Western Brooks Range Research Articles

Collision-deformed Paleozoic continental margin of Alaska — Foundation for microplate accretion

Construction of a map of basement rocks of Alaska has revealed a complexly deformed, arcuate Paleozoic continental margin that is a northwestern extension of the western margin of North America. Three key areas in Alaska (Nation River, Livengood, and Terra Cotta Mountains) have similar sections of Ordovician to Devonian and locally younger Paleozoic siliceous shale and chert, interfingering with limestone and containing various turbidity-current and slope deposits. Microplates moving north from the Pacific basin have collided with this Paleozoic continental margin and deformed and collapsed the original shale-out facies. A similar south-facing collapsed continental margin of middle and late Paleozoic age can be traced across the western Brooks Range of northern Alaska. We believe that this continental margin in the Brooks Range, although it is mainly younger, may be an extension of the same margin in central Alaska. This continental margin has been offset to the Brooks Range along the Porcupine megashear.

Collision-deformed Paleozoic continental margin, western Brooks Range, Alaska

A structural sequence of Carboniferous, Permian, and Triassic argillaceous and cherty rocks in the northwestern Brooks Range, herein named the Kagvik sequence, is a key to the tectonic history of Arctic Alaska. The Kagvik sequence lies structurally below thrust-faulted sheets of coeval carbonate rocks, including the Lisburne Group, and structurally above quartz mica schist and greenstone of the southern Brooks Range. Sedimentary features of the shelly fossil-rich carbonate strata of the Lisburne in the upper thrust sheets indicate deposition in a shallow-water shelf environment. The underlying shale and chert of the Kagvik is an attenuated section about 500 m thick repeated by imbricate thrusts. Ubiquitous pelagic fossils (radiolaria and Nereites trace-fossil assemblages) and volcanic material locally forming andesitic tuff and flows point to an oceanic environment of deposition. The presence of some limestone turbidites interbedded with the ocean-floor sediments of the Kagvik suggests that a carbonate shelf was nearby and that a south-facing continental margin existed during late Paleozoic and early Mesozoic time near the present-day southern Brooks Range. Collapse of this long-standing continental margin appears to be related to collision and accretion of Paleozoic island arcs and microcontinental blocks now represented by the metamorphic terrane along the south flank of the Brooks Range. The structurally higher thrust slices of mafic and ultramafic rocks emplaced on top of the collapsed continental margin represent ophiolitic basement of the Yukon-Koyukuk basin.

Evidence in the Bering Strait region for differential movement between North America and Eurasia

North-trending folds and regional deflection of Cordilleran trends in the Bering Strait region strongly suggest east-west compression and crustal shortening between the North American and Eurasian continents during late Mesozoic or early Cenozoic time. Along the northwest coast of Alaska, the west- and southwest-trending grain of the Cordilleran belt appears to be deflected by a belt of folds and thrust faults that can be traced from Cape Lisburne southward across the Seward Peninsula and Yukon-Koyukuk province to the Kaltag fault. Fold axes are generally overturned to the east, and locally Paleozoic strata are thrust eastward onto Mesozoic strata. Major geologic trends on the north side of the Kaltag fault appear to be offset as much as 130 km to the east. On the Bering-Chukchi shelf, increasing evidence suggests that Cordilleran trends between Alaska and Siberia are buckled into a tight southward-looping orocline. The Paleozoic and early Mesozoic stratigraphic sequence on St. Lawrence Island is nearly identical with that found in the western Brooks Range, 400 km to the north, and distinctive belts of late Paleozoic and Mesozoic mafic volcanic and intrusive rocks and alkalic intrusive rocks appear to be traceable in an arc stretching southward from the Kobuk-Selawik area to St. Lawrence Island and then northward onto the Chukotsk Peninsula. Oroclinal bending in the Bering Strait region and in the eastern Brooks Range appears to have foreshortened the northern part of the Alaska Cordillera by at least 800 km. As neither of these deflections is reflected in the arcuate trends of the Alaska and Aleutian Ranges, the northern part of Alaska may have been decoupled from the southern part along the Kaltag fault. This compressional event in Late Cretaceous or early Tertiary time may be related to opening of the North Atlantic Ocean basin.

