Sedimentary characteristics and evolution of the Lower Carboniferous Akeshake Formation in the Yili Basin

The Lower Carboniferous Akeshake Formation in the Yili Basin can be lithologically divided into three sections from bottom to top,i.e.the clastis interval,clastis with carbonate interval and carbonate interval.Based on detailed field geological investigations and laboratory analyses,six facies and fourteen sub-facies were identified from the Akeshake Formation.Carbonates are important deposits in the Yili Carboniferous marine basin,where they are deposited in hybrid coast facies,restricted-open platform facies,platform-basin facies,platform-edge reef-flat facies,and platform fore-edge slope and basal-slope facies.During the late period of early Carboniferous,the depositional evolution of the Yili Basin was obviously influenced by tectonic activities,paleotopography,sea-level fluctuation,paleoclimate and provenance,and it can be divided into three stages:① A braided delta facies developed in the early period and sediments mainly came from the northern Tianshan area.② Braided delta,hybrid coast,open platform and platform-basin facies developed in the middle period,of which the braided delta and hybrid coast facies are distributed in the Nileke Cement Plant area adjacent to the Tianshan provenance,the hybrid coast facies occurs in the Akeshakegou area in Zhaosu County,while the open platform and platform-basin facies are distributed in the Kekesu Gorge and Qieketaigou areas,respectively.③ The Yili Basin was subsiding with minor input of terrigenous materials in the late period,experiencing a typical carbonate-platform depositional stage when sedimentary facies showed a banded distribution horizontally open to the west and converged to the east,there still existed a hybrid coast facies along the Nileke Cement Plant area because of the adjacence to provenance.