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Field N is situated in Yamal peninsula in Western Siberia. From 19 productive layers, J3-4 layers are the most important for consideration as they contain significant amount of STOIIP. On base of complex analysis of core data and paleogeography, depositional environment of J3-4 layers was defined as tide dominated delta. Sandbodies are presented by channel facies and also by interdistributary facies, formed by levees and crevasse splays. Qualitative and quantitative analysis of log data allowed creation of input data for geological model. 3D facies model was obtained using combination of object modeling and sequential indicator simulator methods. Properties in model were obtained with use of random simulation function and various recalculations. Geological model was transformed into simulation model. Uncertainties, related to sandbody dimensions, model realizations and sandbody distribution in tested well were evaluated using numerous realizations of geomodel. Cumulative oil recovery and rates were evaluated for horizontal and vertical wells, using P10 P50 P90 values.

Exploitation of unconventional resources could prolong the gas production in the North Sea. Low-net-to-gross fluvial intervals may have tough-gas reservoir potential in thin-bedded crevasse splays.To assess economic risks associated to the development of these reservoirs, a numerical model can help to predict the sediment distribution. To this end, simulations were conducted with Delft3D process-based modelling software. Input parameters and the validation data sets for these models are derived from outcrop studies in the present-day Rio Colorado fluvial system in the Altiplano Basin, Bolivia. The grain-size trends of the simulated surface sediments for a single flood event show a trend which is consistent with the validation data. For example, grain-size decreases with increasing distance from the channel, which is in line with the physical concept of decreasing sediment size for decreasing flow energy. This shows that numerical models can be used to support sediment trends and depositional mechanisms of a crevasse splay. The combination of numerical models and discrete field data provides a solid case for sediment distribution predications. However, simulations still have a limited accuracy.

Crevasses and englacial fracture networks route meltwater from a glacier's surface to the subglacial drainage system and thus influence glacial hydraulics. However, rapid fracture growth may also lead to sudden and potentially hazardous structural failure of unstable glaciers and ice dams, rifting of ice shelves, or iceberg calving. Here, we use passive seismic recordings to investigate the englacial fracture network on Glacier de la Plaine Morte, Switzerland. Glacier dynamics and the drainage of an ice-marginal lake give rise to numerous icequakes, the majority of which generate dispersed, high-frequency Rayleigh waves. A wide distribution of events allows us to study azimuthal anisotropy between 10 and 30 Hz in order to extract englacial seismic velocities in regions of preferentially oriented crevasses. Beamforming applied to a 100-m-aperture array reveals azimuthal anisotropy of Rayleigh-wave phase velocities reaching a strength of 8% at high frequencies. In addition, we find that the fast direction of wave propagation coincides with the observed surface strike of the narrow crevasses. Forward modeling and inversion of dispersion curves suggest that the azimuthal anisotropy is induced by a 40-m-thick crevassed layer at the surface of the glacier with 8% anisotropy in shear-wave velocity.

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. The preceding article in the Technology Today Series (Jan. 1989 JPT. Pages12–13) described the general processes of building deltas. The present articlefocuses on the reservoir characteristics of high-energy sand-rich deltas. Theseform when the wave and tidal action is sufficiently strong to rework thedeposits brought in by distributary channels before their burial. The principaltypes of sand bodies and highest-quality reservoir rocks in a delta arechannels and stream-mouth bars. Also present are the sheet sands deposited on adelta front and localized sand bodies left in minor crevasse deposits adjacentto channels during floods. While these bodies have definite areal relationshipsto one another during the process of building a lobe on the delta, theirthree-dimensional relationships become complex as the deltas build seaward orshift positions along the shoreline. Thus, a channel active during a laterbuilding cycle may cut into or may lie over stream-mouth bars, delta-frontsheets, or crevasse sands left earlier. Fortunately, the various sand bodieshave different distributions of permeability and different responses to logs sothat they can be identified. The coarsest sand is left at the base of a channeland silt and clays are left at the top. Typical gamma ray log responses andpermeabilities measured in cores show depth profiles in Fig. permeabilitiesmeasured in cores show depth profiles in Fig. 1a through c in sands leftupstream, onshore, and near mouths of distributary channels, respectively. Thelow gamma ray log responses at the bases show that little clay or shale ispresent. The increased gamma ray log response toward the tops of the channelsreflects the presence of more clay and shale. Similarly, the largest grainsizes and highest permeabilities are measured in cores from the base anddecrease upward. p. 89

