Depositional facies and stratal cyclicity of carbonate successions in the Yingshan and Yijianfang Group (Lower-Middle Ordovician) in Yuejin-Tuoputai Region, Tarim Basin, NW China

Abstract

The Lower-Middle Ordovician Yingshan and Yijianfang Group in the Tarim Basin are overwhelmingly composed of cyclic carbonates. Based on microscopic observation and facies analysis from two borehole sections in Yuejin-Tuoputai area, two main types of facies are recognized: semi-restricted subtidal, open-marine subtidal, and these are further subdivided into six and nine lithofacies in the Yuejin and Tuoputai area, respectively. In general, these facies are vertically arranged into shallowing-upward, meter-scale cycles. These cycles are commonly composed of a thinner 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: semi-restricted subtidal and open subtidal, are further identified. The semi-restricted subtidal cycles, predominating over the middle-upper Yingshan Group and the lower Yijianfang Group, commence with algal bindstone in the bottom and are capped by pelletoid grainstone and bindstone with peloidal grains. In contrast, the open subtidal cycles, dominating the upper Yijianfang Group, are dominated by clastizoic grainstone or clastizoic Bindstone. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, nine larger scale third-order depositional sequences (Sq3-Sq11) 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 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 Sq3–Sq7, is at the rising–descending cycle in Fisher Curve plots. And this indicates the accommodation space keeps steady. The upper sequence set, including Sq8–Sq11, some features can be found that these Fisher Curves of sequences flunctuate strongly in the middle-lower Yijianafang Group while keep steady in the upper part. This indicates a phenomenon, where the accommodation space varies from violent to gentle.