North-central Brooks Range Research Articles

Using Cosmogenic 10Be Exposure Dating and Lichenometry to Constrain Holocene Glaciation in the Central Brooks Range, Alaska

ABSTRACTWe compile new and previously published lichenometric and cosmogenic 10Be moraine ages to summarize the timing of Holocene glacier expansions in the Brooks Range, Arctic Alaska. Foundational lichenometric studies suggested that glaciers likely grew to their Holocene maxima as early as the middle Holocene, followed by several episodes of moraine building prior to, and throughout, the last millennium. Previously published 10Be ages on Holocene moraine boulders from the north-central Brooks Range constrain the culmination of maximum Holocene glacier advances between 4.6 ka and 2.6 ka. New 10Be ages of moraine boulders from two different valleys in the central Brooks Range published here show that maximum Holocene glacial extents in these valleys were reached by 3.5 ka and ca. 2.6 ka, supporting previous studies showing that Holocene maximum, or near-maximum, glacial extents in the Brooks Range occurred prior to the Little Ice Age. However, in-depth reconciliations between glacier extent and local and regional climate are hampered by uncertainties associated with both lichenometry and 10Be dating.

Holocene summer temperature reconstruction from sedimentary chlorophyll content, with treatment of age uncertainties, Kurupa Lake, Arctic Alaska

Quantitative records of pre-industrial temperature changes are fundamental for understanding long-term natural climate variability. We used visible reflectance spectroscopy to measure chlorophyll content (and its derivatives) in a sediment core from Kurupa Lake, north-central Brooks Range, Alaska, to reconstruct summer temperature and the number of annual non-freezing days over the past 5.7 ka. A calibration-in-time approach was used to convert downcore changes in chlorophyll content to the climate variables, and an ensemble approach was used to integrate age and calibration uncertainties. The strongest correlation ( rmedian = 0.69, pmedian = 0.02, RMSEP = 1.9°C) is for summer (June through September) temperature using the 20th Century Reanalysis Project dataset. The chlorophyll-inferred 3-year-mean summer temperature shows that the warmest century (3.0–2.9 ka BP) was about 3.0°C (90% range of the ensemble members = 2.3–4.0°C) higher and that the coldest century (1.4–1.3 ka BP) was about 5.5°C lower (90% range = −7.6°C to −5.0°C) than during the reference period (AD 1961–1990). Century-to-century temperature changes over the past 5.7 ka at Kurupa Lake have been large (90% range = −2.8°C to 3.1°C shifts in centennial mean), including the shift between the 19th and 20th centuries, which was above the 90th percentile of temperature changes across all representations of the reconstruction. In contrast to most Northern Hemisphere temperature reconstructions, Kurupa Lake shows no overall millennial-scale cooling trend. We suggest that increased summer duration (by 4.3 days during the last 6 ka) along with no long-term increase in sea-ice cover over the adjacent Chukchi Sea counter-balanced the influence of decreased insolation intensity on the aquatic productivity in Kurupa Lake.

Permian deposition in the north central Brooks Range, Alaska: Constraints for tectonic reconstructions

Two opposing tectonic models have been offered to explain the regional structural relations in the north central Brooks Range fold‐thrust belt of northern Alaska. The first suggests that rocks of the northern Endicott Mountains were thrust from south to north over the area of the present Mount Doonerak high and are therefore highly allochthonous. The second implies that the rocks of the northern Endicott Mountains were deposited in a basin that lay north of the Mount Doonerak high and later were thrust a short distance southward onto the northern flank of the high and are thus parautochthonous. To provide stratigraphic constraints for these models, this study examines Permian facies of the north central Brooks Range. Permian rocks in the north central Brooks Range comprise a thin (40 to 160 m thick), fining‐upward succession of clastic, storm‐influenced shelf deposits. When the rocks of the northern Endicott Mountains are restored south of the Mount Doonerak area, a minimum distance of 80 km, the Permian deposits grade systematically from distal facies (Siksikpuk Formation) in the southwest to proximal facies (Echooka Formation) in the northeast. Facies trends in the reconstructed Permian basin include, from southwest to northeast, (1) an increase in carbonate content and corresponding decrease in silica content, (2) a general darkening and thickening of shaley intervals, (3) an increase in proximal features of storm beds, including coarser, thicker, more abundant, and more closely spaced beds, and (4) an increase in abundance and diversity of the faunal assemblage with a corresponding decrease in age. These stratigraphic relations imply that rocks of the northern Endicott Mountains are allochthonous and structurally overlie a proximal stratigraphie succession similar to that exposed in the Mount Doonerak area and northeastern Brooks Range.

