Lisburne Group Research Articles

Geological Mapping in Doonerak Fenster, Central Brooks Range, Alaska: ABSTRACT

End_Page 671------------------------------Mapping of the north flank of the Doonerak fenster has traced the Amawk thrust, the sole fault of the Endicott Mountains allochthon, from the North Fork of the Koyukuk River-Mount Doonerak area eastward for more than 40 km (25 mi) to the east plunge of the Doonerak anticline at Koyuktuvuk Creek near the Dietrich River. Mapping has concentrated on the structural style of the area and on the autochthonous or parautochthonous Carboniferous Lisburne Group, Kayak Shale, and Kekiktuk Conglomerate--which are present along most of the anticline--and Triassic Karen Creek Sandstone, Triassic Shublik Formation, and Permian-Triassic Sadlerochit Group--which are present only in the west. This Triassic to Mississippian section closely resembles the coeval autochthonous to parautochthonous Ellesmeri n section of the subsurface to the north and in the Brooks Range to the northeast. The north-dipping Amawk thrust is mapped between outcrops of lower Paleozoic metasedimentary rocks and this subjacent Triassic to Mississippian section. On the west, an unbroken stratigraphic sequence underlies the Amawk thrust; a complete section of Karen Creek, Shublik, Sadlerochit, Lisburne, Kayak, and Kekiktuk is exposed in the canyon of Bombardment Creek. The Kekiktuk Conglomerate unconformably overlies weathered Ordovician-Cambrian mafic volcanic rocks. Well-developed slaty cleavage in Sadlerochit and Lisburne rocks is compatible with northward-thrust transport of the overlying Endicott Mountains allochthon. East of Bombardment Creek, structural complexity increases markedly, and multiple slivers of Kekiktuk, Kayak, Lisburne, and Sadlerochit are mapped below the Amawk thrust. At Falsoola Mountain, near the east end of the fenster, the Lisburne is isoclinally folded. Near Koyuktuvuk Creek, this parautochthonous section can be described as a broken formation. Movement has occurred along incompetent horizons, particularly within the Kayak Shale, resulting in imbrication of the competent beds. This movement can be interpreted as drag resulting from emplacement of the overlying Endicott Mountains allochthon and not from a regionally significant thrust fault. Along most of the north side of the fenster, from Mount Doonerak eastward to Falsoola Mountain, east-trending high-angle longitudinal faults uplift the core of the fenster. These vertical to steeply south-dipping faults have vertical separations of a few meters to over 500 m (1,600 ft), and postdate the Early Cretaceous (probably Neocomian) emplacement of the Endicott Mountains allochthon. This uplift was probably related to the Albian and Late Cretaceous tectonic events interpreted elsewhere in the Brookian orogen. Alternatively, the high-angle faults can be interpreted as part of the floor of a duplex. However, there is no evidence of any large amount of horizontal translation along these faults. End_of_Article - Last_Page 672------------

New Upper Paleozoic and Lower Mesozoic Stratigraphic Units, Central and Western Brooks Range, Alaska

The sequence of rocks described lies primarily in the DeLong and Endicott Mountains in the western and central parts of the Brooks Range, northern Alaska. New names are given to dominantly siliceous and shaly sedimentary rocks of Mississippian through early Middle Jurassic age. The Kuna Formation (new name) consists dominantly of black bedded chert, shale, limestone, and dolomite within the Lisburne Group; some beds are phosphatic, and mafic extrusive igneous rocks occur in a few areas. Paleontologic data suggest an Early Mississippian to Early or Middle Pennsylvanian age for the Kuna Formation. Because the formation intertongues eastward with light-colored shallow-water carbonate rocks of the Wachsmuth and Alapah Limestones of the Lisburne Group, it is also assigned to t e Lisburne Group. Light-colored shallow-water carbonate rocks of other formations of the Lisburne Group were apparently also deposited on the south, but have been telescoped northward by thrust faulting over the darker Kuna Formation. The formation ranges up to 100 m in thickness. The Etivluk Group (new name) is composed of dominantly siliceous beds of the previously named Siksikpuk Formation and overlying Otuk Formation (new name). New paleontologic data suggest that the Siksikpuk is of Pennsylvanian, Permian, and Early Triassic age. The Otuk Formation consists of up to 100 m rhythmically interbedded chert, silicified limestone, and shale previously assigned in this area to the Shublik Formation. A thin but distinctive zone of early Middle Jurassic organic shale and dolomitic limestone ocally present within the top of the Otuk constitutes the Blankenship Member (new name), which is up to 7 m thick. Paleontologic data indicate that the Otuk Formation ranges in age from late Early Triassic to early Middle Jurassic. These dominantly siliceous stratigraphic units appear to be limited to the allochthons of the western and central Brooks Range; they are not known to be present on the autochthonous complex of the northeastern Brooks Range and subsurface Arctic Slope.

