The Vendian succession of northeastern Spitsbergen: Petrogenesis of a dolomite-tillite association     

Ian J. Fairchild  Michael J. Hambrey 《Precambrian Research》1984,26(2):111-167

The sedimentary record of late Precambrian time is magnificently displayed in the highland snowfields of northeastern Spitsbergen (Svalbard). Vendian strata are represented essentially by the Polarisbreen Group which consists mostly of dolostone and includes two dolomitic glacial units. The oldest sediments in the Polarisbreen Group compose the Elbobreen Formation (c. 400 m), which is divided into four laterally-persistent members. The Lower Carbonate Member (E1, 125 m) contains a distinctive basal dark-grey limestone (with microspar-filled synaeresis cracks) suggested to be of lagoonal origin and associated with minor dolostone, shale and chert. Higher parts of the member are dominantly dolostone, partly stromatolitic, with some shale and sandstone; shallow subtidal to intertidal deposition is indicated by the dominance of intraclastic lithologies and relics of anhydrite. Penecontemporaneous dolomite is partially overprinted by microsparry dolomite, thought to be of groundwater origin.The redefined Petrovbreen Member (E2) consists of diamictite and other detrital dolostone. Pronounced thickness variations (2–40 m) are thought to be original depositional features. The member represents the deposits of a short glacial period in which the following depositional processes are inferred: lodgement (massive diamictite), subaqueous meltout (massive and bedded diamictite), ice-rafting (lithologies bearing dropstones, and possibly also diamictite), redeposition by sediment gravity flows (some diamictite and conglomerate; rhythmite and shale), current winnowing (thin tabular conglomerate), subaerial or subaqueous meltwater action (channelled conglomerate and sandstone), periglacial shrinkage (diamictite wedge-fillings).The MacDonaldryggen Member (E3, 230 m) is a monotonous succession of shaly dolostone of lagoonal origin. It grades up into the Slangen Member (E4, 25 m) which consists of subtidal to intertidal dolarenite with anhydrite relics succeeded by fenestral dolostone that was fractured and cemented by saline groundwaters in an emergent environment.The Wilsonbreen Formation (160 m) represents a return to glacial deposition, but this time longer-lasting and with substantial extra-basinal material represented. The Gropbreen Member (W1, 28–107 m) and the Ormen Member (W3, 44–139 m) consist dominantly of dolomitic diamictite with subordinate conglomerate and sandstone and are separated by a Middle Carbonate Member (W2, 3–30 m) which contains distinctive rhythmitic and stromatolitic limestone as well as sandstone. The same depositional processes can be recognised as in the Petrovbreen Member, but the Wilsonbreen Formation is overall of somewhat more continental aspect (lower proportion of rhythmite and dropstone structures). In addition there are: basal breccia and crack-fillings formed by frost-shattering of the underlying cemented dolostone, tabular sandstone thought to be formed by wave reworking of outwash, a striated (terrestrial) cobble pavement, supraglacially-derived breccia horizons, periglacial wedges filled by sand and the W2 assemblage of possible lacustrine origin.The Dracoisen Formation (525 m) represents an abrupt return to non-glacial conditions. An upward-deepening wave-dominated succession of pure dolostone (D1, 20 m) and impure dolostone (D2, 105 m) is succeeded by offshore black shale (D3, 150 m) and then by a very-shallow water succession of evaporite lacustrine aspect with a dolostone containing evaporite relics (D5, 10 m) separating dolomitic sandstone and shale (D4, 80 m and D6, 150 m). The contact with the transgressive Cambrian sandstones above is poorly exposed.Deposition of the succession dominantly under marine conditions is inferred, but it is difficult to rule out a lacustrine environment at any particular horizon. This dolomite—tillite association can be explained by penecontemporaneous (and minor secondary) dolomite formation in marginal environments (with warm climatic indicators at some levels) being sharply interrupted, because of rapid climatic changes, by glacial sediments containing abundant detrital dolomite. Since the latter sediments make up only 17% of the 1080m-thick succession, glacial conditions only occupied a small proportion of Vendian time.  相似文献   

Evaluation of the Dominion Observatory Bronze Pendulum Apparatus     

Angus C. Hamilton 《Geophysical Journal International》1961,6(1):9-28

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Upper Miocene and Pliocene geomagnetic secular variation in the Borgarfjördur area of Western Iceland     