Multistory duplexes with forward dipping roofs, north central Brooks Range, Alaska

The Endicott Mountains allochthon has been thrust far northward over the North Slope parautochthon in the northern Brooks Range. Progressively younger units are exposed northward within the allochthon. To the south, the incompetent Hunt Fork Shale has thickened internally by asymmetric folds and thrust faults. Northward, the competent Kanayut Conglomerate forms a duplex between a floor thrust in Hunt Fork and a roof thrust in the Kayak Shale. To the north, the competent Lisburne Group forms a duplex between a floor thrust in Kayak and a roof thrust in the Siksikpuk Formation. Both duplexes formed from north vergent detachment folds whose steep limbs were later truncated by south dipping thrust faults that only locally breach immediately overlying roof thrusts. Within the parautochthon, the Kayak, Lisburne, and Siksikpuk‐equivalent Echooka Formation form a duplex identical to that in the allochthon. This duplex is succeeded abruptly northward by detachment folds in Lisburne. These folds are parasitic to an anticlinorium interpreted to reflect a fault‐bend folded horse in North Slope “basement,” with a roof thrust in Kayak and a floor thrust at depth/These structures constitute two northward tapered, internally deformed wedges that are juxtaposed at the base of the allochthon. Within each wedge, competent units have been shortened independently between detachments, located mainly in incompetent units. The basal detachment of each wedge cuts upsection forward (northward) to define a wedge geometry within which units dip regionally forward. These dips reflect forward decrease in internal structural thickening by forward vergent folds and hindward dipping thrust faults.

Geologic Framework of a Transect of the Central Brooks Range: Regional Relations and an Alternative to the Endicott Mountains Allochthon

Geologic Framework of a Transect of the Central Brooks Range: Regional Relations and an Alternative to the Endicott Mountains Allochthon

Deformation and Foreland Basin Sedimentation in the North-Central Brooks Range, Alaska: ABSTRACT

Deformation and Foreland Basin Sedimentation in the North-Central Brooks Range, Alaska: ABSTRACT

Seismic images of the Brooks Range, Arctic Alaska, reveal crustal-scale duplexing

An integrated set of seismic reflection and refraction data collected across the Brooks Range, Arctic Alaska, in 1990, has yielded a composite image of this Mesozoic and Cenozoic fold-and-thrust belt that reveals duplexing to lower-crustal depths. Interpretations from this image are as follows. (1) Many terranes and subterranes that were amalgamated in the Late Jurassic to Early Cretaceous extend no deeper than the upper crust (3–10 km). (2) In contrast, crustal duplexing, extending to nearly 30 km depth above a south-dipping basal decollement, has produced latest Cretaceous to Cenozoic antiforms, including the Doonerak antiform in the central Brooks Range and anticlinoria near the northern range front. (3) The duplexing occurs in basement rocks of the North Slope subterrane, which core the antiforms. (4) North-dipping structures in the middle crust of the Yukon-Koyukuk basin and southern Brooks Range may postdate Mesozoic terrane amalgamation and predate or coincide with the duplexing. (5) The thickest crust, 50 km, occurs beneath the north-central Brooks Range, north of the root zone of the basal decollement. The position of the thickest crust may indicate that either the duplexed crust above the decollement was thrust onto and depressed the plate beneath the North Slope or the protracted tectonic history of the Brooks Range has left structures not simply explainable in terms of a single collisional event.

