Norton Basin Research Articles

Thermal History and Hydrocarbon Maturity Study in the Norton Basin, Alaska

Two COST wells in the Norton Basin of Alaska were examined using a one-dimensional quantitative dynamic model (1-D). By inversion of vitrinite reflectance data with the present day thermal gradient in the basin, the paleothermal history of the basin was reconstructed and showed two high thermal peaks, one during the initial stage of basin development (60–50 MaBP) and the other at late Miocene (15–5 MaBP). The early thermal peak corresponded to thinning of the lithosphere and subsidence with upwelling of the hot asthenosphere, and the more recent thermal high was caused by the subsidence and volcanic activity during late Miocene. The Norton Basin is thermally mature with hydrocarbon generation occurring in 15–2 MaBP. Modeling the fluid flow and geopressure development using a two-dimensional fluid flow/compaction model in the basin shows that the directions of fluid movement are both vertical and towards the Yukon Horst, a main horst structure in the basin, suggesting that the hydrocarbons might accumulate in structures around the Yukon Horst and in stratigraphic traps within the basin, which are expected to be common because of the depositional variation of the various facies.

Oil and Gas Developments in Alaska in 1983

Twelve exploratory wells were drilled in Alaskan waters in 1983. No discoveries were announced discoveries as all the wells were suspended or plugged and abandoned. Main industry interest continued to be in the North Slope and Beaufort Sea areas of the state. The most costly wildcat well in North American history was drilled unsuccessfully in the Beaufort Sea. OCS lease sales in the Norton basin and St. George basin of the Bering Sea dominated leasing activity.

Geology of Norton Basin and Continental Shelf Beneath Northwestern Bering Sea, Alaska

The rocks that floor the Norton basin and the northwestern Bering Sea are most likely of Precambrian and Paleozoic age, like those rocks that crop out around the basin. A maximum of 6.5 km of mainly Cenozoic strata lie over basement in the basin. On the basis of the geometry of reflections in seismic data, we believe alluvial fans to be present deep in the basin and to border major basement fault blocks. These fans are the lowest units of the basin fill in many areas and consist of uppermost Cretaceous or lower Paleogene, possibly coal- and volcanic-rich rocks. Mainly clastic nonmarine sedimentary rocks overlie the fan deposits. The Neogene and Quaternary basin rocks apparently were deposited in a marine environment. The basement of the northwestern Bering Sea, west of Norton basin, is 1 km below sea level for the most part, but local structural lows are filled with at most 2 km of strata of probable late Neogene and Quaternary age. The continental shelf in this area is separated structurally from the Seward Peninsula by a zone of small basement antiforms, over which relatively high gravity values have been measured. The Norton basin comprises two structural areas that are separated by a major northwest-striking horst. The first structural area lies west of this horst, where major normal faults strike northwest to form local areas where the basin is as deep as 5 km. The second area, east of the horst, includes major normal faults that strike east and northeast. The deepest part of the Norton basin (6.5 km) lies there. During the Late Jurassic and Early Cretaceous, basement rocks now beneath the Norton basin were affected by the orogeny that formed the Brooks Range. These basement rocks were metamorphosed and thrust, and then eroded deeply nearly 50 m.y. before the basin began to form. In the middle Late Cretaceous, the area of the eastern Seward Peninsula and eastern Norton Sound was subjected to east-west compression and consequent eastward thrusting. The strike-slip Kaltag fault probably formed at this time. If Norton basin formed as a pull-apart basin along this fault, then the basin may be as old as middle Late Cretaceous. However, during the latest Cretaceous and early Paleogene, the compression gave way to regional extension that formed northeast-trending grabens onshore, east of Norton basin We believe that the extensional Norton basin most likely formed in response to this regional extension. Initial deepening of the basin was controlled by major normal faults and occurred rapidly, whereas subsidence was slower and more regional in scale.