Norman D. Watkins  Ian McDougall  Leo Kristjansson 《Geophysical Journal International》1977,49(3):609-632

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On the birtrate of galactic supernovae     

Ian Lerche 《Astrophysics and Space Science》1981,74(2):273-286

The birthrate of galactic supernovae is estimated in three different ways:

1. on the basis of the historical record (eight events) the mean time interval between supernovae, τ, is considered to be in the range τ=60±40 yr;
2. on the basis of an approximate total of 120 supernovae events in hundreds of other galaxies, considered similar to our own, the interval obtained is in the range τ=70±50 yr; (iii) on the
3. on the basis of the 130 supernovae remnants in our own Galaxy, the interval is estimated to be in the range τ=80±30 yr. The three ranges overlap, and we suggest that 70±35 yr represents a more realistic estimate of the rate than some that have previously been made.

The galactic radio supernovae remnants, and their observed systematic brightness gradients perpendicular to the galactic plane, imply a scale height of about 200 pc for the remnant progenitors, and indicate that the galactic magnetic field's scale-height is about 300 pc. Long standing anomalies associated with (a) the young remnant AD 1006, (b) the galactic loops, and (c) faint remnants, are accounted for by the brightness gradient effect, providing independent, and firm, corroborating evidence for the fundamental validity of the remnant method of deducing the galactic supernova birthrate.  相似文献   

Geochemistry of mafic dykes in the Antarctic Peninsula continental-margin batholith: a record of arc evolution   总被引：6，自引：0，他引：6  

Jane H. Scarrow  Philip T. Leat  Christopher D. Wareham  Ian L. Millar 《Contributions to Mineralogy and Petrology》1998,131(2-3):289-305

Mafic dykes of the Antarctic Peninsula continental-margin arc are compositionally diverse, comprising calc-alkaline (dominant), shoshonite, tholeiite, and OIB-like varieties. Their compositions give information about different mafic magma sources tapped during arc evolution. The compositional groups represent partial melts of at least five distinct mantle sources: a low-ɛNd subduction-modified, garnet-bearing, lithospheric mantle (older calc-alkaline); a high-ɛNd subduction-modified, garnet-bearing, lithospheric mantle (shoshonites); a high-ɛNd subduction-modified, spinel-bearing, asthenospheric mantle (younger calc-alkaline); E-MORB-like spinel-bearing asthenosphere depleted by a previous melting event (tholeiites); and within-plate non-subduction modified, garnet- and spinel-bearing, asthenosphere (OIB-like). Slab-derived fluids, subducted sediment, and arc crust also contributed to the magmas. Consideration of previous work in the light of our new compositional and geochronological data enables presentation of a summary of arc evolution. For most of the Cretaceous and Tertiary, the tectonic regime of the Antarctic Peninsula arc was transtensional, and calc-alkaline magmas intruded. An oceanic spreading centre collided with the trench during the Late Cretaceous and induced tectonic changes which caused tapping of different magma sources. A pulse of shoshonitic, tholeiitic, and OIB-like mafic magmatism resulted. Three ridge-trench collisions are now recognized during the history of the arc, in Mid–Late Jurassic, Late Cretaceous, and Early–Mid Tertiary times. Received: 13 January 1997 / Accepted: 5 December 1997  相似文献   

The Predictability of Large Earthquakes: Evidence from an Analogue Model of Earthquake Rupture     

T. Hamilton  J. McCloskey 《Pure and Applied Geophysics》1998,152(1):23-35

—The recurrence behaviour of large earthquakes, in several tectonic settings, has been explained by simple models of stress accumulation and release which assume that the fault stress state is solely a function of the far-field tectonic strain rate. However, the limited dataset of large event recurrence intervals has been a major obstacle to the verification of these and other models. We present the results from a simple analogue model of earthquake rupture and stick-slip which displays power-law frequency-size statistics and involves many cycles of large events. We show that, despite the macroscopic homogeneity of the model, large events do not conform to simple deterministic time- or slip-predictable patterns. However, when the recurrence intervals for large events are divided by the median recurrence interval, the normalized data are composed of two distinct lognormally distributed populations. One population is characterized by events which are strongly clustered in time with relatively short recurrence intervals and low moment release, the other by events which are weakly clustered in time with median-sized recurrence intervals. It is suggested that the long-term recurrence behaviour of large earthquakes, whilst being non-deterministic, may be modelled by a well-defined statistical distribution of recurrence intervals.  相似文献   

