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81.
ALAIN TABBAGH YVES BENDERITTER PIERRE ANDRIEUX JEAN PAUL DECRIAUD ROGER GUERIN 《Geophysical Prospecting》1991,39(8):1083-1097
For over 20 years, powerful VLF transmitters have been used as electromagnetic sources for subsurface investigations in mining exploration. Measurements initially concerned the vertical component of the magnetic field or the inclination of the field and were later extended to measurement of the horizontal electric field in the direction of the transmitter, to determine the resistivity of the terrain. Measurement of the electric field is usually performed with electric lines, grounded or not, with lengths of at least 5 m. This paper presents the concept of a VLF resistivity meter with a very short electric sensor (1 m) and the results obtained with it. This technique improves the measurement of the electric field, which is in principle a point value. It also permits a higher spatial sampling rate and, by closely linking the electric sensor with the magnetic sensor on a lightweight mount, makes it possible for the instrument to be used by a single operator. In addition, transformation of the electric field data, analogous to reduction to the pole in magnetism, is proposed to correct the horizontal deformation of the anomalies created by polarization of the primary field. Comparison with direct current electrical measurements shows highly satisfactory correlations. This transformation, validated for VLF, can be extended to any electrical or electromagnetic method using a uniform primary field, i.e. gradient array in direct current or low-frequency magnetotellurics. We call this verticalization of the electric field. Resistivity measurements and mapping using the VLF frequency range can be applied not only to mining but also to a wide range of shallow geophysical studies (hydrology, civil engineering, etc.) and are not limited to problems concerning the location of conductive targets 相似文献
82.
The Cambro-Ordovician Cap Enragé Formation is interpreted as a deep submarine channel complex of conglomerates, pebbly sandstones and massive sandstones. The formation is up to 270 m thick, and crops out in a coastal belt 50 km long. In general terms, it has previously been interpreted as a deep sea channel deposit, with the channel about 300 m deep, at least 10 km wide and trending south-westward, parallel to the coastal outcrops. Eight facies have been defined in this study and they have been grouped into three major facies associations. In the Coarse Channelled Association, conglomerates with carbonate boulders up to about 4 m are associated with graded-stratified finer grained conglomerates. Facies of this association make up about 25% of all the beds in the formation. The association is also characterized by abundant major channels 1–10 m deep and up to 250 m wide. Excellent outcrop allows the reconstruction of topographic highs (bars) within the channels and the association is interpreted as a braided channel and bar system. The second association, Multiple-Scoured Coarse Sandstones, contains some graded-stratified fine conglomerates, along with massive to structureless coarse and pebbly sandstones, and rare cross-bedded pebbly sandstones. Deep channels are absent, but multiple channelling on the scale of 0.5–1 m is characteristic. In the absence of the very coarse conglomerates and deeper channelling, this association is interpreted as being deposited on topographically higher terrace areas adjacent to the main braid plain. The third facies association, Unchannelled Sandstones, is characterized by massive sandstones with abundant fluid-escape structures, classical turbidites and thin shales. In the absence of any scouring deeper than a few tens of centimetres, this association is interpreted as being deposited on an even higher and smoother terrace, farther from the braid plain. Palaeoflow directions for conglomerate facies indicate fairly consistent south-westward transport, apparently parallel to the base of the Cambro-Ordovician continental slope. Flow directions in the finer-grained facies are rather variable, suggesting complex bar development and overbank spills. Thinning-and fining-upward sequences are present on two scales. The smaller, 1–10 m sequence, is related to channel filling and abandonment. Thicker sequences (10–100 m), with facies of the Multiple Scoured, and Unchannelled Sandstone Associations, may indicate switching of a main channel away from the area and its subsequent burial by marginal terrace and higher terrace deposits. 相似文献
83.
