West Basin Research Articles

A plate‐tectonic model for Late Jurassic Ophiolite Genesis, Nevadan orogeny and forearc initiation, northern California

Recently published age and structural data allow the reconciliation of previously conflicting models for Late Jurassic genesis of the Josephine, Smartville and Coast Range ophiolites, and the Nevadan orogeny in the Klamath Mountains and Sierra Nevada. The resulting model is consistent with the mode of initiation, location and geometry of the Great Valley forearc basin, and with the lack of a significant forearc basin west of the Klamath Mountains. The Coast Range ophiolite formed by backarc spreading west of an east‐facing intraoceanic arc. Soon thereafter, a remnant arc was calved off the west side of this arc, and the Smartville ophiolite formed by backarc (interarc) spreading. During this time, the Sierran phase of the Nevadan orogeny began as the intraoceanic arc encountered the west‐facing continental‐margin arc of North America. An east‐west‐trending calcalkaline dike swarm in the Sierra Nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision. Simultaneously, a new subduction zone was initiated west of the collision (suture) zone, and this new trench propagated southward, thus trapping the Coast Range ophiolite in the new forearc area south of the Klamath area. Intense deformation in the Sierran region resulted from this collision, and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted. Continued westward relative movement of the North American arc was permitted north of the Sierra Nevada owing to the lack of a colliding intraoceanic arc. The result was the westward rifting of the continental‐margin arc by intraarc spreading, which formed the Josephine ophiolite in the Klamath area. The Klamath phase of the Nevadan orogeny resulted from contraction of the west‐facing intraoceanic arc and Josephine backarc basin beneath the continental margin. Basal sediments of the Great Valley forearc basin were derived primarily from the sutured arc/ophiolite terranes, and were deposited on top of the Coast Range ophiolite, the southern edge of the Klamaths, and the western side of the Sierra Nevada. A new (late Mesozoic) magmatic arc was superposed across the previously accreted terranes, and formed the primary sediment source for the Cretaceous forearc basin.

Mesozoic and Cenozoic regional tectonics and metallogenesis in Mainland SE Asia

The major Mesozoic tectonic event in mainland SE Asia was the late Triassic Jndosin­ ian orogeny. Deformation and uplift in the Main Range Malaysia and northern Thailand, emplacement of syn- to late-tectonic two-mica granites of the Central Tin Belt, and the recently reported imbricate thrusting in Bangka Island, can best be explained by early Triassic collision of the Shan-Thai block foreland with the Indochina block to the east. Syncollision east-verging back-thrusts can explain the narrow gap between the Shan-Thai block and magmatic arc. The Mogok Belt of Central Burma was metamorphosed and elevated above a major collision-related west-verging foreland thrust. Regional stress related to foreland thrusting initiated eastward subduction of oceanic lithosphere at a 1urassic spreading centre in a small ocean basin west of the Shan-Thai block. Subduction of the spreading system and ophiolite detachment were followed by eastward subduction of the remnant ocean basin. A continental fragment, exposed in the Jndoburman Ranges, collided with Burma in the latest Jurassic, and-was underthrust beneath the ophiolite. East-verging thrusts of the Shan Scarps and Thai­ Burma border were related to the collision, and caused partial melting and emplacement of anatectic granites of the Western Tin Belt. Generation of mineralized granites in both the Central and Western Tin Belts may have been related to expulsion of fluids from a common source tectonically buried in the late Triassic and Cretaceous. The Cretaceous heating event well-documented in the Main Range granites was possibly related to their position above converging Benioff zones at a depth of a few hundred kilometres. Cenozoic relaxation of compressive stress probably permitted isostatic rise of the Shan Plateau. Dextral movement on the Sagaing transform fault was accompanied to the east, in northern Thailand, by crustal thinning, emplacement of basaltic magma at depth, and uplift of the Khorat Plateau. Hydrothermal activity related to the sub-surface intrusions and crustal extension caused widespread antimony and fluorite mineralization. Similarities in the regional tectonic setting of northern Thailand ~o that of the Great Basin in western USA suggest the possibility that epithermal gold-silver mineralization could occur at depth. Subduction of Indian Ocean floor beneath the Jndoburman Ranges collision belt began in the Aptian, with magmatism in the Burma volcanic arc and probably Its continuation through Sumatra. Olistostromes of probable Campanian age in the Indoburman Ranges are interpreted as debris flows formed by diapiric rise and extrusion of mud-matrix melanges. Bodies of jadeite and schist were possibly elevated in serpentinite melange diapirs.