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.

New Information on Lisburne Group (Carboniferous and Permian) in Naval Petroleum Reserve Alaska: ABSTRACT

New Information on Lisburne Group (Carboniferous and Permian) in Naval Petroleum Reserve Alaska: ABSTRACT

Lisburne Group (Mississippian and Pennsylvanian), Potential Major Hydrocarbon Objective of Arctic Slope, Alaska

The Lisburne Group, a thick carbonate-rock unit of Mississippian and Pennsylvanian age, is one of the most widespread potential reservoir-rock units in northern Alaska. A comprehensive review of the Lisburne in the subsurface of the eastern Arctic Slope indicates attractive reservoir characteristics in a favorable source and migration setting where numerous trapping mechanisms appear to be available. Evaluation of this group as a potential exploration objective is particularly timely because of impending offshore sales in the Beaufort Sea and current exploration programs under way in the Prudhoe Bay area and the Naval Petroleum Reserve. Dolomite and sandstone have been identified as reservoir rocks. Oolitic grainstone is a common rock type, but observations indicate little reservoir potential owing to complete void filling by calcite cement. The most important reservoir rock as judged by thickness, areal extent, and predictability is microsucrosic (10 to 30µ) dolomite of intertidal to supratidal origin. It is present throughout the Lisburne and is most abundant near the middle of the sequence. Northward it decreases in thickness from 1,000 ft (300 m) to less than 100 ft (30 m). Porosity of the dolomite as determined in selected wells averages between 10 and 15% and attains a maximum of slightly more than 25%. Net thickness of reservoir rocks (i.e., rocks with greater than 5% porosity) ranges in these wells from 40 to 390 ft (40 to 120 m). Oil shows are common, and drill-stem tests have yielded as much as 1,600 bbl/day of oil and 22 MMcf/day of gas in the Lisburne pool of the Prudhoe Bay field and as much as 2,057 bbl/day of salt water outside the field area. The occurrence of dolomite over such a large area makes its presence in the offshore Beaufort Sea and adjacent Naval Petroleum Reserve 4 fairly certain. The presence of sandstone as thick as 140 ft (40 m) in the middle and upper part of the Lisburne in two coastal wells suggests that larger areas of sandstone may be found on the north in offshore areas. Shows of oil and gas and a saltwater flow of 1,470 bbl/day have been recorded from this sandstone facies. Shales of Permian and Cretaceous ages unconformably overlie the Lisburne, providing adequate sealing beds above potential reservoirs. Impermeable limestone (completely cemented grainstone) and thin beds of shale may serve as seals within the Lisburne, but the possibility of fractures in these units may negate their sealing capability. The most favorable source rock for Lisburne hydrocarbons appears to be Cretaceous shale that unconformably overlies the Lisburne east of Prudhoe Bay. This shale is reported to be a rich source rock and is the most likely source for the entire Prudhoe Bay field. A source within the Lisburne or within the underlying Kayak Shale is postulated for oil shows in the southernmost Lisburne wells. This postulated source may be in a more basinal facies of the Lisburne and may be similar to dark shale in the upper Lisburne in thrust slices to dark shale in the upper Lisburne in thrust slices in the Brooks Range. Coal in the underlying Endicott Group is a possible source for dry gas. At present, much of this coal probably is in a gas-generating regime downdip from the Prudhoe Bay field. Stratigraphic traps involving the Lisburne Group may have resulted from widespread Permian and Cretaceous unconformities. Structural traps related to normal faulting may be present along the trend of the Barrow arch, and faulted anticlines are numerous in the foothills of the Brooks Range. Combination traps are possible along the trend of the Barrow arch.