Geology of Norton Basin, Northern Bering Sea: ABSTRACT

The rocks that floor the Norton basin are most likely of Precambrian and Paleozoic age, like those that crop out around the basin. A maximum of 6.5 km of mainly Cenozoic rocks lies over basement in the basin. I believe that alluvial fans are present deep in the basin and border major basement fault blocks. These fans are the lowest units of the basin fill in many areas and probably consist of uppermost Cretaceous and Paleogene, coal- and volcanic-rich rocks. Mainly clastic nonmarine sedimentary rocks overlie the fan deposits. The Neogene and Quaternary basin rocks apparently were deposited in a marine environment. The Norton basin comprises two structural areas that are separated by a major northwest-striking horst. The first structural area lies west of this horst, where major normal faults strike northwest to form local areas where the basin is as deep as 5.0 km. The second area, east of the horst, includes major normal faults that strike east and northeast; the deepest part of the Norton basin (6.5 km) lies there. During the Late Jurassic and Early Cretaceous, basement rocks now beneath the Norton basin were affected by the orogeny that formed the Brooks Range. These basement rocks were metamorphosed and thrust, and then eroded deeply nearly 50 m.y. before the basin began to form. In the middle Late Cretaceous, the area of the eastern Seward Peninsula and eastern Norton Sound was subjected to east-west compression and consequent eastward thrusting. During the latest Cretaceous and early Paleogene, the compression gave way to regional extension that formed northeast-trending grabens onshore, east of Norton basin. I believe that the Norton basin most likely formed in response to this regional extension. Initial deepening of the basin was controlled by major normal faults and occurred rapidly, whe eas later subsidence was slower and more regional in scale. End_of_Article - Last_Page 569------------

Petroleum Geology of Norton Basin, Alaska

Basement rocks beneath the main part of the Norton basin were deformed and heated during the Late Jurassic and Early Cretaceous to the extent that these rocks were not capable of generating hydrocarbons when the basin formed during the latest Cretaceous or early Paleogene. Consequently, source rocks for oil, if they exist, are most likely to be within the basin fill. If the Norton basin began to form 65 m.y. ago, subsided at a nearly constant rate, and had an average geothermal gradient of between 35 and 45°C/km, then rocks as young as late Oligocene are in the oil window (vitrinite reflectance between 0.65 and 1.30%). The appearance on seismic sections of reflections from rocks in and below the calculated oil window suggests that these rocks were deposited in a nonm rine environment. Thus, gas and condensate are the most likely hydrocarbons to be present in the basin. Because of their shallow depth of burial, Neogene (possibly marine) rocks are not likely to be thermally mature anywhere in the basin. Deep parts of the basin formed as isolated fault-bounded lows; consequently, the volume of mature rocks makes up at most 11% of the total basin fill. Numerous potential traps for hydrocarbons exist in the Norton basin; the traps include fractured or weathered basement rocks in horsts, strata in alluvial fans on the flanks of horsts, and arched strata over horsts.

A Summary of Interacting, Surficial Geologic Processes and Potential Geologic Hazards in the Norton Basin, Northern Bering Sea

Future developers of the Bering Sea epicontinental shelf will have to cope with a variety of interacting or synergistic surficial geologic processes and potential hazards. These include moderate tectonism and resultant faulting, thermogenic gas seepage, seafloor gas cratering, sediment liquefaction, ice gouging, sour depression formation, storm-sand deposition, and large-scale bedform migration. Introduction Studies of potential geologic hazards on the Norton basin seafloor in the northern Bering Sea (Fig. 1) were conducted by the USGS to evaluate oil and gas lease tracts for Outer Continental Shelf (OCS) leasing. The data base for this evaluation included 9000 km of high-resolution geophysical tracklines (Fig. 2), 1,000 grab samples, 400 box cores, and 60 vibracores. In addition, hundreds of camera, hydrographic, and current meter stations have been occupied during the past decade by USGS, National Oceanic and Atmospheric Admin. (NOAA), and U. of Washington oceanographic vessels. The northern Bering Sea is a broad, shallow epicontinental shelf region covering 200 000 km of subarctic seafloor between northern Alaska and the U.S.S.R. (Fig. 3). The shelf can be divided into four general morphologic areas:the western part - an area of undulating, hummocky relief with glacial gravel and transgressive-marine sand substrate;the southeastern part - a relatively flat, featureless plain with fine-grained, transgressive-marine sand plain with fine-grained, transgressive-marine sand substrate;the northeastern part - a complex system of sand ridges and shoals with fine - to medium-grained sand substrates; andthe eastern part - a broad, flat marine reentrant (Norton Sound) part - a broad, flat marine reentrant (Norton Sound) with Holocene silt and intercalated very fine-grained storm-sand substrate underlain by gas-rich, peaty Pleistocene mud. A detailed discussion of Pleistocene mud. A detailed discussion of bathymetry and geomorphology of the northern Bering Sea is given by Hopkins et al. The northern Bering Sea is affected by a number of dynamic factors: winter sea ice, high winds, storm waves, and strong currents (geostrophic, tidal, and storm). The sea is covered by pack ice from November through May. A narrow zone of shorefast ice (sea ice attached to the shore) develops around the margin of the sea during winter months; around the front of the Yukon River delta, shorefast ice extends 50 km offshore. During the open-water season, the sea is subject to occasional strong northerly winds. In the fall, strong south-southwesterly winds cause high waves and storm surges along the entire west Alaskan coast. Throughout the year a continual northward flow of water is present with currents intensifying on the east side of strait areas. Although diurnal tides are very minor (less than 0.5 m), intense tidal currents are found in shoreline areas and within central Norton Sound. Potential geologic hazards discussed in our report include faulting, thermogenic gas seepage, biogenic gas-charged sediment and cratering, sediment liquefaction, ice gouging, current scouring, storm-sand migration, and mobile bedform dynamics. These geologic hazards may pose problems for the future development of Norton basin resources.