Cycling of iron, manganese, silica, phosphorus, calcium and potassium in two stratified basins of Shagawa Lake, Minnesota

Geochemical studies of Fe, Mn, Si, P, Ca, and K were made in two basins of Shagawa Lake, Minnesota during summers 1977–1979. Within the metalimnion, upward and laterally diffusing Fe(II) oxidizes and co-precipitates a large fraction of soluble P. Nevertheless, the soluble Fe P ratio decreases while progressing upward through the oxidation zone. The mean paniculate Fe Si ratio is 3.0 ± 0.5 in the thermocline, but because of the stoichiometric excess of dissolved Si over Fe, and preferential associations between Fe and P, the oxidation of Fe has a negligible effect on the dissolved SiO 2 profiles. The lake sediments are net sources of Ca, K, and P during summer stratification. These elements are transported upward through the thermocline and exported from the lake. Despite the very active sediment release of dissolved SiO 2, and upward transport through the thermocline, the lake acts as a net sink for biogenic opal in summer. The prominent soluble Mn maxima in the thermocline of the east basin result from antecedent sediment release (Mn 2+) and vertical transport (primarily in the west basin), followed by dispersion and oxidation in the epilimnion, and finally downward settling and dissolution within the east basin thermocline. Rhodochrosite solubility does not influence Mn 2+ profile development. Very large percentage increases in epilimnetic Fe, Mn, Si, and P occur simultaneously during episodes of wind-activated entrainment of metalimnion water. These increases are in stoichiometric agreement with the entrained water mass, provided account is also taken of the preformed slurries overlying the metalimnetic shelf sediments in mid-summer. The bathylimnetic soluble Fe P ratio remains low since the cultural inputs of P were terminated in early 1973. This geochemical stability is explained by the large reserves of potentially mobile P in the upper post-cultural sediments. Phosphorus release from these sediments and transport phenomena account for the persistent eutrophic condition of the lake during mid-late summer, despite the 80% reduction in external P loading.

Deformational and sedimentary processes in trench slope basins of the western Sunda Arc, Indonesia

A detailed geophysical survey of the Sunda Trench lower slope reveals the structure and stratigraphy of trench slope basins west of Nias Island. Structural ridges bound the basins and generally strike parallel to the trend of the arc. The ridges on the lower part of the trench slope are formed of folded trench strata and are bounded on their seaward margins by landward-dipping thrust faults. The basins near the toe of the slope are narrower and contain less sediment than basins higher on the slope. Deformation occurs across the entire slope; near-surface strata in the shallowest basin are tilted landward and are cut by a recent thrust fault. A progressive increase in the intensity of deformation with depth is evident. The landward dips and numerous onlapping relationships at the seaward edges of the basins indicate that the seaward margins of the basins are being relatively uplifted with respect to the basin strata. Basins near the outer-arc ridge receive terrigenous turbidites that are delivered down submarine canyons from Nias Island. A small submarine fan has developed at the mouth of a canyon in the shallowest basin. Local slumping of hemipelagic slope sediment forms a minor contribution to the overall sediment fill. Slumping is more important near the toe of the slope, because basins there are isolated from terrigenous sediment input.

