Neruokpuk Formation Research Articles

Detrital geochronology of pre-Mississippian strata in the northeastern Brooks Range, Alaska: Insights into the tectonic evolution of northern Laurentia

The Arctic Alaska terrane of northern Alaska and Yukon is one of several exotic terranes in the North American Cordillera with putative early Paleozoic connections to the northern Caledonian-Appalachian orogen. The U-Pb and 40 Ar/ 39 Ar isotopic data from detrital minerals in pre-Mississippian sedimentary units of the northeastern Brooks Range are presented here to investigate the consequences of Caledonian orogenesis on sediment dispersal trends and the paleogeography of northern Laurentia. Neoproterozoic–Cambrian siliciclastic rocks of the informal Firth River group and the Neruokpuk Formation were most likely deposited along a passive margin that sourced Archean and Paleoproterozoic basement rocks of the Canadian shield and reworked Mesoproterozoic and younger sedimentary units. These strata are overlain by a Lower Ordovician–Lower Devonian succession of fine-grained siliciclastic turbidites, herein referred to as the Clarence River group, which records a prominent shift in provenance most likely associated with the onset of the Caledonian-Appalachian orogeny in northeast Laurentia. U-Pb detrital zircon age populations of ca. 470–420 and 990–820 Ma, along with 40 Ar/ 39 Ar detrital muscovite ages of ca. 470–430 Ma, support provenance connections with the East Greenland Caledonides, Pearya, and Svalbard. Partially reset 40 Ar/ 39 Ar ages in these sedimentary successions are linked to low-grade metamorphism associated with the Early–Middle Devonian Romanzof orogeny, a poorly understood tectonic event in the Brooks Range that is possibly associated with the emplacement of an allochthonous oceanic assemblage, herein named the Whale Mountain allochthon.

Provenance and paleogeography of the Neruokpuk Formation, northwest Laurentia: An integrated synthesis

The Neruokpuk Formation is a Neoproterozoic and Cambrian turbiditic succession in northwesternmost Yukon (Canada) and northeastern Alaska (USA), part of a latest Proterozoic to Early Devonian slope and basin succession that is correlated in detail with strata in Selwyn Basin of the northern Canadian Cordillera. It includes quartz-lithic sandstone, locally containing altered detrital feldspar and muscovite indicating that a metamorphic source contributed detritus to the unit. The muscovite yields disturbed Ar-Ar spectra suggesting ages of 1800–1900 Ma. Detrital zircon distributions are dominated by 1800–2000 Ma grains with subsidiary populations of 1000–1600 Ma, 2300–2500 Ma and 2600–2800 Ma grains, consistent with a hybrid provenance dominated by a Laurentian cratonic source. Additional populations are derived from recycled Mackenzie Mountains and possibly Wernecke Supergroups. Integrating the geochronology with the regional stratigraphic setting, structural history, and geochemistry leads to the conclusion that the Neruokpuk Formation was deposited near its present location as part of the autochthonous northwest Laurentian continental margin. Therefore, the eastern part of Arctic Alaska, underlain by the Neruokpuk Formation, has a history that is distinct from the allochthonous western part(s) of the Arctic Alaska terrane. However, the rest of Arctic Alaska is structurally and stratigraphically linked to the eastern part by Late Devonian time.

The pre-Mississippian "Neruokpuk Formation," northeastern Alaska and northwestern Yukon: review and new regional correlation: Reply

Since the early 1900's, regional reconnaissance in Alaska and the Yukon has failed to resolve the stratigraphy and structure of the pre-Mississippian Formation. Its age and distribution have been defined and redefined as new data have slowly accumulated. In most recently published reconnaissance maps of the Yukon, the Neruokpuk includes nearly all of the pre-Mississippian strata in the British Mountains and is assigned a Precambrian age. In contrast, approximately half of contiguous strata in adjacent Alaska are interpreted as early Paleozoic in age and are excluded from the Neruokpuk. Recent detailed studies in the Firth River area of the Yukon have documented intense small-scale imbrication of fossiliferous Lower Cambrian to Devonian(?) units that were previously mapped as Precambrian Neruokpuk.A remarkable similarity between the lithologies of lower Paleozoic rocks in the British Mountains and the Selwyn Basin 1000 km to the southeast is strengthened by biostratigraphic ties in Lower Cambri...