Seismic images of the Brooks Range fold and thrust belt, Arctic Alaska, from an integrated seismic reflection/refraction experiment

We describe results of an integrated seismic reflection/refraction experiment across the Brooks Range and flanking geologic provinces in Arctic Alaska. The seismic acquisition was unusual in that reflection and refraction data were collected simultaneously with a 700 channel seismograph system deployed numerous times along a 315 km profile. Shot records show continuous Moho reflections from 0–180 km offset, as well as numerous upper- and mid-crustal wide-angle events. Single and low-fold near-vertical incidence common midpoint (CMP) reflection images show complex upper- and middle-crustal structure across the range from the unmetamorphosed Endicott Mountains allochthon (EMA) in the north, to the metamorphic belts in the south. Lower-crustal and Moho reflections are visible across the entire reflection profile. Travel-time inversion of PmP arrivals shows that the Moho, at 33 km depth beneath the North Slope foothills, deepens abruptly beneath the EMA to a maximum of 46 km, and then shallows southward to 35 km at the southern edge of the range. Two zones of upper- and middle-crustal reflections underlie the northern Brooks Range above ~ 12–15 km depth. The upper zone, interpreted as the base of the EMA, lies at a maximum depth of 6 km and extends over 50 km from the range front to the north central Brooks Range where the base of the EMA outcrops above the metasedimentary rocks exposed in the Doonerak window. We interpret the base of the lower zone, at ~ 12 km depth, to be from carbonate rocks above the master detachment upon which the Brooks Range formed. The seismic data suggest that the master detachment is connected to the faults in the EMA by several ramps. In the highly metamorphosed terranes south of the Doonerak window, the CMP section shows numerous south-dipping events which we interpret as a crustal scale duplex involving the Doonerak window rocks. The basal detachment reflections can be traced approximately 100 km, and dip southward from about 10–12 km near the range front, to 14–18 km beneath the Doonerak window, to 26–28 km beneath the metamorphic belts in the central Brooks Range. The section documents middle- and lower-crustal involvement in the formation of the Brooks Range.

Solid Bitumen at Atigun Gorge, Central Brooks Range Front: Implications for Oil Exploration in the North Slope Fold and Thrust Belt

The Atigun Gorge area of the north-central Brooks range is a structurally complex region in which a sequence of north-verging duplex structures involving Paleozoic and Mesozoic Ellesmerian continental margin deposits are structurally overlain by a south-verging thrust of Brookian foreland basin deposits of Albian age. The resulting structural triangle zone is marked by numerous small-scale thrusts involving Permian and Triassic strata in which solid bitumen, occupying fissures up to 10 cm wide and several meters in length, has been found. The presence of aromatics in the odorless, black material was confirmed by ultraviolet fluorescence following extraction in dichloromethane. The occurrence of solid bitumen at Atigun Gorge adds to a growing inventory of hydrocarbon-filled fractures found mostly in Cretaceous rocks in the Brooks Range foothills. These occurrences are consistent with a model of hydrocarbon generation beneath the northern margin of the Brooks Range. The regional distribution of vitrinite reflectance isograds suggests that the northern margin of the Brooks Range and the adjoining foreland basin deposits of the North Slope have experienced similar thermal histories. The 0.6% vitrinite reflectance isograd intersects the land surface along the southern margin of the foreland and the 2.0% isograd lies within the northern part of themore » range. Although these relations suggest the possibility of petroleum resources at shallow depths beneath the Brooks Range foothills, they also indicate that a considerable amount of differential uplift has occurred, probably resulting in redistribution and some leakage of any oil and gas accumulations.« less

Facies Comparison of Autochthonous and Allochthonous Permian and Triassic Units, North-Central Brooks Range, Alaska: ABSTRACT

Eight stratigraphic sections of Permian and Triassic rocks have been studied over a 30 km by 150 km area in the Endicott and Philip Smith Mountains of the central Brooks Range. Six of the sections are located on the Endicott Mountains allochthon, and the remaining two are parautochthonous columns in the Mount Doonerak area. The sections record a facies transition between the autochthonous Sadlerochit Group and Shublik Formation of the northeastern Brooks Range and the characteristically siliceous rocks of the allochthonous Siksikpuk and Otuk formations of the western Brooks Range. Laterally continuous and bioturbated beds of fine-grained sandstone, siltstone, and shale dominantly compose the Permian sequence, whereas the Triassic rocks consist of black shales, thin rhythmically bedded siliceous mudstones, and fossiliferous limestones. When the allochthonous sections are restored to a position south of the Mount Doonerak area, a general shallowing trend from southwest to northeast becomes evident within the reconstructed marine basin. To the south and west, the Permian sediments show a marked increase in silica content, with the occurrence of barite and a corresponding decrease in the thickness of the basal, coarser grained clastics. The Triassic formations also document an increase in silica and the presence of barite to the south and west, while becoming signi icantly sooty and phosphatic to the north and east. Ongoing petrographic and micropaleontologic studies of the field data will clarify these general paleogeographic relationships. End_of_Article - Last_Page 655------------

