美国Bryce峡谷古新统-始新统红色层古土壤微形态特征及其指示意义

RED PALAEOSOL MICROMORPHOLOGICAL CHARACTERISTICS AND THEIR INDICATIVE SIGNIFICANCE OF PALEOCENE-EOCENE STRATUM IN BRYCE CANYON,UNITED STATES

地层中的古土壤记录了古气候环境演化的重要信息,然而年代越老的地层由于受到后期持续的地质作用,使得古土壤面貌发生很大改变,增加了识别难度。美国Bryce峡谷广泛出露的古新统-始新统Claron组一直被认为是湖相石灰岩,却呈现整体氧化红色,并显示诸多古土壤特征。文章主要采用偏光显微镜和扫描电子显微镜来探究Bryce峡谷Claron组中古土壤特征,并结合X射线衍射特征分析矿物物相,结果表明:Bryce峡谷Claron组红色层是一系列成土深浅程度不同的古土壤层,表现为干旱土和淋溶土;地层中的碳酸盐主要是淀积在土壤下部的成土碳酸盐,碳酸盐富集也表现为土壤淀积的钙结核等钙积层,从土壤学看是钙积干旱土特征而非水中沉积的石灰岩层;深浅不同的红色层代表了粘化层和钙积层等土壤层次变化;不同颜色层代表着不同的形成环境:褐红色层形成于气候比较暖干时期,而灰白色层的形成环境比褐红色层更加湿润,可能每年雨季会形成临时性积水洼地,粉红色层的湿润程度介于褐红色层和灰白色层之间。综合上述分析,本研究不支持Claron组是终年积水古湖沉积环境的看法。

Paleosol is a significant archive of paleoclimate and palaeoenvironment,which is a quality material to study past climate and environment.However,the difficulty of identification of the paleosol from the ancient strata is increased due to subsequently continuous geological processes,in which some paleosol features have been changed to a great extent.The Claron Formation from Paleocene to Eocene,widely exposed in Bryce Canyon,USA,have been considered as lacustrine limestone for a long time,but shows a generally red color of oxidation and characteristics of paleosol,indicating the deposit of paleosol occurred in the stratum.Taking the paleosol in the stratum as limestone to indicate the change of climate and environment,the reconstruction of paleoenvironment in this period is illogical.Therefore,the goal of this study is to identify the paleosol from the Claron Formation in Bryce Canyon,and then to discuss its forming environment based on characteristics of paleosol.The study area is located in Bryce Canyon National Park,western United States,37.5°~37.8°N,112°~112.3°W,and at altitudes of 2011~2775 m.The Claron Formation unconformably overlies the upper Cretaceous strata and is the most widely exposed strata in Bryce Canyon.Detailed field observation and sampling of the Claron Formation at Sunrise Point(37°37’42″N,112°09’46″W)and Sunset Point(37°37’21″N,112°09’57″W)was carried out,and 52samples were finally obtained according to the soil layers and different colors.Soil micromorphology is an effective method to identify paleosol from sedimentary rock.In this paper,the characteristics of paleosol of the Claron Formation in Bryce Canyon are mainly studied by polarized light microscope and scanning electron microscope,and the mineral facies are analyzed by X-ray diffraction.It was found in the field that evident palaeosol characteristicswell-preserved root traces,nest of hymenoptera,calcium nodules,reticulate structure,calcic horizons and argillic horizons-occurred in red layers of the stratum.Furthermore,characteristics of rhizoliths,carbonate-filled wormholes,soil aggregates and optical clays were clearly observed by polarized light microscope,which are completely consistent with the field characteristics.Therefore,both the macroscopic morphological and micromorphological characteristics indicate that paleosol sequences are developed greatly in the red layer of Claron Formation.The illuviation of the carbonate in the strata is occurred uasually under argillic horizon,and the enrichment of carbonate is in the form of calcium nodules or calcic horizons rather than limestone with horizontal beds deposited under water.The color of paleosol in the strata was founded to be between 5YR~10R,and the change of reddish color was founded to be related the change of soil layer such as argillic horizons and calcic horizons.Combined with X-ray diffraction,it was founded that minerals of the red layer of Claron Formation are dominated by goethite,hematite,kaolinite and other minerals formed in the process of soil formation,and the primary minerals of red layer are mainly quartz and almost no feldspar,mica and other minerals that are subject to weathering,which indicates that the strata underwent continuous process of chemical weathering and soil formation.Comparing the characteristics of paleosol in the Claron Formation with the modern soil,two types of paleosol,aridisols and alfisols,were identified from the strata.In the maroon layer,characteristics of alfisols were observed obviously,and kaolinite and hematite were founded to be present,indicating the high precipitation and high evaporation coexisted in this period,and the maroon layer was formed under a warmer and drier climate.While the offwhite layer was identified that corresponds to a relatively warm and humid environment,or even corresponds to a higher rainfall,which leads to the probable formation of emporary stagnant water depressions during the rainy season each year.Under this condition,hematite become unstable and the red color gradually decreased.The degree of wetness in the pink layer is between that of the maroon layer and offwhite layer.The surface textures of quartz grains indicates that both eolian deposit and near-source slope deposit occurred in the strata,and red paleosol developed to varying degrees after deposition,which is quite different from the lacustrine deposit.In conclusion,the evidence presented in this study do not support the view that the Claron Formation are the sedimentary environment of a year-round ancient lake.