| The Cretaceous is a special period in the geological history,during which the tectonic-climate events are of great significance to the present pattern of the environment.Previous studies on the Cretaceous are mainly from marine sediments,but a few of studies from terrestrial sediments.Therefore,it is an important way to comprehensively understand the evolution of Cretaceous global tectonic-climate pattern by strengthening the study of climate change information recorded in continental strata.The Cretaceous strata are well developed in the Basins in the Northwest China.Among these,the Jiuxi Basin is located in the western part of the Hexi Corridor,and is well-studied.In this paper,high-density sampling of the Lower Cretaceous sedimentary rocks in the Hongliuxia section of the Jiuxi Basin and the color and magnetic susceptibility(MS)values were obtained.Combined with lithology,sedimentary facies,magnetic mineral analysis,and analysis for the causes of variation of color index and magnetic susceptibility,the climatic change in the study area during the Early Cretaceous are discussed.The main conclusions are as follows:1.The characteristics of color and MS measurements:L*ranges between 41.18 and 84.26with a mean value of 59.34,a*and b*varies between-1.94 and 27.44(4.60 averaged),and between 5.16 and 34.76(15.84 averaged),respectively.Meanwhile the?lfvaries between0.07×10-8and 60.96×10-8m3/kg(10.92×10-8m3/kg averaged),and?hfis between 0.07×10-8and56.77×10-8m3/kg(10.70×10-8m3/kg averaged),the?fd%(frequency-dependent mass magnetic susceptibility)ranges betweeb 0 and 24.44%(1.84%mean).The variation trends are opposite between L*versus a*,b*,and between?fd%versus?lfand?hf.2.The variation characteristics of MS and color of sedimentary rocks on the profile:When the clastic rock particles are fine(such as mudstone),the a*,b*and MS are relatively high,but L*is lower;when the clastic rock particles are coarse(such as coarse sandstone),a*,b*and MS are lower,L*is higher.The a*,b*and MS of the red and brown sedimentary rocks are higher,the L*are lower,and the a*,b*and MS of the gray-white sedimentary rocks are lower and the L*are higher.When the sedimentary environment is braided and meandering fluvial facies,a*and b*are higher.When the sedimentary environment is unstable(such as alluvial fan),MS is higher;when the deposition environment is stable,the color value and MS are generally lower.In addition,the superparamagnetic material content of the deep lake-semi-deep lake phase is higher.3.The species and contents of magnetic minerals are different in different sedimentary facies.In the alluvial fan facies the magnetic minerals are mainly magnetite,maghemite,and a small amount of hematite appearing in red and brown samples.The brown siltstone contains pyrrhotite or iron hydroxide(goethite,lepidocrocite).Magnetic minerals of fan-delta facies are mainly magnetite and maghemite,a small amount of pyrrhotite or lepidocrocite,goethite content.The magnetic minerals of deep lake and semi-deep lake facies are mainly maghemite,magnetite,and a small amount of goethite,lepidocrocite.Magnetic minerals of Braided river facies are maghemite,magnetite,hematite and a small amount of pyrrhotite or iron hydroxide(eg,lepidocrocite).The magnetic minerals in the meandering fluvial sedimentary rocks are mainly maghemite,magnetite,and a small amount of hematite and goethite.4.The correlation between the color index and TFe,Mn,Si O2and Ca CO3is analyzed.It was found that color is affected by different elements in different depositional environments.In general,a*and b*are mainly influenced by Fe,L*is influenced by Ca CO3and Si O2.a*is also affected by Mn in fan delta facies,deep lake facies,semi-deep lake facies.5.According to sedimentary facies and variations of color indexes and MS,the climate change recorded on the sections can be divided into four stages:the first stage is an alluvial facies,a*and the?lffluctuate relatively slightly and the average value is relatively high,indicating a warm and semi-arid climatic condition.The second stage encompasses alluvial and fan-delta facies,a*fluctuates slightly(average minimum),?lffluctuates strongly(average largest),compared with the first stage,the temperature decreases and the humidity increases.These suggests a cold and humid climatic condition.The third stage is a semi-deep or deep environment,compared with the previous two,?lfaverage minimum,a*average higher.The representative climate status should be a warm and arid climate with a gradual increase in temperature and the precipitation will decrease.The fourth stage is fluvial facies,a*averaged is the largest along the section,and the?lfaveraged is lower than that of the second stage,representing warm and humid climate environment.In the other hand,there are some short-term climatic fluctuations within each climate phase.The climate change in the Hongliuxia area of the Jiuxi Basin was basically governed by global climate change. |
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