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Depositional facies and stratal cyclicity of dolomites in the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, NW China

  • The Upper Cambrian Lower Qiulitag Group in the Tarim Basin, NW China, is overwhelmingly composed of cyclic dolomites. Based on extensive field investigations and facies analysis from four outcrop sections in the Bachu-Keping area, northwestern Tarim Basin, four main types of facies are recognized: open-marine subtidal, restricted shallow subtidal, intertidal, and supratidal facies, and these are further subdivided into ten lithofacies. In general, these facies are vertically arranged into shallowing-upward, metre-scale cycles. These cycles are commonly composed of a thin basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: peritidal and shallow subtidal cycle, are further identified. The peritidal cycles, predominating over the lower-middle Lower Qiulitag Group, commence with shallow subtidal to lower intertidal facies and are capped by inter-supratidal facies. In contrast, theThe Upper Cambrian Lower Qiulitag Group in the Tarim Basin, NW China, is overwhelmingly composed of cyclic dolomites. Based on extensive field investigations and facies analysis from four outcrop sections in the Bachu-Keping area, northwestern Tarim Basin, four main types of facies are recognized: open-marine subtidal, restricted shallow subtidal, intertidal, and supratidal facies, and these are further subdivided into ten lithofacies. In general, these facies are vertically arranged into shallowing-upward, metre-scale cycles. These cycles are commonly composed of a thin basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: peritidal and shallow subtidal cycle, are further identified. The peritidal cycles, predominating over the lower-middle Lower Qiulitag Group, commence with shallow subtidal to lower intertidal facies and are capped by inter-supratidal facies. In contrast, the shallow subtidal cycles, dominating the upper Lower Qiulitag Group, are capped by shallow-subtidal facies. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, six larger-scale third-order depositional sequences (Sq1-Sq6) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thicker-than-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average peritidal cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lower-order sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq1-Sq3, is characterized by peritidal facies-dominated sequences and a progressive decrease in accommodation space, indicating a longer-term fall in sea level. In contrast, the upper sequence set (Sq4-Sq6) is characterized by subtidal facies-dominated sequences and a progressive increase in accommodation space, indicating a longer-term rise in sea level.show moreshow less

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Metadaten
Author:Yanqiu Zhang, Daizhao Chen, Xiqiang Zhou, Zenghui Guo, Wenwen Wei, Maria Mutti
DOI:https://doi.org/10.1007/s10347-014-0417-1
ISSN:0172-9179 (print)
ISSN:1612-4820 (online)
Parent Title (English):Facies : an international journal of palaeontology, sedimentology, geology
Publisher:Springer
Place of publication:New York
Document Type:Article
Language:English
Year of first Publication:2015
Year of Completion:2015
Release Date:2017/03/27
Tag:China; Dolomites; Facies; Sequences; Stratal cyclicity; Tarim Basin; Upper Cambrian
Volume:61
Issue:1
Pagenumber:24
Funder:National Science and Technology Special Project of China [2011ZX0500803]; National Basic Research Project (973 Project) [2012CB214802]
Organizational units:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Erd- und Umweltwissenschaften
Peer Review:Referiert