SUBSISTENCE AGRICULTURE AND NUTRITION IN THE BIMIN VALLEY, OKSAPMIN SUB‐DISTRICT, PAPUA NEW GUINEA

The agricultural economy of the Mountain Okspeaking people of Oksapmin and Telefomin districts has been very little studied. There has been work by Barth (1971; 1975), Morren (1977; 1979; 1981)and Hyndman (1982; 1984) among the Baktaman, Miyanmin and Wopkairnin, respectively ; but all three are fringe groups of hunter-gatherer-cultivators, inhabiting forested terrain at lower altitudes and at much lower population densities than are typical of the highlands region. The highlanders inhabiting the mountain valleys and karst basins west of the Strickland Gorge and between the Sepik and Fly headwaters have been much less researched, except from the points of view of art, ritual and social organization. A better understanding of the agricultural economy of this region is now needed, since its problems of infant malnutrition, lack of cash cropping opportunities and selective out-migration are becoming increasingly prominent (Bourke, 1979; Cape, 1980, 1981; Morren, 1981; Heywood & Nakikus, 1982). The region is also of interest since it is now undergoing an Ipomoean Revolution, or transition from taro to sweet potato, which elsewhere in the Highlands was accomplished two or three hundred years ago (Golson, 1982; Bayliss-Smith, 1985). In this paper some preliminary data are presented for the Bimin-Kuskusmin group occupying the Bimin Valley, south of Oksapmin.

Institutional Capacity And The Resolution Of A Commons Dilemma

This article concerns the dynamic process of resolving a commons dilemma without an externally imposed solution. We focus on two approaches: a model by Lewis and Cowens (1983) that yields a cooperative private arranghent that incorporates voluntarily chosen public institutions as instruments facilitating a resolution of the commons dilemma. The conditions necessary to Lewis and Cowen's result–a Itresolution without institutions–are contrasted with Ilinstitutional capacity” conditions treated as variables that may take on values enhancing the possibility of resolution. This latter approach yields certain advantages: less extreme assumptions, greater descriptive relevance, and the possibility of a variety of actual resolutions. A description of the case of West Basin in Southern California offers an example of the interaction of institutional capacity with participants' actions to produce a successful resolution of a commons dilemma.

Orinoco and Amazon River sediment input to the eastern Caribbean Basin

Carbonate and mineralogic analyses of piston cores from Aves Ridge, eastern Caribbean Basin, reveal climatically modulated relationship among Quaternary carbonate deposition and terrigenous sediment influxes from the Orinoco and Amazon Rivers. Low carbonate content and dark sediment color, extending into the Pleistocene, suggest considerable dilution of pelagic components by terrigenous sediment on the western flank of Aves Ridge, which is characterized by a southwest-thickening sedimentary wedge. Contrary to typical trends observed in the more distal areas of the central basin west of Aves Ridge, carbonate content is lowest during interglacial climates and highest during glacial climates. A high clay-mineral input correlates with interglacial climate; however, the Orinoco tracer mineral pyrophyllite is low during interglacials and high during glacials. The observed trends suggest that Amazon sediment influx to the eastern Caribbean is highest during interglacials, or times of high sea level. The Amazon influx is promoted by deposition on the shelf and westward coastal transport of sediment during high sea level, and reduced by shelf-edge deposition and loss to the Amazon Canyon during low sea level. The Orinoco influx is similarly modulated, but it undergoes a relative (and possibly absolute) increase during low sea level because of proximity to the depositional site. The terrigenous dilution produces the reversal of carbonate trends. However, total sediment accumulation rates on southernmost Aves Ridge remain higher during glacials, possibly as a result of increased carbonate production caused by upwelling.

Lacustrine Deposits in Rifted Deep Basins of Yellow Sea: ABSTRACT

The central Yellow Sea is a typical intracratonic rifted basin that consists of 4 major depressions bounded by aligned listric faults along horst blocks of uplifted basement (Kunsan, West Kunsan, Yellow Sea subbasins, and Central Trough). The depressions are half grabens caused by pull-apart extensional stresses. Core analysis and micropaleontologic study indicate that more than 5 km of lacustrine sediments were accumulated in the central part of the West Kunsan basin. Two distinctive sedimentary successions are recognized in the core descriptions: alternation of reddish-brown siltstones and sandstones containing evaporites and marlstones, and an overlying progradational sequence including minor limestone beds in the lower part of the sequence. The progradational sequence is interpreted as lacustrine deltaic deposits. Abundant palynofloral occurrence of freshwater green algae, Pediastrum, and absence of marine fauna such as dinoflagellates are also supporting evidence for a lacustrine environment. The lithofacies and tectonic framework of the Yellow Sea are very similar to those of Cretaceous acustrine sediments of the Korea Peninsula onshore and Pohai coastal basin in China. End_of_Article - Last_Page 262------------