Stratigraphy of Endicott Mountains and Picnic Creek Allochthons, Killik River and Chandler Lake Quadrangles, North-Central Brooks Range, Alaska: ABSTRACT

Geologic mapping in the Killik River and Chandler Lake quadrangles has delineated rocks of at least three of the major allochthons found in the De Long Mountains of the western Brooks Range: the Brooks Range (Endicott Mountains), Picnic Creek, and Copter Peak allochthons. Rocks characteristic of parts of the Tpnavik River and Nuka Ridge allochthons are also present. The Endicott Mountains allochthon forms the main crest and mountain front of the central Brooks Range. It is the structurally lowest of the allochthons in the region. Major rock units on the allochthon are: Upper Devonian Hunt Fork Shale and Noatak Sandstone, Upper Devonian-Lower Mississippian Kanayut Conglomerate, and Mississippian Kayak Shale of the Endicott Group; Alapah Limestone and Kuna formation of the Lisburne Group; and Permian Siksikpuk Formation and Triassic Otuk formation of the Etivluk group. Lower Cretaceous coquinoid limestone and, in some places, orogenic sediments of the Okpikruak Formation cap the allochthon. Total stratigraphic thickness of the Endicott Mountains allochthon is over 2,000 m (6,500 ft). Rocks of the Picnic Creek allochthon are widespread in the disturbed belt of the Brooks Range foothills, and structurally overlie the Endicott Mountains (Brooks Range) allochthon in the Killik River quadrangle and quadrangles to the west. The allochthon is not recognized with certainty east of the Killik River quadrangle. Major rock units on the Picnic Creek allochthon are: Upper Devonian(?) Hunt Fork Shale, Lower Mississippian Kurupa sandstone tongue (new name) of the Noatak Sandstone, and Mississippian Kayak Shale of the Endicott Group; Carboniferous Akmalik chert (new name) of the Lisburne Group; and Pennsylvanian Imnaitchiak chert (new name) and Otuk formation of the Etivluk group. Orogenic sediments of the Lower Cretaceous Okpikruak Formation form the top of the Picnic Creek al ochthon. Total stratigraphic thickness of these rock units is not over 1,000 m (3,200 ft). Plant fossils from a number of localities in the Kurupa sandstone are dated as Early Mississippian (Tournaisian-Visean) by B. A. Thomas and R. A. Spicer and have close affinities to material from eastern USSR. Palinspastic restoration of the Picnic Creek allochthon southward to a position south of the Endicott Mountains allochthon restores the Kurupa sandstone, the Akmalik chert, and the Imnaitchiak chert to their proper basin position. In the reconstructed basin, the Kurupa sandstone appears to represent the distal, southern end of the Kanayut-Noatak coarse-clastic wedge. The Akmalik chert and Imnaitchiak chert represent basinal equivalents of the Alapah Limestone-Kuna formation of the Lisburne Group and Siksikpuk Formation of the Etivluk group. Detailed stratigraphic and paleontologic studies of the Mississippian through Triassic rocks on both allochthons are in progress. End_of_Article - Last_Page 671------------

Age and Correlation of the Otuk Formation, North-Central Brooks Range, Alaska: ABSTRACT

