Cyclostratigraphicanalysis Of The Ediacaran Doushantuo Formation,South China

The Ediacaran Period(635-541 Ma)was perhaps one of the most important time intervals in the Earth's sedimentary environment and biota evolution history.The Doushantuo Formation(DST Fm)almost accounts for 90% of the Ediacaran period,which are divided into 4 Members(DST-1,DST-2,DST-3 and DST-4),during which occurred three significant carbon isotope negative excursions(EN1,EN2,EN3),and EN3(Shuram/Wonoka)spans the upper DST-3 and the entire DST-4,which is the longest duration of any recorded negative carbon-isotope excursion in the Earth's history.It is an important project to understand the timing of onset,durations and the relationships of these biota evolution events,particularly to resolve the controversial potential causes of the carbon-isotope excursions,of the Gaskiers glaciation and of the appearances of Ediacaran biota.Thus it is an interesting and necessary project to figure out them in the Earth science system research.At present,there are many controversiesfor the characteristics and origin of these geological,biological and extreme climate events due to the lack of high-precision geological time scales in the DST Fm.Based on astronomical orbital theory,we selected the Jiulongwan section and the Sixi section in Yichang city of South China as the research object,focusing on the DST Fm at the Jiulongwan section and Sixi section,and we applied variety of geochemical elements and magnetic susceptibility(MS)as the paleoclimate proxies to study the cyclostratigraphic analysisof the DST Fm,and establish the continuous high-resolution astronomical time scale.Based on Condon et al.reportedthe absolute radioactive U-Pb dating(635.2 ± 0.6 Ma,632.5 ± 0.5 Ma,and 551.1 ± 0.7 Ma)for the three layers of volcanic ash in the DST Fm,and then we obtained a continuous astronomical time scale for ~50 Myr,and which provides accurate age constraints for the lithostratigraphy,isotopic chemical stratigraphy and biostratigraphy in DST Fmduring the Ediacaran period.Based on this,the astronomical orbits-forcing might contribute to a comprehensive understanding of the geological evolution history of the region and the global carbon cycle change.Our results has provided some new insights into the systematic study of the cyclostratigraphic analysisof the DST Fmat the Jiulonogwan section and the Sixi section during the Ediacaran period,which aredivided into four aspects as the follows:?Using factor analysis and hierarchical clustering methods tocompare and analyze the rock element content and MS data series.There are different of the paleoclimate significance indicated by many paleoclimatic proxies such as MS,Ti,Ca and Rb/Sr,etc.We focus on the 21.1 mof the DST Fm at the Sixi section,and these eight paleoclimate proxies dividedinto three categories(Factor1,Factor2 and Factor3)by mathematical analysis methods.Due to their internal correlation,our resluts shows that Factor1 as an indicator for terrigenous detritus input,including Fe,Zr,Ti,K,and Rb/Sr as products of terrestrial sediments to reconstruct paleoclimatic changes;Factor 2 indicates the marine biological provenance/paleoproductivity,which contains Ca and Mn and used as marine sedimentary proxies;Factor 3 refers to MS mainly from the contribution of magnetic minerals,and related with clay mineral content,which can indirectly reflect terrigenous weathering.Therefore,choosing the reasonable paleoclimate proxies provide us with reliable database to guarante our data series.? Astronomical time scale for the lower the DST Fmin early Ediacaran,South China.The Jiulongwan section is a well-studied reference section for these Ediacaran events.We choose Ca content and Fe/Ti as the paleoclimate proxies in the lower of the lower DST Fm(22.3 m).Based on the average sedimentation ratio as 1.83 m/Myr and combined with field outcrop observation.Spectral analysis of geochemical data through the lower DST Fm shows 27 predominant ~0.9-m cycles that correspond to405-kyr long eccentricity cycles.An 11.16 Myr-long astronomical time scale has been constructed for the lower DST Fm and providestime constrains of ~1.6 Myr for the base DST Fm cap carbonate(EN1)basing on the most stable 405-kyr long eccentricity cycle tuning.Using the U-Pb age of 635.2 ± 0.6 Ma for the volcanic ash bed at the top of cap carbonate,we proposed a 636.8 Ma age for the base of the DST Fm,~1.6 Myr for the duration of DST-1(cap carbonate,EN1),and ~2 Myr cyclemight be an important factorto drive the ?13C and ?18O during the early Ediacaran.?Cyclostratigraphic analysis of the middle-upper DST Fm(84.9 m)at the Jiulongwan section,and established a continuous high-resolution astronomical time scale of ~29.7 Myr,and provided a precise duration of EN3.Based on the propsed average sedimentation ratio as 2 m/Myr for the lower DST Fm,cyclostratigraphic analysis of geochemical data through the middle-upper DST Fm(84.9 m)indicates that sedimentary cycles are interpreted to long-and short-eccentricity(405-kyr,135 to 100 kyr),obliquity(35 to 25 kyr)and precession(19 to 16 kyr)periods,and a total of 73 sedimentary cycles with 405-kyr long-eccentricity cycles were calculated.A ~29.7Myrof astronomical time scale has been constructed basing on tuning to the 405-kyr long-eccentricity for the DST Fm at the Jiulongwan section,the Member DST-4(black shale)of the top DST Fm spans ~7.5 Myr and the negative ?13C excursion(EN3/Shuram/Wonoka)has duration of ~20 Myr.Therefore,due to a U-Pb date of 551.1 ± 0.7 Ma near the top of the DST Fm,we propose that the base of DST-4 is at 558.6 ± 0.8 Ma,that the projected basal age of DST-3 is 579.3 ± 0.8 Ma,and that the onsets of EP2 and EN3 were ca.578.1 ± 0.8 Ma and 571.1 ± 0.8 Ma,respectively.Both ?13C excursions were younger than the termination of the Gaskiers glaciation(~581 Ma).Our results provide important time constraints for the internal strata division in the Ediacaran periods.?Astronomical time scale for the middle DST Fm at the Sixi section.We observed prominent astronomical cycles on the middle the DST Fm(22.1 m)at the Sixi section outcrop.The fluctuation of MS and Fe content show obvious Milankovitch cycles,and contains more than twenty-one 405-kyr long-eccentricity to reconstructa continuous and high-resolution "floating" astronomical scale of ~8.46 Myr.Based on the anchor ages from the boundary age of 579.3 ± 0.8 Ma of DST-3/DST-2 at the Jiulongwan section,we established an absolute age model of 584.9-576.4Ma for the middle DST Fm at the Sixi section.The duration of the fourth ?13C negative excursion(SN4)is ~5.3 Myr(584.6-579.3 Ma).