TL stratigraphy of pre-Weichselian loess from NW Europe using feldspar coarse grains

A new feldspar relative TL dating method is proposed that enables dating of pre-Weichselian loess older than 130 ka, by applying the additive γ dose technique and the Mejdahl's exponential extrapolation to alkali feldspar coarse grains. The method is applied to loess deposits from various reference loessic sections of NW Europe. Our relative TL age estimates are consistent with the information provided by the regional stratigraphy; furthermore, they lead to a clear separation of the pre-Weichselian periods of loess deposition in the time period 130 ka–300 ka.     

The shape of loess particles reviewed

An important property of loess is a tendency to collapse on loading and wetting (hydroconsolidation) which can have serious consequences worldwide for civil engineering projects. This paper describes the use of Monte Carlo and other analytical techniques to predict the shape of naturally occurring loess particles. Randomly generated particles are classified according to Zingg shape categories: disc, sphere, blade and rod. By assuming a uniform distribution for the basic particle, average relative dimensions are calculated for the blade category, into which most loess particles have been shown to fall.     

The loess of North-East Essex, England

A thin mantle of cover-loam over much of North-East Essex has been recognised as consisting largely of loess. The cover-loam represents the intermixing, to a varying degree, of a layer of loessial silt with a thin layer of underlying sand which is also of likely aeolian origin. The heavy mineral content of the coarse silt fraction of North-East Essex loess is generally similar to that of last glaciation age loesses elsewhere in Eastern England, Belgium and the Netherlands. This suggests the North-East Essex loess is part of a single loess sheet deposited over Eastern England and parts of Western Europe. Nevertheless, detailed examination of the heavy minerals content from all of these areas reveals slight areal variations especially in the proportion of hornblende. These differences show the coarse silt from North-East Essex loess to have closest affinities with that from Norfolk. A distant source for the loess within the present North Sea Basin is proposed on textural evidence. Loess accumulation in North-East Essex probably commenced in the few thousand years leading up to the maximum extent of Devensian ice (about 18,000 years B.P.) and may have continued to about 14,000 years B.P.     

结构性黄土吸力的试验研究

利用轴平移法量测吸力技术,进行了等吸力三轴剪切试验,研究人工制备结构性黄土吸力与周围压力的关系;利用土吸力量测仪进行了原状结构性黄土吸力的量测,研究了结构性黄土吸力随含水量及饱和度的变化关系,随着三轴剪切试验的周围压力的增大,吸力逐渐减小;原状结构性黄土的含水量(饱和度)对吸力有显著的影响,随着土的含水量的增加吸力减小。      

The Lateglacial of North-west Europe

Book reviewed in this article:
J. J. Lowe, J. M. Gray & J. E. Robinson (eds.): Studies in the Lateglacial of North-west Europe     

Stability of loess

Lutenegger, A.J. and Hallberg, G.R., 1988. Stability of loess. Eng. Geol., 25: 247–261.

The natural stability of loess soils can be related to fundamental geotechnical properties such as Atterberg limits, water content and void ratio. Field observations of unstable conditions in loess deposits in the upper midwest, U.S.A. show relationships between instability and the in situ moisture content and the liquidity index of the loess. Unstable loess can attain natural moisture contents equal to, or greater than, its liquid limit. Implications of these observations for applied engineering works are described.     

黄土弹塑性损伤本构模型基本构架研究

黄土由于特殊的结构性,其变形和强度受到力和水的共同影响,因此对于黄土的力学研究必须同时考虑力和水的作用。根据热力学和连续损伤力学理论以及黄土在加载和增湿作用下的能量转换过程及结构破损过程分析,提出了黄土的损伤变量、加载损伤和增湿损伤的定义以及它们之间关系,并提出由塑性、加载损伤和增湿损伤三部分组成可以考虑加载和增湿作用的黄土弹塑性损伤本构模型的基本构架,为建立黄土的结构性模型提供一种新的思路。     

The origin of silt particles in the loess question

Many researchers consider loess as solely the product of glacial activity. Investigations throughout South America, however, have demonstrated that loess and loess-like sediments can be formed during geological periods when glaciation was absent. Loess deposits may be derived from volcanic action, from weathering processes, and as a result of pedogenic activity. Production of silt, regardless of the geologic process, can result in loess formation under suitable climatic conditions.     

