Arctic Margin Research Articles

Out-of-sequence, basement-involved structures in the Sadlerochit Mountains region of the Arctic National Wildlife Refuge, Alaska: Evidence and implications from fission-track thermochronology

Fission-track thermochronology and structural analysis set limits on the timing and nature of structural development of the Sadlerochit Mountains, along the southern edge of the coastal plain in the Arctic National Wildlife Refuge (ANWR) of northeastern Alaska. The Sadlerochit Mountains are the northernmost part of the north-vergent Brooks Range fold-and-thrust belt and lie close to the Arctic continental margin. Thermochronology results indicate that sedimentary rocks exposed within Ignek Valley, south of the Sadlerochit Mountains, were subjected to two episodes of rapid cooling from elevated paleotemperatures at ca. 45 Ma and at some time since ca. 31 Ma, whereas similar-aged rocks exposed along the northern flank of the Sadlerochit Mountains cooled rapidly at ca. 45 Ma and ca. 27 Ma. Combined with five additional analyses from the Beli Unit #1 well, located northwest of the Sadlerochit Mountains, the thermochronology results indicate that the Sadlerochit Mountains region was progressively heated during Late Cretaceous through middle Eocene time, after which two major episodes of rapid cooling occurred in the middle Eocene at ca. 45 ± 3 Ma (±2σ) and in the late Oligocene at ca. 27 ± 2 Ma (±2σ). These episodes of rapid cooling are interpreted to have occurred in response to kilometer-scale erosional denudation resulting from rapid (≤5 m.y.) uplift due to structural thickening during the emplacement of thrust sheets in a basement-involved duplex. Initially, at least one thrust sheet was probably emplaced to the north of the Sadlerochit Mountains at ca. 45 Ma. Subsequently, at ca. 27 Ma, (1) the Sadlerochit Mountains thrust sheet was probably emplaced out of sequence behind the earlier-emplaced thrust sheet(s), and (2) basement-involved deformation formed structures beneath the coastal plain to the north. Both of these events occurred far within the continent, >1200 km from the southern Alaska convergent plate boundary. These results indicate that maximum burial, and hence peak hydrocarbon generation, occurred prior to middle Eocene time, before the formation of potential traps in and immediately north of the Sadlerochit Mountains. However, (1) hydrocarbons generated from these rocks could have migrated updip into existing stratigraphic traps prior to structural deformation, and (2) hydrocarbons generated later in more distal parts of the basin could have migrated updip into subsurface structures formed in middle Eocene and late Oligocene time north of the Sadlerochit Mountains and along strike to the east and west.

Late Pleistocene and Holocene meltwater events in the western Arctic Ocean

Accelerator mass spectrometer 14 C dated stable isotope data from Neogloboquadrina pachyerma in cores raised from the Mendeleyev Ridge and slope provide evidence for significant influx of meltwater to the western Arctic Ocean during the early part of marine oxygen isotope stage 1 (OIS 1) and during several intervals within OIS 3. The strongest OIS 3 meltwater event occurred before ca. 45 ka (conventional radiocarbon age) and was probably related to the deglaciation at the beginning of OIS 3. Major meltwater input to the western Arctic Ocean during the last deglaciation coincides closely with the maximum rate of global sea-level rise as determined from the Barbados sea-level record, demonstrating a strong link between the global record and changes in the central Arctic Ocean. OIS 2, which includes the last glacial maximum, is very condensed or absent in the cores. Abundance and δ 13 C values for N. pachyderma in the middle part of OIS 3 are similar to modern values, indicating high productivity and seasonal ice-free areas along the Arctic margin at that time. These records indicate that the Arctic Ocean was a source of heat and moisture to the northern polar atmosphere during parts of OIS 3.

Detrital zircon geochronology of the Adams Argillite and Nation River Formation, East-Central Alaska, U.S.A.