STRUCTURAL GEOMORPHOLOGY OF THE WEST AUSTRALIAN BASIN

The structure and morphology of the floor of the West Australian Basin and adjacent areas are described. Four main levels of the floor are identified according to geographic zonality and morphology. Structural-geomorphological elements of the basin's rim are subdivided into two groups: those formed simultaneously with the formation and evolution of the continental margin and those formed during intensive subsidence and fracturing of the outer part of the basin.

Crustal structure of the continental shelf off northwest Britain from two‐ship seismic experiments

Two‐ship multichannel seismic profiles, using expanding spread and constant offset source‐receiver configurations, were shot with explosive charges and a 16.4 ℓ air gun to investigate the structure of the continental shelf off northwest Britain. A long‐range (69 km) expanding spread profile reveals that the crystalline basement off northern Scotland is covered by sedimentary sections up to 2.5 km thick, and is divisible into two seismic units with velocities of 6.1 km/s and 6.6 km/s. Prominent supercritical Moho reflections indicate a crustal thickness of 26.7 km. Strong sedimentary and basement refractions, together with oblique reflections from the vicinity of the Moho, have been profiled at a constant air gun‐receiver offset of 10 km across a sedimentary basin west of the Orkney Islands. On the Outer Hebridean shelf to the west of mainland Scotland, the metamorphic basement lies within 250 m of the sea floor. P- to S-wave conversion occurs at the basement surface; [Formula: see text] gives a Poisson’s ratio of 0.27–0.31 at depths of 300–1 000 m. Marked changes in mode conversion efficiency are observed on constant offset profiles and are attributed to variations in the velocity structure of an uppermost low‐velocity (4.9 km/s) layer of weathered basement. The deeper crustal velocity appears uniform with depth, although there is some evidence of significant lateral velocity changes (6.28–6.61 km/s). In contrast to the shelf north of Scotland, reflections from within the basement and from near the base of the crust are recorded only sporadically on constant‐offset profiles. A strong event at 10.4 s two‐way reflection time appears to have arisen from a seismic discontinuity within the upper mantle. The differences in seismic character of the basement on the two‐ship profiles suggest significant variations in crustal structure within the Caledonian foreland of northern Britain.

Crustal Structure of the Continental Shelf off Northwest Britain from Two-Ship Seismic Experiments

Two-ship multichannel seismic profiles, using expanding spread and constant offset source–receiver configurations, were shot with explosive charges and a 16.4 I air gun to investigate the structure of the continental shelf off northwest Britain. A long-range (69 km) expanding spread profile reveals that the crystalline basement off northern Scotland is covered by sedimentary sections up to 2.5 km thick, and is divisible into two seismic units with velocities of 6.1 km/s and 6.6 km/s. Prominent supercritical Moho reflections indicate a crustal thickness of 26.7 km. Strong sedimentary and basement refractions, together with oblique reflections from the vicinity of the Moho, have been profiled at a constant air gun–receiver offset of 10 km across a sedimentary basin west of the Orkney Islands. On the Outer Hebridean shelf to the west of mainland Scotland, the metamorphic basement lies within 250 m of the sea floor, P- to S-wave conversion occurs at the basement surface; Vp/Vs gives a Poisson’s ratio of 0.27–0.31 at depths of 300–1000 m. Marked changes in mode conversion efficiency are observed on constant offset profiles and are attributed to variations in the velocity structure of an uppermost low-velocity (4.9 km/s) layer of weathered basement. The deeper crustal velocity appears uniform with depth, although there is some evidence of significant lateral velocity changes (6.28–6.61 km/s). In contrast to the shelf north of Scotland, reflections from within the basement and from near the base of the crust are recorded only sporadically on constant offset profiles. A strong event at 10.4 s two-way reflection time appears to have arisen from a seismic discontinuity within the upper mantle. The difference in seismic character of the basement on the two-ship profiles suggests significant variations in crustal structure within the Caledonian foreland of northern Britain.

