Facies analysis and depositional evolution of Lower-Middle Ordovician carbonates in the Shuntuoguole Low Uplift of Tarim Basin (NW China)

Abstract

Abstract The depositional evolution of the Early to Middle Ordovician carbonate platform in the Shuntuoguole Low Uplift (Tarim Basin, NW China) is controversial, leading to difficulty in understanding the distributional pattern of depositional facies and its controls and in petroleum exploration. Integrated with geophysical analyses, detailed sedimentological observations were carried out on the Lower-Middle Ordovician carbonate successions of the Shuntuoguole Low Uplift, in order to recognize the depositional facies and sequences and to reveal the carbonate platform evolution. The results show that a carbonate ramp system developed in the Shuntuoguole Low Uplift during the Early-Middle Ordovician. In such a depositional system, twelve types of lithofacies were recognized, which are arranged into two facies belts (or associations), including peritidal to semi-restricted subtidal and open-marine subtidal facies. Based on the spatiotemporal distribution of lithofacies (or facies), seven third-order depositional sequences were recognized in the Lower-Middle Ordovician carbonate successions. Individual sequences are composed of a transgressive package in the lower part and a regressive one in the upper part. And the sequence boundaries are commonly characterized by transitional zones without remarkable subaerial exposure features. And the Lower-Middle Ordovician carbonate successions were subdivided into three depositional phases during the long-term evolution of the carbonate platform, which were primarily characterized by semi-restricted to open-marine subtidal facies, peritidal through semi-restricted to open-marine subtidal facies, and open-marine subtidal facies, respectively. The correlation of relative sea-level (or accommodation spaces) changes derived from the vertical stacking pattern of facies with the global sea-level curve of Haq and Schutter (2008) and sea-level oscillations of coeval successions worldwide suggests that the eustatic sea-level fluctuation was an overriding factor controlling the development and evolution of carbonate ramp in the study area during the Early-Middle Ordovician. Meanwhile, regional or local tectonic subsidence and other environmental factors (e.g., paleotopography) also exerted an influence on the deposition processes at this time interval.