The pre-Mississippian "Neruokpuk Formation," northeastern Alaska and northwestern Yukon: review and new regional correlation: Discussion

The pre-Mississippian "Neruokpuk Formation," northeastern Alaska and northwestern Yukon: review and new regional correlation: Discussion

The pre-Mississippian "Neruokpuk Formation," northeastern Alaska and northwestern Yukon: review and new regional correlation

Since the early 1900's, regional reconnaissance in Alaska and the Yukon has failed to resolve the stratigraphy and structure of the pre-Mississippian Neruokpuk Formation. Its age and distribution have been defined and redefined as new data have slowly accumulated. In most recently published reconnaissance maps of the Yukon, the "Neruokpuk" includes nearly all of the pre-Mississippian strata in the British Mountains and is assigned a Precambrian age. In contrast, approximately half of contiguous strata in adjacent Alaska are interpreted as early Paleozoic in age and are excluded from the Neruokpuk. Recent detailed studies in the Firth River area of the Yukon have documented intense small-scale imbrication of fossiliferous Lower Cambrian to Devonian(?) units that were previously mapped as Precambrian Neruokpuk.A remarkable similarity between the lithologies of lower Paleozoic rocks in the British Mountains and the Selwyn Basin 1000 km to the southeast is strengthened by biostratigraphic ties in Lower Cambrian, Lower Ordovician, and Lower Silurian strata. This correlation between basin facies suggests that shelf and slope facies of upper Proterozoic through lower Paleozoic strata may also be correlatable between the two areas. The paleogeographic implications of these correlations indicate that pre-Mississippian strata in Arctic Alaska and the Yukon are part of a single Arctic–Pacific continental margin.The Neruokpuk Formation name should be restricted in Canada to the quartzite-dominant unit, contiguous with similar strata to which the restricted Neruokpuk definition applies in Alaska. The current broad definition, based on an assumed Proterozoic age but including many rock types, should be discontinued.

Possible Hydrocarbon Habitat of the Bulge, Alaska and Yukon Territory

Bedrock geology of the northernmost Bulge of the Rocky Mountain Cordillera consists of units ranging in age from the Proterozoic to the Recent. Concerted LANDSAT imagery, field mapping, and CDP seismic interpretation indicates that there are several thick, unconformity-bounded and areally distinct depositional mega-sequences in northern Alaska and Yukon Territory. Analyses of the Arctic National Wildlife Refuge (ANWR), 1002 area, subsurface suggest the presence of several large structures. However, seismic resolution is insufficient to determine the stratigraphy with a high degree of confidence. The oldest sediments in the Bulge are the northerly derived Katakturuk dolomite and the southerly derived, predominantly clastic Neruokpuk Formation. Tests of these units immediately outside ANWR produced oil, gas, and water from vugs and fractures. Both the Katakturuk and Neruokpuk are overlain by dissimilar but thick and areally limited Cambrian-Devonian sediments with undetermined reservoir potential. Middle and Upper Ellesmerian crop out around the periphery of the coastal plain and are found in the subsurface. Their presence and reservoir development in the structures of the 1002 area depend upon the extent of Lower Cretaceous truncation. Two dissimilar locally derived breakup megasequence sandstones having limited lateral extends overlie older units. They have increasing regional importance as commercial oilmore » and gas reservoirs. Very thick, southerly derived, Brookian clastics overstep this area. They contain the largest endowment of the in-place hydrocarbons in Alaska and the Yukon. Their commercial development is incipient.« less

Style and Age of Tectonism of Sadlerochit Mountains to Franklin Mountains, Arctic National Wildlife Refuge (ANWR), Alaska: ABSTRACT

