TRIASSIC STAGE BOUNDARY DEFINITION USING AN INTEGRATED BIOSTRATIGRAPHY, MAGNETOSTRATIGRAPHY, CHEMOSTRATIGRAPHY AND GEOCHRONOLOGY IN MARINE STRATA OF GUANDAO SECTION, NANPANJIANG BASIN, SOUTH CHINA
Conodonts enable broad delineation of stages, substages, and numerous biozones from the Changhsingian Stage of the Late Permian through the Carnian Stage of the Late Triassic. We use the following appearances and disappearances of conodont taxa as indicators of stage and substage boundaries: Changhsingian- Griesbachian, first Hindeodus parvus; Griesbachian-Dienerian, last H. parvus, first Neospathodus dieneri; Dienerian-Smithian, last Ns. dieneri, Ns. cristagalli, first Ns. pakistanensis, Novispathodus waageni; Smithian-Spathian, first Ns. crassatus, Spathicuspus spathi, Spathian-Aegean, first Chiosella gondolelloides, Cs. timorensis; Aegean-Bithynian, last Triassospathodus homeri, first Nicoraella germanica, Ni. kockeli, Bithynian-Pelsonian, last Neogondolella regalis, Pelsonian-Illyrian, last Paragondolella bulgarica, first Pg. excelsa; Illyrian-Fassanian, first Budurovignathus trumpyi, Ng. trammeri; Fassanian-Longobardian, first Bv. mungoensis; Longobardian-Cordevolian, first Quadralella polygnathiformis.
The section contains numerous magnetic reversals with predominantly normal polarity zones in the Smithian-Spathian Aegean-Bithynian, Longobardian-Cordevolian, and predominantly reversed polarity in the upper Griesbachian-Lower Smithian, Upper Spathian, and Pelsonian. Large amplitude carbon isotope excursions characterize the Lower Triassic and basal Anisian. The overall pattern in magnetic reversals is consistent with that of European sections and the carbon isotope excursions have been correlated globally. Several volcanic tuff horizons bracket the P-T boundary and Spathean-Aegean boundary in Guandao section and across the basin. High resolution U-Pb analysis from zircons in volcanics provide a robust age date of 247.2 Ma for the Spathean-Aegean Triassic boundary.