the
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Doonerak fenster, thanks to special permission granted from the National Park Service. (B) 2AM
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alpenglow on Mount Doonerak after an early June snow storm. (C) Hoiland searching for coarse-
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grained siliciclastic lithologies to date the Apoon assemblage with zircon U-Pb geochronology.
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The W-trending ridge in the background was the location of our first camp. (D) The Amawk
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Thrust (Mull et al., 1987) sits between the light grey carbonate and siliciclastic rocks of the
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Ellesmerian sequence in the footwall and the darker brown units of the Hunt Fork Shale of the
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Endicott Mountains allochthon in the hanging wall. (E) Cook tent partially buried in snow after a
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small blizzard engulfed camp for ~24 hours. In the background is the North Fork of the Koyukuk
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River. (F) Olive-green massive tuffaceous horizon with calcite veins in penetratively-deformed
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grey-black phyllite of map unit Pzp of the Apoon assemblage. Hammer for scale is 32 cm in
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length. (G) Massive gabbro sampled for zircon U-Pb geochronology from map unit Pzv near the
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confluence of Karillyukpuk Creek and Clear River. This was also the location of our second
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basecamp.
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