Mississippian Unconformities in Northern Alaska Related to Antler Tectonic Pulses: GEOLOGIC NOTES

The Lisburne Group of northern Alaska, a sequence of Carboniferous carbonate rocks, contains several subtle subregional unconformities and abrupt vertical depositional discontinuities. Two of these are present in about the same stratigraphic position as regional Mississippian unconformities in the western United States. An unconformity within the Wachsmuth Limestone of the Lisburne Group is present in central Brooks Range outcrops in lower or basal Meramecian carbonate and shale. It can be traced as a sharp contact for 70 mi (112 km) along the Brooks Range front. Correlations between outcrops suggest erosional thinning of 105 ft (32 m) in 11 mi (18 km) over part of its area of outcrop. Another subregional unconformity is within subsurface Chesteran carbonate rocks of the Arctic Slope. Low-angle truncation occurs at the top of a dolomite zone in most of the deep Arctic Slope wells. In the Cordilleran region of the western United States there was orogenic and epeirogenic unrest during early End_Page 435------------------------------ Meramecian and again during Chesteran times. Tectonic pulses of the Antler orogeny are reflected in the stratigraphic record outside of, as well as within, the orogenic belt. Evidence for Antler epeirogenic unrest previously had been reported as far north as Idaho and Montana with a questionable extension into western Canada. The Mississippian unconformities in northern Alaska may be evidence of mild epeirogenic unrest related to the Antler orogeny as far north as the Brooks Range and Arctic Slope, Alaska.

Environmental and Time Significance of Local Guide Fossils, Lisburne Group, Northeast Alaska: ABSTRACT

Environmental and Time Significance of Local Guide Fossils, Lisburne Group, Northeast Alaska: ABSTRACT

Carboniferous Section and Unconformity at Mount Doonerak, Brooks Range, Northern Alaska

The Lisburne Group and lower Paleozoic basement rocks at Mount Doonerak are in the core of a large doubly plunging anticline that is flanked by thrust faults that dip away toward the north and south. The Carboniferous section at Mount Doonerak is more than 400 m thick. The Kayak Shale, 120 m thick, contains Tournaisian fossils in its upper beds. The basal sandstone member of the Kayak Shale is similar lithologically to the Kekiktuk Conglomerate and lies on lower Paleozoic volcanic rocks. The Wachsmuth Limestone is 111 m thick and is early to middle Visean (Meramecian) in age. The Alapah Limestone is 150 m thick and is late Visean-early Namurian (Meramecian and Chesterian) in age. The Wahoo Limestone, 40 m thick, is zone 20 (Morrowan, Pennsylvanian) in age. The upper part of the Alapah Limestone and the Wahoo Limestone lithologically resemble beds of similar age in the Romanzof Mountains and in the subsurface of northeastern Alaska.

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.

Carboniferous Carbonate Depositional Models, Preliminary Lithofacies and Paleotectonic Maps, Arctic Alaska