Origin of Gasoline-Range Hydrocarbons and Their Migration by Solution in Carbon Dioxide in Norton Basin, Alaska

Carbon dioxide from a submarine seep in Norton Sound, Alaska, carries a minor component of gas- and gasoline-range hydrocarbons. The molecular and isotopic compositions of the hydrocarbon gases and the presence of gasoline-range hydrocarbons indicate that these molecules are derived from thermal alteration of marine and/or nonmarine organic matter buried within Norton basin. In the gasoline-range hydrocarbons, individual cyclic and branched-chain molecules are much more abundant than straight-chain hydrocarbons. This distribution suggests that the hydrocarbon mixture is an immature, petroleumlike condensate of lower temperature origin than normal crude oil. The submarine seep provides a natural example in support of a carbon dioxide solution transport mechanism thought to be operative in the migration of hydrocarbons in certain reservoirs.

Origin of Gasoline-Range Hydrocarbons and Their Migration by Solution in Carbon Dioxide in Norton Basin, Alaska: ERRATUM

Origin of Gasoline-Range Hydrocarbons and Their Migration by Solution in Carbon Dioxide in Norton Basin, Alaska: ERRATUM

Resource Potential and Plate-Margin Geology of Frontier Basins of North Pacific and Bering Sea: ABSTRACT

Few of Alaska's offshore frontier basins have been explored by drilling. However, regional geologic data and tectonic considerations can be used to assess the likelihood that commercial volumes of oil and gas are present in the basins. Basins of the northern margin of the Gulf of Alaska, and the contiguous Aleutian Ridge on the west, have formed along the Aleutian subduction zone, a tectonic terrane 3,600 km long that separates the Pacific and North American plates. The eastern gulf shelf is underlain by Cenozoic deposits that are as much as 10 km thick, but adequate reservoir beds are thought to be absent in Neogene and younger beds. However, the discovery that potential reservoir and source beds of early Tertiary age underlie the continental slope enhances the oil and gas prospects of the eastern gulf margin. Basins of the central gulf shelf (Kodiak Island area) contain upper Cenozoic beds that are as much as 5 to 6 km thick. These beds are broadly deformed and unconformably overlie more deformed rocks of Paleogen and Cretaceous age. Grabens (unusual for the gulf margin) filled with 6 to 8 km of Neogene and younger beds are present beneath the western gulf shelf (Sanak Island). Publicly available data imply that reservoir and End_Page 1279------------------------------ source beds adequate to form large hydrocarbon deposits are probably absent from the central and western gulf. Farther west the lower Tertiary igneous core of the Aleutian Ridge is overlain by broadly deformed Neogene and younger deposits. These beds are 2 to 4 km thick in summit basins, and probably much thicker below the Aleutian terrace along the ridge's southern flank. Although these basins include diatomaceous and turbidite sequences, the probable abundance of readily altered volcanic detritus cautions against optimistic expectations of large quantities of oil and gas along the Aleutian Ridge. Five extensive (25,000 sq km) basins filled with as much as 15 km of mostly Cenozoic beds are present beneath the Beringian shelf, and, therefore, north of the Aleutian subduction zone. Except near Siberia, the deposits in these basins are little deformed. Elongate St. George and Navarin basins, along the southern or outer edge of the shelf, have formed on a collapsed foldbelt of miogeoclinal rocks that include beds of Jurassic and Cretaceous age. Subsidence of the foldbelt occurred after subduction of oceanic crust ceased beneath the Beringian margin (60 to 70 m.y. ago) and shifted south to the Aleutian Ridge. In contrast, Norton basin, which underlies the inner or northern edge of the shelf, is floored by Paleozoic and older rocks of Brooks Range affinity that subsided in response t Cenozoic strike-slip faulting in western Alaska. A speculative reading of the geologic history of the Beringian basins implies that some of them could harbor commercial volumes of oil and gas. South of the Beringian margin, the abyssal floor (3 to 4 km) of the Bering Sea basin is underlain by 4 to 10 km of undeformed deposits chiefly of Cenozoic age. Drilling results, and the detection of deep-water bright spots (VAMPs), suggest that hydrocarbon deposits (of unknown volume) occur in the basin. Its basement of Lower (?) Cretaceous oceanic crust was presumably separated from the north Pacific by the formation of the Aleutian Ridge in latest Cretaceous or earliest Tertiary time. Since early Mesozoic time, the evolution of the structural framework of the north Pacific margin has been controlled by the subduction of more than 10,000 km of oceanic lithosphere. However, recognition that segments of the margin are underlain by deeply submerged miogeoclinal rocks of Mesozoic and early Tertiary age, and the results of DSDP drilling at Pacific margins, attest that the evolution of Alaskan and Bering Sea margins is not adequately described by models of accretionary tectonics or back-arc spreading. Little understood aspects of subduction and post-subduction tectonics that cause and control marginal uplift and subduction are thought to hold important clues to the economic potential of the frontier basins of the north Pacific and Bering Sea regions. End_of_Article - Last_Page 1280------------