Allochthonous Triassic rocks of the north-central Brooks Range thrust belt were originally mapped as part of the Middle to Upper Triassic Shublik Formation. Recently, these strata were named the Otuk formation. Detailed paleontologic studies of 11 measured sections more precisely document the age of the Otuk and show that its base is older than the base of the Shublik and that its top is younger than the top of the Shublik. Megafossils (pelecypods and ammonites) and microfossils (radiolaria, conodonts, and foraminifers) indicate an age range of Early Triassic (Dienerian-Smithian or older) to Middle Jurassic (Bajocian). The lithology consists of 120 m (390 ft) of interbedded, very fine-grained rocks (shale, limestone, and chert) representative of very slow deposition, belo wave base in an open marine environment. The Otuk formation does not contain suitable reservoir rocks, but organic geochemical data indicate that the shales are possible oil source rocks. The Otuk formation is disconformable with both the underlying Permian (Wolfcampian-Guadalupian) Siksikpuk Formation and overlying Lower Cretaceous (Valanginian) coquinoid limestone and shale. These unconformities are correlative with similar unconformities in the northeastern Brooks Range and subsurface of the North Slope. Thus, the Otuk formation is a condensed, deeper water, more distal equivalent of the Ivishak and Shublik Formations, Karen Creek Sandstone, and lower Kingak Shale of the northeastern Brooks Range and equivalent subsurface units of the North Slope. End_of_Article - Last_Page 657------------

Paleomagnetic Results from the Sadlerochit and Shublik Mountains, Arctic National Wildlife Range (ANWR), and Other North Slope Sites, Alaska: ABSTRACT

Carboniferous through Triassic sedimentary units exposed in the Shublik and Sadlerochit Mountains were sampled in an attempt to obtain reliable primary magnetic components. Reliable pre-Cretaceous paleomagnetic poles from this area would greatly advance the understanding of the rotation and latitudinal displacement history of the North Slope. Carbonate rocks of the Carboniferous Lisburne Group were drilled in south-dipping units of Katakturuk Canyon, Sadlerochit Mountains, and in the north-dipping Fire Creek section, Shublik Mountains. Magnetic cleaning involved stepwise thermal demagnetization to 550°C. Principal component analysis of the demagnetization results defines two major components of magnetization. The secondary component is steep and down (inc = 87°), but the characteristic component (325°C-500°C) is reversed. The secondary magnetization postdates Cretaceous and younger folding, whereas the characteristic component was acquired before folding. The components may have recorded two phases of overprinting: a Late Cretaceous into Cenozoic normal overprint and a predeformation remagnetization epi ode during a time of reverse polarity. However, the reverse component more likely is primary remanence. If so, it would suggest little latitudinal displacement but 40° of clockwise rotation with respect to North America. The Devonian Nanook Limestone, sampled in the Shublik Mountains, also reveals two major components of magnetization; however, the characteristic component is isolated at blocking temperatures greater than 500°C and is shallower in inclination than expected from the Devonian reference pole for North America. The recovery of the reversed characteristic component in this study is a significant result by itself. It is good evidence that at least part of the northeast Brooks Range has escaped the thorough Cretaceous normal-polarity overprinting that has been observed in the north-central Brooks Range. We hope that analyses of additional samples from the Katakturuk Dolomite, Nanook Limestone, Lisburne Group, Sadlerochit Group, and Shublik Formation in ANWR and from the Triassic and Jurassic Otuk Formation in the east-central foothills will also discriminate pre-Cretaceous magnetizations and will provide constraints on the time they were set. End_of_Article - Last_Page 676------------

Phosphatic Glauconitic Sandstone and Oncolite Deposition at the Upper Paleozoic Base of Etivluk Group, North-Central Brooks Range, Alaska: ABSTRACT

Carboniferous stratigraphy of the Picnic Creek allochthon in the central Brooks Range is dominated by bedded cherts and shales. In the Killik River quadrangle, bedded black cherts of the Lisburne Group are overlain by a thin diagnostic clastic unit composed of sandstone and conglomerate. The sandstone is a thin (0.35-m), laterally extensive, planar, laminated litharenite with an average Q:F:L of 40:17:43 and a Qp:Lv:Ls of 12:8:80. The provenance is interpreted to be a recycled orogen dominated by uplifted sedimentary sequences with minor plutonic, metamorphic, and volcanic sources. The presence of glauconite (7%) and authigenic phosphate (18%) indicates deposition in a shelf environment. This phosphatic sandstone forms part of the matrix in a conglomerate at one locality. The conglomerate is lenticular (2 m × 10 m), crudely graded, and very poorly sorted, and it contains black chert ripups. Clasts are composed of oncoids (70%), chert (22%), shale (5%), and limestone (3%). Barite preferentially replaces all clasts except chert and part of the matrix. The oncolites are SS-type mode C hemispheroids, indicating formation in a continuously agitated shallow to intertidal marine environment. Sedimentologic and petrographic observations suggest that the phosphatic glauconitic sandstone developed in a shelf environment, and the oncolitic conglomerate is a debris flow off a nearby carbonate platform that transported shallow-water material out onto the shelf. Preservation of unaltered echinoderm fragments and calcareous algal oncolites clearly indicates deposition above the CCD. Radiolarians from immediately above the clastics include spongy tetrahedral Latentifistulidea, which suggests that sandstone and conglomerate deposition probably occurred in the Morrowan (Early Pennsylvanian). End_of_Article - Last_Page 678------------