Two distinct environmental suites can be delineated for the shallow-marine Carboniferous rocks of Arctic Alaska. One is a marine transgressive suite, typified at its base by a paralic clastic sequence followed by nearshore shales and siltstones, and overlain by spiculitic carbonate rocks. This transgressive clastic-marine carbonate sequence is associated with regional Mississippian transgression. Developed above this is a carbonate platform sequence characterized by very low terrigenous clastic content and shallow-water cyclic carbonate sedimentation. The outer margins of the shelf did not support reef-forming organisms. Carboniferous sedimentation in Arctic Alaska represents a northward marine transgression over an irregular surface with residual stable positive areas. In the northeast, the Sadlerochit high was not submerged until early Chester time. In the west-central foothills region, the Nuka high is a long, linear, east-trending positive area in the subsurface; it may have been the source for the Mississippian feldspathic clastic rocks of the Nuka Formation. The Nuka high may have remained a mildly active positive element during the Mississippian. A northeast-trending negative area, the Canning sag, stretches from the Prudhoe Bay region southwest to the Sagavanirktok River region of the Brooks Range, where there is a 3,000-ft-thick accumulation of shallow-water Carboniferous carbonate rocks. The Lisburne Group, at many outcrops in the Brooks Range, was deposited as a complex of cyclic shallow-water carbonate sediments in an arid or semiarid climate. A typical cycle grades from marine ooid-crinoid grainstone to supratidal microdolomite. Composite cyclic carbonate sequences may be thousands of feet thick in outcrop. Outcrop studies indicate that thick sequences that have been dolomitized by reflux brines may have reservoir qualities in the subsurface of the north slope.

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.

Carboniferous Colonial Rugose Corals, Biostratigraphy, and Paleoecology, Lisburne Group, Arctic Alaska: ABSTRACT

The Lisburne Group of arctic Alaska contains coral faunas of Osagian (Early Mississippian) to Atokan (Middle Pennsylvanian) age. Beds of Osagian age have a small fauna of solitary and tabulate corals. Beds of Meramecian and very earliest Chesterian ages contain a large fauna of Ekvasophyllum spp., Faberophyllum spp., Diphyphyllum klawockensis Armstrong, D. nasorakensis Armstrong, Lithostrotion (Siphonodendron) dutroi Armstrong, L. (S.) sinuosum (Kelly), L. (S.) warreni Nelson, L. (S.) lisburnensis Armstrong, Lithostrotion reiseri Armstrong, Lithostrotionella niakensis Armstrong, L. banffensis (Warren), L. mclareni (Sutherland), L. birdi Armstrong, L. pennsylvanica (Shimer), Thysanophyllum astraeiforme (Warren), T. orientale Thomson, Sciophyllum lambarti Harker and McLaren and S. alaskaensis Armstrong. Corals are rare in beds of younger Chesterian age; they include Lithostrotionella aff. L. mclareni (Sutherland), Lithostrotion (S.) ignekensis Armstrong, Syringopora spp., and a few solitary corals. Pennsylvanian (Atokan) beds of the Lisburne Group contain Lithostrotionella wahooensis Armstrong, Corwenia jagoensis Armstrong, a thick-welled syringoporoid, and Michelinia sp. The Lisburne Group limestones are cyclic and were deposited on a slowly subsiding carbonate platform. Colonial corals of Meramecian and Atokan ages are present in carbonate rocks associated with shallow-water shoaling facies. The scarcity of corals in carbonate rocks of Osagian, Chesterian, and Morrowan ages is attributed to regional temperature or salinity changes that inhibited their growth. Beds of Atokan age contain more calcareous algae and Foraminifera, indicating warmer waters. Paleoecologic analysis of the carbonate beds associated with the colonial corals of Atokan age indicates that the corals lived in clear, agitated water between oolitic tidal flats. Carboniferous corals are not known to have formed reef-like masses in arctic Alaska. End_of_Article - Last_Page 768------------

Revised Age and Structural Interpretations of Nuka Formation at Nuka Ridge, Northwestern Alaska: GEOLOGIC NOTES

New collections of foraminifers and macrofossils from Nuka Ridge, the type area of the enigmatic Nuka Formation, demonstrate that the type Nuka Formation is not a continuous stratigraphic sequence as originally described. Paleontologic evidence indicates the presence of a structurally repeated sequence at Nuka Ridge consisting of Upper Mississippian (Meramec and younger) strata overlain by Permian beds which, in turn, are succeeded by Triassic(?) and Cretaceous rocks. Mississippian calcareous mudstone and limestone units are correlated with the Lisburne Group; microcline-bearing arkose units with Mississippian and Permian macrofossils and microfossils represent the Nuka Formation; units of chert are inferred to be of Permian and Triassic age; and wacke at the top of the sequence resembles unnamed wacke assigned to the Cretaceous elsewhere. These new data and their interpretations indicate that the type section of the Nuka Formation is inadequate. The standard for the Nuka Formation is therefore referred to a newly measured section of the middle arkosic unit of the old section.