MR. T. D. LATOUCHE'S letter (page 39) is very interesting as a more than usually independent contribution (for the reason given therein) to the interesting question of glacial erosion, and as showing how similar (allowing for the difference in size) are the phenomena of the Himalayan and the Alpine glaciers. But I think that moulins, as a rule, are not likely to be very important agents in the formation of the rock-basins in which lakelets and tarns are often lodged. So far as my experience goes, the range over which the moulin-torrents can act is very restricted; for the crevasse, which gives the opportunity to the water, is generally formed very nearly at the same part of the glacier. Thus after the moulin has travelled for a very short distance down the glacier, a new crevasse opens out behind it and cuts off the torrent. I have frequently seen four or five dry shafts in advance of the working moulin. The lateral range also of the moulin must be small. Hence I think that the giant's-kettle (as is usually supposed) more accurately represents the ordinary product of a moulin. An excellent illustration is afforded by the well-known “glacier-garden” at Lucerne. I think, also, that the rock-basins, of which we speak, are more commonly found insituations where moulins would not be numerous or large, viz. in cwms and corries. It is, however, true that in certain undulating rock districts, as parts of Scandinavia and the Scotch Highlands, lakelets are common. The form of these, however, does not appear to bear much relation to the hollow produced by a moulin. So that I doubt whether we can regard a moulin as an agent of primary importance in the production of an ordinary rock-basin, though it may sometimes be a minor contributory. As I have more than once discussed the question of the probable cause of the formation of tarns as well as of large lake-basins, it is needless to repeat what has appeared in print.

Piezoelectric quartz resonators, because of their mechanical durability and their small decrement, have found many well‐known uses, particularly in frequency stabilization and in the construction of electric filters for high frequency channels, but their possibilities in connection with general laboratory use have not been realized. A piezoelectric ultramicrometer has been described permitting linear displacements to be measured to 10 9cm under ordinary laboratory conditions. Work has been continued on the application of this method to the precise measurement of the thermal coefficients of expansion of fine wires, the diameter of fibers, the effect of small temperature and pressure changes on the dielectric constants of gases, the estimation of moisture content in thin solid films, and the dielectric constants of thin solid and liquid films.

Lower delta plains are primary sites for the deposition, accumulation, and preservation of macrodetrital plant parts. Chacaloochee Bay, an interdistributary estuary in the Mobile Delta, Alabama, is being infilled actively by crevasse-splay sedimentation. Within this system are a variety of sites in which plant macrodetritus has accumulated and has the potential of being preserved. Aerial plant parts constitute laminar leaf-litter beds within various sediments, in addition to extensive accumulations of relatively pure organics (peats). The leaf-litter beds in the crevasse-channel distributary mouth bar and a detrital peat shoal have been examined with respect to the distribution of accumulated plant parts and their size fractionation. Whole leaves recovered from the sediment-water interface of the crevasse channel provide an insight as to the provenance of the accumulated plant litter. The vast majority of leaves being incorporated into the interdistributary bay sediments have not originated from the surrounding vegetational communities. Rather, most recovered leaves originated from communities existing at least 13 km north of the depositional site. The taphonomic processes responsible for the transport, deposition, and alteration of the accumulated macrodetritus include heterochronous leaf fall, fluctuations in bedload and suspension-load transport, rates of periodic flooding, redistribution by tidal and wind-generated surface currents, biodegradation, and bioturbation.

Reynisdjup Channel is a conduit across the insular rise for turbidity currents from southern Iceland. It has a large meander, at depths of 2,000 to 2,200 m, that was examined with a deep-tow vehicle. Erosive turbidity currents are creating cut-off channels by spilling across several parts of the meander: (1) a crevasse at the outside of the bend may cause channel avulsion; (2) chute cut-off has isolated a scroll bar; and (3) the 8-km-wide meander neck has been dissected by narrow spillways and by “mega-flutes” as much as 1 km wide and 30 m deep.

Sedimentary stratification, foliation, and crevasse traces (including those of healed crevasses) are well displayed at the surface of the small, steep glacier Charles Rabots Bre. Two main flow units, cropping out as convex downglacier arcuate systems of stratification, are recognizable, but others, related to the irregular distribution of the accumulation areas, also contribute significantly to the outcrop pattern. Between the flow units and at the margins of the glacier, strong, steeply dipping longitudinal foliation crops out parallel to longitudinally striking stratification. Much of this foliation is believed to form as a result of shearing and recrystallization in the planes of favorably oriented sedimentary layers. The persistence of crevasse traces below the sets of marginal and transverse crevasses in an area of strong ablation indicates that fracture occurs to greater depths in temperate glacier ice than is generally believed.

