Cenozoic growth of West Kunlun Mountains and tectono-sedimentary evolution of adjacent SW Tarim Basin-New spatial model based on seismic data

<p>Kunlun Mountains, SW part of the Tarim Basin and S edge of the Bachu Uplift in central Asia collectively form the northernmost segment of the vast Cenozoic deformation zone and associated depositional areas formed in course of the India – Euroasia collision. Five seismic transects from the SW Tarim Basin (Yechang - Hotan area) calibrated by deep wells were used in order to assess lateral variations of a structural style and syn-tectonic sedimentation in this part of the basin. Pre-Cenozoic substratum of SW Tarim Basin is formed by crystalline basement covered by Paleozoic strata, with important mid-Cambrian evaporites (Awatage Formation) that served as first, deep detachment level. Cenozoic sedimentary infill consists of several kilometers of shallow water to terrestrial clastics with Paleogene evaporites of the Bashiblake Formation at their base. Paleogene evaporites acted as a second, shallow detachment. Mid – late Miocene to Quaternary wedging along the front of the Kunlun Mts., associated with formation of a large-scale duplex consisting of slivers built of Precambrian to Permian rocks, resulted in progressive, laterally variable uplift of the S margin of the Tarim Basin documented by well-preserved growth strata that have been also described in the field. Jade Anticline, large intra-basinal structure that is located in the central part of the Tarim Basin, previously interpreted as a regional wrenching zone, was reinterpreted as a thin-skinned syn-depositional “fish tail” structure, detached in the Paleogene evaporites and formed in Quaternary above local basement elevation. Northernmost late Miocene compressional deformations have been recognized along the S edge of the Bachu Uplift in its Western and central segment. They formed due to complex interplay of thick-skinned basement reverse faulting responsible for regional elevation of basement blocks, and two types of thin-skinned thrusting: southward directed thrusting detached within the mid-Cambrian evaporites and northward directed thrusting detached within the Paleogene evaporites. Compressional deformations along the S edge of the Bachu Uplift are diminishing and eventually disappearing towards the East. All these findings point to significant transfer of compressional stresses into the far foreland of the W Kunlun Mountains and laterally variable tectonic coupling between the Tibet Plateau and central part of the Tarim Basin.</p><p>Seismic data used in this study was kindly provided by China National Petroleum Corporation (PetroChina). IHS Markit is thanked for providing academic license of Kingdom seismic interpretation software.</p>

Characteristics and dolomitization of the Lower Qiulitage Group reservoir in Bachu Uplift, Tarim Basin, China

The characteristics of the Lower Qiulitage Group in Bachu uplift, which originated in the Upper Cambrian period, are not clear. Based on four core observations, identification of 40 thin sections, and geochemical analysis of samples, this study concludes that crystal dolomite reservoirs developed in the Lower Qiulitage Group in Bachu uplift. Intercrystalline pores and dissolved pores formed the main reservoir space, while dissolution pores and fractures were developed in the middle and bottom part of the formation. The reservoir features low porosity and low permeability, and the fine-medium crystalline dolomite reservoir has the best pore structure. According to a petrochemical analysis, the Lower Qiulitage Group reservoir experienced intense dolomitization, with slightly higher content of trace elements Mn and Sr and negative oxygen isotropy. Tectonic hydrothermal dolomitization developed locally under the influence of base faults, and vug pores were filled with saddle dolomites and siliceous filling. The presence of rare earth elements Ce and Eu presented positive anomaly characteristics. It suggested that the CO2 acid dissolution caused by hydrothermal fluids may be a new mechanism for the increase in pores in deeply buried reservoirs, which is worth further study.

Taphocoenoses and diversification patterns of calcimicrobes and calcareous algae, Ordovician, Tarim Basin, China

Despite the crucial role of epibenthic primary producers (cyanobacteria, green and red algae), no diversity curves for calcimicrobes and calcareous algae are available to assess the pyramiding paleoecology characterizing the Ordovician biodiversification episode. A total of 24 taxa of calcimicrobes and calcareous algae are identified from a Dapingian to lower Katian succession of carbonate sedimentary rocks exposed at the Leyayilitag ridge, Bachu Uplift, Tarim Basin, northwest China. Calcimicrobes (14 taxa), Dasycladales (seven taxa), Bryopsidales (one taxon), and Cyclocrinales (two taxa) contribute to five distinct taphocoenoses characterizing a suite of carbonate mounds. In stratigraphic order, these are calathid sponge mounds, algal calcimicrobial mounds, algal mounds, algal reefs, and calcimicrobial mounds. Within the lower Katian Belodina confluens Zone, the diversity increases substantially from around 5 to more than 20 taxa per 2 Ma. This increase in diversity is based on new calcimicrobes (Bija, Ortonella, Garwoodia, Hedstroemia, Rothpletzella, Phacelophyton, Rauserina) and the diversification of Dasycladales and Cyclocrinales. By comparison, the global diversity of calcimicrobes and calcareous algae (derived from literature data) started to increase earlier, namely within the late Darriwilian Pygodus serra Zone (offset of about 4 Ma). This offset might be due to the peculiar lithology of the Sandbian Tumuxiuke Formation (condensed section of red nodular limestones bounded by disconformities). However, a similar temporal offset is recorded for calathid sponge mounds; therefore, the Tarim tectonic microplate (Tarim Block) might display an endemic–anachronistic character. The diversity curves of Ordovician benthic primary producers (calcimicrobes, calcareous algae) are similar to those recorded by some fossil groups, in particular eleutherozoan echinoderms.