Geologic Setting and Source Depth of the Norton Basin Gas Seep

The Norton basin gas seep was discovered in 1976. Seismic reflection records across the seep zone show two types of acoustic anomalies indicative of gas-charged sediment: reflector pull-downs and abrupt reflector terminations. The seeping gas apparently originates from a secondary hydrocarbon accumulation trapped at the basin margin by a combination of structural and stratigraphic features. Introduction A field program for measuring concentrations of low-molecular-weight hydrocarbons in the waters of the southern Chukchi Sea and the northern Bering Sea (Norton Sound) was conducted Sept 1976 by the Pacific Meteorological and Environmental Pacific Meteorological and Environmental Laboratory (NOAA). During this survey, a plume of hydrocarbon-rich water was discovered in Norton Sound. Fig. 1 shows the isopleths of ethane concentration in the near-bottom waters. The elevated hydrocarbon levels in the plume could be detected for 150 km downcurrent from what apears to be a source or sources on the seafloor about 45 km south of Nome, AK. Preliminary dynamic modeling estimates of the initial gas-phase composition predict methane/ethane and ethane/propane ratios of 24 and 1.7, respectively, assuming the hydrocarbons were introduced by bubbles from a submarine seepage of natural gas. The low ethane/propane ratio indicates gas from a petroleum source rather than from nonassociated or biogenic natural gas. A detailed discussion of the geochemistry of the seep gases is given by Cline and Holmes and Kvenvolden et al.; Nelson et al. discuss bottom sediment geochemistry and possible seafloor locations of the seep. Unsubstantiated reports indicate the existence of natural hydrocarbon seeps near the mouth of the Inglutalik River (Fig. 1) in the northeastern part of Norton Bay, and in the Sinuk River Valley (Fig. 1) about 35 km northwest of Nome. The source rocks for the Inglutalik and Sinuk seeps have not yet been identified definitely. A mantle of Quaternary deposits rests on Cretaceous rocks of the Yukon-Koyukuk geosyncline at the Inglutalik River site. Hopkins (in Miller et al.) reports that much of the trench formed by the Sinuk and Stewart river valleys may be underlain by infolded, unmetamorphosed Cretaceous or Tertiary sediments. Miller et al. also mention a reported occurrence of oil shale on Besboro Island northwest of Unalakleet (Fig. 1). Limited prospecting for oil was conducted at Hastings Creek near Cape Nome (Fig. 1) in 1906 and 1918. Two wells were drilled during the 1906 operation; one showed a trace of oil and the other encountered flammable gas at a depth of 37 m. The gas was at sufficient pressure to blow a 550-kg stem 23 m up the hole. The two holes drilled in 1918 reached depths of 45 and 65 m without encountering any hydrocarbon shows; another attempt in 1919 reached 107 m with similar negative results. The impetus for these drilling episodes was the repeated sighting of oil-like films on the lagoons near Nome and Cape Nome, and a beach, foam thought to be paraffin, brought in by the onshore winds. paraffin, brought in by the onshore winds.