The stress intensity factor K at the upper tip of a vertical, liquid-filled crack in the earth's crust is large for relatively small crack (K>3MPa m/sup 1/2/ when the crack length is larger than about 40 m). Consequently, the large water-filled cracks considered for use in the extraction of geothermal heat might be expected to pop to the earth's surface. in this paper it is shown that a freely slipping horizontal joint (as well as a not so freely slipping joint) can stop the upward motion of a large liquid-filled crack. If the joint is tightly fitting, there is not limit to the length of a stopped crack. If slip on a joint results in the joint being propped open, the maximum crack length of a stopped crack is given by the crevasse expression: total crack length =..pi..T/2(rho-rho')g, where T is the tectonic stress, g is the gravitational acceleration, and rho-rho' is the difference between the rock and liquid density. The results can also be applied to the problem of magma transport in the earth's crust. In particular, laccolith emplacement is favored when the nonhydrostatic tectonic stress component is compressive in character.

Crevasses are subject to rotation and bending according to the velocity field through which they travel. The objective of this study is to determine to what extent the velocity field can be inferred from measurements of the resulting shapes of crevasses. A quantitative model of crevasse deformation is developed, based on the following assumptions: (1) each crevasse is assumed to open perpendicularly to the principal extensional regional strain-rate, (2) the crevasse forms when the principal extensional strain-rate exceeds some specified critical value, and (3) velocity gradients are constant over the area of interest. The first two assumptions are reasonable and the third is necessary for an analytic solution of flow trajectories. The crevasse is carried along, rotated, and bent, and may continue to increase in length. Calculations are made for different velocity fields, and velocity fields are sought that produce crevasses similar to those found in three different areas of Ice Stream B. Hook-shaped crevasses occur just outside the chaotic zone at the ice-stream margin. These are similar to the curved marginal crevasses often found in the accumulation zone of valley glaciers. They are successfully modelled by combining strong lateral shear with slow flow of ice from the ice ridge into the ice stream. The curvature at the most sharply bent part of the crevasse is found to be a useful measure and, together with measurements of ice flow from the ridge, can be used to infer the rate of lateral shear. This rate compares favorably with the single measurement obtained so far (Bindschadler and others 1987). A pattern of splaying crevasses develops on the ice stream down-glacier of its narrowest part. These crevasses are similar to longitudinal crevasses found in the ablation zone of many valley glaciers. Models with linear variation in velocity cannot reproduce the observed pattern. However, we have been able to simulate higher-order variations by joining together successive linear models. The observed crevasse pattern is successfully produced if the side shearing varies as the third power of distance from the center of symmetry of the crevasse pattern. Such a variation is expected for a linear gradient in side-drag stress and a third-power constitutive relation for ice. The observed crevasse pattern is thus consistent with side drag varying linearly across the ice stream. The third example is the rotation of transverse crevasses, which occur in trains on the main part of the ice stream. This rotation is due to side shearing but its magnitude is also affected by turning of the flow line and by normal strain-rates. It is therefore possible to reproduce the observed pattern for a wide range of velocity fields, and so measurements of the orientation of transverse crevasses provide only an upper limit on side shearing within the main body of the ice stream. There are many other examples of crevasse patterns on Ice Stream Β and on other glaciers that can be studied in this way. We propose that important constraints can be placed on velocity gradients and on the flow dynamics by using quantitative modelling of crevasse shapes.

AT the station maintained from September 1935 to June 1936 near the centre of the Western Ice Cap in North-East Land (80° N., S 20° 30 E., approximately) by the Oxford University Arctic Expedition 1935–36, it has been found that, contrary to expectation1, the ice-cap is not frozen to any great depth. While a shaft was being excavated in the firn beneath the station, a concealed crevasse was discovered—there was absolutely no trace of it at the surface, even during the period of summer thaw—and investigated to a distance of some hundreds of feet in the horizontal direction. At a depth of some 70 ft. below the surface was a lake of water varying in depth from 3 ft. to 6 ft. The bed of the lake was of ice.