ABSTRACT The Cambrian Adams Argillite and the Devonian Nation River Formation are two sandstone-bearing units within a remarkably complete Paleozoic stratigraphic section in east-central Alaska. These strata, now foreshortened and fault-bounded, were originally contiguous with miogeoclinal strata to the east that formed as a passive-margin sequence along the northwestern margin of the North American continent. Seventy-five detrital zircon grains from the Adams Argillite and the Nation River Formation were analyzed in an effort to provide constraints on the original sources of the grains, and to generate a detrital zircon reference for miogeoclinal strata in the northern Cordillera. Thirty-five single zircon grains from a quartzite in the Adams Argillite yield dominant age clusters of 1047-1094 (n = 6), 1801-1868 (n = 10), and 2564-2687 (n = 5) Ma. Forty zircons extracted from a sandstone in the Nation River Formation yield clusters primarily of 424-434 (n = 6), 1815-1838 (n = 6), 1874-1921 (n = 7), and 2653-2771 (n = 4) Ma. The Early Proterozoic and Archean grains in both units probably originated in basement rocks in a broad region of the Canadian Shield. In contrast, the original igneous sources for mid-Proterozoic grains in the Adams Argillite and 430 Ma grains in the Nation River Formation are more difficult to identify. Possible original sources for the mid-Proterozoic grains include: (1) the Grenville Province of eastern Laurentia, (2) the Pearya terrane along the Arctic margin, and (3) mid-Proterozoic igneous rocks that may have been widespread along or outboard of the Cordilleran margin. The 430 Ma grains may have originated in: (1) arc-type sources along the Cordilleran margin, (2) the Caledonian orogen, or (3) a landmass, such as Pearya, Siberia, or crustal fragments now in northern Asia, that resided outboard of the Innuitian orogen during mid-Paleozoic time.

Arctic margin gravity highs remain puzzling

Many passive continental margins, particularly in the Arctic (Figures 1 and 2), are festooned by chains of elongate positive gravity anomalies overlying, in part, marginal oceanic crust. To this day, no simple unifying explanation has emerged to account for these highs. Variously modeled by mantle doming, crustal thinning, dense crust, basement rises, postbreakup sediment depocenters, and topographic effects, these anomalies epitomize nonuniqueness in geophysical inversion. It seems unlikely that all the highs share a common origin, but it seems even more unlikely that many disparate processes produce gravity anomalies so similar to each other.Upon consideration of several hypotheses, a phase change explanation seems the most plausible for Arctic margin gravity highs (AMGHs). The better studied highs are spatially correlated with sediment depocenters (Figure 2), which however by themselves account for only a fraction of the gravity anomalies (for example, Figure 3). Thus, a reason has to be found as to why dense or thinned oceanic crust (or dense mantle) is found underneath sediment loads. Essentially, the phase change explanation relies on the added lithostatic pressure under sediment piles to “densify” some of the buried oceanic crust, for example, by conversion to garnet granulite or even eclogite [Neugebauer and Spohn, 1981 ]. More well‐designed seismic experiments are needed, however, to delineate the velocity (and hence density) structure of the underlying crust to verify the depocentermascon association and test the phase change hypothesis and others discussed below.

Present heat flow and paleo-geothermal regime in the Canadian Arctic margin: analysis of industrial thermal data and coalification gradients

Calculations of the present geothermal gradient and terrestrial heat flow were made on 156 deep wells of the Canadian Arctic Archipelago. Corrected bottom hole temperature (BHT) data and drill stem test (DST) temperatures were used to determine the thermal gradients for sites for which the quality of data was sufficient. Thermal gradients evaluated for depths below the base of permafrost for the onshore wells and below sea bottom for the offshore wells were combined with the estimates of effective thermal conductivity to approximate heat flow for these sites. The present geothermal gradient is in the 15–50 mK/m range (mean = 31 ± 7 mK/m). Present heat flow is mainly in the 35–90 mW/m 2 range (mean = 53 ± 12 mW/m 2). Maps of the present geothermal gradient and present heat flow have been constructed for the basin. The analysis of vitrinite reflectance profiles and the calculation of logarithmic coalification gradients for 101 boreholes in the Sverdrup Basin showed large variations related in many cases to regional variations of present terrestrial heat flow. Paleo-geothermal gradients estimated from these data are mostly in the range of 15–50 mK/m (mean = 28 ± 9 mK/m) and paleo-heat flow is in the 40–90 mW/m 2 range (mean = 57 ± 18 mW/m 2) related to the time of maximum burial in the Early Tertiary. Mean values of the present heat flow and paleo-heat flow for the Sverdrup Basin are almost identical considering the uncertainties of the methods used (53 ± 12 versus 57 ± 18 mW/m 2, respectively). Present geothermal gradients and paleo-geothermal gradients are also close when means are compared (31 ± 7 versus 28 ± 9 mK/m respectively). A zone of high present heat flow and a paleo-heat flow zone coincide in places with the northeastern-southwestern incipient rift landward of the Arctic margin first described by Balkwill and Fox (1982). Correlation between present heat flow and paleo-heat flow for the time of maximum burial in the earliest Tertiary suggests that the high heat flow zone has prevailed since that time.

