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1.
Olive green clays likely to be bentonitic in composition have been mineralogically and chemically studied. They occur in association with other lacustrine sediments at Lake Manyara. Radiocarbon dates from four diatom horizons indicate ages ranging from 12 Ka to 135 Ka suggesting a Mid-Holocene age. Middle Pleistocene age have been assigned to the ridged oncolites of Lake Manyara. The olive green coloured clays in the Manyara basin are known to occur in association with other lake beds including phosphorite deposits, stromatolites, bioturbated silty clays, partly silicified marls, conglomerates and olive green coloured opal beds. The results presented herein are from the olive green coloured clays. The olive green clays (bentonite?) are a result of devitrification or alteration of volcanic ashes and/or pyroclasts. The green clays occur in different forms as they are separated from each other by other lacustrine sediments. The alteration might have taken place in slightly different environments in terms of salinity and alkalinity. One of the top layer is friable and shows conchoidal fractures when dry. The other beds below in the lacustrine sequence are cemented with calcite and some dolomite as well as zeolites. The lowermost layer in the sequence is friable and shows cracks filled with coarse crystalline calcite. Mineralogically the bentonite is composed of the clay minerals illite, illite-smectite mixed layer clays, and chlorite. Other authigenic minerals include various zeolites (analcime, clinoptilolite, erionite and some traces of mordenite), opal, and fluorapatite. The clays have magnesium contents varying from 3.01% to 7.43%. The calcium contents vary widely due to presence or absence of one of the two minerals calcite or apatite. Trace elements like Ba, Ce, Sr, Zr are equally attributed to the presence of calcite and apatite. The formation of the illite-smectite mixed layer clays in an alternating manner with other lake sediments depicts different episodes of volcanic eruptions in the area. The mineralogical composition of smectites, zeolites, and opal in the green clays suggests a deposition of pyroclasts and volcanic ashes in a closed lake system with fluctuating levels. Due to evaporation alkalinity and salinity levels were fluctuating. The clays might have been bentonite which have undergone illitisation, a phenomena noted in other neighbouring rift basins. 相似文献
2.
Arantxa Luzón Angel González Arsenio Muñoz Belén Sánchez-Valverde 《Journal of Paleolimnology》2002,28(4):441-456
The lacustrine facies from two sections (Candasnos and Fraga) ofthe Oligocene-Miocene Torrente de Cinca lithostratigraphic Unit in thecentral part of the Ebro Basin (Spain) have been analysed to determine theinfluence of orbital parameters in lacustrine sedimentation. The unit ispredominantly composed of limestones and marls, and represents a shallowlacustrine freshwater system. The sedimentological features of the faciesstudied demonstrate that the lower part of the Candasnos section representsoffshore lacustrine subenvironments whereas the upper part, and the whole ofthe Fraga section, characterise marginal lacustrine areas. Series of stratalthickness variations of limestone, marl, and limestone/marl couplets fromboth sections have been analysed using spectral analysis. This shows thatinformation corresponding to periodic cycles only appears in the offshorefacies, that is to say, in the lower part of Candasnos section, and disappearsin the marginal facies where non-periodic cycles exist. Furthermore, thespectral analysis of the offshore facies highlights the existence of a peak inthe power spectrum with a period of around 7 (6.8 to 7.8) that can berecognised in the field as shallowing-upward lacustrine sequences.Magnetostratigraphic data from the Candasnos section allow us to establish atime span of 2,808 years for the limestone/marl couplet from the lower partof this section, and between 19,000 and 22,000 years for the periodic cycleidentified, thus representing the climatic precession cycle. Shallowingsequences from marginal areas do not correspond with any periodiccycle. 相似文献
3.
Richard L. Reynolds Joseph G. Rosenbaum Josh Rapp Michael W. Kerwin J. Platt Bradbury Steven Colman David Adam 《Journal of Paleolimnology》2004,31(2):217-233
Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter. 相似文献
4.
STÉPHANE POCHAT JEAN VAN DEN DRIESSCHE VALÉRIE MOUTON FRANÇOIS GUILLOCHEAU 《Sedimentology》2005,52(4):809-825
Lacustrine environments are an excellent indicator of continental palaeoclimate. In particular, the sedimentary record of waves in lakes may be used to constrain atmospheric palaeocirculation. Wave ripples have been identified in a Permian lacustrine basin (the Salagou Formation, 260–250 Ma, Lodève Basin) located in the southern French Massif Central, part of the western European Hercynian mountain chain. Wave ripple patterns are interpreted with regards to hydrodynamics and water palaeodepth. It is shown that, in the case of the Salagou Formation, wave ripple orientations were controlled by the direction of the prevailing palaeowind. The Late Permian wind blew from between north and 20° east of north, possibly over several millions of years and certainly throughout the period of deposition of about 2000 m of strata in the Lodève Basin. Permian lacustrine sedimentation is widespread and well preserved on the Earth's surface and so wave ripple data may help constrain numerical modelling of the Earth's past climates, especially with regards to Permian times outside of desert regions. 相似文献
5.
