LINE CURRENT FILTERING OF FIXED-LOOP TRANSIENT ELECTROMAGNETIC DATA1

The VLF filtering technique of Karous and Hjelt has been applied to fixed-loop step-response transient electromagnetic data. This allows the data measured in each channel to be converted to an equivalent current-density pseudosection. For a conductive half-space, the maximum value of the equivalent current density starts near the transmitter loop and migrates outwards as a function of delay time. The rate of migration tends to increase as a function of delay time, with the increase being faster for a surficial conductive layer than it is for a half-space. Theoretical and field examples show that the currents tend to be more persistent in the relatively conductive areas, so that a pseudosection which is the average of the current densities at all delay times will highlight the more conductive zones. In resistive ground, it is not so critical to average the pseudosections as a particular delay time may give a better idea of the conductivity structure. For example, the latest possible delay time will reveal the most conductive features.     

Phase Petrology and Mineral Chemistry of Coexisting Amphiboles from Telemark, Norway

Electron microprobe analyses of coexisting amphiboles, garnet,and cordierite in specimens collected from a single outcropprovide the basis for phase diagrams thought to be representativeof fixed external conditions in the sillimanite zone. A projectionthrough the compositions of quartz and plagioclase is employedto characterize the mineral facies and to facilitate comparisonswith other metamorphic terrains. Limiting recalculations whichbracket the possible range of Fe⁺³ contents in calcic amphiboleanalyses by electron probe permit an additional projection throughthe composition of magnetite, a ubiquitous phase. From thesedata the apparent non-ideality exhibited by various combinationsof coexisting cumming-tonite, anthophyllite, gedrite, and calcicamphibole is explained by intracrystalline cation ordering.     

Zones of Progressive Regional Burial Metamorphism in Part of the Tasman Geosyncline, Eastern Australia

A succession of metamorphic zones can be recognized along theMolong Geanticline, an ancient rise within the Tasman Geosynclineof Eastern Australia. In order of increasing grade, the zonesin the volcanogenic rocks are: (1) an albite-quartz zone, containingneither prehnite, pumpellyite, actinolite, nor biotite; (2)a prehnite zone; (3) a prehnite-pumpellyite zone; (4) an actinolitezone; and (5) a biotite zone. Zeolite associations are not developed.Zones 4 and 5 lie within the greenschist facies. The rocks alongthis geanticline are generally only slightly deformed, lackingcleavage. The grade of this metamorphism within the Molong Geanticlineincreases partly in accordance with increasing stratigraphicdepth and partly increases eastwards towards the adjacent HillEnd Trough. Within the Trough, rocks show metamorphic assemblagesakin to those of Zones 4 and 5 as defined within the MolongGeanticline, but deformation is more extreme and a regionalcleavage is characteristic. In the region about the Hill EndTrough and the adjacent geanticlines there is an apparent variationfrom essentially static burial metamorphism of the geanticlinesthrough transitional types to tectonic dynamothermal metamorphismof the trough. Both metamorphisms are envisaged as having arisenduring the same general period of metamorphism, but deformationof the geanticlines was much less than deformation in the interveningHill End Trough.     