Arctic margin gravity highs: Deeper meaning for sediment depocenters?

Morphologic, gravity, and seismic reflection/refraction data from ca. 10,000 km of Arctic passive continental margins suggest that the numerous oval free-air gravity anomalies, their +50–150 mGal extrema typically located just landward of shelf breaks, are caused by combinations of rapidly deposited Plio-Pleistocene glacial marine sediment loads, older post-breakup sediments, and perhaps causally related density anomalies (mascons) in the underlying oceanic crust. Dispersed seismicity associated with some gravity highs may reflect ongoing brittle, flexural adjustment to the loads. Multi-channel-seismic-controlled depocenter models for several prominent highs (including the Hornsund gravity high re-examined here) suggest that sediments alone do not suffice to explain the gravity highs, unless depocenter seismic velocities have been significantly underestimated. A flexural backstripping model for the Hornsund anomaly only roughly replicates observed gravity. Subjacent 'mascons', if present below some depocenters, may be caused by (1) anomalous subsidence of initially formed dense/thin crust; (2) depocenter blanketing of early-formed crust, mitigating hydrothermal fracturing and related density reduction; or (3) metastable phase transitions, converting basalt/gabbro to denser phases (Neugebauer–Spohn hypothesis), while cracks close or fill under the increased pressures and temperatures.

Late-Weichselian ice sheets in Arctic and Pacific Siberia

Abstract Interpretation of Space images coupled with field data on glacial geomorphology and glacial geology of Northern Eurasia have provided compelling evidence for a continuous Late Weichselian ice sheet covering the entire Arctic margin of the continent. In addition to the Scandinavian ice sheet, three ice sheets of the same order — the Karan, East Siberian and Beringian — are identified within the major Eurasian ice sheet. Also, there is evidence and other considerations suggesting a former ice sheet in the Sea of Okhotsk. Ice-spreading centers of the ice sheets were situated on the continental shelves off the Siberian coasts, thus we can term the ice sheets ‘Siberian’. So far, the conventional stratigraphic approach to the problem of Siberian glaciation has proven fruitless and yielded nothing but uncertainty, while geomorphological studies have discovered a wealth of glacial landforms in Arctic Siberia. It was these landforms and their spacing that made the existence of former Siberian ice sheets evident. The East Siberian ice sheet rested on the shelves of the eastern Laptev and East Siberian Seas, and its flow was directed radially, including up the Yana and Indigirka Rivers. The Beringian ice sheet was grounded on the Chukchi-, Bering-, and Beaufort Sea shelves, overriding the Chukchi and Seward Peninsulas from the north and flowing through the Bering Strait. It was continued by a thick floating ice shelf buttressed by the submarine Aleutian-Commander Ridge. The Okhotsk ice sheet was grounded on the Okhotsk shelf. Specific gemorphic signatures of marine ice sheets — the ‘glacially cut corners’ — have been identified on all promontories jutting out into the Bering and Okhotsk Seas. The Beringian and Okhotsk ice sheets are consistent with the results of deep-sea drilling in the adjacent North Pacific during Leg 145 of D/V JOIDES Resolution.

Introduction to Special Section: The Trans‐Alaska Crustal Transect (TACT) Across Arctic Alaska

This special section of the Journal of Geophysical Research addresses the composition and structural evolution of the lithosphere in northern Alaska. Investigations reported in this section were mainly undertaken as part of the Trans‐Alaska Crustal Transect (TACT), an integrated geological and geophysical transect of the entire Alaskan lithosphere along a north‐south corridor undertaken from 1984 to 1992 (Figure 1). The onshore segment of the transect approximately follows along the route of the trans‐Alaskan pipeline; the offshore segment extends across the continental margin in the Gulf of Alaska to the Pacific plate. The TACT line is unique in that it provides a coordinated onshore/offshore geological and geophysical traverse of the North American plate in Alaska from the active convergent Pacific margin to the passive Arctic margin of the continent.

