Measurement And Paleoclimatic Significance Of Color And Clay Minerals Of Sediments Of Liupanshan Group

The Cretaceous is a significant transition of global tectonic and environmental system, which is the nearest typical greenhouse Earth state. As a mirror of "greenhouse climate" phase in earth history, it has important implications to recognize the causes of the past climate change and current global warming and to predict the future climate trends through studying the evolution of climate and environment in this period.The Cretaceous nonmarine sediments of the Liupanshan Group in Liupanshan basin, located in the central of China, have well documented the information of early Cretaceous climate. Based on the high-density sampling and measurement of the color, chemical elements and minerals and clay minerals of the sediments of the Liupanshan Gr. which has been already strictly constrained by high-density magnetostratigraphy, high-resolution color sequence ranging 128.11-112.97Ma, and clay mineral assemblages.ranging 125-112.97Ma were obtained for the first time. It is found that the brightness values (L*) of alluvial and fluvial sediments in the lower portion of the Liupanshan Gr. was low and gradually increased, the red (a*) and yellow value (b*) were high and decreased gradually, there was no conspicuous variation of color values in the sediment of a shallow lake environment. The L* fluctuations is related to the cement of the rock, and a* to the hematite content within the rocks. The rocks are characterized by a clay assemblage of illite, kaolinite, chlorite and illite/smectite mixed layer, Which is a mass amount of illite with the crystallinity of 0.15-0.37 and Chemical index of 0.22-0.46. Comprehensive analysis of color, clay mineral assemblages and the relationship with lithology and sedimentary environment, it is indicated that climate recorded by sediment were overall relatively warm, and was divided into two stages by the boundary at 125Ma. The early stage (128.11-125Ma) with the low L*, high a* and b*.The Lithology is the purplish red, visual color of sandstone in the upper Sanqiao Formation through Heshangpu Formation, indicative of a relatively humid and hot climate. The second stage (125-112.97Ma) with the shallow lake and semi-deep lake facies in Liwaxia Formation through Madongshan Formation, indicative of a relatively dry and warm climate. The characteristics of low L*, high a* and b* has a little change. The variation of illite content was decline-level-rise, but Kaolinite is reversely varied. The dolomite and calcite content was more-less-more. Combination of elements (K2O＋Na2O＋CaO)/Al2O3 ratio, Dolomite and Illite content reveals the two drought events during 124-122.8Ma and 114.8-113.8Ma.Analysis of color together with global climate trend in Cretaceous, it is founded that the early Cretaceous climate transition at 125Ma revealed by the record of the sediments of the Liupanshan Gr., which is approximately equivalent to the transition of global sea-surface temperature at 123.3 Ma, but it was about 1.7 Ma in advance. Meanwhile the temperature trend at 113.7Ma changed from decreasing to increasing, corresponding to the temperature transition(temperature trend change from decreasing to increasing) at about 112.2Ma founded in Cretaceous marine sediments, indicative of the land systems were more sensitive to global change than marine systems.