• 论文 •

### 靖远曹岘黄土的形成时代及显微结构特征

1. 西北大学地质系黄土研究室, 西安 710069
• 收稿日期:1994-07-01 修回日期:1994-12-01 出版日期:1995-11-15 发布日期:1995-11-15
• 基金资助:
国家自然科学基金

### FORMATION AGE AND MICROFABRIC CHARACTERISTICS OF LOESS IN CHAOXIAN, JINGYUAN, GANSU, CHINA

Lei Xiangyi

1. Loess Research Section of Geology Department. Northwest Univeristy. Xian 710069
• Received:1994-07-01 Revised:1994-12-01 Online:1995-11-15 Published:1995-11-15

Abstract: The Thickest loess deposits in the world are observed at the Chaoxian village, Jingyuan, Gansu, China. Developed on the 6th terrace of Yellow River and having a typical loess-paleosol sequence. this loess section is about 505 m thick. with 33 layers of paleosols. According to the paleomagnetic data, the Jingyuan section contains Brunhes Normal Polority Zone and Matsuyama Reversal Polority Zone. BIM boundary is located in L8 (8th layer of loess from top downward). The Jaramillo (J) Normal Polority Subchron corresponds to the interval from L11 to S12 Jingyuan loess began to form at 1. 40 MaBP. The rate of loess deposition in Jingyuan loess section is about 36 cm/ka.The component particles of loess in Jingyuan are less than 0. 25 mm. with about 65%-68% silts (0. 050 0.01 mm). The average quantity of particles which are less than 0. 002 mm is about 7. 0%. Its main chemical composition is SiO2 and Al2O3 the contents of which are about 57. 21% and 11. 90% respectively. A obvious characteristic of the chemical component is that content of Fe2O3 is smaller (2. 87%). while the content of CaO is higher (more than 8. 31 %). The above-mentioned fact shows that the paleo-conditions in Jingyuan may be arid-cold during the formation of loess.Under the scanning electron microscope (SEM). the microfabrics of the loess in Jingyuan change from the top to the bottom gradually from the weak-cementation fabric group to the semi-cementation one, and then to the cementation fabric group, and correspondingly, the loess engineering properties are also getting better. For instance, coefficients of collapsibility obviously diminish,while initial pressure of collapsing gradually increases. It indicates that the mechanical properties of loess are closely related to the microfabric characteristics.The Jingyuan section has been correlated with a composite oxygen isotope (δ18O) record. In the past 0. 9 Ma, 9 loess-paleosol groups coincide with 9 climatic cycles of oxygen isotope record. which fluctuate within a wide range and a long-period, and from 1. 38 Ma to 0. 9MaBP, the climatic cycles of δ18O record fluctuated within a narrow range and a short-period, being coincidence with the thin loess and closely bedded paleosols in the Jingyuan section.

Key words: loess, paleosol, microfabric

• P642.131