Abstract

Precambrian sequences of the Aksu region, which is situated near the northwestern margin of the Tarim Block, are composed of the Mesoproterozoic Aksu Group and Neoproterozoic Qiaoenbulak and Wushinanshan Groups. Whole-rock Rb-Sr isochron ages of 962 Ma and 944 Ma indicate that the protolith of the Aksu Group is older than about 960 Ma. The Qiaoenbulak Group comprises a large thickness of clastic rocks, representing turbidite deposits on a submarine fan. The Wushinanshan Group is made up of glacial deposits of the Umainak Formation, red sandstone of the Sugiatbulak Formation and littoral carbonate rocks of the Qigebulak Formation.

Two types of diamictite exist in the Qiaoenbulak and Wushinanshan Groups. The lower diamictite (Qiaoenbulak), previously described as a tillite, consists mainly of debris flow deposits. The upper diamictite (Wushinanshan) is a continental glacial deposit. Although they seem similar in appearance, appreciable differences in clast composition, roundness and sorting, as well as in facies association and contact relationships, allow the two to be distinguished.

Introduction

Precambrian sequences including the Mesoproterozoic Aksu, Neoproterozoic Qiaoenbulak and Wushinanshan Groups are exposed in the Aksu region along the northwestern margin of the Tarim Block (Fig. 7.1). Precambrian tillites were first recognized there by Zhang Zhenhua and Gao Zhenjia et al. of Geological Brigade No. 13 of the Ministry of Geology (Zhang et al., 1957). Later, several investigations concentrating on the Precambrian sequences were performed by Gao et al. (1982, 1983, 1985), Gao and Qian (1985) and Wang and Gao (1986). These researchers described two layers of diamictite and proposed a glacial origin for both. In 1986 the authors of this paper had an opportunity to investigate the Precambrian sequences in the Aksu region.