四川峨眉山龙门洞三叠系SEDBA数据库及其应用

APPLICATIONS OF SEDBA TO THE TRIASSIC STRATA IN LONGMENDONG,MT.EMEI,SICHUAN,CHINA

摘要:

本文是峨眉山龙门洞三叠系SEDBA数据库地球化学资料在化学地层和海平面变化及层序地层学等方面的应用。化学单元组的划分完全符合岩性地层组的界限,一些化学单元则与岩性段不同。许多化学单元与沉积体系域相一致。根据碳酸盐岩的碳氧同位素分布的群聚特征(图3),结合薄片鉴定,划分了碳酸盐岩的三种沉积和浅埋藏成岩环境:T₁j¹和T₁j³段为海相正常盐度环境沉积,其他多数岩段为超盐度环境。碳氧同位素在垂向上具周期性变化(图4)。无蚀变的均匀细粒的碳酸盐岩的δ¹⁸O有与沉积环境相应的周期性变化,其δ¹⁸O作为海平面变化的一种地球化学响应,较好地提供了本区相对海平面变化及其与区域构造沉降速率和沉积物供应速率间相互关系的判据。认为T₁j³和T₁l³是两次与全球海平面上升有关的沉积,其δ¹⁸O大幅下降与宏观的海侵沟蚀面伴随,而T₁j¹段的相对海平面上升则是由于区域构造沉降速率大于碎屑供应和碳酸盐生产速率而引起的、并因两种速率频繁变化造成碎屑岩-灰岩的高频韵律互层。本文还对一些层段及海进海退层序中的界面性质作了论述。

Abstract:

Applications of the geochemical data in the SEDBA of the Triassic in the Longmendong section,Mt. Emei to chemical strata and to sea-level Changes and sequence stratigraphy are pre-sented in the paper. The division of the chemical unit sets accords fully with the boundary Of the lithostratigraphic groups,Some of chemical units are different from the division of lithos-tratigraphic members. Many of chemical units accord with the depositional system tracts. Three depositional and shallow buried diagenetic environments of the carbonates are divided based on the regional clustering features in the scatter diagram of the distribution of the δ¹⁸O and δ¹³C of carbonates;combined with thin-section identification. Deposition of T₁j¹ and T₁j³ members occurred in normal saline environment,majority of other lithostratigraphic members deposit in hypersaline environment. There are the cyclic changes in the vertical curve change of δ¹⁸O and δ¹³c. The δ¹⁸O values of fine and denes carbonate rocks without alteration possess the corresponding cyclic variations related to the depositional environments. The δ¹⁸O as a geo-chemical responses to the sea-level change or eustacy,provides the criterion for the relationship of global sea-level Change and the rate of regional tectonic subsidence and the rate of clastic ma-terial supply. It is believed that marine transgressions in T₁j³ and T₂l³ member are due to global sea-level rising;meanwhile,the smaller transgression or relative sea-level rising is ralated to the larger rate of regional tectonic subsidence than the rate of clastic material supply and carbonate Production,and the rhythmic interbeds consisting of sandstones and limestone beds are also due to the frequent variations in the preceding two rates. Besides,the origin of the rocks in some members and beds,and properties of some boundary lines or interfaces have been briefly ex-pounded in the paper.