The pre-Tertiary rocks north of the Franklin Mountains and south of the coastal plain in ANWR can be subdivided into two major structural units: (1) basement--Neruokpuk Formation, Nanook Limestone, and Katakturuk Dolomite--and (2) Lower Mississippian to Upper Cretaceous sedimentary rocks. Basement rocks underwent intense deformation prior to deposition of Lower Mississippian rocks; locally the contact is structural. Crustal shortening at the structural level exposed was accommodated primarily by concentric folding. Axial planes of major folds generally strike N70°-90°E and dip 50°-80°S, indicating north vergence, and can be subdivided into two groups: east-northeast trending and east trending. Major folds plunge subhorizontally and continue laterally for up to 10 mi (15 km). Exposed reverse faults show relatively small amounts of throw (< 7,500 ft or 2.5 km). Relative shortening decreases from greater than 41% at the northern margin of the Franklin Mountains to less than 16% in the Sadlerochit Mountains, amounting to approximately 14 mi (22 km) over 39 mi (64 km) of a north-south transect. Reverse/thrust faults are interpreted to sole out along a basal decollement. This detachment probably lies between the Neruokpuk Formation and overlying basement carbonate rocks between the Franklin and Sadlerochit Mountains, and ramps upward under the coastal plain. Field structural data of the study area, and well and seismic data of the area west of ANWR, constrain the principal post-Mississippian deformation, a phase of north-south compression, to have occurred between the mid-Eocene and the present. East-trending folds suggest two deformational events. Relative timing of these events, based on field data, is equivocal. End_of_Article - Last_Page 668------------

Pre-Mississippian Accreted Terranes of Northeastern Brooks Range, Alaska: ABSTRACT

Low-grade metamorphic rocks (Neruokpuk Formation, sensu lato) underlying a regional angular unconformity in the northeastern Brooks Range have been interpreted as a conformable Precambrian to Devonian stratigraphic succession. The pre-Mississippian rocks include not only miogeoclinal quartzites and carbonates but also a variety of eugeoclinal lithologies such as radiolarian cherts, argillites, and graptolitic shales; mafic to intermediate volcanic rocks; and volcanogenic gray-wackes. Fossils of Cambrian, Ordovician, and Silurian age have been identified in these lithologies. Many of these units are fault-bounded and may be interpreted as pre-Mississippian tectono-stratigraphic terranes. Several of the terranes are depositionally overlapped by Middle(?) Devonian clasti rocks and intruded by plutonic rocks depositionally overlain by Mississippian and younger rocks, but which yield equivocal middle Paleozoic age dates. Similar relationships are exposed in the Doonersk anticlinorium in the central Brooks Range. These features are interpreted to indicate that the lower Paleozoic rocks of the eastern Brooks Range were tectonically assembled by accretionary processes along an active continental margin prior to Middle Devonian time. Subsequent uplift and erosion occurred prior to deposition of the Mississippian to Neocomian (Ellesmerian) passive margin sequence. End_of_Article - Last_Page 670------------

Reservoir Description of Endicott Field, Prudhoe Bay, Alaska: ABSTRACT

Located about 2 mi offshore and several miles east of Prudhoe Bay, the Endicott field contains about 1.4 billion reservoir bbl of oil and 0.5 billion reservoir bbl of gas. Hydrocarbons occur within Mississippian fluvial sandstones of the Kekiktuk formation, which unconformably overlies the Neruokpuk Formation and grades upward into the Kayak and Itkilyariak formations. Stratigraphy is subdivided into three lithofacies that, from the base upward, reflect deposition in a swamp/lacustrine/flood plain environment (zone 1), a braided stream system (zone 2), and a meandering stream system (zone 3). Sediment dispersal was from a northerly source. Endicott field structure defines a southwesterly plunging antiform that is bounded to the north, northeast, and southwest by major normal faults and is truncated to the northeast by the Lower Cretaceous Unconformity (LCU). Shales overlying the LCU and shales of the Kayak and Itkilyariak formations form the reservoir cap. Reservoir properties within the hydrocarbon column vary by zone with zones 3 and 2 typified by an average net/gross-porosity-water saturation-permeability of 37%-18%-22%-500 md and 88%-22%-13%-1,100 md, respectively. In contrast, zone 1 quality is very poor. Reservoir sands are compositionally very mature and exhibit an enhanced pore network. Diagenetic minerals include quartz along with lesser kaolinite and carbonate. Gas is present from about 9,500 ft (2,850 m) to 9,855 ft (2,958 m), oil is down to 10,180-10,200 ft (3,054-3,060 m), and tar accumulations are down to 10,400 ft (3,120 m) subsea. Average oil gravity is 23° API. Geochemical data indicate that the tar accumulations originated through a physical deasphalting process. Cenozoic imbibition resulted in water overriding tar. End_of_Article - Last_Page 656------------