RIPPLE MARK INDICES AND THEIR USES

The following dimensionless parameters (two of them well-known and five of them new) are defined for determination of ripple mark geometry: ripple index (RI), ripple symmetry index (RSI), continuity index (CI), bifurcation index (BI), straightness index (SI), and two different parallelism indices (PI₁ and PI₂). In general, RI = 15 or less indicates wave or water current origin; RI = 17 or more indicates wind or swash origin. RSI = 1.5 or less indicates wave or swash types; RSI = 3 or more indicates wind or water current types. CI = 15 or more suggests wind or wave origin; CI = 10 or less suggests water current origin. BI = 10 or more suggests wave varieties; BI = 1 or less suggests wind varieties. SI = 10² or more indicates wind or deep-water wave types; SI = 15 to 10² indicates wind or wave types; SI =4 or less indicates current types. PI1 = 7 or more suggests wave origin; PI1 = 1 or less suggests water current origin. PI2 = 0.4 or more is probably the result of swash or water current action. PI2 = 0.2 or less is probably the result of wind or wave action. Longitudinal ripple marks (such as rib-and-furrow) and deformed or modified varieties (such as flat-topped tidal-flat ripple marks and nearly- flat-topped intermittent creek ripple marks) have been excluded, inasmuch as (1)they are commonly easy to identify from their appearance, and (2)they are difficult to measure with ordinary methods. Plots of two indices against each other on coordinate paper can be particularly useful; the best combinations are RI vs. RSI, and RI vs. PI1, although several other pairs are almost as good. Where all seven parameters can be obtained, the confidence one can have in the interpretation is close to 98%. The effects of current bias, or depth bias, on wave-type ripple marks, extend to both the symmetry (RSI) and to sediment-transport interpretations. Unless the investigator is reasonably sure that no such bias is present (i.e., RSI = 1.0 instead of some significantly higher value such as 1.5), wave-type ripple marks cannot be used to determine direction of either wave approach or sediment transport. If no such bias is present, wave-type ripple marks still cannot be used to determine precise sediment transport direction. If RSI = 1.0 precisely, it is not even necessary that the ripple crests parallel the waves that formed them. The same restrictions apply to the interpretation of micro-crossbedding (that is, ripple mark internal structure). Despite these seemingly severe limitations, general geometry commonly permits a reliable interpretation, and hence ripple marks can provide a great deal of useful data for paleogeographic interpretations. The swash-zone variety of ripple marks includes two sub-types: those modified by a small but unmistakeable hydraulic jump, and those not so modified. The RI can be used to distinguish between these two, even when they were not observed to form.     

Zircon and whole-rock Nd-Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera

Graptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛ_Nd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; T_DM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (<30%) component that was likely derived from a high‐μ (U/Pb) terrane.     

Biosedimentological zonation of Boundary Bay tidal flats, Fraser River Delta, British Columbia

Boundary Bay tidal flats on the inactive southern flank of the Fraser Delta have surface sediments consisting almost entirely of very fine to fine, well to very well sorted sands which show a gradual fining-shorewards trend. Five floral/sedimentological zones form distinct biofacies. These are, from the shoreline seaward, the saltmarsh, algal mat, upper sand wave, eelgrass and lower sand wave zones. The lower limit of the saltmarsh lies at a constant level above which the maximum duration of continuous exposure rises abruptly from ～ 12 to 40 days. Similarly, at the lower limit of the algal mat zone the maximum duration of continuous exposure jumps from 1 to ～ 2 days, and at the upper limit of the eelgrass zone from ～ 0·5 to ～ 0·8 days. These correlations between exposure and zonation are suggested to be causal. In the algal mat and eelgrass zones microtopography of biogenic origin, only a few centimetres high, creates lateral heterogeneity within the zonal biofacies. In the upper sand wave zone, very low amplitude (a～ 0·1 m) symmetrical sand waves (λ～ 30 m) of probable storm-wave origin have a similar effect. In the lower sand wave zone, sand waves (a～ 0·5 m, λ～ 60 m) are formed by tidal currents or wave action and physical sedimentary structures dominate over biogenic ones. The densities of the following macrofaunal organisms which produce distinctive biogenic sedimentary structures were determined on two surveyed transects: Callianassa californiensis and Upogebia pugettensis, both thalassinidean shrimps; three polychaete worms, Abarenicola sp., Spio sp. and Clymenella sp.; the bivalve Mya arenaria and the gastropods Batillaria attramentaria and Nassarius mendicus. Callianassa excavate unlined temporary feeding burrows, whereas Upogebia build mud-lined permanent dwelling burrows. Upogebia are restricted to below mean sea-level where continuous exposure is < 0·5 days, whereas Callianassa extend up to a level, just below mean higher high water, where maximum continuous exposure rises abruptly from 4 to 9 days. This difference in range is probably due to the latter's greater anoxia tolerance—a necessary adaptation for life in an unlined feeding burrow.     

