Rock Magnetism Characteristics And High Resolution Geological Time Scale Of Lower-Middle Permian Carbonates In South China

Middle Permian is a critical period in geological history. The eruption of Emeishan large igneous province which was considered as the trigger of the Paleozoic-Mesozoic double mass extinction launched at this time. The establishment of high-resolution time scale in Middle Permian is all important for understanding the onset time and cause of the mass extinction and the eruption of Emeishan large igneous province. However, the precision of time scale for Middle Permian is seriously limited due to the lack of high precision radiometric ages (The only zircon age currently available, comes from an ash bed that～37.2m below the base of the Capitanian stage). According to the fact that it is difficult to obtain accurate absolute ages, it’s necessary to consider other dating methods. In recent decades, as the understanding and research of the theory that Earth’s orbital parameters can affect climate change, a great number of achievements have been achieved in aspects of dating and establishment of high-resolution geological time scale by astronomical cycles. In this study, a high resolution float point time scale(FPTS) has been established for the Wordian stage in Shangsi section and the Capitanian stage in Tieqiao section by astronomical cycles dating based on the research of sedimentology and rock magnetism.The largest transgression since Late Paleozoic had begun in South China during the Early Permian. That formed a rigorously isolated platform-type marine environment which was rare in the geological history. We investigated the characteristics of rock magnetism and sedimentology for Lower-Middle Permian carbonates at Tieqiao section, Guangxi area, South China. The magnetic susceptibility (MS) data sets of Tieqiao section are mostly negative, indicating the dominant magnetic components are diamagnetic calcite and chert, with subordinate paramagnetic, ferrimagnetic and antiferromagnetic minerals. Laboratory measurements on rock magnetism suggest that the magnetic minerals are mainly paramagnetic minerals (mainly are illite) with a small amount of ferrimagnetic minerals and antiferromagnetic minerals, including soft magnetic magnetite and titanomagnetite, hard magnetic hematite and goethite. The proportion of hard magnetic component increased from bottom to top in the section profile. Hard magnetic minerals are dominant in the upper part of the Capitanian stage. The characteristics of rock magnetism near the Guadalupian-Lopingian boundary showed a pronounced shift. The magnetic minerals are changed from hard magnetic minerals to soft magnetic minerals and then returned to hard magnetic minerals. The MS increased first and reduced later, but the content of magnetic particles showed a contrary tendency. All these transitions were finished in4m strata around the Guadalupian-Lopingian (G-L) boundary that synchronizes with the regressive and subsequent transgressive at Later Guadalupian, also the shift of paleotemperature of seawater. These transitions probably indicated a short-lived (0.1myr) special event at the G-L boundary. In addition, a large number of pyrite framboids were found in Lower Chihsia formation, confirming the existence of reduction of magnetic minerals.Astronomical cycles are used to dating the Capitanian stage based on detailed research of rock magnetism and biostratigraphic work at Tieqiao section. The MS data sets of Capitanian stage are mostly negative (diamagnetic), suggesting that the Laibin area was isolated from terrigenous sources during most of the Capitanian. A sudden increase in the MS signal at the top of J.altudaensis just below the Guadalupian-Lopingian (G-L) boundary is recorded. The coincidence of the increase with the onset of the Emeishan Large Igneous Province (LIP) eruptions argues strongly for volcanic effects controlling the MS in this part of the section. The thermomagnetic susceptibility measurements indicate that the dominant magnetic constituents in this study are paramagnetic and subordinate ferrimagnetite minerals. Spectral (time-series) analysis of the MS data was performed using Multi Taper Method (MTM) and Fourier Transform (FT) analysis. Five frequencies that are consistent with Milankovitch-band orbital forcing were compared to this data set:eccentricity E1and E2(～‖100and～405kyr, respectively), obliquity02(-44.10kyr), and precession P1(～17.70kyr) and P2(-20.95kyr). Interpretation of these results indicates that the duration for the Capitanian in the Tieqiao section was～3.85myr. In turn, the mean sediment accumulation rate (SAR) for the whole Capitanian was～2.9cm/kyr. In addition, graphic comparison of the Tieqiao MS data to a floating point time scale(FPTS) for the Upper Wordian throughout Capitanian time allows estimates of the duration for conodont zones identified from the section, these ranging from～10.3kyr to～2.43myr before the extinction of individual conodont species. In turn, timing of the onset age of Emeishan LIP eruptions was estimated, yielding an age of～262.67Ma, with the start of these eruptions at～1.42myr below the G-L boundary, and it is also the onset time of Paleozoic-Mesozoic mass extinction event.We chose the Wordian stage of Shangsi section in Guangyuan, Sichuan province as the object of astronomical cycles dating because biostratigraphic works were well done by previously researchers. The MS data sets of Wordian in Shangsi section also are mostly negative, suggesting that the Guangyuan area was isolated from terrigenous sources during most of the Wordian. Thermomagnetic susceptibility measurements indicate that the magnetic carriers in this study mainly are paramagnetic with subordinately magnetite. Spectral (time-series) analysis of the MS data was performed using MTM and FT analysis. Five frequencies that are consistent with Milankovitch-band orbital forcing were compared to this data set:eccentricity E2(～405kyr,～100kyr), obliquity01(-35.00kyr) and O2(～44.00kyr), and precession P1(～17.60kyr) and P2(-20.9kyr). A FPTS was built for Wordian of Shangsi section by graphic comparison, with resolution of～200kyr. Interpretation of spectral analysis indicates that the duration for the Wordian in the Shangsi section was～2.84myr. In turn, the mean sediment accumulation rate (SAR) for the whole section (from upper Roadian to lower Capitanian) was～1.65cm/kyr.