Canada Basin, Arctic Ocean: Evidence against a rotational origin

The most widely accepted model for the opening of Canada Basin invokes 66° of counterclockwise rotation of Arctic Alaska and Chukotka away from the Canadian Arctic in Early Cretaceous time. Late Paleozoic structural trends and paleogeography have been used in support of the rotation hypothesis. Recent refinements in the ages of Paleozoic tectonic events in Arctic Alaska, Yukon, and the Canadian Arctic Islands provide new controls on correlations of late Paleozoic paleogeography and raise doubts about whether the Paleozoic tectonics of the Arctic Alaska‐Yukon region necessitate a rotational reconstruction of Arctic Alaska against the Canadian Arctic Islands. A rotational restoration of Arctic Alaska requires the Alaskan and Canadian margins to be conjugates of comparable age and evolution. The rift‐drift transition age for the Alaskan margin is most likely Hauterivian (Early Cretaceous), but for the Canadian Arctic margin it is most likely post‐Albian (mid‐Cretaceous). Crustal structure data from the Beaufort Sea continental margin in Canada define a rifted margin segmented by fracture zones which constrain the kinematics of ocean spreading to be northwestward, perpendicular to that required by the rotation hypothesis but subparallel to that suggested by seismic velocity anisotropy in the upper mantle. The Alaska‐Chukotka rotation hypothesis also fails to account for up to 600 km of continental overlap upon restoration of 66° of rotation and the absence of any accommodating contractional structures in northern Yukon and adjacent Northwest Territories. Because the Alaska‐Chukotka rotation hypothesis fails to account for much of the available data, senous doubt is cast on its viability. An existing multistage tectonic model for the evolution of Canada and Makarov basins is summarized as an example of a model which can account for the existing data.

Palaeoceanography on the European arctic margin during the last deglaciation

Abstract To reveal the palaeoceanographic and palaeoclimatic evolution related to the disintegration of the Barents Sea and Fennoscandian ice sheets, high-resolution sediment cores from the continental margin off western Svalbard, western Barents Sea and northern Norway were investigated. The location of these cores is below the axis of the Norwegian Current and beyond, but close to, glaciated continental areas. Hence they should sensitively reflect the palaeoceanography of the northernmost Norwegian Sea. Between, 14.5–19.5 and 22.5–29 14 C ka, a high abundance of planktonic foraminifera and a small content of subpolar species indicate seasonally ice-free and slightly warmed surface water. This phenomenon is related to the advection of surface water of North Atlantic origin. The onset of the deglaciation is characterized by a marked low oxygen event dated to between 15 and 13 ka, interpreted to reflect a surface water freshening produced by meltwater and reduced oceanic mixing. After the onset of deglaciation, the surface ocean warmed in three rapid steps: (1) around 12.5 ka, indicated by increased abundances of planktonic foraminifera; (2) at 10.2 ka and at (3) 10–9.6 ka. The two latter steps are reflected by the increased abundance of subpolar planktonic foraminifera.

Caledonian and precambrian history in Svalbard: a review, and an implication of escape tectonics

This article contains two parts, a review of geochronological data of the Svalbard basement, and a hypothetical application of the concept of escape tectonics on the Caledonian-Franklinian mobile belts of the Arctic. All Caledonian and older isotopic ages obtained from Svalbard are compiled and synthesized into a sequence of events from Proterozoic to middle Paleozoic times. The areal distribution of the rocks of various ages are considered in conjunction with the present understanding of the lithostratigraphy of the archipelago. Caledonian tectonothermal events in the Middle Silurian and Middle Ordovician are well defined by isotopic age determinations and stratigraphie evidence. An oceanic suture zone represents the early Caledonian subduction event. The Late Proterozoic-Early Ordovician low-grade metasediments occupy ca. 60% of the pre-Devonian basement of Svalbard. Early Paleozoic and latest Proterozoic isotopic age groups have no clear geological correlatives, however, they may reflect regional geological events recognized in other parts of Arctic margins. The Grenvillian and Svecofennian events have been well documented by the isotope systems and these data have made it possible to distinguish Late, Middle and Early Proterozoic rock units in Svalbard. The Middle and Early Proterozoic rocks occupy ca. 40% of the whole basement, and most of the crystalline basement has been formed essentially during these two tectonothermal events. These basement rocks are separated into three orogen-parallel structural units by steeply dipping faults in Svalbard. In the second part of this article, these faults and other orogen-parallel faults in the western part of the Arctic Caledonian-Franklinian mobile belts are summarized and considered in the context of “escape tectonics”.

