Sedimentology and magnetostratigraphy of the Tierekesazi Cenozoic section in the foreland region of south West Tian Shan in Western China

Abstract

The geology of Tian Shan provides an excellent example for understanding the intracontinental orogeny in the context of Indian–Eurasian convergence. Previous studies leave much space in basinfill deposition process to be assessed in the regions west to the Talas–Fergana fault (TFF). We improve the understanding by conducting new investigations on sedimentology and magnetostratigraphy in the Tierekesazi section of the foreland region of south West Tian Shan. Four lithofacies have been identified, (i) marine lithofacies from the Aertashi to Bashibulake Formations, (ii) lacustrine to fluvial (plain) lithofacies from the Keziluoyi to the middle Pakabulake Formations, (iii) alluvial sand–gravel sheet lithofacies in the upper Pakabulake Formation, and (iv) conglomerate lithofacies from the Atushi to Xiyu Formations. Magnetostratigraphic analysis, accompanied with biostratigraphic correlation, indicates that four lithofacies cover age intervals of ca. 65Ma to 34Ma, ca. 22.1Ma to 12Ma, 12Ma to 5.2Ma, and 5.2Ma to approximately present (?), with the sediment accumulation rates increasing from ca. 2.4/3.3–3.5 (compacted/decompacted) cm/ka in the lithofacies (i), to 12.3/16–17cm/ka in the lithofacies (ii), to 16.3/19.5–20.6cm/ka in the lithofacies (iii), and finally to >22.8/>22.8cm/ka in the lithofacies (iv). These results suggest three episodes of sedimentary events.Combined with previous results, these episodes of sedimentary events are attributed to tectonic activities that are widespread along south Tian Shan. We speculate that the Oligo-Miocene boundary event more directly and likely marks the initial underthrusting of the Tarim block beneath the south Tian Shan. The mid-Miocene and Mio-Pliocene boundary events, although approximately synchronous between the regions east and west to the TFF, have different structural expressions in the two regions. Such difference is proposed to cause the dextral slipping of the TFF, and more fundamentally, likely be driven by the northward indentation of the Pamir at this time.