Lake Mixing and Its Relationship to Epilimnetic Phosphorus in Shagawa Lake, Minnesota

Using thermometry, the mixing regime was intensively studied in each of the two principal basins of Shagawa Lake. After the thermocline had migrated past the interbasin sill depth (6 m), it migrated more rapidly in the larger, windswept western basin than in the smaller, sheltered eastern basin. The metalimnetic eddy conductivities (Kz) were also larger in the west basin, ranging from 0.025 cm2∙s−1 during intervals of low wind energy up to 0.25 cm2∙s−1 during intervals bracketing powerful cold fronts. The bottom temperature increased rapidly in the west basin in response to mixing, reducing total water column stability and increasing susceptibility to a sudden "turnover." Because of these warm bottom temperatures and drainage basin influence, density currents sometimes carry cold water into the benthic zone of the west basin following heavy, cold rains in midsummer. Epilimnetic phosphorus concentrations in Shagawa Lake depend on linkages among weather, basin morphometry, and water column redox. Time series analysis of 8 yr of record reveals the special importance of two factors. First, the anoxic zone below the mixed layer must include an important fraction of the metalimnetic shelf sediments in the western basin (6–10 m zone). Second, the mixed layer must migrate downward and directly entrain water overlying the anoxic sediments. Time series analysis excludes the possibility of sediment release of phosphorus within the epilimnetic zone as the principal control on phosphorus in the pelagic mixed layer. For lakes with similar morphometry and climatic setting, and a previous history of sewage inputs, lake mixing complicates the trophic adjustment of the lake to a reduction in external phosphorus loading.

Quantitative and Qualitative Characteristics of Urban Discharge to Small River Basins in the South West of Sweden

The paper deals with the qualitative and quantitative characteristics of urban discharge. Ratios for urban discharge and recipient flow during different time intervals are presented and discussed. The quality of the urban discharge is illustrated through pollutographs.

Multiple glaciations of the Cordon del Plata, Mendoza, Argentina

Evidence exists for four glaciations in the Río Blanco basin west of Mendoza, Argentina. Morphology, superposition of tills, soil-profile development, loess thickness, and boulder weathering have been the techniques most useful in mapping the tills. Glaciers of Vallecitos (= Wisconsinan) age extended to 2600 m and left distinctive moraines. Tills of two pre-Vallecitos glacier advances cover the floor of the valley from 2600 m to below 2100 m, and remnants of one of the tills extend nearly to the junction with Río Mendoza (1400 m) 13 km below the lowest Vallecitos moraines. These deposits were considered to be mudflows rather than tills by Polanski; however, the quartz sand grains examined with SEM have surface textures characteristic of glacial abrasion. The sediments, thus, are more likely glacial than mudflow deposits. One still older till caps ridges as much as 200 m above the present valley floor. Vallecitos glaciers did not smooth the walls of the wide valleys through which they flowed; the valleys had been enlarged by the larger ice tongues during one or more of the earlier glaciations. Frost shattering of the rhyolite and quartzite has altered much of the distinctive glacial valley shape, and talus lies between valley walls and Vallecitos lateral moraines. Holocene glaciations seem to be in phase with those of the Northern Hemisphere, and the Vallecitos glaciation coincided with oxygen isotope stage 2 and the Wisconsin of North America. The next older glaciation may correlate with oxygen isotope stage 6 and the Illinoian glaciation of North America. A till that underlies it is difficult to correlate and is clearly much older, but postdates the last major uplift of the range. It may represent isotope stage 12, and the period that followed it was more moist than other interglacial ages in this area. The oldest, Los Mesones, may correlate with Mercer's “Greatest Glaciation” of Patagonia, 1.0 to 1.2 m.y. ago. Although it is beyond the established oxygen isotope stages, a long cold period is present on the curves at this time, nd it coincides with one of the glaciations of the “Nebraskan—Kansan” complex of North America.