Microfaunal Evidence for Elevated Pliocene Temperatures in the Arctic Ocean

The migration of thermophilic marine Ostracoda into the Arctic Ocean during the Pliocene indicates that winter and summer ocean temperatures around Arctic margins were ≥ 0 °C and > 3 °C, respectively, and that ice‐free conditions existed for most or all of the Arctic. By at least 3.5–3.0 Ma, probably earlier, the opening of the Bering Strait allowed marine organisms to migrate through the Arctic Ocean, mostly from the Pacific Ocean. Migrant taxa such as Cythere, Hemicythere, and Neomonoceratina are known from Pliocene deposits of Alaska and Canada as well as Neogene deposits of the North Pacific and Atlantic oceans. On the basis of ecological and Zoogeographic information on ostracode species from more than 800 modern “core top” samples for the North Atlantic, North Pacific, and Arctic Oceans, we determined winter and summer temperature tolerances for migrant taxa to be at or above about 0 °C and 3 °C. This suggests ice‐free summers, and probably, a perennially ice‐free Arctic Ocean in some regions. Elevated water temperatures in the Arctic Ocean between 3.5 and 2.0 Ma is supported by evidence for late Pliocene increased meridional heat transport in the North Atlantic Ocean.

Glaciomarine environments in Canada: an overview

The present understanding of Canada's glaciomarine environments owes much to the remarkable role played by the scientists of the Geological Survey of Canada. Their efforts have led to the review and partial revision of three scientific paradigms: (1) There is a mechanical rather than a climatic control of the collapse of a tidewater ice sheet; (2) ice sheets were mostly grounded on Canada's continental shelves (rather than with floating ice shelves); (3) ice-loaded glaciomarine sediments are sometimes indistinguishable from deposits of till. A proposed stratigraphic framework for Canadian glaciogenic sequences can be quantified, allowing insights into ice sheet dynamics. For instance, the arctic margin of the Wisconsinan ice complex appears to have generated comparatively little meltwater, ice margin retreat being principally by iceberg calving. Surprisingly, the Atlantic margin of the Wisconsinan ice complex appears to have transported larger quantities than its Pacific counterpart. This is contrary to the present postglacial sediment yields discharged onto each margin. Glaciogenic sedimentation rates are shown to vary with the distance from a sediment source and the delivery rate of sediment. Glaciogenic accumulation rates are dependent on basin history and basin shape. Numerical examples include (1) the determination of accumulation rates from carbon stratigraphy; (2) the evaluation of the flux of sediment from a fjord to the open shelf during the retreat phase of an ice sheet; and (3) the application of a basin fill model to predict the styles of sedimentation within a fjord.

Meltwater along the Arctic margin of the Laurentide Ice Sheet (8-12 ka) : Stable isotopic evidence and implications for past salinity anomalies

We report a new time series of stable isotopic values for the past 12 ka measured in two species of shallow-water marine bivalves collected from raised marine deposits along the coast of Arctic Canada. Each sample ( n = 60) had been radiocarbon dated previously. The results indicate that between 11.8 and 10 ka the δ 18 O of the shells ranged between 0.25???? and -6????, but values increased after 9 ka as the flux of meltwater to the Arctic Ocean decreased and as the Canadian Arctic channels opened to allow the present surface circulation to become established. Prior to about 9 ka, glacial meltwaters from the northern margin of the Laurentide Ice Sheet must have been entrained in the surface waters of the Arctic Ocean and directed southward to the North Atlantic through Fram Strait. This source of meltwater must be included in late Quaternary models of forcing of the North Atlantic thermohaline circulation. Glacial isostatic uplift of the sills within the Canadian Arctic channels implies that outflow through Fram Strait increased during the Holocene.

