排序方式： 共有21条查询结果，搜索用时 218 毫秒
1.
2.
Tremolite-hornblende relationships are reported for high gradeand relatively low grade zones within the Barrovian type SkagitSuite, using rocks that range widely enough in composition toqualify as potential hosts of any calcic amphibole from practicallyA1-free to A1-rich types. Out of over 100 samples analysed bymicroprobe, 22 representative analyses are listed, with end-membercalculations. In the sillimanite grade core of the Skagit Suite, calcic amphibolesfrom various, commonly metasomatized metamorphic ultramaficsand genetically related hornblendites, from amphibolites, schistsand gneisses, and from some metamorphosed impure dolomites showcontinuous solid solution between tremolite and highly aluminoushornblendes ranging from almost Fe-free to moderately Fe-richtypes. A1total/A1IV is nearly constant and approximates 1?44.The second group of calcic amphiboles studied is from the lower-mediumrange of the epidote amphibolite facies (comprehensively defined),that is, from near and above the oligoclase isograd. Host rocksare variously metasomatized meta-peridotites, and amphibolitesand schists. There is a large compositional gap between analysesof tremolites and of moderately to highly aluminous hornblendes.A1total/A1IV approximates 1?73 both in the tremolite and hornblendefields, as against 1?44 at high grade. Fe-poor hornblendes,such as are stable at high grade, were not found at the lowergrade. At both grades, Al shows good overall correlation withNa+K, with A occupancy, and with Ti (with more Ti at high grade). The compositional gap between tremolite and hornblende analysesfrom the lower grade rocks does not necessarily define a solvusbecause no tremolite-hornblende pairs were found. Rather, theanalyses provide outer limits on the possible width of a solvusat this grade (T). However, certain data suggest that a truemiscibility gap not only exists but probably is not a greatdeal narrower than the gap between the actual analyses. Besides,the proposed restriction on A1/Fe ratios at this grade wouldreduce the probability of finding tremolite-hornblende pairs. 相似文献
3.
Subaerial unconformities are used widely for palaeoenvironmental and palaeogeographic reconstructions, sequence stratigraphy and petroleum reservoir assessments. Recognition and interpretation of these unconformities, particularly those with associated palaeosols, may be problematic in Lower and Middle Palaeozoic carbonate successions because of the collective effect of limited land plant development, superficial similarities between some pedogenic and marine features, and overprinting by later diagenesis. The isolated Judy Creek reef complex in the Lower Frasnian Swan Hills Formation in west‐central Alberta, Canada, contains two subaerial unconformities, R0.5 and R4, which formed as a consequence of relative sea‐level falls of at least regional scale. Deposits beneath these unconformities have distinctive palaeosol and palaeokarst features. The lower unconformity, R0.5, occurs at the top of a progradational reefal phase of stromatoporoid rudstones–floatstones and peloidal packstones–grainstones and has been recognized in at least one other isolated Swan Hills reef complex (Snipe Lake). Palaeosol–palaeokarst profiles beneath this unconformity extend as deep as ca 2 m below the unconformity. These profiles are characterized by the presence of small rhizoliths, laminar calcretes, ferroan dolomite glaebules, desiccation cracks, breccias, green shale and solution vugs. The upper unconformity, R4, occurs at the top of a backstepping phase of reef growth and has been correlated widely between isolated reefs and carbonate banks on both the western and eastern shelves of the Central Alberta Basin. Palaeosol–palaeokarst profiles, extending as deep as ca 9·5 m beneath the R4 unconformity, are distinguished by abundant, sub‐horizontal desiccation cracks filled with green shale, occurring in peloidal wackestones–packstones. Comparison of the R0.5 and R4 profiles indicates that the major intrinsic controls on the development and modification of the profiles are parent‐material lithology, particularly the prior degree of induration and particle size; the low topographic relief at the top of the reef interior; and limited vegetation of the exposed reef top due to unfavourable growth conditions and geographic isolation. In addition to climate, the major extrinsic controls are the extent of relative sea‐level fall, estimated to be 2·5 to 3 m and 13 to 14 m associated with the R0.5 and R4 unconformities, respectively, and the degree of shoreface erosion during the ensuing marine transgression, estimated to be up to 3 m. This study highlights the complex interplay of mainly physical and chemical processes influencing the formation of subaerial unconformities in carbonate environments during the Devonian, before major evolutionary innovations among vascular land plants led to more intense pedogenesis. 相似文献
4.
5.
6.