饱和黄土的性质与非饱和黄土流变模型

对西安地区相关数据统计分析，把西安市饱和黄土、软黄土的工程性质与一般黄土进行详细对比，指出它们之间的工程性质差异。同时重点介绍推导非饱和黄土加载体积流变模型过程以及模型的适用条件，对该模型的应用作了分析。     

The pre-weichselian glaciations of North-West Europe

In North-West Europe no evidence has been found for glaciations older than the Elsterian. The Elsterian seems to consist of at least two separate ice advances with a minor interval in between. During the Elsterian in the area south of the Baltic Sea over 400 m deep channels were cut into the underlying substratum by a combination of glacial erosion and meltwater activity. The channels were active until the Late Elsterian and were subsequently filled with meltwater deposits and glaciolacustrine silt and clay. During the Saalian no comparative channels were being formed. Three different ice advances can be distinguished within the Saalian, the stratigraphical correlation of which is to be discussed. No convincing evidence was found for any interglacial within the Saalian sequence. There are good reasons to assume that the ice advances were only separated from each other by minor ice-free intervals.     

Problems of loess chronology

Loess as a typically subaerial, loess deposit has long been regarded a Pleistocene sediment and its date of formation has been put to ca 600 ka BP. Recently, the beginning of the Pleistocene is identified in the official concord reached — at 1.6 Ma BP, while not infrequently others propose 2.4 Ma BP. Until the fifties the whole loess (loess-paleosol-sand) sequence could be referred into the shorter Pleistocene period, which —according to the climatic claendar by Milankovitch — consisted of nine cold and eight warm intervals. Some supporters of the longer Pleistocene chronology hold that the beginning of loess formation can be put as far back as 1.6 – 2.4 Ma BP. Loess formation intervals are usually correlated with the cold stages (nos 2, 4, 6 etc.) of the Emiliani oxygen isotope timescale, while soil formation is believed to correspond to the warm stages (nos 1, 3, 5, 7 etc.). The typical loess, i. e. loess proper, are not older than stage no 23 (Jaramillo event — 0.96 Ma BP). Previous to this date climatic conditions had not generally favoured loess formation and paleosols formed one above the other with clay, loam or carbonate intercalations.     

The precambrian of West Africa

Consequent upon continued active field investigations and the advent of radioactive dating of samples from the area, the time is thought opportune to attempt a synthesis of Precambrian correlations throughout West Africa. Some isotope age determinations would, at first glance, appear to necessitate radically new concepts of stratigraphic successions and correlations. For example, the Ahaggar Suggarian and the Anti-Atlas Precambrian I suggest age values consonant with Upper or then Middle Precambrian times; the Older and Younger Granites of Nigeria appear to have been emplaced in Lower Palaeozoic and Jurassic times respectively. These and other examples trend towards younger ages than had hitherto been surmised for many stratigraphic units. However, it is thought premature at this time to place too much emphasis on isotope age values obtained for these ancient rocks of West Africa.Field studies to date have been more successful in establishing successions rather than correlations, whereas isotope age determinations have achieved little in either respect.Several well-known series, once placed in the Precambrian, e.g. Falemian, Akjoujt, Bakel, are now thought to be rather Cambro-Ordovician in age. On the other hand, for other familiar units, e.g. Ahnet Purple Series, Buem Formation, the stratigraphic position remains problematical.

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Zusammenfassung Nach fortgesetzten aktiven Feldforschungen und mit dem Erscheinen von radioaktiven Datumsbestimmungen von Proben aus dem Gebiet, scheint es an der Zeit, eine Synthese der präkambrischen Verhältnisse in ganz Westafrika zu unternehmen.Einige isotopische Altersbestimmungen scheinen auf den ersten Blick eine vollkommen neue Auffassung der stratigraphischen Sukzessionen und Korrelationen nötig zu machen. So lassen zum Beispiel das Ahaggar Suggarien und das Anti-Atlas Präkambrium I Alterswerte vermuten, die mit dem oberen und mittleren Präkambrium übereinstimmen; die älteren und jüngeren Granite in Nigeria scheinen im unteren Paläozoikum bzw. im Jura übergelagert worden zu sein. Diese und andere Beispiele zeigen einen Trend zu jüngerem Alter, als bisher für viele stratigraphische Einheiten angenommen wurde. Jedoch wird es für verfrüht gehalten, den isotopischen Alterswerten für diese alten Gesteine in Westafrika eine zu große Bedeutung beizumessen.Feldstudien waren bisher erfolgreicher bei der Herstellung von Sukzessionen als bei der Herstellung von Korrelationen, während die isotopischen Altersbestimmungen für beides wenig erbracht haben.Einige wohlbekannte Reihen, z. B. Falemien, Akjoujt und Bakel, die zuvor ins Präkambrium gelegt wurden, werden nun dem Alter nach eher dem Kambro-Ordovicium zugeordnet. Andererseits bleibt für andere bekannte Reihen, z. B. die Ahnet Purple Series und de Buem Formation, die stratigraphische Position problematisch.