Late Triassic succession of North American halobiid bivalves and their circum-Pacific correlation

The thin-shelled pteriid bivalve Halobia is widely distributed in Upper Triassic (Carnian-mid Norian) marine strata throughout the circum-Pacific region with significant occurrences along the western and arctic margin of North America, northeast Russia, China, Japan, Indochina, islands of the western Pacific, and New Zealand. Halobiids are commonly associated with deep water, black shale facies typically devoid of macrobenthos. This facies occurrence and a thin-shelled, flat-valved morphology suggests an autecology (possibly pseudoplanktic) in contrast to modern benthic pteriids. High speciation and extinction rates of halobiids not only provide an excellent means for dating rocks lacking ammonoids, but they give an independent view of rapidly evolving lineages during Carnian-Norian time.A composite halobiid succession with nearly complete coverage from the earliest Carnian through the late mid Norian is constructed from numerous North American sequences and localities with ammonoid control. Although differences in nomenclature rather then endemism make correlation of the North American succession with other circum-Pacific regions difficult, similar (and quite possibly homologous) morphologies allow limited correlation between these regions. The early Carnian is typified by H. zitteli, which occurs in predominantly boreal strata of Canada and Siberia. By later Carnian time, a variety of similar morphotypes (probably conspecific) belonging to subspecies of H. superba, H. radiata, and several other species are known throughout the circum-Pacific. The earliest Norian Kerri ammonoid zone is recognized by the first appearance of H. beyrichi in North America, and H. styriaca in Asia. In North America and the rest of the circum-Pacific, the latter part of the early Norian is marked by a rapid diversification of forms, including H. cordillerana and H. fallax. By late mid Norian time diversity reached a maximum 5 species. The mid Norian species H. fallax, H. halorica, and H. plicosa are particularly wide-spread; they occur throughout the circum-Pacific where they coexisted with the earliest Monotis. As in the Alpine-Mediterranean region, the remaining halobiids went extinct by the end of the mid Norian.Halobiid speciation and extinction rates exceed other Late Triassic macrofauna except ammonoids. The average North American species duration is fewer than three ammonoid zones (ca. 2.5 MY). The late early Norian is a period of pronounced diversification, whereas the middle early Norian and late middle Norian are times of accelerated extinctions. Instead of catastrophic extinction at the Carnian-Norian boundary, North American halobiids exhibit complex taxonomic turnover. Similar diversity patterns are recognizable in other circum-Pacific halobiid sequences.

Siliceous sponge communities, biological zonation, and Recent sea-level change on the Arctic margin: Ice Island results: Discussion

Siliceous sponge communities, biological zonation, and Recent sea-level change on the Arctic margin: Ice Island results: Discussion

Siliceous sponge communities, biological zonation, and Recent sea-level change on the Arctic margin: Ice Island results: Reply

Bathymetric profiles, bottom-camera photographs, surface-sediment samples, and biological samples were obtained through a 44 m long hydrohole in the Canadian Ice Island as it drifted over the Canadian polar margin from western Ellesmere Island to Meighen Island. Continuous bathymetric profiles show a 300 km wide shelf with a narrow, dissected, shallow (< 200 m) inner shelf and a deep (300–800 m), sloping outer shelf. Extensive areas of siliceous sponge communities are present on the seafloor beneath the polar pack ice on the central shelf. A clear biological zonation is evident: above 130 m, large living sponges support a diverse benthic community and form reef mounds up to 10 m high; from 130 to 150 m, there are few living sponges, but extensive mats of sponge spicules are present; from 150 to 300 m, there are few, small living sponges, and radiocarbon dating of mud-covered spicule mats indicates that formerly extensive sponge communities have died during the past 500–2000 years. This zonation may reflec...

