新疆克孜勒苏群的沉积环境探讨——兼论塔里木盆地西部白垩系最低海相层位

AN APPROACH TO THE DEPOSITIONAL ENVIRONMENT OF THE CRETACEOUS KIZILSU GROUP: THE LOWERMOST MARINE HORIZON OF THE CRETACEOUS IN THE WESTERN TARIM BASIN

本文根据新发现的自生海绿石矿物、微量元素硼含量、碳氧稳定同位素分析以及岩性和沉积特征,进一步论证了新疆克孜勒苏群下亚旋回上部和上亚旋回存在海相层,其下亚旋回上部为塔里木盆地西部白垩系的最低海相层位,推测时代为早白垩世巴列姆期。

The Kizilsu Group is composed of brownish red and greyish green massive quartzitic sandstones, qusrtzose graywacke and basal conglomerate, exibiting well-developed cross-bedding, representing two subcycles of sedimentation and totalling about 1300m in thickness. For about thirty years, this group has been considered as of terrestrial origin. In 1987 some marine trace fossils were found in abundance, and fine preservation in horizons from the upper part of the lower subcycle to the upper subcycle of the group. Recently, based on the mineralogical, geochemical and sedimentary features, some new evidence indicating marine facies has been found by the author in the same horizons. 1. Endogenic glauconite is common, ranging in content from 1％ to 5％. 2. The content of the trace-element boron in the clay minerals is about 137—260 ppm, averaging 194 ppm. 3. Based on the data of M. L. Keith and J. N. Weber (1964), the maximum stable carbon isotope value for Cretaceous fresh-water carbonates is reasonably considered to be -1.8—-2.2‰, and any values higher than this should indicate a marine origin. The average value of δ^(13)C is 1.42‰ for the carbonates in horizons from the upper part of the lower subcycle to the upper subcycle of the group, and -2.74‰ for those in the lower part of the lower subcycle. 4. In the quartzitic sandstone and conglomerate from the upper part of the lower sub- cycle and the upper subcycle, sand grains and gravels are well sorted and of high psephicity and have less cement. Stable minerals such as quartz and chert predominate and large crossbeddings are well developed. From the facts mentioned above it may be concluded that some marine strata exist in the upper part of the lower subcycle to the upper subcycle of the group, and the upper part of the lower subcycle is the lowermost Cretaceous marine horizon in the western Tarim Basin, which may belong to the Barremian.