GEOFFREY S. BOULTON P. NEIL CHROSTON JACK JARVIS 《Boreas: An International Journal of Quaternary Research》1981,10(1):39-51
A sparker survey was undertaken of the sea area inshore of the peninsula of sleat and the islands of Eigg and Muck in Western Scotland. This revealed major submarine moraines across the mouths of Loch Nevis and Loch Ailort, which help define the margin of a major glacier readvance phase, presumed to be equivalent to the late-glacial Loch Lomond Readvance. Formation names are suggested for the seismic para-stratigraphy. West of the moraines, there is a till (Minch Para-formation) resting on bedrock, overlain by a stratum (Muck para-formation) with well-defined internal layering parallel to the substratum. The till is presumed to have been deposited by an ice cap which at its maximum reached the western edge of the continental shelf at some time after 27,000 B.P. The Muck para-formation probably represents a glaciomarine unit deposited during the retreat of this ice cap, and has been deeply eroded. Above this erosion surface occur a series of sediments which infill local basins, and which appear to be of Flandrian age (Arisaig para-formation). East of the moraines in Loch Nevis and Loch Ailort the draped sediments are missing and the till is overlain by apparent equivalents (Nevis para-formation) of the marine sediments of Flandrian age to the west 相似文献
7.
F. RICHARD HAUER JILL S. BARON DONALD H. CAMPBELL KURT D. FAUSCH STEVE W. HOSTETLER GEORGE H. LEAVESLEY PETER R. LEAVITT DIANE M. MCKNIGHT JACK A. STANFORD 《水文研究》1997,11(8):903-924
The Rocky Mountains in the USA and Canada encompass the interior cordillera of western North America, from the southern Yukon to northern New Mexico. Annual weather patterns are cold in winter and mild in summer. Precipitation has high seasonal and interannual variation and may differ by an order of magnitude between geographically close locales, depending on slope, aspect and local climatic and orographic conditions. The region's hydrology is characterized by the accumulation of winter snow, spring snowmelt and autumnal baseflows. During the 2–3-month ‘spring runoff’ period, rivers frequently discharge > 70% of their annual water budget and have instantaneous discharges 10–100 times mean low flow. Complex weather patterns characterized by high spatial and temporal variability make predictions of future conditions tenuous. However, general patterns are identifiable; northern and western portions of the region are dominated by maritime weather patterns from the North Pacific, central areas and eastern slopes are dominated by continental air masses and southern portions receive seasonally variable atmospheric circulation from the Pacific and the Gulf of Mexico. Significant interannual variations occur in these general patterns, possibly related to ENSO (El Niño–Southern Oscillation) forcing. Changes in precipitation and temperature regimes or patterns have significant potential effects on the distribution and abundance of plants and animals. For example, elevation of the timber-line is principally a function of temperature. Palaeolimnological investigations have shown significant shifts in phyto- and zoo-plankton populations as alpine lakes shift between being above or below the timber-line. Likewise, streamside vegetation has a significant effect on stream ecosystem structure and function. Changes in stream temperature regimes result in significant changes in community composition as a consequence of bioenergetic factors. Stenothermic species could be extirpated as appropriate thermal criteria disappear. Warming temperatures may geographically isolate cold water stream fishes in increasingly confined headwaters. The heat budgets of large lakes may be affected resulting in a change of state between dimictic and warm monomictic character. Uncertainties associated with prediction are increased by the planting of fish in historically fishless, high mountain lakes and the introduction of non-native species of fishes and invertebrates into often previously simple food-webs of large valley bottom lakes and streams. Many of the streams and rivers suffer from the anthropogenic effects of abstraction and regulation. Likewise, many of the large lakes receive nutrient loads from a growing human population. We concluded that: (1) regional climate models are required to resolve adequately the complexities of the high gradient landscapes; (2) extensive wilderness preserves and national park lands, so prevalent in the Rocky Mountain Region, provide sensitive areas for differentiation of anthropogenic effects from climate effects; and (3) future research should encompass both short-term intensive studies and long-term monitoring studies developed within comprehensive experimental arrays of streams and lakes specifically designed to address the issue of anthropogenic versus climatic effects. © 1997 John Wiley & Sons, Ltd. 相似文献
8.
9.
10.
ABSTRACT. The city of Amarillo, Texas, is unusual in that more than 5,000 art objects in the form of signs are displayed on individual properties. These signs represent a unique partnership between the public and a wealthy individual, Stanley Marsh 3, who subsidizes them. Through a field survey of 723 signs and a questionnaire mailed to 98 residents with signs in their yards, we explored use of the signs for communal and individual expression. The field survey found a higher concentration of signs in low‐ and moderate‐income neighborhoods and in Hispanic areas than in high‐income and non‐Hispanic neighborhoods. The questionnaire revealed that residents used signs for both individual and communal expression and that most residents with signs liked them. Dissatisfaction among a small percentage of residents with signs suggested that the vast number of signs may have compromised their initial uniqueness. 相似文献