STRATIGAFIC ASSIGNMENT OF EOLIAN AND FLUVIAL SEDIMENTS IN UDYN SAYR, GOBI DESERT, MONGOLIA USING AN INDICATOR OF DEFECT CENTERS IN QUARTZ COMPOSED OF SAND PARTICLE

A large number of dinosaur fossils preservation have been occurred in the Upper Cretaceous eolian and fluvial sediments in the Mongolian Gobi desert, well known as one of most-important dinosaur localities in the world. Furthermore, vertebrate fossils have been extensively found from the Djadokhta Formation in the Gobi desert, where any of tephra and micro-fossils as a geochronological indicater has not been confirmed so far. Therefore, a stratigraphic assignment of the eolian and fluvial sediments should be required to proceed in paleontological investigation on the evolution of the dinosaurs by comparison with those reported elsewhere in the world as well as geological study in these areas. The previous works suggested that cathodoluminescence (CL) features of quartz grains in the eolian sediments characterize crystal-chemical proper of the quartz whereas the electron spin resonance (ESR) features provide the information on detect centers in the quartz in among the fossil-bearing localities of Alag Teg, Tsugrikin Shireh, Bayan Dzag and Udyn Sayr. Furthermore, we have conducted to characterize both eolian and fluvial sediments in addition to previous studies. The present study focuses on a comparative characterization of in ESR and CL features of the quartz grains collected from eolian and fluvial facies in Udyn Sayr.

Quartz is one of the most abundant minerals on the surface of the Earth, and has been used for dating by an ESR method, which can detect the electrons trapped at lattice defects in quartz, created by natural radiation and accumulated in the mineral through a geological-time scale. Based on the previous works showing that ESR and CL signals are useful in fingerprinting the eolian and fluvial sediments, the ESR signals have been employed for the assignments of the quartz grains extracted from the terrestrial sediments. Quartz grain were took out from the sediments in the following procedures: chemical treatment with 50% HCl, 20% HF, separation with heavy-liquid of sodium polytungstate (SPT), and an isodynamic-magnetic separation. A portion of each sample was irradiated at 2.5 kGy by ⁶⁰Co gamma rays, and subsequently heated at 300℃ for 15 minutes before ESR measurement at room temperature for the E₁’ center. The results for ESR and CL feature will be presented for a further discussion.