中国过去2000年湖泊沉积记录的高分辨率研究:现状与问题 总被引:22,自引:1,他引:22
过去2ka地球环境的变化规律和原因是全球变化研究的关键内容之一,与其它自然记录相比,湖泊沉积具有储存信息量大,时间分辨率高和地理覆盖面广的优势。中国80年代后期以来已陆续开展了全新世湖泊沉积与气候,环境演化研究,并有迅速发展之势,对于过去2ka湖泊沉积的高分辨率研究,需要重视以下几方面问题,即提高核素测年的精度,建立环境指标的转换函数,注意早期化学成岩作用,选择分层湖和年层沉积,探讨气候要素有效指 相似文献
6.
The Cuu Long Basin (Mekong Basin) is a rift basin off southern Vietnam, and the most important petroleum producing basin in the country. However, information on petroleum type and characteristics has hitherto been largely unavailable to the public. This paper presents petroleum geochemical data on nine oil samples from four different producing fields in the Cuu Long Basin: the Dragon (Rong), Black Lion (Sutu-Den), Sunrise (Rang ?ong) and White Tiger (Bach Ho) Fields. The oils are highly paraffinic with bimodal normal alkane distributions and show moderate pristane to phytane ratios and a conspicuous hyperbolic decrease in abundance with increasing carbon number of hopane homologues from C30 to C35. The TPP-index of Holba et al. (Holba, A.G., Dzou, L.I., Wood, G.D., Ellis, L., Adam, P., Schaeffer, P., Albrecht, P., Greene, T., Hughes, W.B., 2003. Application of tetracyclic polyprenoids as indicators of input from fresh–brackish water environments. Organic Geochemistry 34, 441–469) is equal to 1 in all samples which in combination with tricyclic triperpane T26/T25 ratios >1 and the n-alkane and hopane distributions mentioned above provide a strong indication of an origin from lacustrine source rocks. This is supported by the absence of marine C30 desmethyl steranes (i.e. 24-n-propylcholestanes) and marine diatom-derived norcholestanes. Based on the overall biological marker distributions, the lakes probably belonged to the overfilled or balanced-fill types defined by Bohacs et al. (Bohacs, K.M., Carroll, A.R., Neal, J.E., Mankiewicz, P.J., 2000. Lake-basin type, source potential, and hydrocarbon character. An integrated sequence-stratigraphic–geochemical framework. AAPG Studies in Geology 46, 3–34). The oils were generated from source rocks at early- to mid-oil-window maturity, presumably Oligocene lacustrine shales that are present in the syn-rift succession. Oils from individual fields may, however, be distinguished by a combination of biological marker parameters, such as the oleanane index, the gammacerane index, the relative abundance of tricyclic terpanes, the proportions of diasteranes and 28-norspergulane, complemented by other parameters. The oils of the Cuu Long Basin show an overall similarity to the B-10 oil from the Song Hong Basin off northern Vietnam, but are markedly different from the seepage oils known from Dam Thi Nai on the coast of central Vietnam. 相似文献
7.
The postglacial sediment record of Lake Manitoba is composed dominantly of silty clay to clayey silt with little variation in most physical, chemical, and mineralogical properties. Distinct in the sedimentary sequence of this large lake, however, are four zones that have a low moisture content, blocky to pelletal structure, and gleyed colour. All of these zones formed during the warmest and driest postglacial period in the region, 9500 to 4500 years ago. Although several possible hypotheses can be formulated regarding the origin of these zones including changes in sedimentation rate, clay mineralogy, lake chemistry, and depth of winter freezing, the most likely explanation is that they represent pedogenic horizons. Formation by pedogenesis during dry or extremely low water conditions is not mutually exclusive of the other hypotheses, and several are favoured by low water conditions. 相似文献
8.
9.
Jean-Franois Pastre Agns Gauthier Sbastien Nomade Paul Orth Ana Andrieu Franck Goupille Herv Guillou Stphane Kunesch Stphane Scaillet Paul Randall Renne 《Comptes Rendus Geoscience》2007,339(16):987-997
The Alleret maar (Massif Central, France) provides a long lacustrine sequence (40.6 m) attributed to the early Middle Pleistocene. Sediment, pollen and diatoms analysis of its upper part (AL2 core, 14.6 m) indicates two temperate phases marked by high lake levels, forest development and vegetation expansion. They are separated by a cold period during which lake level drops, coarse sediment input increases and steppic and xerophilous plants develop. Pollen data suggests that this sequence belongs to the upper part of the Cromerian complex. These results are in agreement with the 557 ± 3 ka (±12 ka, including all errors) 40Ar/39Ar age obtained from an interbedded tephra layer emitted by the Mont-Dore/Sancy strato-volcano and establish that this sequence probably covers the MIS 15 substages. 相似文献
10.
Braid-delta depositional systems are widely developed in most continental basins in China. Research indicates that, for different types of braid delta, the facies sequence and association, which are cr... 相似文献