Basement Rocks of Barrow Arch, Alaska, and Circum-Arctic Paleozoic Mobile Belt

Silurian fossils from steeply dipping slate and graywacke underlying the Prudhoe Bay oil field, and Ordovician and Silurian fossils from similar rocks in the Barrow test wells indicate that rocks of about the same lithology and age as the Neruokpuk Formation in the Brooks Range-Barn Mountains area continue west in the subsurface and form the basement of the Barrow arch. This belt of highly deformed, weakly metamorphosed, predominantly siliceous strata, including volcanic rocks, is part of an early Paleozoic geosyncline that was deformed, intruded by Silurian and Devonian granite, and deeply eroded by the end of the Devonian. Similar rocks affected by the same orogeny form the Innuitian fold belt in the Canadian Arctic Islands, but the connection between the two fold belts is obscured by rocks of mainly younger age in the Mackenzie River delta region and by water along the west side of Banks Island. South of the Mackenzie River delta, the Richardson trough with its graptolitic shale and chert section separates carbonate strata of the Yukon shelf of northern Yukon Territory and east-central Alaska from the Mackenzie carbonate platform lapping onto the Canadian shield. Now that the Neruokpuk Formation in the Barn Mountains is known to include graptolitic shales and cherts of the same age as, and of partly similar lithology to, those in the nearby Richardson trough, it seems likely that deep-water pelagic sedimentation continued from the Neruokpuk geosyncline into the Richardson trough. The argillaceous rocks of the Richardson trough, unlike those of the Neruokpuk geosyncline or Cordilleran geosyncline, grade downward into argillaceous limestone that lies on a Cambrian platform section. In the Ogilvie Mountains at the south end of the trough, shales interfinger with limestones, indicating that the trough was constricted into narrow channels and was separated partly from the Cordilleran geosyncline farther south. These relations indicate that the Richardson trough was not a basin of oceanic type or proportions but may be an aulacogen that developed across the Yukon shelf from an Arctic sea-floor-spreading center.

Carboniferous Biostratigraphy, Prudhoe Bay State 1 to Northeastern Brooks Range, Arctic Alaska

The 3,000-ft-thick Carboniferous section in Atlantic Richfield/Humble Prudhoe Bay State 1 is divided into two groups. The older rocks in the section, Endicott Group, are 1,200 ft thick and unconformably overlie argillites of the pre-Mississippian Neruokpuk Formation. Within the Endicott Group, the Kekiktuk Conglomerate is 600 ft of gray to dark-gray sandstone, shale, and thin-bedded coal, deposited in a paralic environment. The overlying Kayak(?) Shale is 620 ft of quartzitic and calcareous red sandstone, siltstone, shale, and arenaceous to ooid limestone; it represents transition to marine sedimentation and contains microfossils of Zone 16s (Chesterian). Overlying the Endicott Group, the Lisburne Group is 1,770 ft thick. Within the Lisburne Group, the Alapah L mestone, 1,050 ft thick, includes arenaceous bryozoan-echinoderm wackestone and packstone to lime mudstone and microdolomite with gypsum. The age of the Alapah Limestone ranges from Zones 16s through 18 (Chesterian). The overlying Wahoo Limestone, 720 ft thick, is a series of cyclic glauconitic-bryozoan-echinoderm-ooid packstone and grainstone capped by lime mudstone and arenaceous microdolomite. These are interpreted to represent cyclic deposition of tidal-flat lime mud developed directly over ooid tidal bars. The age of the Wahoo Limestone ranges from Zone 20 to Zone 21 (Pennsylvanian). In comparison with the section penetrated by this well the outcrop data 75-112 mi southeast show that the Endicott Group is diachronous because, in the Franklin Mountains, it has a lithology similar to that found in the Prudhoe well but contains (Meramecian) Zone 11 marine microfossils. During the Meramecian the Sadlerochit Mountains were high; the Endicott Group is either absent or less than 50 ft thick here. The Alapah Limestone of Prudhoe Bay State 1 is more dolomitic and arenaceous than its equivalent in the Franklin or Sadlerochit Mountains. The Wahoo Limestone of the Prudhoe Bay subsurface is similar in lithology and in environments of deposition to the outcrop sections in the Sadlerochit Mountains.

