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1.
Lateral moraines constructed along west to east sloping outlet glaciers from mountain centred, pre-last glacial maximum (LGM) ice fields of limited extent remain largely preserved in the northern Swedish landscape despite overriding by continental ice sheets, most recently during the last glacial. From field evidence, including geomorphological relationships and a detailed weathering profile including a buried soil, we have identified seven such lateral moraines that were overridden by the expansion and growth of the Fennoscandian ice sheet. Cosmogenic 10Be and 26Al exposure ages of 19 boulders from the crests of these moraines, combined with the field evidence, are correlated to episodes of moraine stabilisation, Pleistocene surface weathering, and glacial overriding. The last deglaciation event dominates the exposure ages, with 10Be and 26Al data derived from 15 moraine boulders indicating regional deglaciation 9600 ± 200 yr ago. This is the most robust numerical age for the final deglaciation of the Fennoscandian ice sheet. The older apparent exposure ages of the remaining boulders (14,600-26,400 yr) can be explained by cosmogenic nuclide inheritance from previous exposure of the moraine crests during the last glacial cycle. Their potential exposure history, based on local glacial chronologies, indicates that the current moraine morphologies formed at the latest during marine oxygen isotope stage 5. Although numerous deglaciation ages were obtained, this study demonstrates that numerical ages need to be treated with caution and assessed in light of the geomorphological evidence indicating moraines are not necessarily formed by the event that dominates the cosmogenic nuclide data. 相似文献
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Only a few very young landforms are the result of currently operating geomorphic processes. Because the time scale for landscape evolution is much longer than the time scale for late Cenozoic climate changes, almost all landscapes are palimpsests, written over repeatedly by various combinations of climate-determined processes. Relict glacial and periglacial landforms are widely identified in mid-latitude regions that have been traditionally described as having been shaped by the “normal” processes of fluvial erosion. Less confidently, deeply weathered regolith and associated relict landforms in the middle and high latitudes are attributed to early Tertiary warmth. However, assemblages of geomorphic processes specific to certain climatic regions, like faunal and floral assemblages, cannot be translated across latitude, so in spite of the many books about the geomorphology of specific modern climate regions, there are few sources that discuss former warm high-latitude, or cold low-latitude, low-altitude geomorphic processes that have no modern analogs. Students and teachers alike who attempt to interpret landforms by extrapolating modern climatic conditions to other latitudinal zones will find their outlook broadened, and they become better prepared to consider the geomorphic impacts of global climate change. 相似文献
3.
东海残留沉积区舟山ＱＺ５０４孔未发现上海地区普遍存在的全新世暗绿色硬粘土层。但通过对该孔微体古生物组合特征的分析与对比，划分了全新世与更新世的更线，反映了气候的变化，证实了滚流对本区的影响。 相似文献
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由于缺少有效钻孔资料,对于南海扩张的时间一直存在较大的疑问.在南海三大海盆中,西北次海盆面积最小、磁条带特征不明显,因此对其扩张年代的争议最大.最新采集的高密度(小于10 km测线间距)船测地磁资料清晰地显示了西北次海盆磁条带的存在.在OBS和多道地震资料的约束下,利用船测地磁资料,本文对西北次海盆的地壳年龄进行了重追踪.根据定量的比较,西北次海盆的主体扩张始于35.8 Ma(C16n,2n),在34.7 Ma(C15)时其西南部开始扩张,扩张最终同时终止于33.2 Ma(C13n),整体的全扩张速率在40~50 mm/a之间.这表明南海的扩张可能首先起源于西北次海盆,在其结束扩张后,东部次海盆才开始打开(约30 Ma).得益于数据精度和密度的提高,利用化极后的磁力异常以及反演的磁化强度可以对西北次海盆进行二级中脊段的划分.我们共划分出六个中脊段和一个明确的转换断层.中脊的分段性与OBS反演的地壳厚度的变化相一致.转换断层东侧,中脊主体分为四个中脊段,每个中脊段长度均在30 km左右.转换断层西侧,存在一个长约50 km的中脊段和一个不确切的中脊段.中脊段上磁化强度的变化幅值和中脊段长度在整体上成正比.每个中脊段中央的磁化强度弱于中脊段两端的磁化强度,这与扩张速率相近的大西洋中脊的磁化强度特征一致. 相似文献
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Simonini R Ansaloni I Bonini P Grandi V Graziosi F Iotti M Massamba-N'siala G Mauri M Montanari G Preti M De Nigris N Prevedelli D 《Marine environmental research》2007,64(5):574-589