Facies relationships in Pleistocene outwash gravels, southern Ontario: a model for bar growth in braided rivers 总被引:1,自引:1,他引:1
Facies relationships in Pleistocene braided outwash deposits in southern Ontario demonstrate the presence of a large braid bar with adjacent side channel. The core of the bar is up to 6 m high, and consists of crudely horizontally stratified gravels. Downstream from the core is the bar front facies, consisting of large gravelly foresets up to 4 m high, rounded off in many places by reactivation surfaces. Upstream from the core is the bar stoss side facies consisting of several sets (individually up to 35 cm thick) of tabular cross-bedding, arranged in coarsening-upward sequences. The stoss side—core—bar front relationships are continuously exposed in one 400 m long quarry face which is cut almost parallel to the palaeoflow direction. A transverse quarry face shows the side channel facies, which consists of trough cross-bedded sands. Gravel layers can be seen to finger from the main gravelly bar into the sandy side channel, but they do not reach the base of the channel. This surprising relationship indicates that gravel moved only in the topographically higher parts of the system. After deposition in the side channel, and growth upstream and downstream from the bar core, the entire system aggraded. Crudely horizontally stratified, and imbricated gravel sheets were laid down as a bar top facies. Grain size analyses indicate strongly bimodal distributions, implying that much of the sand in the spaces between pebbles and boulders filtered in after the gravel had been deposited. This interpretation is strengthened by velocity calculations—mean velocities in excess of 300 cm/s would be needed to roll the gravel as bed load, but at such a velocity, a large amount of sand would be transported entirely in suspension. In a final section of the paper, our results are combined with other work on braided systems in an attempt to formulate a more general facies model. 相似文献
84.
D. W. POWELL 《Geophysical Prospecting》1967,15(2):208-220
The symmetry shown, by the function describing the magnetic anomaly due to a dyke or vertical fault step leads to a method by which an observed profile may be adjusted to conform with that symmetry. By making these adjustments a minimum the ideal dyke or fault profile closest to the observations is constructed. In three of four possible situations this is interpreted by simple geometry. Parallelpiped models may be transformed into two dykes for symmetry analysis. Depth limited models are not so amenable, even in special cases. 相似文献
85.
JOHNSON TIM E.; GIBSON ROGER L.; BROWN MICHAEL; BUICK IAN S.; CARTWRIGHT IAN 《Journal of Petrology》2003,44(5):789-813
Metapelitic rocks in the aureole beneath the Bushveld Complexpreserve evidence for both high- and low-aH2O anatexis. Theaureole is characterized by an inverted thermal structure inwhich suprasolidus rocks potentially interacted with an H2O-richvolatile phase derived from underlying, dehydrating rocks. Atlower grade (T < 700°C) the rocks contain fibrolite matsand seams that record local redistribution of volatiles. Incongruentreactions consuming biotite produced small quantities (<1mol %) of liquid and peritectic cordierite that remained trappedwithin the mesosome. Larger volumes of melt (34%), preservedas coarse-grained discordant leucosomes, were produced by congruentmelting following a structurally focused influx of H2O. Subhorizontalvolatile-phase flow was concentrated within thin ( 相似文献
86.
What status for the Quaternary? 总被引:3,自引:0,他引:3
PHILIP L. GIBBARD ALAN G. SMITH JAN A. ZALASIEWICZ TIFFANY L. BARRY DAVID CANTRILL ANGELA L. COE JOHN C. W. COPE REW S. GALE F. JOHN GREGORY JOHN H. POWELL PETER F. RAWSON PHILIP STONE COLIN N. WATERS 《Boreas: An International Journal of Quaternary Research》2005,34(1):1-6
The status of the Quaternary, long regarded as a geological period effectively coincident with the main climatic deterioration of the current Ice Age, has recently been questioned as a formal stratigraphic unit. We argue here that it should be retained as a formal period of geological time. Furthermore, we consider that its beginning should be placed at the Gauss-Matuyama magnetic chron boundary at about 2.6 Ma, rather than at its current position at about 1.8 Ma. The Quaternary would be formally subdivided into the Pleistocene and Holocene epochs. The global chronostratigraphical correlation table proposed is enclosed at the back of this issue. 相似文献
87.
