AGE CALIBRATION FOR THE BOUNDARY BETWEEN LOWER AND MIDDLE TRIASSIC BY STRONTIUM ISOTOPE STRATIGRAPHY IN EASTERN SICHUAN

Abstract

In Southern China, Geologists normally take the volcanic ash bed (with the thickness from 10 to 50 cm) as the boundary for marine Lower and Middle Triassic (i. e. the boundary between Anisian and Olenekian). The volcanic ash is just interbeded in evaporates (gypsum and anhydrite) of the Jialingjiang Formation (below the volcanic ash) and the Leikoupo Formation (above the volcanic ash), and has been alternated to illite clay stone now in Sichuan Province and Chongqing Municipality. In order to determine the age of the volcanic ash bed (also for the boundary between the Jialingjiang Formation and the Leikoupo Formation), we measured the 87Sr/86Sr ratios of marine sulfate samples near the boundary collected from Eastern Sichuan. The XRD analysis results, Sr, Mn, Ca, Mg, Si and SQ42- concentrations indicate that the sulfate generally contains information on the original seawater strontium isotope composition. According to the principle of strontium isotope stratigraphy, we attempt to calibrate the ages of the boundary between the Jialingjiang Formation and the Leikoupo Formation. The 87Sr/86Sr ratios of the samples with the thickness range 50cm above and 30 cm below the volcanic ash are from 0.708243 to 0.708312, and the ages calibrated are from 240.8Ma to 242.5 Ma. The erupting time of the volcano is about 242.5Ma by SIS age of top and foot samples. The results of age calibration shows that the volcanic ash as the boundary between Lower and Middle Triassic is reasonable. A new age for the boundary between Lower and Middle Triassic has been provided in this paper.