Regional Geology of Northern Yukon1

ABSTRACT Structurally the northernmost Yukon is part of the Eastern Cordilleran System, and in many respects it is an eastward extension of Brooks Range and the adjacent structural provinces of northern Alaska. Comparisons may also be made between the geological history of the northern Alaska - northern Yukon region and that of the Canadian Arctic Islands, a fact that has led to some speculation concerning the possible contiguity of these two areas throughout much of Phanerozoic time. This report will deal with British and Barn Mountains, Old Crow Plain and Dave Lord Ridge. Richardson Mountains will not be discussed in detail. The Ordovician, Silurian and possibly much of the Cambrian are represented east of Dave Lord Ridge by approximately 10,000 ft of fine-grained clastic sediments, the Road River Formation. Rocks of similar age in British and Barn Mountains and Brooks Range are strongly deformed metasediments (Neruokpuk Formation). Deposition of the Road River Formation took place in a basin named the Richardson Trough, which may have been a southward extension of the Franklinian Geosyncline of the Canadian Arctic Islands. A major tectonic episode, the Ellesmerian Orogeny, affected northern Yukon, northern Alaska and the Arctic Islands in several separate phases during the Devonian. The Neruokpuk underwent low-grade regional metamorphism at this time and several granite stocks were emplaced, including that at Mount Sedgwick in British Mountains. Thick syntectonic and post-tectonic clastic wedges were formed nearly everywhere near the present Alaskan and Canadian Arctic coasts, except within the Yukon, as a result of a mid-Devonian pulse of the Ellesmerian Orogeny. Between the Early Mississippian and the Late Triassic the Lisburne Group and the Sadlerochit and Shublik formations were deposited. The Lisburne and Sadlerochit are two of the most important reservoir rocks at Prudhoe Bay. Lisburne sediments, mainly carbonates, are confined (within the report area) to British and Barn Mountains. The Sadlerochit Formation (coarse-grained, porous clastic deposits) is well developed in the Dave Lord Ridge area but is thin or absent in northeastern British Mountains. The Shublik is typically developed in British Mountains. The principal sediment source area for northern Alaska and northern Yukon during the Cambrian to Jurassic period appears to have been located off the 1 Manuscript received February 9, 1972. 2 Shell Canada, P.O. Box 880, Calgary, Alberta. Present address: Institute of Sedimentary and Petroleum Geology, Calgary. This paper has developed from a field and photogeological study carried out between July 1970 and March 1971 while the author was in the employ of J. C. Sproule and Associates Ltd. The project was carried out for Charter Oil Company Ltd. Both these companies are to be thanked for granting me permission to publish the material. C. W. Drew provided an invaluable preliminary photogeological interpretation of the project area. R. Cech was my very energetic field assistant. The ideas presented herein have benefited considerably from discussions with D. K. Norris, I. Tailleur and W. O. Kupsch, all of whom have read the manuscript. Finally, grateful thanks are due to Shell Canada Limited for providing me with drafting and typing services. L. H. Taylor drew the illustrations and Mrs. J. Barnes typed the final manuscript. End_Page 81------------------------- present Arctic Coast and may have been part of the same source area that contributed detritus to the Canadian Arctic Islands region. Jurassic, Cretaceous and Tertiary sediments are almost exclusively clastic in northern Yukon. They were derived from several sources including an ancestral Brooks Range and Campbell Uplift of the Inuvik area. The Cretaceous marks a major shift in sediment dispersal patterns, particularly in northern Alaska, as a result of large-scale changes in the distribution of land and water masses at this time. These may have been caused by major continental-drift movements in the area of the Arctic Ocean Basin. A major tectonic episode, the Eurekan Orogeny, took place between Middle Cretaceous and Early Tertiary times in northern Yukon (and in Alaska and the Arctic Islands), involving all but the most recent sediments. No igneous activity has been recorded in connection with the Eurekan Orogeny in northern Yukon but most of the structural deformation of the post-Devonian rocks dates from this time.