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Résumé Par suite des recherches actives et prolongées sur le terrain et de la venue d'échantillons radio-actifs datés de cette région, le temps semble opportun pour tenter une synthèse de corrélations précambriennes de toute l'Afrique Occidentale.A première vue, quelques déterminations d'âge absolu semblent nécessiter de nouvelles idées de successions et de corrélations stratigraphiques. Par example, l'Ahagar Suggarien et l'Anti-Atlas Précambrien I suggèrent des valeurs d'âge conforme au temps Précambrien supérieur ou moyen; les Vieux Granites (Older Granites) et les Granites Jeunes (Younger Granites) du Nigéria semblent avoir été placés respectivement dans le Paléozoique inférieur et le Jurassique. Ces examples-ci, et encore d'autres, tendent à indiquer pour beaucoup d'unités stratigraphiques des âges plus jeunes qu'on n'a supposés jusqu'à présent. Mais il est encore prématuré d'appuyer trop sur les valeurs d'âge absolu obtenus pour ces anciens terrains de l'Afrique Occidentale.Jusqu'à présent, des recherches sur le terrain ont réussi à établir des successions plutôt que des corrélations, pendant que les déterminations d'âge absolu n'ont eu que peu de succès des deux côtés.Quelques séries bien connues, qui jadis étaient placées dans le Précambrien, p. ex. le Falmien, la Série de l'Akjoujt et les Quartzites de Bakel sont à présent considérés d'être plutôt de l'âge Cambro-Ordovicien. Par contre, la position stratigraphique reste problématique pour d'autres unités bien connues, comme pour la Série Pourprée de l'Ahnet, la formation de Buem, et d'autres.

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Methodology of loess palynology

Basic climatic indexes for loess deposits may be determined by the floristic method of palaeoclimatic reconstruction. Spectra of loess pollen are selective, thus may or may not reflect adequately the original composition of the vegetation. Presence of a steppe may be unambiguously stated based on pollen of taxa characteristic for a steppe cenose or of taxa zonally restricted to steppes. Family ranked taxa may often not be regarded as indicating a steppe, because inferior taxa included may be of different cenotic requirements. In order to determine the vegetation and other conditions during the deposition of loess it is important to take into consideration the palaeofloristic data as well.     

Non-classical types of loess

The purpose of this contribution is to describe the sequence of physical and chemical processes resulting in the sediment-type named loess, a fine-grained sediment deposit of universal occurrence. Owing to historical causes, loess has been (and still is) implicitly linked to glacial/periglacial environments among most naturalists. However it is known today that most eolian dust is deflated from tropical deserts. Hence, that sequence of processes is more comprehensive than the former narrow cold scenario. Six examples of different “non-classical” cases (from South America and Europe) that fit well to the loess definition are developed: 1) volcanic loess in Ecuador: pyroclastic eruptions/valley wind/mountain praire/silica structuring; 2) tropical loess in northeastern Argentina, Brazil and Uruguay: deflation of river and fan splays/savanna/iron sesquioxide structuring; 3) gypsum loess in northern Spain: destruction of anhydrite/gypsiferous layers in a dry climate/valley wind/Saharian shrub peridesert/gypsum structuring; 4) trade-wind deposits in Venezuela and Brazil: deflation in tidal flats/trade wind into the continent/savanna/iron hydroxide structuring; 5) anticyclonic gray loess in Argentina: continental anticyclone on plains/anti-clockwise winds and whirls/steppe/carbonate structuring. All these non-classical types conform to the accepted loess definitions and they also share the most important field characteristics of loess such as grain size, friability, vertical or sub-vertical slopes in outcrops, subfusion and others. Other cases can probably be recognized when systematically scrutinized.