地质因素对源岩评价结果的影响及其相对贡献量大小模拟     

庞雄奇  陈章明 《沉积学报》1998,16(1):50-57

源岩评价是油气资源评价的基础。目前,基于源岩排烃过程模拟结果可对源岩品质、属性和有效性进行综合定量评价。本文采用盆地模拟技术和不确定性分析方法对影响源岩评价的十三种地质因素的作用大小及其相对贡献量进行评价,同时探讨各种地质因素作用下源岩品质评价指数,属性评价指数和有效性评价指数的变化情况及分布概率。  相似文献   

Nutrient uptake in a highly turbid estuary (the Humber, United Kingdom) and adjacent coastal waters     

Peter J. Shaw  Duncan A. Purdie  Pedro S. de Frietas  Andrew P. Rees  Ian Joint 《Estuaries and Coasts》1998,21(4):507-517

Surveys were conducted in April and June 1995 to quantify the uptake of dissolved nutrients in a highly turbid estuary (the Humber, United Kingdom) and to determine the factors controlling nutrient uptake rates. A combination of isotope labelling methods were used in conjunction with on-deck incubation techniques to estimate the uptake of dissolved nutrients (PO₄ ^3?, NH₄ ⁺, NO₃ ^?, and urea) in surface samples collected from coastal waters. Similarly, isotope labelling and laboratory incubgation techniques were employed to estimate dissolved nitrogen uptake (NH₄ ⁺, NO₃ ^?, and urea) in surface samples collected from the estuary mouth. Nutrient uptake rates were at the low end of ranges for coastal and estuarine environments reported in the literature. Concentrations of chlorophyll and the availability of photosynthetically active radiation were identified as potentially important factors controlling the uptake rates of nutrients. Uptake rates of dissolved nitrogen in the Humber mouth appeared to be related to the location of smapling sites. Depletion rates of dissolved nutrients in situ were estimated on the basis of integrated water column nutrient uptake rates and indicated assimilation of up to 16% of nutrients in the entire water column. Estimated depletion rates did not indicate preferential loss of any of the nutrient species investigated.  相似文献   

A palaeolimnological study of Tugulnuit Lake, British Columbia, Canada, with special emphasis on river influence as recorded by chironomids in the lake's sediment     

Andrea Rück  Ian R. Walker  Richard Hebda 《Journal of Paleolimnology》1998,19(1):63-75

Sediments from Tugulnuit Lake in the Okanagan Valley of British Columbia, Canada, were examined for chironomid assemblages. The chironomid stratigraphy obtained encompasses the last 4000 to 5000 years and suggests a warm and fairly stable climate typical for a temperate lake at low- to mid-elevation. This is indicated by the even distribution of warm-water taxa, such as Cladopelma, Dicrotendipes, Polypedilum, Pentaneurini, Stempellina, Stempellinella/Zavrelia and Pseudochironomus throughout the core. Very few cold-water taxa occurred in the sediments. However, stream inputs have had a major impact on Tugulnuit Lake. Sandy sediments and the appearance of Simuliidae and stream-inhabiting chironomid taxa (e.g., Brillia/Euryhapsis, Eukiefferiella/Tvetenia, Rheocricotopus) indicate that a stream intruded into the current lake's basin ca. 3800 yr Before Present (BP). Sediments deposited prior to, and after, the stream's intrusion show a distinctly different chironomid assemblage exhibiting chironomid taxa more typical for lentic habitats. This result indicates that chironomids can serve to detect past stream influences on lake environments. Thus, rheophilic chironomids preserved in lake cores provide a new alternative for reconstructing stream palaeoenvironmental records.  相似文献   

Time-dependent structure in ultraviolet absorption lines of the rapid rotators HD 64760 (B0 Ib) and HD 93521 (O9.5 V)     

Ian D. Howarth  Townsend  Clayton  Fullerton  Gies  Massa  Prinja  & Reid 《Monthly notices of the Royal Astronomical Society》1998,296(4):949-960

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