The long-term effects of sand extraction on macrozoobenthic communities were investigated in an offshore area in the Northern Adriatic Sea characterised by relict sands formed during the last Adriatic post-glacial transgression. Surveys were carried out before, during and 1, 6, 12, 18, 24 and 30 months after extraction at three impacted and seven reference stations. The operations did not influence the physical characteristics of the sediment, but they caused almost complete defaunation at dredged sites. Univariate and multivariate analyses highlighted that the macrozoobenthic community responses to the dredging operations were (1) a rapid initial recolonisation phase by the dominant taxa present before dredging, which took place 6-12 months after sand extraction; (2) a slower recovery phase, that ended 30 months after the operations, when the composition and structure of the communities were similar in the dredged and reference areas. This pattern of recolonisation-recovery fits well with the commonly encountered scenario where the substratum merely remains unchanged after marine aggregate extraction. 相似文献
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抚顺东部地区紫苏花岗质岩石形成于晚太古宙，以包体形式产于黑云长英质片麻岩中。许多特征均说明，它并非由紫苏北岗质岩浆直接结晶形成的，而是源岩在部分熔融与分凝过程中产生的残留相、后经麻粒岩相变质作用及构造变形改造所致的再造岩石。 相似文献
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新疆东准噶尔喀姆斯特晚古生代沉积记录:物源和沉积作用研究 总被引：4，自引：0，他引：4
东准噶尔喀姆斯特下泥盆统阿拉比也巴斯他乌组和下石炭统卡姆斯特组代表陆壳增生不同阶段的沉积响应.碎屑岩碎屑组成模式和地球化学分析结果表明阿拉比也巴斯他乌组形成于大洋-活动大陆过渡型构造环境,物源区主要为发育在过渡型地壳之上的岩浆岛弧;卡姆斯特组形成于活动大陆型构造环境,物源区主要为大陆岛弧环境的切割岩浆弧.沉积相、相组合及生物生态等沉积特征显示两组的沉积环境分别为海底斜坡和海底扇中扇-外扇盆地平原.结合区域构造分析和地层对比研究,下泥盆统阿拉比也巴斯他乌组海底斜坡沉积是东准噶尔构造带早泥盆世弧后盆地沉积响应的主要记录,卡姆斯特组海底扇-海底平原沉积则主要记录了东准噶尔复合地体早石炭世晚期弧间残余海盆的沉积响应.两套沉积响应记录的环境演化受控于中亚型造山带复杂的造山作用. 相似文献
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Relict soils provide insights into Quaternary soil formation and erosion on the Edwards Plateau of central Texas and into soil-forming processes in karst terranes. Late Quaternary climate-driven soil erosion produced a mosaic of thick and thin soils on the Edwards Plateau landscape. Thick soils on uplands of the Edwards Plateau are interpreted to be relicts of a formerly more extensive soil cover that was eroded during the late Pleistocene to middle Holocene. The relict, thick soils are silicate-rich and most commonly overlie the relatively silicate-poor Cretaceous Edwards Limestone, which supports the idea that the thick soils did not form from weathering of the underlying limestone. Other potential sources of silicates for the relict soils include dust, alluvial sediments, and the Del Rio Clay, a stratigraphically higher but locally eroded clay-rich unit. Here we investigate the geographic distribution, texture, clay-sized mineralogy, rare earth element geochemistry, and neodymium isotope composition of the relict soils. We have found that the relict, thick soils are deeply weathered soils that occur dominantly over the Edwards Limestone and have a high clay content and a neodymium isotope composition that is similar to that of the Del Rio Clay. Thus, we propose that in situ weathering of the Del Rio Clay, along with partial weathering of the upper portion of the underlying Edwards Limestone produced thick chert- and clay-rich soils over resistant limestone. In areas like the Edwards Plateau, where pure limestones are interbedded with clay-rich strata, the overlying clay-rich strata must be considered as a possible silicate source to soils on pure limestone bedrock. 相似文献
10.
B.W. Goodfellow A.P. Stroeven C. Httestrand J. Kleman K.N. Jansson 《Geomorphology》2008,93(3-4):213-232
Relict surfaces contain information on past surface processes and long-term landscape evolution. A detailed investigation of relict non-glacial surfaces in a formerly glaciated mountain landscape of northern Sweden was completed, based on interpretation of colour infrared aerial photographs, analysis in a GIS, and fieldwork. Working backwards from landscape to process, surfaces were classified according to large- and small-scale morphologies that result from the operation of non-glacial processes, the degree of weathering, regolith characteristics, and the style of glacial modification. Surfaces were also compared in the GIS according to elevation, slope angle, and bedrock lithology. The study revealed five types of relict non-glacial surfaces but also two types of extensively weathered glacial surfaces that were transitional to relict non-glacial surfaces, illustrating spatially variable processes and rates of non-glacial and glacial landscape evolution. Rather than being static preglacial remnants, relict non-glacial surfaces are dynamic features that have continued to evolve during the Quaternary. The classification provides hypotheses for landscape evolution that can be field tested through, for example, terrestrial cosmogenic nuclide studies and geochemical analyses of fine matrix materials. The classification may be applicable to relict non-glacial surfaces in other formerly glaciated landscapes. 相似文献