Tectonic and Geodynamic Setting of Oil and Gas Basins of the Soviet Union (1)

Within the territory of the Soviet Union and its offshore economic zone are about 70 sedimentary basins containing oil and gas. The basins include almost all basin types described in present-day plate-tectonic classifications, namely (1) intracontinental and pericontinental rifts, suprarift syneclises, and zones of pericratonic downwarps; (2) ancient passive margins of continents with adjacent overthrust fold system; (3) modern passive margins of continents; (4) zones of convergence of lithospheric plates (i.e., zones of subduction of oceanic plates below continental plates); and (5) zones of collision of continental lithospheric plates. Basins of the first type include the Pripyat'-Dnieper-Donets aulacogen, the Viluy aulacogen of Siberia, the huge West Siberian supra-aulacogen basin, the analogous Yenisei-Khatanga basin, and others. The second type includes the Volga-Urals basin with the Ural foredeep, Azov-Kuban' basin and Terek-Caspian basin with the Greater Caucasus foredeeps, and others. The difference between these two types is rather arbitrary, because the passive margins of continents also undergo the rift stage of development, though their main stage of oil and gas accumulation corresponds to the post-rift stage. The third type is represented by the poorly studied prospective basins of the Arctic margin of the Soviet Union. The fourth includes numerous but relatively small basins of the far eastern and northe stern Soviet Union, of which the East Sakhalin, West Kamchatka, and Anadyr basins have proved to contain oil and gas. Rifting also played a role in the formation of the back-arc group of these basins. Basins in central Asia (Fergana, South Tadjik) and the Transcaucasia region (South Caspian) belong to the fifth basin type. The central Asian basins lie within the area of secondary, epiplatform orogenesis, whereas the Transcaucasian basins are with the area of primary, epigeosynclinal orogenesis. So, far, the only type of basin not identified within the territory of the Soviet Union is the pull-apart basin. The location and distribution of oil and gas deposits in the section of a basin, prevailing types of traps, and scale of potential resources are all features influenced by the geodynami type of the basin.

Late Holocene benthic formainifera beneath perennial sea ice on an Arctic continental shelf

The Canadian Ice Island Project allows geological, biological and oceanographic studies of regions on the Arctic margin that are beneath the perennial pack ice. High-resolution seismic profiles, grab sampling and bottom photography resulted in the discovery of siliceous sponge reefs on the Axel Heiberg Shelf. Sponge colonies interspersed with mudflats form a variety of biotopes for a rich benthic life. The formainifera assemblage is characterized by large numbers of specimens, high species diversity, predominance of calcareous specimens, and a nearly complete lack of living fauna. The perennial sea ice cover is the main controlling factor of the environment. Low regional runoff and rapid freezing of leads in the ice may explain periodic high bottom salinity, reduced detrital deposition, and low organic matter production. The surface foraminifera assemblage is of a subfossiliferous nature which represents an accumulation of dead tests over approximately 1000 years. Small-scale biotope differences and the resulting patchy distribution of species evolved from the environmental stability of the area. The species composition is unique when compared with other known shallow-water Arctic regions and shows a slight affinity with the Pacific fauna, suggesting a migration through the Bering Strait.

Climatic effects of reduced Arctic sea ice limits in the Giss II General Circulation Model

In this paper we present results of an atmospheric general circulation model (GCM) experiment in which Arctic sea ice limits were substantially reduced in all months. March sea ice limits were set equivalent to modern September limits, and all sea ice was removed in September. Sea ice coverage for other months varied between these two extremes. This climate sensitivity experiment makes predictions about mean northern hemisphere atmospheric conditions (including temperature, pressure, wind patterns, and precipitation) consistent with these boundary constraints. The major effects of reduced sea ice limits are observed in winter. They include large regional warming of the circum‐Arctic region, northward migration of the Icelandic low pressure system, and strengthening of the Azores high. Changes in net heating over the North Atlantic Ocean suggest that increases in sea surface temperatures and salinities in this region would also accompany reductions in Arctic sea ice limits. In the wind field, a weakening of the polar easterlies and an intensification of cyclonic circulation over the Norwegian‐Greenland Sea suggest that surface water exchange between the Atlantic and Arctic would increase when sea ice limits are reduced. However, zonally averaged changes in strength of the westerlies or upper level jet stream are minimal. The late Pliocene cooling of the North Atlantic Ocean and North American Arctic margin may have been linked in part to the development of perennial sea ice cover.