Structure and Tectonic History of the Western Brooks Range, De Long Mountains and Lisburne Hills, Northern Alaska

This study outlines the structure and tectonic history of the De Long Mountains, the western end of the Brooks Range and the Lisburne Hills, an area extending from the Chukchi Sea to long 156 ° W., 250 mi eastward. Seven distinct, but roughly coeval, stratigraphic sequences from Upper Devonian to Lower Cretaceous, can be recognized. The Brooks Range and Ivotuk Hills Sequences characterize the Brooks Range and its foothills, although the Brooks Range Sequence is also present in the De Long Mountains. The De Long, Nuka Ridge, and Ipnavik Sequences are confined to the De Long Mountains. The Lisburne Hills Sequence is characteristic of and confined to the Lisburne Hills. The Brooks Range and De Long Mountains constitute two distinct tectonic provinces; compressional tectonics identify the former, and gravity-slide tectonics, the latter. The two styles of deformation are considered complementary and were developed during the orogeny. Each component tectonic unit, that is, thrust-sheet or slide-sheet, consists of only one stratigraphic sequence, which has enabled the relative allochthonicity of the sheets to be ascertained. The gravity slide-sheets have been dislocated at least 60 to 70 mi in the De Long Mountains. The orogeny was initiated in the Jurassic Period; the early phase was marked by the emplacement of an ophiolitic suite marginal to the geanticline. Flysch trough subsidence had commenced by earliest Cretaceous time and flanked a geanticline centered on the present Baird Mountains area. Subsidence continued throughout the Cretaceous Period with successive maxima accompanied by migration northward of the axes of subsidence. Compressional overthrusting of the Brooks Range and Ivotuk Hills Sequences developed in association with these sequences that were downwarped with the subsiding troughs. Where downwarping was most prolonged, or most strongly developed, gravity sliding took place, specifically in the region of the present De Long Mountains and Lisburne Hills. These gravity slides form the bulge of the De Long Mountains and are postulated to override the overthrust Brooks Range “spine.” These movements reached a climax in the Late Cretaceous with the final uplift and isostatic adjustments occurring in the Late Cretaceous, early Tertiary. Accompanying this final orogenic phase was the development of large-scale transcurrent faulting and secondary thrusting.

Mesozoic Tectonics and Correlations in Yukon-Koyukuk Province, West-Central Alaska: ABSTRACT

The Yukon-Koyukuk province of west-central Alaska is a broad, wedge-shaped depression of Mesozoic and early Cenozoic rocks bordered on the west, north, and southeast by metamorphic rocks of Paleozoic and possibly older age. The province is not a simple sedimentary basin, as formerly believed, but a highly mobile tract that was subjected to repeated volcanism and plutonism during Cretaceous and early Tertiary times. Sedimentation was largely volcanogenic in character and confined, mostly, to a relatively short interval in mid-Cretaceous. Recent mapping on St. Lawrence Island suggests that the Mesozoic and early Cenozoic rocks of the Yukon-Koyukuk province swing in a broad arc across the Bering Sea and western St. Lawrence Island to join the Okhotsk volcanic belt of northea tern Siberia. Three different structural trends are recognizable among the intensely deformed pre-Tertiary rocks of the Yukon-Koyukuk province: East-west trends in the northern part of the province and adjoining Brooks Range, north-south trends along the western edge of the province from the western Brooks Range to the Yukon delta, and northeast-southwest trends along the Yukon and Koyukuk valleys. The province is transected between Ruby and Unalakleet by the Kaltag fault, a major strike-slip fault with 40-80 mi of probable right-lateral offset. Major structural discontinuities also occur along the Kobuk fault zone at the northern edge of the province, but the evidence for large-scale strike-slip displacement is inconclusive. End_of_Article - Last_Page 2500------------

Mesozoic Stratigraphy, Sedimentology, and Paleogeography of Northern Yukon, Canada: ABSTRACT