Sonobuoy Refraction Measurements from Norton Basin, Northern Bering Sea: ABSTRACT

Recent discovery of thermogenic gaseous hydrocarbons seeping from the seafloor 45 km south of Nome, Alaska, suggests that the underlying Norton basin may be an important future petroleum province. The results of 38 sonobuoy refraction profiles obtained in 1977 and 1978 show that Norton Sound and Chirikov basin are underlain by a single sedimentary trough approximately 130 km wide and 350 km long; the basin axis trends west-northwest and extends from Stuart Island to a point 100 km west-southwest of King Island. Although average depth to basement is only 2.5 km, two deeper areas, containing up to 5.5 km of sedimentary section, were discovered 75 to 90 km northwest of the Yukon River delta. Norton basin is floored by an acoustic basement whose compressional velocity is 5.5 to 6.5 km/sec. The basin fill consists of three major units distinguishable on the basis of their compressional velocities; unconformities probably separate each of these units. The basal unit, with a velocity of 4.9 km/sec, is present only in the deeper parts of the basin. A thick (2 to 3 km) section has velocities ranging from 2.3 to 3.7 km/sec and lies on this lower unit and on acoustic basement. Compressional velocities in the 1.2 km-thick upper unit range from 1.6 to 2.1 km/sec. The lower two units are probably Cretaceous and lower to middle Tertiary marine and nonmarine rocks lying on a basement complex of Paleozoic and Mesozoic igneous, metamorphic, and sedimentary rocks similar to those mapped n Seward Peninsula and St. Lawrence Island. The upper unit probably consists of upper Tertiary and Quaternary sedimentary rocks and sediment. End_of_Article - Last_Page 468------------

Review of Petroleum Geology of Anadyr and Khatyrka Basins, USSR

Two sedimentary basins bordering the Bering Sea in northeastern Siberia, the Anadyr and the Khatyrka, have been the targets of recent Soviet exploration for oil and gas. Drilling in the Anadyr basin has produced noncommercial quantities of gas and condensate from Tertiary volcaniclastic rocks. The Anadyr basin lies due west of Norton basin, a large unexplored sediment-filled Tertiary basin that underlies Norton Sound. Geologically, Anadyr basin lies between a late Mesozoic belt of volcanic and plutonic rocks similar to the Sierra Nevada of California and a complex foldbelt on the south that is structurally and lithologically similar to Franciscan terrane of the California Coast Ranges. Less extensive exploration in the Khatyrka basin has discovered only noncommercial quantities of oil and gas. The Khatyrka basin extends offshore between Capes Rubicon and Navarin near the northwest end of the Navarin basin on the Bering Shelf, and contains Tertiary sediment deposited within a structurally complex Mesozoic foldbelt that parallels the edge of the continental shelf.

Submarine Seepage of Natural Gas in Norton Sound, Alaska

Unusual concentrations of dissolved two- to four-carbon alkanes were observed in the waters in Norton Sound in a localized area approximately 40 kilometers south of Nome, Alaska, in 1976. The hydrocarbons were identified in the near-bottom waters downcurrent for more than 100 kilometers from a sea-floor point source. Preliminary dynamic modeling estimates of the initial gas phase composition predict methanelethane and ethanelpropane ratios of 24 and 1.7, respectively, assuming the hydrocarbons were introduced by bubbles. The low ethanelpropane ratio is indicative of gas from a liquid petroleum source rather than from nonassociated or biogenic natural gas. Preliminary data on the structural geology of Norton Basin lend support to the interpretation based on the hydrocarbon plume. Unconformably truncated strata dip basinward from the seep locus; acoustic anomalies and numerous steeply dipping faults in the immediate vicinity of the seep are corroborating evidence that shallow gas- or petroleum-charged sediments and strata coincide with avenues for migration of mobile hydrocarbons to the sea floor. These factors, taken in concert with the sedimentological regime, the recent revision (increase) of basin depth estimates, and the highly localized hydrocarbon source, strongly suggest a thermogenic rather than a recent biogenic origin for these gaseous compounds.