Mineralogy and geochemistry of epigenetic features in metalliferous sediment, Atlantis II Deep, Red Sea

Finely laminated and undisturbed except in the area of the southwest basin; in this part, bedding, resedimentation, and brecciation are common. In addition, some sediment cores contain vein minerals precipitated in open fissures which crosscut unlithified metalliferous sediments. These veins are the conduits by which new hot brine vents into the Deep. Vein mineralogy is dominated by anhydrite, talc, smectite, pyrite, sphalerite, and chalcopyrite. Vertical zonation due to cooling and reaction of the incoming brine as it rises through the metalliferous sediment. Near the Southwest Basin- west basin transition, metalliferous sediment has been locally recrystallized to a hematite-magnetite-pyroxene assemblage, probably due to the intrusion of basalt into the metalliferous sediments.--Modified journal abstract.

Sand Bodies on Muddy Shelves: A Model for Sedimentation in Western Interior Cretaceous Seaway, North America: ABSTRACT

The continental shelf on the western margin of the Cretaceous Interior seaway was a muddy surface which bore abundant northwest-southeast trending sand bodies, up to 20 m (65 ft) thick and many km long (Medicine Hat, Mosby, Shannon, Sussex, Duffy Mountain, and Gallup Sandstones). These features resemble the storm-built or tide-built sand ridges reported from the modern Atlantic continental shelf, or from the Southern Bight of the North Sea. However, whereas modern sand ridges may rise from the Holocene transgressive sand sheet through overlying Holocene mud deposits to be exposed at the present sea floor, no modern examples are known where sand ridges are completely encased in mud, as the Cretaceous examples seem to have been. Hydrodynamical theory suggests that special circumstances may make it possible to build sand bodies from a storm flow regime whose transported load consists of sandy mud. Under normal circumstances, such a transport regime would deposit little clean sand. The sea floor is eroded as storm currents accelerate, but erosion ceases when the boundary layer becomes loaded with as much sediment as the fluid power expenditure will permit (flow reaches capacity). Deposition of the graded bed occurs as the storm wanes; the resulting deposit is liable to consist of a sequence of thin shale beds with basal sand laminae. However, slight topographic inequalities in the shelf floor may result in horizontal velocity gradients so that the flow undergoes acceleration and deceleration in space as well as in time. Fluid dynamical theory predicts deceleration of flow across topographic highs as well as down their down-current sides. The coarsest fraction of the transported load (sand) will be deposited in the zone of deceleration, and deposition will occur throughout the flow event. Relatively thick sand deposits, 20 to 50 cm (8 to 20 in.) can accumulate in this manner. Enhancement of initial topographic relief results in position feedback; as the bed form becomes higher, it extracts more sand from the transported load during each successive storm. Individual storm beds may tend to fine upward (waning current grading), but the sequence as a whole is likely to coarsen upward, reflecting increasing perturbation of flow by the bed form as its amplitude increases. Stability theory suggests that the end product of these processes should be a sequence of regularly spaced sand ridges on the shelf surface. However, sand bodies are localized in stratigraphic position and lateral distribution within Cretaceous shelf deposits. Upward-coarsening sequences are a widespread phenomenon in the Western Interior Cretaceous System, and the sand bodies appear to constitute localized sand concentrations within more extensive sandy or silty horizons. Especially widespread upward-coarsening sequences appear to be due to the close coupling between activity in the overthrust belt to the west and sedimentation in the foreland basin. In the proposed sequence of events, a thrusting episode increases relief in the source terrane as well as the load on the crust. Sedime tation at first dominates over subsidence, and initially the shelf on the western margin of the basin becomes shallower. As it does so, intensified wave scour on the shelf floor increases the amount of bypassing, which results in the deposition of increasingly coarser sediment, culminating in a sandy horizon. As relief in the hinterland wanes, subsidence overtakes sedimentation and the shelf subsides. Renewed thrusting begins the cycle anew. In a second mechanism for the formation of upward-coarsening sequences, tectonic uplift affects parts of the shelf as well as the hinterland. The initiation of Sevier or Laramide structural elements beneath the shelf, and the remobilization of other, older structures, creates submarine topographic highs. These highs cause slight sand enrichment over broad sectors by means of the process described above. The development of sand-enriched areas on the shelf floor by both mechanisms leads to the flow-substrate feedback behavior that builds large scale, elongate bodies of clean sand. End_of_Article - Last_Page 556------------