Itkilyariak Formation: New Mississippian Formation of Endicott Group, Arctic Slope of Alaska

The Itkilyariak Formation of Mississippian age has been recognized for at least 300 mi (480 km) along a linear trend both on the surface and in the subsurface of the northern Arctic Slope, Alaska. Typical lithologies in the formation are red and maroon sandstone, conglomerate, breccia, and limestone interbedded with maroon and greenish-gray shale and light-gray quartzitic sandstone. The Itkilyariak is gradational with the overlying Alapah Limestone of the Lisburne Group. The base of the formation is probably an unconformity. Outcrop thickness is as much as 150 ft (45 m), but over 500 ft (150 m) of redbeds in a similar stratigraphic position have been found in some wells. Irregular distribution suggests that the formation is related to the truncation of pre-Mississippian c astic rocks. The formation is thought to grade in a short distance southward to the normal marine shales and carbonates of the Endicott and Lisburne Groups, and is believed to have been deposited near the Mississippian shoreline. A Late Mississippian fauna has been collected from a surface exposure.

Prudhoe Bay--Greatest Geologic Explosion of Our Time: ABSTRACT

Prudhoe Bay field, potentially the largest oil discovery on the North American continent, is on the flat-lying arctic coastal plain of Alaska. The structure of the field resulted from 2 periods of deformation. The Lisburne and Sadlerochit (Carboniferous-Triassic) productive structure is a westward-plunging anticline superimposed on a stable ancestral shelf. The anticline broadens eastward until it is progressively truncated by a major unconformity. The geologic structure is complicated by 2 fault patterns cutting rocks of pre-Cretaceous age: (1) normal, NW-SE-trending faults on the south flank, and (2) high angle stair-step faults parallel with the axial plane of the fold on the north flank. The structure of the field, contoured with the top of the Kuparuk River (Cretaceous) as datum, is a pronounced eastward-plunging structural nose. Two major depositional sequences have been recognized in northern Alaska: (1) an upper sequence composed of orogenic rocks with poor reservoir characteristics, and (2) a lower sequence ranging in age from Mississippian to Early Cretaceous which contains most of the Lisburne Group. Accumulation was controlled by structure and porosity-permeability variations. The middle and main producing zones are sandstones of Permian to Triassic age which compose a gigantic stratigraphic trap abruptly truncated and overlapped by impervious Cretaceous shales. The upper producing zones are confined to the western part of the field and consist of discontinuous lenticular sandstones of Late Jurassic and Early Cretaceous ages. A model concept of field-wide unitization will be in effect before production commences in the mid-1970s. End_of_Article - Last_Page 346------------

Foraminifera and Rugose Coral Zones of Mississippian-Pennsylvanian Lisburne Group, Brooks Range, Arctic Alaska: ABSTRACT

Thin-section studies of outcrop samples from the Brooks Range show that the Lisburne Group ranges in age from Osagean (Early Mississippian) to Atokan (Middle Pennsylvanian), and can be divided into 15 microfossil zones (Bernard Mamet's zones 8 through 21). Although the foraminiferal fauna is impoverished, the resolution of the foraminiferal zones is finer than zones based on rugose corals. Also, the Foraminifera are in a much wider range of carbonate paleoenvironments than are the rugose corals. Colonial rugose corals are relatively abundant in the Lisburne Group in shelf carbonates of Meramecian and Atokan ages. These beds can be subdivided by use of corals into faunal zones and used for regional correlation within the Cordillera of North America. In the Brooks Range, Lisburne Group shelf carbonates of Osagean, Chesteran, and Morrowan ages are relatively poor in colonial rugose corals and are zoned exclusively by Foraminifera. End_of_Article - Last_Page 559------------