The Neruokpuk Formation of the Barn Mountains and Driftwood Hills, Northern Yukon; its Age and Graptolite Fauna

The pre-Carboniferous racks of the Bam Mountains consist of a very thick, highly deformed and dislocated sequence of mildly metamorphosed elastics and carbonates assignable to the Neruokpuk Formation. The Driftwood Hills, for the most part, are composed of dark shales possibly assignable to the Neruokpuk Formation. Graptolites collected from a number of thin shale beds in the two areas are correlative with the Arenigian, Llanvirnian, Ashgillian, ?Llandoverian, Ludlovian, and Siegenian or Emsian Stages. It is suggested that complete, or nearly complete Ordovician and Silurian sedimentary sequences are present, and that these sediments represent geosynclinal equivalents of cratonic sediments to the south and southwest.

Ordovician to Devonian History of Northern Yukon and Adjacent District of Mackenzie

ABSTRACT The eastern half of the study area, the Mackenzie and Anderson River Plains area, is underlain by uniform sequences of carbonates and relatively minor shales, cherts and evaporites, and is marked by widespread, regional disconformities. Middle Ordovician, Upper Silurian, and at least the lower Lower Devonian sediments are completely absent from the area. The western portion that is, roughly the northern part of Yukon -- was, as a result of more local tectonic instability, characterized by more rapid facies changes, development of troughs, ridges and basins, and more localized disconformities and unconformities. The Richardson Trough, more or less conforming in outline to the area of the present Richardson Mountains, was a deeper-water, graptolite-shale trough from before earliest Ordovician through to at least early Emsian; was apparently positive between late Emsian and early Givetian; and, finally, was an area of rapid sedimentation throughout the remaining Devonian. South of but in linear continuity with the Richardson Trough, a structure recognized for the first time and termed the Bonnet Plume High may have been emergent throughout the Ordovician, Silurian and the earliest part, at least, of the Devonian. The area west of the present Richardson Mountains, north of the Ogilvie Mountains, and south of British and Barn Mountains, and termed the Yukon Stable Block, was one of platform-carbonate deposition. The Block was more or less diagonally dissected by an essentially NE-SW-trending structure, the Dave Lord High, which was positive from Late Ordovician through to at least mid-Early Devonian times. At times what was probably a subaqueous extension of this High joined with an area of carbonate development west of and beneath the Mackenzie River delta, and appears to have restricted the northern end of the Richardson Trough. The Neruokpuk Formation of the British Mountains, in the extreme northwestern portion of the region, is considered to be at least Early Ordovician to Late Silurian in age, because of correlation with rocks of the same formation in Alaska, and because of its lithologic similarity to rocks of that age range in the Barn Mountains. These rocks are interpreted as miogeosynclinal equivalents of the more stable platform suites to the south and southeast. Intrusion of two small, granitic masses is believed to have occurred at about Early Devonian time in the Barn Mountains area. End_Page 321------------------------ During late Middle and early Late Devonian times, the sedimentation pattern of the entire region changed, leading to the deposition of black, siliceous shale and localized reefs. This was followed during later Frasnian and Famennian time by strong uplifting in and northwest of the British and Barn Mountains regions, and the deposition of widespread and locally great thicknesses of clastic sediments to the southeast. These are characterized by coarse, greywacke-type sediments in the northwest, grading southeastward to finer sandstones and siltstones, and minor carbonates.