Fold-Nappes and Polyphase Thrusting in North-Central Brooks Range, Alaska: ABSTRACT

Ongoing study involving inch-to-the-mile mapping of a geologic transect along the Itkillik-Koyukuk Rivers is providing new information on the structural style and kinematic development of the central Brooks Range. The principal structures controlling north-directed structural telescoping are three detachments. These detachments are stratigraphically controlled and occur in: (1) Devonian Hunt Fork Shale, (2) Mississippian Kayak Shale, and (3) Permian and Triassic shale. The detachments and the subsidiary thrusts that branch from them form a thrust complex that developed in two stages. During the first stage, duplexes consisting of horses of Mississippian Lisburne Group platform carbonates formed where the Kayak Shale detachment ramped up to Permian and Triassic beds, imbri ating the intervening Lisburne Group. In the second stage, alpine-style fold nappes involving Devonian Kanayut Conglomerate and elements of the underlying Hunt Fork Shale were emplaced sequentially from south to north on thrust faults solely in the Hunt Fork Shale. Thrust faults that emplace the fold nappes merge with the previously formed detachment in the Kayak Shale. Complex deformation associated with both stages include: (1) ramping of duplexes in Lisburne Group rocks over previously imbricated Lisburne Group rocks; (2) regional folding of imbricated Lisburne Group rocks; (3) folded individual subsidiary faults within the duplexes of Lisburne Group rocks; (4) warping, folding, and thrusting of the Kayak Shale detachment associated with generation and emplacement of the underlying fo d nappes; and (5) faults that cut through the hinge area of fold nappes and thrust the trailing limbs of the fold nappes over the overturned leading limbs of the fold nappes. End_of_Article - Last_Page 667------------

Geologic Significance of Doonerak Structural High, Central Brooks Range, Alaska

Rocks in the Doonerak anticline resemble correlative sequences in the northeastern Brooks Range more than they resemble the directly adjacent sequence in the north-central Brooks Range. Mafic dikes in the anticline have potassium-argon ages of about 380 m.y. (Devonian) and 470 m.y. (Ordovician). They intrude low-grade metamorphic rocks and associated volcanic rocks that are therefore probably of Cambrian or Ordovician age. These early Paleozoic units are overlain unconformably by Carboniferous to Triassic strata. The Doonerak anticline is bounded on both the north and south by faults that can be interpreted in at least two ways. A structural-window hypothesis postulates that the two faults are actually parts of the same fault and reflect long-range thrusting from south to north over the Doonerak high. This fault would constitute a sole thrust under the north-central Brooks Range, and all Upper Devonian through Triassic rocks there would be allochthonous. Consequently, both late Paleozoic and Mesozoic strata might be preserved at depth beneath the stacked thrust sheets of that area. An alternative hypothesis favored by the writers calls for two separate faults, the southern one thrusting Middle and lower Upper Devonian rocks northward onto the phyllitic rocks on the south flank of the anticline. A distinct northern fault is interpreted as a south-directed thrust that moved Upper Devonian and younger rocks southward over the Carboniferous rocks on the north flank of the anticline. This interpretation implies that the rocks of the north-central Brooks Range are mainly autochthonous and that the rocks directly beneath them would be like Middle Devonian and older strata now present in the core of the Brooks Range, south of the Doonerak anticline.

Late Pleistocene glacial chronology of north-central Brooks Range, Alaska

Stratigraphic sections and radiocarbon dates serve as a basis for defining four substages in the late Pleistocene Itkillik glaciation which are correlated with the four substages of the Wisconsin glaciation of central North America. Three post-Itkillik advances, restricted to the highest parts of the range, are correlated with post-hypsithermal advances elsewhere in the North American Cordillera.