| Red beds,an important geological vestige resource,is a hotspot in the fields of sedimentology,geology,paleoclimatology and so on.The Claron Formation spanning the Paleocene to Eocene,widely exposed in the Southwest Utah,United States,is an ideal material for understanding the paleoclimate and paleoenvironment during Paleocene-Eocene,which consists of interbedding of many different shades of red layers and off-white layers.The Claron Formation exposed in the Markagunt Plateau,Paunsaugunt Plateau,Table Cliffs Plateau and other areas has been extensively investigated,from which,evidently,detailed geological data for the widely distributed Claron Formation are available for us.However,there are still great disputes on the understanding of sedimentary environment of the Claron Formation.According to the lithologic changes,field sedimentary characteristics,trace fossils and so on,the Claron Formation was considered to be lacustrine sediments.However,we found that there are unrecognized paleosol and significant aeolian deposition characteristics in the strata,which,at present,have been rarely reported,indicating that it is unreasonable to consider the Claron Formation as lacustrine deposits.Therefore,based on previous geological surveys and biological dating,this paper takes the Claron Formation exposed in Bryce Canyon National Park in the United States as the research object,and detailed field investigations and sampling were carried out in Sunrise point and Sunset point.soil micromorphology,grain size,environmental magnetism,diffuse reflectance spectrum,geochemistry and X-ray diffraction were conducted to recognize the paleosol characteristics and analyze paleoenvironment of the Claron Formation,and main understandings were obtained as follows:(1)Root traces,rhizoliths,hymenoptera nest remains and wormholes were distinctly observed from the macroscopic and microscopic morphological characteristics.Furthermore,calcium nodules,reticulate structure,rust spot and sliding rubbing surface,which indicate soil structures formed in an alternate dry-wet soil formation process,and the soil genetic layer composing of the argillic horizon(Bt)and the calcic horizon(Bk),were clearly observed in the strata.The above characteristics are evidence of paleosol,which indicate that the red layers of the Claron Formation are a series of paleosol sequences with varying degrees of soil formation.By comparing the characteristics of paleosol identified in the strata with those of modern soil,the paleosol can be divided into aridisols and luvisols.In addition,light brown layers and root traces were also found in the off-white layer,which indicates that the off-white layer has undergone the process of surface pedogenesis.(2)Typical subaerial sedimentary structure and significantly aggradation characteristics were observed in red layers of the strata,totally different from subaqueous characteristics.Moreover,the surface texture of quartz of the red samples were found to be characterized by large and small mechanical saucer-shaped pits,round pits,deep troughs and precipitation of SiO2,all of which are features of eolian deposit.Therefore,the above evidence shows the origin of aeolian deposition of the red layers.Furthermore,the grain size components of the Sunrise point and Sunset point were unmixed by the non-parametric grain-size end-member analysis model,and six end members with different sedimentary significance were obtained:soil-forming components;components of hydrostatic sedimentation formed in soak/ephemeral lake;components transported by high altitude air streams over long distances;components of dust settling after storms;alluvial-diluvial components;and near-source or far-source components transported by storms.End members 3,4 and 6 were identified as eolian components,with the first modal particle size of 5.33 μm,11.93 μm and 42.34 μm,respectively.And we can conclude eolian deposits was a dominating and stable source for the strata from the variation of proportion of each end member in the profile.(3)The results of environmental magnetic and diffuse reflectance spectra of the Sunrise point and Sunset point show that the Claron Formation contains mainly hematite and goethite.Hematite is the main iron mineral in the maroon layer;the content of hematite in the pink layer is significantly lower than that in the maroon layer,and the content of goethite is relatively high;while the content of both goethite and hematite in the off-white layer are very low,and there are a certain proportion of lepidocrocite in the off-white layer.The maroon layer was found to be characterized by the aluminum-substituted hematite,kaolinite,and luvisols in red color,which indicate that the maroon layer was formed in a period with the concurrence of high rainfall and high evaporation.The off-white layer was formed in a temporary ponding environment,in which the migration and transformation of iron minerals occurred,which corresponded to the increase of rainfall generating the inflow of surface runoff into the basin.The physical and chemical properties of the pink layer show the transitional characteristics between the maroon layer and the off-white layer,indicating that the degree of dry and wet of the pink layer was between that of the maroon layer and the off-white layer.(4)Based on the above analysis,we conclude that the red beds of the Claron Formation in Bryce Canyon are composed of eolian sediments,alluvial-diluvial deposition and soak/ephemeral lake deposition.On account of the strata forming in the foreland basin,the terrain is featured with being high around and low in the middle,which implies the changes of sedimentary environment are greatly affected by climate variability.The maroon layer was formed in the surface oxidation environment,mainly by eolian dust deposition;the off-white layer was mainly deposited under water,with a significant increase in rainfall,and there still was surface oxidation environment during the deposition period;the pink layer consists of alternate layers of eolian sediments,alluvial-diluvial deposition and soak/ephemeral lake deposition. |
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