Data derived from reconnaissance field work in 1962 and published data indicate that the Mesozoic succession is divided into 5 major cycles which reflect important events in the geologic history of the region: (1) Late Triassic transgression across a low shelf; (2) widespread Jurassic transgression and development of the Richardson trough; (3) earliest Cretaceous regression and basin filling that ended with local minor(?) uplift and erosion; (4) Albian transgression and development of the early Late Cretaceous regression; and (5) Late Cretaceous and early Tertiary nonmarine sedimentation in local tectonic basins. Within each of these major cycles several diastems and unconformities are present which increase in magnitude toward the basins' margins. During Jurassic and earliest Cretaceous times, the Richardson basin was filled with marine shale and bordered on the east by a landmass (probably mountainous) which shed sand westward. The sands now form a series of Lower, Middle, and Upper Jurassic and Lower Cretaceous nearshore sandstone bodies. During the earliest Cretaceous some sand in the Babbage River region was derived from the underlying Mesozoic and upper Paleozoic clastic sediments in the western Brooks Range geanticline. The earliest Cretaceous clastics filled the basin so that nonmarine conditions prevailed in the northernmost (Blow River) part. Moreover, slight uplift and erosion took place at that time. By the end of Aptian time, much of the eastern landmass had been peneplaned. The tectonic and sedimentary patterns changed drastically during Albian to early Tertiary times. Sediments were derived from a rising cordillera which formed the western and southern margins of the new foreland basin and developed across the previous eastern Jurassic and Lower Cretaceous landmass. A thick succession of largely silty Albian shale was deposited in this basin. The thin Upper Cretaceous sedimentary sequence consists of nonmarine strata (Eagle Plain Formation) and marine shale. Much of the deformation and uplift of the Richardson Mountains took place during middle Late Cretaceous time. During the late stages of the cordillera deformation on the south and southwest, several small, partly fault-bounded basins developed in which thick sequences of coarse clastic materials wer deposited. A similar but thicker Mesozoic succession appears to extend beneath the Mackenzie delta. One of the best prospective hydrocarbon zones is the sequence of well-sorted sandstones near the base of the Lower Cretaceous, if these sandstones do not grade to shale on the north. End_of_Article - Last_Page 2497------------

Large-Scale Thrusting and Migrating Cretaceous Foredeeps in Western Brooks Range and Adjacent Regions of Northwestern Alaska: ABSTRACT

Large-scale, low-angle thrust sheets mapped within a 30,000-sq-mi region have been grouped tentatively into five tectonic units, each characterized by multiple northward thrusting of a distinctive rock sequence. Rock sequences which distinguish the tectonic units in the De Long Mountains are informally designated as follow: 1. Foothills unit (northernmost and structurally lowest): Permian(?), Triassic, and Jurassic chert, shale, and limestone, (b) Early Cretaceous (Valanginian) shale with Buchia-bearing coquinoid limestone, and (c) late Early Cretaceous (early? Albian) graywacke and shale. 2. Wulik unit: (a) Mississippian dark carbonate, chert, and shale, (b) Permian(?), Triassic, and Jurassic(?) varicolored chert, shale, and limestone, (c) Valanginian quartzitic sandstone and shale containing Buchia, and (d) early? Albian graywacke and shale. 3. Kelly unit: Subunit A: Devonian and Mississippian detrital rocks; Subunit B: (a) Mississippian light carbonate and terrigenous rocks, (b) Permian(?) and Triassic chert and shale, and (c) earliest Cretaceous (Berriasian and Valanginian) graywacke and shale. 4. Ipnavik unit: (a) Devonian carbonate, (b) Mississippian dark chert, shale, and carbonate intruded by numerous mafic sills (Jurassic?), and (c) Berriasian graywacke and shale. 5. Misheguk unit: (structurally highest): (a) Jurassic(?) tabular, mafic igneous complex several thousand feet thick, and (b) Devonian carbonate. The five internally thrust tectonic units are juxtaposed and separated by important low-angle thrusts, some of which have a minimum northward displacement of more than 20 mi. Cumulative northward displacement of all thrusts in the region may exceed 150 mi. The major episode of thrusting was probably Aptian to early(?) Albian; however, three other phases of less intense Cretaceous deformation also are postulated. All four episodes were accompanied by deposition in adjacent foredeeps (exogeosynclines), which are progressively younger northward. The ages of the preserved foredeep rock sequences are: (1) Berriasian and Valanginian; (2) early(?) Albian; (3) later Albian (Nanushuk Group); and (4) Late Cretaceous (Colville Group). The contrast between the north-trending grains of the Lisburne Peninsula and the southwest-trending structures of the De Long Mountains may have resulted from latest Cretaceous or Tertiary bending of the regional structural grain. End_of_Article - Last_Page 567------------