Foraminiferal Biostratigraphy and Carboniferous Microfacies, Northern Alaska and Yukon: ABSTRACT

Eight major carbonate types are widespread in the Lisburne Group of northern Alaska and Yukon: (1) bryozoan-crinoidal recrystallized packstone; (2) bryozoan-crinoidal mudstone and wackestone; (3) crinoid-pelletoid recrystallized packstone and grainstone; (4) pseudo-oolitic grainstone; (5) oolitic grainstone; (6) bryozoan grainstone; (7) spiculite packstone; and (8) dolomitic limestone and dolomite. These facies are present in the Wachsmuth, Alapah, and Wahoo Limestones, and the Kogruk and Nasserak Formations. The most favorable facies for foraminifers is the pseudo-oolitic grainstone which abounds in the Wahoo Limestone. Crinoid-pelletoid recrystallized packstones and grainstones also yield a good archaediscid fauna. Endothyrid assemblages are present in the bryozoan-crinoidal wackestones of the Wachsmuth Limestone. Spiculites are usually devoid of foraminifers; hence zonation of the Lisburne Group in the western part of Alaska must be based mostly on macrofaunal evidence. Twelve consistent foraminiferal assemblages can be recognized in the northern part of the American Cordillera: (1) a late Tournaisian (Zones 8 and 9) tournayellid assemblage (Septatournayella-Tournayella); (2) and early Visean (Zones 10-11) Earlandia facies (first appearance of Globoendothyra); (3) an early middle Visean (Zone 12) Eoendothyranopsis spiroides microfauna; (4) a late middle Visean (Zone 13) Eoendothyranopsis pressa-rara assemblage; (5) an early late Visean (Zones 14-15) Brunsia facies; (6) a Meramec-Chester boundary assemblage (Zone 16i), characterized by the elimination of many Meramecian elements such as Eoforschia, Eoendothyranopsis, etc.; (7) a latest Visean (Zone 16s) Neoarchaediscus-Planospirodiscus assemblage; (8) an earliest Namurian (Zone 17) Asteroarchaediscus fauna; (9) an early Namurian (Zone 18) Biseriella assemblage; (10) a middle Namurian (Zone 19) Quasiarchaediscus?-Eosigmoilina? assemblage; (11) a late Namurian (Zone 20) Lipinella-Millerella-Globivalvulina assemblage; and (12) Zone 21, the youngest assemblage recognized in the Lisburne Group, characterized by the outburst of numerous fusulines including Eoschubertella and Pseudostaffella. End_of_Article - Last_Page 2494------------

Mississippian Dolomites from Lisburne Group, Killik River, Mount Bupto Region, Brooks Range, Alaska

Allochthonous Osagian and Meramecian shelf carbonate rocks of the Lisburne Group that crop out from the Killik River west to Mount Bupto are extensively dolomitized and silicified. The carbonates were deposited as open-marine echinoderm-bryozoan wackestone, packstone, and grainstone with a thick sequence of interbedded carbonate mudstone in a shallow, restricted, marine to supratidal environment. Sequence of major diagenetic events in the dolomite is silicification of limestone to form chert nodules, followed by dolomitization of remaining limestone. Petrographic studies of the dolomites indicate that they have (1) dolomite rhombs with cloudy centers and clear rims, (2) voids formed by the partial dissolution of monocrystalline crinoid fragments, and (3) preserved organic skeletal voids. These features indicate dolomitization of limestone by hypersaline brines deficient in CO2. Field and stratigraphic relations suggest reflux dolomitization by hypersaline brines formed in a supratidal environment. The dolomite porosity, apparently not related to an ancient erosion surface or recent weathering processes, is thought to have been caused by the dolomitization process. Some pores contain anthroaxolite. Dolomite reservoir porosity may be present in the subsurface of the Brooks Range foothills and the North Slope from the Killik River to the Kiligwa River.