The Commonwealth Trans-Antarctic Expedition, 1955–8, observed that the Filchner Ice Shelf was traversed by a straight east-west zone of brecciated ice—the ‘Great Ice Chasm’—about forty miles south of Shackleton. Analysis of the crevasse patterns at the eastern end of the Chasm indicates that the brecciated zone probably resulted from a wrench- or tear-faulting movement, in which the northern side moved westwards relative to the southern. The frictional drag caused by the strong westward current of the Weddell Sea on the base of the northern part of the floating ice-shelf is considered to have been largely responsible for the formation of the Chasm.

Excellent laboratory models of glaciers were produced by pouring a mixture of molding plaster and water down a prefashioned trough of nonabsorbent material. The plaster models simulate the dynamics of glacier flow and accurately display many surface and internal structures common to real glaciers, including crevasse and flow patterns, icefalls and ogives, shears, and folds. Internal and surface reference lines depict the nature of the deformation produced by flow. The technique is successful as a teaching aid and shows definite potential as a research tool.

Reports 1955 and 1957 investigations of this glacier in the Chugach Mts. near Valdez. Snow depths (less than expected), accumulation, and ablation (greatest on warm, windy days) were studied at 3180-9500 ft on the main and east branches. Variations in snow depth with altitude and location on the glacier were also examined. Accumulation-ablation ratios, compared for various parts of Columbia and for some other Alaskan glaciers, indicate that the main branch is gaining and the east branch losing strength. The beginning of a new crevasse, also a large ice avalanche were seen in 1957.

A crevasse believed to be of average size, located in the coastal belt of the northwest Greenland icecap, has been studied. In 1955, it was at least 300 m long and 15 m deep. At its widest point it was three meters broad, becoming narrower at the ends, top, and bottom. It was strongly bridged by snow, firn, glazing, and porous masses of vapor crystals. Ice shelves and wide cracks in the ceiling and floor which were observed may indicate at least three periods of widening, separated by one or more seasons of surface drift-bridging and bottomwater freezing. The bottom-water level depended upon an equilibrium between supply and drainage, which fluctuated with weather. No other changes were noted during two months of observations. Some interesting analogies between this crevasse and a natural limestone cave have been noted.

Sedimentary structures in a sandstone in the Weald Clay are described. They are groove casts, possibly formed by plant fragments being moved by water, air-heave structures, scour structures, load casts, syndepositional faults and folds, trace fossils and ripple structures. From their study it is concluded that the sandstone was deposited by the sudden inrush of sediment-laden water; and a comparison is suggested with the crevasse sands of modern deltas.

The Jardine-Crevasse Mountain area contains the only known economic metal deposits associated with Precambrian rocks in Montana. Previous field investigations indicate that (1) the ores are probably magmatic hydrothermal, (2) the Crevasse Mountain granite was the source of the ore solutions, and (3) the emplacement of the granite ended the Precambrian history in the area. The gold-bearing sulfide ores and sulfurbearing phases from the country rocks and from ore-alteration products have been investigated by sulfur isotopic analyses in order to determine the genesis of the ores and the other sulfur-bearing phases. Ore sulfides range from + 1.0 to + 4.4 permil (8 S34), disseminated sulfides in the country rocks from + 1.6 to + 3.3 permil (8 S34), and alteration products from the ores show: melanterite + 3.7 permil (8 S34) and native sulfur + 4.7 permil (8 S34). The data are consistent with a magmatic hydrothermal origin for both the ore sulfides and the disseminated sulfides, and the higher 8 S34 values for the alteration products indicate that post-ore events have only been of local importance. Introduction The gold-arsenic-tungsten ores and their host and source rocks in the Jardine-Crevasse Mountain area, Southwestern Montana have been investigated by sulfur isotopic variations and by petrographic and mineragraphic studies. The area lies along the north boundary of Yellowstone National Park in Park County, Montana. This area is in the extreme southwest part of the Beartooth-Absaroka uplift. Some of the geology and the sample locations are shown in Figure 1. Early investigations indicated that the ores of the area were not typical of other deposits in Montana as they are the only ones not related to any Tertiary event(s). Seager (1944) has published the only comprehensive study of the area to date and Brown (1965) has recently substantiated many of Seager's earlier conclusions as well as investigating the area by isotopic means. This content downloaded from 207.46.13.159 on Sat, 22 Oct 2016 04:46:00 UTC All use subject to http://about.jstor.org/terms Genetic SigniJc J.X£IE CMSE ..:, ,. ' p , , , ',/01,; ...... / \ ApAs A ,,,; :, i 10N a X ; v QU 2ZJ.iRY ';' ( _t= = 1 1 ALi j A; tg os ( tY s *