Quantitative Simulation of the Hydrothermal Systems of Crystallizing Magmas on the Basis of Transport Theory and Oxygen Isotope Data: An analysis of the Skaergaard Intrusion

Application of the principles of transport theory to studiesof magma-hydrothermal systems permits quantitative predictionsto be made of the consequences of magma intruding into permeablerocks. Transport processes which redistribute energy, mass,and momentum in these environments can be represented by a setof partial differential equations involving the rate of changeof extensive properties in the system. Numerical approximationand computer evaluation of the transport equations effectivelysimulates the crystallization of magma, cooling of the igneousrocks, advection of chemical components, and chemical and isotopicmass transfer between minerals and aqueous solution. Numerical modeling of the deep portions of the Skaergaard magma-hydrothermalsystem has produced detailed maps of the temperature, pressure,fluid velocity, integrated fluid flux, ¹⁸O-values in rock andfluid, and extent of nonequilibrium exchange reactions betweenfluid and rock as a function of time for a two-dimensional cross-sectionthrough the pluton. An excellent match was made between calculated¹⁸O-values and the measured ¹⁸O-values in the three principalrock units, basalt, gabbro, and gneiss, as well as in xenolithsof roof rocks that are now embedded in Layered Series; the latterwere evidently depleted in ¹⁸O early in the system's coolinghistory, prior to falling to the bottom of the magma chamber.The best match was realized for a system in which the bulk rockpermeabilities were 10^–13 cm² for the intrusion, 10^–11cm² for basalt, and 10^–16 cm² for gneiss; reaction domainsizes were 0.2 cm in the intrusion and gneiss and 0.01 cm inthe basalts, and activation energy for the isotope exchangereaction between fluid and plagioclase was 30 kcal/mole. The calculated thermal history of the Skaergaard system wascharacterized by extensive fluid circulation that was largelyrestricted to the permeable basalts and to regions of the plutonstratigraphically above the basalt-gneiss unconformity. Althoughfluids circulated all around the crystallizing magma, fluidflow paths were deflected around the magma sheet during theinitial 130,000 years. At that time, crystallization of thefinal sheet of magma and fracture of the rock shifted the circulationsystem toward the center of the intrusion, thereby minimizingthe extent of isotope exchange between rocks near the marginof the intrusion at this level. For comparison, similar calculationswere also made for pure conductive cooling; it was found thatthe rate of crystallization of the magma body was not changed.The solidified pluton cooled by a factor of about 2 faster inthe presence of a hydrothermal system. Transport rates of thermal energy out of the intrusion and oflow-¹⁸O fluids into the intrusion controlled the overall isotopeexchange process. During the initial 150,000 years, temperatureswere high and reaction rates were fast; thus, fluids flowinginto the intrusion quickly equilibrated with plagioclase. However,the temperature decreased between 120,000 and 175,000 yearsand caused a decrease in reaction rates and an increase in theequilibrium fractionation factor between plagioclase and fluid.Consequently, during this time period fluids in the intrusiontended to be out of equilibrium with plagioclase. After 175,000years temperatures had decreased sufficiently that reactionrates became insignificant, but convection rates were largeenough to redistribute fluid and enlarge the regions where fluidand plagioclase were out of equilibrium. By 400,000 years, thepluton had cooled to approximately ambient temperatures, andthe final ¹⁸O values were ‘frozen in’. Reactionsbetween hydrothermal fluid and the intrusion occurred over abroad range in temperature, 1000-200 °C, but 75 per centof the fluid circulated through the intrusion while its averagetemperature was >480 °C. This relatively high temperatureis consistent with the observation that only minor amounts ofhydrothermal alteration products were formed in the naturalsystem, even where several per mil shifts in ¹⁸O were detected. The relative quantities of fluid to rock integrated over theentire cooling history were 0.52 for the upper part of intrusion,0.88 for the basalt, 0.003 for the gneiss, and 0.41 for theentire domain. Almost all of the fluid flowed into the intrusionfrom the basalt host rocks that occur adjacent to the side contactsof the intrusion. Convection transferred about 20 per cent ofthe total heat contained in the gabbro upward into the overlyingbasalts; the remaining 80 per cent of the heat was transferredby conduction.