Relationship of Roberts Mountains Thrust to Oil and Gas Exploration in Nevada: ABSTRACT

The Roberts Mountain thrust is the oldest and probably the largest of many eastward-moving regional thrust plates that make up much of the Basin and Range province. The fault can be traced from the state of Washington through central Nevada into California. Movement commenced during Late Devonian-Early Mississippian time (Antler orogeny), probably as a result of plate collision and subduction farther west along the margin of the eastern Klamath geanticlinal arc. Ordovician to Devonian age, eugeosynclinal, highly organic (2 to 5%), dark-colored shales, siltstones, cherts, and limestones--the western siliceous facies--perhaps originally totaling 20,000 ft (6,100 m) are thrust over eastern-facies carbonate and clastic shelf deposits. Movement along the thrust continues sporadically into middle Permian, by which time much of the eugeosynclinal rocks were eroded eastward as a flysch. These clastic deposits of Mississippian, Pennsylvanian, and Permian age are deposited along with shallow-marine units in local basins and sags. Outcrops of thin-bedded, oil-bearing shales (25 gal/ton) and black cherts of the western siliceous facies Vinni Formation are present at Roberts Mountain. This unit is a potential source, seal, and possible reservoir rock, averaging several thousand feet in thickness, which floors many of the Miocene valley basins west of the leading edge of the thrust. Oil potential is considered to be good although fewer than 15 wells have been drilled in the area. Many had oil shows but none has tested the Vinni or the overlying Cretaceous-Eocene sedimentary units at depth. Free oil has flowed from perforations at approximately 7,200 ft (2,195 m) from possibly Mississippian or Devonian dolomites in the Amoco-Getty 3 Blackburn Unit, (Sec. 8, T27N, R52E, Pine Valley, Eureka County, Nevada). This s the first substantial oil recovery from the Paleozoic rocks in any valley basin other than Railroad Valley in the Nevada portion of the Basin and Range. Oil appears to be of Paleozoic source. In some valleys the overlying Cretaceous and Tertiary units may provide additional source, seal, and reservoir rocks. East of the thrust trace, in several valley basins, Paleozoic source and reservoir rocks are present and intertongue with the flysch of the Roberts Mountain thrust. In some valleys, these units are also overlain by sedimentary Cretaceous and Tertiary rocks which may also be potential source, reservoir, and seal for any oil accumulations. Noncommercial Tertiary and Paleozoic free oil and gas have been recovered in several test wells drilled in T29N, R55E, Huntington Creek valley basin, Elko County, Nevada. Local horst and graben structures, combined with Tertiary truncation similar to that found productive at Trap Spring and Eagle Springs fields, are present in valley basins both east and west of the thrust. End_of_Article - Last_Page 564------------