Significance of Recently Discovered Cambrian Fossils and Reinterpretation of Neruokpuk Formation, Northeastern Alaska: GEOLOGICAL NOTES

The Neruokpuk Formation in northeastern Alaska is a complex sequence of slightly metamorphosed rock units that lies below the major sub-Carboniferous unconformity. Cambrian fossils found in one of these units provide the first paleontologic evidence for the age of any part of the Neruokpuk in Alaska. The quartz semischists in the type area of the Neruokpuk near Lake Peters are considered to be Precambrian. Other fossiliferous strata lie unconformably above the schist and beneath the Cambrian beds. Sedimentologic, structural, or radiometric evidence indicates that both Taconic and Acadian orogenic episodes are represented in northeastern Alaska and northern Yukon.

Paleozoic Sequence in Eastern Brooks Range, Alaska

The sequence described lies in the eastern Brooks Range in northeastern Alaska. Rocks of late Paleozoic age predominate but rocks as young as Jurassic are present. Rocks of Devonian and possibly older age occur in two east-trending belts. In the southern belt the Skajit Limestone, 500-2,600(?) feet thick, formerly believed to be of Silurian age, is now determined to be of Middle(?) and Late Devonian age. An unnamed unit of limestone and siltstone (Upper Devonian), 0-2,500(?) feet thick, rests on the Skajit. The limestone-siltstone unit intertongues with and is overlain by an unnamed unit of slate and sandstone (Upper Devonian) 2,000-4,500 feet thick, which locally rests disconformably on the Skajit. The Kanayut Conglomerate (Upper Devonian), 0-5,000 feet thick, conformably overlies the slate-sandstone unit and probably grades into it laterally. Rocks in the northern belt comprise the four members of the Neruokpuk Formation of Late(?) Devonian or older age; a quartzite-schist member more than 4,000 feet thick, a limestone member 1,000± feet thick, a limestone-phyllite member 1,000± feet thick, and a phyllite-chert member, as much as 3,300 feet thick, which overlies the other members, locally with strong angular unconformity. The Kekiktuk Conglomerate, a new formation of Devonian or Mississippian age, 0-400 feet thick, unconformably overlies the Neruokpuk Formation. The formations of Mississippian age occur in both belts. The Kayak Shale (Lower Mississippian) and Kayak(?) Shale (Lower(?) and Upper Mississippian), 400-1,200 feet thick, rest on the Kanayut and Kekiktuk Conglomerates and, locally, on the Neruokpuk Formation. The Lisburne Group comprises three formations. The Wachsmuth Limestone (Lower Mississippian), 0-950 feet thick, disconformably overlies the Kayak Shale in the western part of the area. The Alapah Limestone (Upper Mississippian), 600-1,950 feet thick, rests disconformably on the Wachsmuth Limestone. Where the Wachsmuth is absent, the Alapah rests conformably on the Kayak(?) Shale. The Wahoo Limestone (a new formation of Pennsylvanian(?) and Permian age), 0-1,400 feet thick, overlies the Alapah with probable disconformity. The Siksikpuk Formation (Lower(?) Permian), is more than 600 feet thick and consists mostly of shale. It rests unconformably on the Wahoo Limestone at Galbraith Lake and on the Alapah Limestone west of Galbraith Lake. East of Galbraith Lake the Sadlerochit Formation (Permian and Triassic) overlies the Wahoo limestone. The Sadlerochit is more than 1,000 feet thick, and consists of sandstone, siltstone, and shale. Unconformities suggest uplift during early Late Devonian time and folding during Late Devonian to Early Mississippian time. The regional unconformity between Mississippian and Permian rocks diminishes eastward and deposition may have been continuous through Pennsylvanian time in part of the eastern Brooks Range.