摘要
多种地质资料表明，陕西北部的三趾马红粘土主体形成于半干旱草原疏林环境中。５．３ＭａＢ．Ｐ左右所对应地层记录了半湿润森林景观。红粘土形成期间古气候周期性波动是其重要特征之一，使得淋溶粘土层与钙质淀积结核层交替出现。化石埋藏特点与含化石岩性的差别显示了不同层位有着不同的沉积环境。气候干湿、冷暖的交替不断改变着本区动植物的面貌。
Hipparion red clay,known as Baode Formation or Lantian Formation in north China is a special type of sediment characteristic of northeastern Asia.Opinions about its genesis differ widely,ranging from residual to eolian interpretations.Outcrops of Hipparionred clay are well exposed in the Wangdaifu Ridge.Laogaochuan,Fugu,Shanxi Province.The authors of the present paper gathered the samples and analyzed the features of main and trace elements,stable isotope of oxygen and carbon,composition of clay minerals,granularity and sporopollen,made the oriented thin section,observed the forms of minerals under the SEA.All of these provide many a proof to explain its forming environment. The Wangdaifu Ridge profile,more than 40m thick,is subdivided into two parts:the lower is yellowish red fine sandstone and siltstones, and the upper is massive brownish red clay and silty clay.Below each of clay beds,there exists a layer of greyish calcareous concretions or calcareous mudstones,consisting mainly of secondary calcite.The clay beds are composed mainly of clay minerals,most of which are garrulous montmorilloniteor illite.Fragmented particles are scarce and all covered with clay minerals.Primary calcite are very few and even disappear in some clay beds.Micro particles have a good optical orientation character under the microscope. Clay minerals of the strata consist mainly of montmorillonite,illite and chlorite.The assemblages,which indicate a relatively hot and dry environment,is consistent with that of lowrainfall grasslands of veldts in temperate zone.The conclusion differs drastically from the previous suggestion that Hipparion red clay formed in a humid and hot environment.As a result of weathering caliche in clay bed was eluviated and the content of calcite decrease sharply.Water is necessary for the eluviation of caliche.Therefore,the changes of the Ca content in different clay beds suggested a fluctuate evolution of paleoclimate and the increasing of rainfall. Oxygen and Carbon stable isotopic composition of secondary carbonate in the profile were well dated.The features of δ 13 C and δ 18 O curves,which showing a sew toothed shape,indicating an alternate dry and humid climate. As a sensitive proxy of the environmental history,vegetation may renew obviously by the changing of precipitation and temperature.The studies on the temperature of environment and the changes of rainfall indicate that the climate in this time changes from dry of semiarid to subhumid,which lead to the alternation of plants from prairie or veldt to forests.The procession hase been confirmed by the analysis of sporopollen.The assemblage of sporopollen in the lower part of the profile mainly is herb such as Artemisiaepollenites minor,Brevitriolpites punctatus,Compositoipollenites sp.,Chenopodipollis minoretc.,while in the upper part,appeared not only herb like Artemisiaepollenites sellularis,Graminidites sp.,Graminidites sp.,Gemmatricoporites sp.etc.,but also some woody plants such as Caryapollenites sp.,Betulaceoipollenites bituitus etc..Being the higher of food chain,animals will evolve by the changing in environment and edibles. Hipparion red clay in the northern part of Shanxi and Shanxi,with steady distribution and approximate or similar thickness and lithologic characteristics,is composed mainly of red,brownish red and yellowish red clay or silty clay,calcareous concretion beds are developed below the clay.These prove the fact that there is not much difference in their forming environment.Therefore,the conclusions drown from the Wangdaifu Ridge section ,Fugu,are of referential values in large regions.
出处
《沉积学报》
CAS
CSCD
北大核心
1998年第4期50-55,共6页
Acta Sedimentologica Sinica
基金
中国科学院西安黄土与第四纪地质国家重点开放实验室资助
