Cyclostratigraphy Of The Lower Triassic In The West Pingdingshan Section, Chaohu, Anhui Province

While there has been a great deal of research and discussion on sedimentary cycles following Milankovitch's pioneering research,the concept of cyclostratigraphy was not introduced formally until 1988 by Fisher and Premoli-Silva at an international conference in Italy.Subsequently, cyelostratigraphy rapidly developed into an area of research with great potential for earth sciences.Climate change due to Sun-Earth orbital forcing is one of the most important principles in cyclostratigraphy because it underlies the determination of high-resolution ages,from which the effects of local events can largely be eliminated.Cyclostratigraphy analysis allows considerable improvements to chronostratigraphic resolutinon by integrating various stratigraphic features, especially cyclicity,with biostratigraphic and absolute ages to establish an "astronomical chronologic chart".The West Pingdingshan Section in Chaohu,Anhui Province,as one of the candidates for the Induan-Olenekian boundary stratotype,has been made extensively study in rently years.The research is mostly refer to biostratigraphy,isotopic chemostratigraphy,magnetostrtigraphy and sedimentology.Ammonoid zone and Bivalve Assemblage zone had been saved very good in the early Triassic stratigraphy in this region and are compared well with other sections.Therefore, making this section significant not only for regional Chinese strtigraphic studies,but also in the quest to delineate a global Triassic chronostratigraphy.The Lower Triassic of the West Pingdingshan Section of Chaohu shows well defined mudstone/limestone cycles,seemingly corresponding to Milankovitch periods of orbital forcing. To clarigy the nature of these cycles quantitatively,we carefully measured and described the lithologies bed by bed,as well as collecting samples at 2-cm intervals for a magnetic susceptibility study.Our cyclostratigraphy study yielded a resolution in the order of tens-thousand of years for sub-meter scale cycles in the West Pingdingshan Section.With spectral and wavelet analysis of magnetic susceptibility of rocks and the observation of lithological vavriety,the cyclostratigraphical study for the Induan strata at the West Pingdingshan Section achieves the following results:1.The bundles and bundle sets identified in lithology at the field outcrops are proved by the wavelet analysis on the magnetic susceptibility data,resulting in a 4-5:1 ratio between the cycles and subcycles.The spectral analysis of the magnetic susceptibility data shows that the first and second predominant cycles are 0.76 m and 3.41 m,respectively,coinciding with the lithologic and wavelet analyses.Thus the cycles are believed a proxy of the orbital forcing Milankovitch cycles during the Triassic.2.The Induan sequence at the West Pingdingshan section is composed of 12 cycles and 56 subcycles in lithology,and the same composition of cycles is presented by the magnetic susceptibility data according to the wavelet analysis.The spectral analysis of the susceptibility data yields two predominant frequencies during the Induan separately at 11.6 and 54.13.Provided these cycles reflect the Earth orbital eccentricity and precession,the age-range of the Induan would be 0.983-1.171 Ma.Supposing the Permian-Triassic boundary aged at 252.6 Ma Mundil et al., 2004),the Induan-Olenekian boundary defined by the conodont Neospathodus waageni at the West Pingdingshan Section is around 251.5 Ma.3.The application of the sedimentary accumulation mode(Schwarzacher,2000)indicates that the tectonic setting and deposition were constant in Chaohu throughout the Induan,so that it is a perfect sequence for various stratigraphical studies.4.The mode about the form of bundle sets lithological cycles in Chaohu region,has been constructed and thought that the limstone/clay bundle sets reflect alternating period of wet and dry climate controlled by precession period in continent.However,changes are only effect to the interior of bundles sets.The change in the lithology of bundle set group and the relationship of bundle sets with each other are affected by higher scale astronomical orbital period,such as eccentricity period.