Depth-to-magnetic source analysis of the Arctic Ocean region

Approximately 400,000 line kilometers of high quality, low level Arctic aeromagnetic data collected by the Naval Research Laboratory, the Naval Oceanographic Office and the Naval Ocean Reseach and Development Activity from 1972 through 1978 have been analyzed for depth to magnetic source. This data set covers much of the Canada Basin, the Alpha Ridge, the central part of the Makarov Basin, the Lincoln Sea, the Eurasia Basin west and south of the 55°E meridian and the Norwegian-Greenland Sea north of the Jan Mayen Fracture Zone. The analysis uses the autocorrelation algorithm developed by Phillips (1975, 1978) and based on the maximum entropy method of Burg (1967, 1968, 1975). The method is outlined, examples of various error analysis techniques shown and final results presented. Where possible, magnetic source depth estimates are compared with basement depths derived from seismic and bathymetric data. All major known bathymetric features, including Vesteris Bank and the Greenland, Molloy and Spitsbergen fracture zones, as well as the Mohns, Knipovich and Nansen spreading ridges and the Alpha Cordillera appear as regional highs in the calculated magnetic basement topography. Shallow basement was also found under the northeastern Yermak Plateau, the Morris Jesup Rise and under the southern (Greenland-Ellesmere Island) end of the Lomonsosov Ridge. Regional magnetic source deeps are associated with such bathymetric depressions as the Canada, Makarov, Amundsen, Nansen, Greenland and Lofoten basins; more localized magnetic basement deeps are found over the Molloy F.Z. deep and over the Mohns, Knipovich and Nansen rift valleys. A linear magnetic basement deep follows the extension of Nares Strait through the Lincoln Sea toward the Morris Jesup Rise, suggesting the continuation of the Nares Strait or Wegener F.Z. into the Lincoln Sea. A sharp drop in the regional magnetic source depths to the southeast of the Alpha Ridge suggests the Alpha Ridge is not connected to structures in northwest Ellesmere Island as previously postulated from high altitude aeromagnetic collected by Canadian workers. A regional deep under the east Greenland shelf west of the Greenland Escarpment suggests the presence of 5–10 km of post-Paleozoic sediments.

Uranium in the Niger-Nigeria Younger Granite Province

AbstractIn Niger, uranium occurs in upper Palaeozoic and lower Mesozoic continental sedimentary basins west of the Aïr mountains, but the source of the uranium had not been identified. Geochemical studies and fissiontrack observations on alkaline ignimbrites preserved in two Palaeozoic anorogenic centres in Aïr show that uranium is concentrated in the matrix and on secondary iron-oxide coatings surrounding lithic and crystal fragments. Based on variable Th/U ratios and degree of oxidation, it is concluded that the original ignimbrite field was enriched in uranium, but that a considerable proportion was leached during the weathering of the volcanic pile. Tectonic uplift, anorogenic magmatism, followed by weathering and erosion of the volcanic cover, with sedimentation in nearby continental basins, have all contributed to the development of uranium mineralization in Niger. The petrological and geochemical similarities between the Palaeozoic ring complexes in Niger and the Nigerian Mesozoic ring structures suggest that sedimentary uranium deposits may also be found in Nigeria if the tectonic and sedimentological controls were favourable.Enriched concentrations of uranium have been discovered in the exposed granitic roof zones of the Nigerian subvolcanic centres, with Th/U ratios approaching unity. Thus local vein deposits of uraninite, as well as dispersed uranium in recent sedimentary horizons, could be discovered particularly in the drainage systems entering the Chad Basin.

The mineralogy and provenance of deep-sea sediments recovered by gravity corer off Sierra Leone, West Africa

The mineralogy of eight gravity cores recovered between Sierra Leone and 25°W longitude has been examined in an investigation of the provenance of the recent sediments in the eastern Equatorial Atlantic. X-ray analyses show that the principal mineral components are calcite, quartz, feldspar, kaolinite, illite and montmorillonite. One core taken from the upper continental slope off Freetown also contains gibbsite, a product of intense tropical weathering. Biogenic material forms the dominant component of most sections of the cores but it is clear from the abundance of quartz, kaolinite and freshwater diatoms that an aeolian supply of continental detritus has been important in the formation of the recent sediment cover. A significant contribution from volcanic sources can be recognised to the west of the Sierra Leone Rise. The occurrence of calcareous turbidites in two cores from the St. Paul's Fracture Zone and from the basin west of the Sierra Leone Rise appears to be a reflection of recent tectonic activity.