The effect of Mn on mineral stability in metapelites 总被引:17,自引:2,他引:17
E. M. MAHAR J. M. BAKER R. POWELL T. J. B. HOLLAND & N. HOWELL 《Journal of Metamorphic Geology》1997,15(2):223-238
Calculations based on a KMnFMASH petrogenetic grid derived using an internally consistent thermodynamic dataset indicate that the principal effect of the presence of Mn in average subaluminous pelite compositions is to stabilize garnet to higher and lower pressures and temperatures over a wide range of bulk compositions. Garnet-bearing fields expand to lower temperatures and pressures with the addition of Mn, and garnet appears as an extra phase at low pressures. The addition of Mn also increases the number and extent of four AMnFM phase assemblages and stabilizes five AMnFM phases along univariant reactions. The KMnFMASH system predictions for typical subaluminous pelite bulk compositions match the sequence of isograds and assemblages observed in the Barrovian zones. The sequence of assemblages observed in the Stonehaven section can also be predicted if there is variation in bulk composition within the stratigraphic section. Mn appears to be less important in producing the sequence of isograds and garnet-absent assemblages in the low-pressure Buchan zones. The addition of Mn to the calculations does not change the sequence of isograds that are predicted to be stable in a regional metamorphic terrane, but the P – T position of these isograds does change. In particular, the predicted temperature of the garnet-in isograd is lowered by as much as 100 °C by the addition of Mn to KFMASH. Mn also increases the range of metapelite bulk compositions that develop the assemblages traditionally identified as metapelite isograds. 相似文献
88.
Melting Experiments on a Sanidine Phlogopite Lamproite at 4-7 GPa and their Bearing on the Sources of Lamproitic Magmas 总被引:2,自引:0,他引:2
Suprasolidus phase relations at pressures from 4 to 7 GPa andtemperatures from 1000 to 1700C have been determined experimentallyfor a sanidine phlogopite lamproite from North Table Mountain,Leucite Hills, Wyoming. The lamproite is silica rich and hasbeen postulated to be representative of the magmas which wereparental to the Leucite Hills volcanic field. Near-liquidusphases above 5 GPa are pyrope-rich garnet and jadeite-rich pyroxene.Below 5 GPa, jadeite-poor pyroxene is the only near-liquidusphase. Near-solidus assemblages consist of clinopyroxene, titanianpotassium richterite and titanian phlogopite with either potassiumtitanian silicate above 5 GPa or potassium feldspar below 5GPa. The potassium titanian silicate is a newly recognized high-pressurephase ranging in composition from K4Ti2Si7O20 to K4TiSi8O20.It coexists with coesite at pressures above 6 GPa at 11001400C.A previously unrecognized K-Ba-phosphate is a common near-solidusphase. The phase relationships are interpreted to suggest thatlamproites cannot be derived by the partial melting of simplelherzolitic sources. However, it is proposed that sanidine phlogopitelamproites an derived by high degrees of partial melting ofancient metasomatic veins within a harzburgiticlherzoliticlithospheric substrate mantle. The veins are considered to consistof phlogopite, KTi-richterite, KBa-phosphate andKTi-silicates. KEY WORDS: lamproilte; experimental petrology; upper mantle
*Corresponding author 相似文献
89.