Foraminiferal Zonation and Carbonate Facies of Carboniferous (Mississippian and Pennsylvanian) Lisburne Group, Central and Eastern Brooks Range, Arctic Alaska

The Lisburne Group carbonate rocks of the central and eastern Brooks Range contain foraminiferal assemblages assigned to zones of late Tournaisian (Osage) to early Moscovian (Atoka) age. Representatives of both Eurasiatic and American cratonic microfaunas permit correlation with the original Carboniferous type sections in western Europe as well as with the standard Mississippian and Pennsylvanian sequences in the Mid-Continent region of North America. Correlation anomalies in the lower part of the sequence are discussed.

Carboniferous Deposits of Northeastern Asia, Alaska, and Yukon: ABSTRACT

The Carboniferous of northeastern Asia is represented by widespread deposits of all its subdivisions. The most complete paleontologic representation of the Tournaisian is in regions of widespread calcareous and carbonate rocks--on the margins of the Kolyma and Omolon massifs, and the Siberian platform. One can distinguish the lower Tournaisian, including 2 superhorizons (the superhorizon with Quasiendothyra kobeitusana and the Kassinskiy superhorizon). The upper Tournaisian Endothyra tuberculata zone is present. West of the massifs and in the Momskiy-Polousnyy area, the Tournaisian is composed of argillaceous-siliceous rocks. The Visean can be subdivided into 3 zones: (1) Palaeospiroplectammina diversa, (2) Eoendothyranopsis armakiensis, and (3) Archaediscus-Cravenoceras-Homoceras; these can be correlated approximately with the Malinov, Yasnaya-Polyana-Okhkiye, and Serpukhov deposits of the Russian platform. On the margins of the Kolyma and Omolon massifs, the Visean is represented by limestones; in Verkhoyansk, the Momskiy-Polousnyy areas on the Uelen massif, and in central Chukotsk, it is composed of argillaceous-siliceous deposits with interbedded limestone. In terrigenous deposits of the Upper Carboniferous of northeast Asia, one can distinguish the Planospirodiscus minimus zone, Orulganites triangulium-bilicatus zone, Actubites lenensis zone, and Camarophoria pentameroides zone. The first 2 zones are approximately comparable with lower Bashkirian and upper Bashkirian-lower Moscovian, and the last 2 with upper Moscovian-Gzhelian. In Alaska, in the Brooks Range, the lower Tournaisian is represented by the Kayak Shale. Upper Tournaisian-Visean is composed of the limestones of the Lisburne Group. The lower part of the Lisburne Group contains upper Tournaisian brachiopods. In the eastern part of the Brooks Range, the Alapah Limestone with a Visean fauna overlies the eroded surface of the upper Tournaisian limestone. Argillaceous rocks containing the goniatites Beyrichoceras micronotum (Phill) and Goniatites crenistria Phill are in the middle part of the Alapah. These fossils correlate with the B2-P1c zones of England (upper part of Yasnaya-Polyana and the lower part of the Okhkiye deposits of the Russian platform). The top of the Carboniferous is represented by the Wahoo Formation, which cons sts of limestone and dolomite with an Upper Carboniferous (Pennsylvanian) fauna. In east-central Alaska and in the British Mountains of the northern Yukon Territory, the Carboniferous and lower part of the Upper Carboniferous are represented by the bituminous shale and limestone of the Calico Bluff Formation. In the middle of that formation there are species of Pseudoendothyra that closely resemble those of the Serpukhov beds of the Russian platform. In the upper part, lower Baskhirian species of Pseudostaffela are found. The overlying Lower Limestone unit includes Moscovian fusulinids. The similarity of the carbonate deposits of the Brooks and Verkoyansk Ranges, and massifs of northeast Asia, and the correlation of argillaceous deposits of the Calico Bluff Formation with the argillaceous rocks outside the middle massifs of the northeast Asia are established. End_of_Article - Last_Page 2470------------