Detailed Geophysical Study of Northwest Norton Basin, Bering Sea Shelf, Alaska: ABSTRACT

Detailed Geophysical Study of Northwest Norton Basin, Bering Sea Shelf, Alaska: ABSTRACT

Newly Discovered Cenozoic Basins, Bering Sea Shelf, Alaska

Recent geophysical studies over the Bering shelf have outlined the basic structure of three previously unknown Cenozoic basins, Norton basin and Pribilof and Zhemchug depressions. The upper part of the crust beneath the Bering shelf consists of a stratified sequence of generally undeformed Cenozoic deposits overlying a basement of Mesozoic sedimentary, volcanic, and plutonic rocks, and older Paleozoic sedimentary and metamorphic units. The average thickness of the Cenozoic deposits is approximately 1.0 km. However, they are more than twice that thick where the basement has been downfolded or downfaulted to form the major intrashelf basins (such as Bristol, Anadyr, and Norton basins) that typically underlie major bays and gulfs, and the narrower, fault-controlled outer shelf basins (such as Pribilof and Zhemchug depressions) that trend parallel with the continental margin. Norton basin underlies the northern part of the Bering shelf north of St. Lawrence Island. It is an east-west trough filled with terrestrial and neritic deposits of Paleogene age and younger, as much as 2.0 km thick. Minor folds and offsets in this fill are present along the flanks of the basin. Pribilof and Zhemchug depressions underlie the outer shelf and the northeast-trending canyon heads of Pribilof and Zhemchug Submarine Canyons, respectively. These depressions in the basement rocks are grabens and/or half grabens, and are filled with stratified neritic deposits of Paleogene age and younger, as much as 2.0 km thick. The Cenozoic fill is deformed above faults that offset the basement and the sea floor near the Pribilof Islands.

AN AEROMAGNETIC PROFILE FROM ANCHORAGE TO NOME, ALASKA

A total‐intensity profile was obtained on a 500‐mile flight by a U. S. Geological Survey airplane from Anchorage to Nome, Alaska, on May 4, 1954. The average flight altitude was 6,000 ft above sea level except over the Alaska Range where the flight altitude was 9,000 ft. This profile crossed eight of the major tectonic elements of Alaska at right angles to their trend and gives valuable regional information in an area where other geophysical and geological information is scarce or lacking. The profile has a net gradient downward to the northwest, most of which is ascribed to the component of the earth’s main magnetic field along the flight traverse. The great variety of magnetic anomalies which are superimposed on this gradient originate from variations in lithology along the traverse. All the magnetic anomalies, except a large one over the Yukon River, are caused by magnetic rocks at or near the surface. The magnetic profile may be divided into four major segments and nine subsegments, each having a characteristic magnetic pattern. Most of these can be related to a tectonic unit. The large plutons of the Talkeetna geanticline are clearly defined by a group of anomalies having the highest amplitudes of any on the profile. The Matanuska geosyncline to the east is represented by a 25‐mile section of sloping profile consistent with a thick sedimentary section but indicating that the geosyncline is comparatively narrow near Anchorage. The 200‐mile central magnetic segment is relatively free from all but very minor anomalies. This segment includes the Alaska Range geosyncline, the Tanana geanticline, and the Kuskokwim geosyncline; showing only slight magnetic contrasts between each of these elements. The two geosynclines either have thick Mesozoic sedimentary sections or have underlying crystalline rocks which are low in magnetic susceptibility at shallow depths. The rocks of the geanticline have a low but not negligible magnetic susceptibility and are predominantly Paleozoic sedimentary rocks. A single 300‐gamma anomaly on the west edge of the central segment is caused by a small, mafic intrusive body in the Paleozoic metamorphic rocks of Mt. Hurst. West of this anomaly the profile consists of a series of small sharp anomalies which are probably caused by Paleozoic metavolcanic rocks of the Ruby geanticline. The second largest anomaly on the profile is in the Koyukuk geosyncline over the Yukon River. The source is calculated to be more than a mile deep and may be an intrusive body at least 15 miles wide. This anomaly is flanked by 20‐mile sections of flat or sloping profile which indicate areas of thick sedimentary rocks, particularly in the region west of the Yukon River. The 150‐mile Norton Sound magnetic segment on the western end of the profile consists of many closely spaced anomalies produced by rocks which are either volcanic or similar to the Seward complex. Of the four Cenozoic basins or lowlands crossed by the profile, three are underlain by rocks of moderate to high magnetic susceptibility at shallow depths. These are the Cook Inlet basin, part of which overlaps rocks of the Talkeetna geanticline, the Innoko basin of central Alaska which overlies the rocks of the Ruby geanticline, and the Norton basin, in which sedimentary deposits are thin or absent. The fourth, the Minchumina basin, is underlain by the low‐susceptibility rocks at the Tanana geanticline, which are also probably close to the surface.