Sedimentology and tectonic implications of Cretaceous fan-delta conglomerates, Queen Charlotte Islands, Canada 总被引:1,自引:0,他引:1
ROGER HIGGS 《Sedimentology》1990,37(1):83-103
The Honna Formation, of Coniacian age, consists of several hundred metres of polymictic clast-supported conglomerate associated with sandstone and mudstone. Five conglomerate facies are recognized: ungraded beds; inverse graded beds; normal graded beds; inverse-to-normal graded beds; and parallel-stratified beds. These facies are interpreted as the deposits of subaqueous cohesionless debris flows and/or high-density turbidity currents. The depositional environment was a deep-water, gravelly fan that draped a fault-controlled, basin-margin slope. The fan is inferred to have passed upslope directly into an alluvial fan (unpreserved); hence, the name fan delta can be applied to the overall depositional system. This type of fan delta, of which the Brae oilfield in the North Sea is an example, is defined here as a deep-water fan delta. The lack of a shelf is in marked contrast to other types of fan delta. Three facies associations are recognized in the Honna Formation: subaqueous proximal-fan conglomerates, distal-fan turbiditic sandstones, and pro-fan/interfan mudstones with thin sandy turbidites. The proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km; such a long subaqueous gravel belt lacks a known modern analogue. The distal fan was an unchannelled sandy extension of the proximal gravel belt. It is postulated that the Honna Formation accumulated in a foreland basin which migrated westwards from the Coast Mountains where the Wrangellia-Alexander terrane was colliding with North America. In this model, the Honna fan delta was sourced by a (west-verging) thrust sheet whose sole-thrust was the Sandspit Fault immediately to the east. Deep-water fan deltas appear to develop preferentially when eustatic sea-level is relatively high, so that the‘feeder’ alluvial fan is small, and gravelly throughout. In petroleum exploration and field development, care should be taken to distinguish deep-water fan deltas from base-of-slope (canyon-fed) submarine fans, because the two systems differ significantly in terms of coarse-sediment distribution. 相似文献
90.
Discordant Iron-Rich Ultramafic Pegmatites in the Bushveld Complex and their Relationship to Iron-Rich Intercumulus and Residual Liquids 总被引:4,自引:1,他引:4
The iron-rich ultramafic pegmatites comprise a suite of coarse-grainedrocks that form discordant bodies within the layered sequenceof the Bushveld Complex. These pegmatites, which are considerablymore abundant than is generally realized, provide evidence forthe differentiation of iron-rich residual melts. The pegmatitesare composed largely of iron-rich olivine and clinopyroxene,together with Ti-magnetite and ilmenite. Feldspar is characteristicallyabsent, but paradoxically the pegmatites preferentially replaceanorthositic cumulates. Two subgroups are recognized, olivine-clinopyroxenepegmatite and Fe-Ti oxides pegmatite. With increased stratigraphicheight the pegmatites become richer in Fe-Ti oxides. Thus, olivine-clinopyroxenepegmatite is prevalent in the Upper Critical and Lower MainZones, whereas Fe-Ti oxide pegmatite is restricted to the UpperMain and Upper Zones. Zoned pegmatite, with a core of Fe-Tioxide pegmatite, is transitional between the two subgroups. New whole-rock and electron microprobe analyses of olivine-clinopyroxenepegmatite from the Upper Critical and Lower Main Zones provideconvincing evidence that their composition is directly relatedto height. Cryptic compositional variations are analogous tothose displayed by the layered cumulates, but for a given heightthe pegmatites are always more evolved. Compositions of clinopyroxenein the pegmatites reflect a near-linear relationship with height,whereas cumulus pyroxenes display upward iron-enrichment trendscomplicated by replenishment and reaction with trapped intercumulusliquid. The olivine-clinopyroxene pegmatite formed by magmatic replacementof earlier-formed cumulates in response to infiltration of iron-richmelts. Suitably differentiated melts comprised intercumulusand residual liquids derived from thick anorthosite layers.The absence of feldspar, although not fully understood, is attributedto an immiscible relationship between dense, iron-rich meltsand light, silica-alkali-rich liquids. The latter infiltratedupward to be reincorporated into the resident magma. The iron-richmelts, however, drained down into the crystallizing cumulatepile. Channelling along early-formed fractures and joints wassignificant, locally resulting in huge pipe-like bodies of pegmatite. The iron-rich melts became increasingly differentiated withheight, partly in response to the fractional crystallizationof more evolved cumulates. The olivine-clinopyroxene pegmatitesare related to infiltration of Fe-Ti oxide-rich silicate melt,whereas Fe-Ti oxide pegmatite is ascribed to Fe-Ti oxide liquid,as originally argued by Bateman (1951). The Bushveld Complexfollowed the Fenner trend of almost uninterrupted iron enrichment.Evidence of pronounced iron enrichment is, however, manifestedin the discordant iron-rich ultramafic pegmatites several thousandsof metres below the height at which iron-rich cumulates areobserved. 相似文献