Objective The original contact relationship between lithostratigraphic groups is the key evidence to record and prove the relationship between the two adjacent lithostratigraphic units, and is also the necessary and sufficient condition to establish and name the stratigraphic units. Since the formation of the Late Carboniferous Hala’alate formations and the Aladeyikesai in the Baogutu stratigraphic area of West Junggar, it is believed that "the lithostratigraphic units contact relationship is unclear ".

Methods We have identified two sets of sedimentary formations with distinct lithology, which are easy to identify and distinguish. Both of them are from the core of the Carboniferous system of well 424 and well 581 of Ke−Bai oilfield, which are adjacent to the south of the type section, and are mainly composed of "(upper) marine fine clastic rock/(lower) volcanic breccia". The core records and logging electrical datas confirm that the two formations are conformable contact. The upper and lower sets of strata can be compared with the lower clastic rock section of the Aladeyikesai Formation and the top section of the Hala’alate Formation from the type section of the Hashan area.

Results Moscovian sporopollen assemblages from marine fine clastic rocks of two wells were obtained, such as Noeggerathiopsidozonotriletes−Protohaploxypinus−Hamiapollenites; this fine clastic rocks were conformable contact by the volcanic rocks with the zircon U−Pb age of 306.2±5.8 Ma. Combined with the zircon U−Pb ages (303~295 Ma) of the volcanic rocks in this formation in Hashan region, the main body of the Aladeyikesai Formation is placed in the Gzhelian Stage of the Upper Carboniferous.

Conclusions Combined with the 310.5 Ma zircon U−Pb age obtained in the volcanic rocks of the adjacent wells in the east of well 581, this volcanic−dominated Hala’alate Formation is placed in Kasimovian−Moscovian Stage of the Upper Carboniferous. The clear original conformable contact relationship support the old−new relations between the two stratigraphic units, and also provide new data for the formation sedimentary sequence and sedimentary basin evolution of the Carboniferous.