Sequence Stratigraphy and High-Frequency Cycles of the Carboniferous Xiaohaizi Formation in Southwestern Tarim

Establishing the sequence stratigraphic framework and high-frequency cyclic architecture of the Carboniferous in southwestern Tarim is important for regional stratigraphic correlation, reconstruction of Late Carboniferous carbonate platform evolution, and prediction of favorable reservoirs. Based on integrated analyses of lithology, well logs, biostratigraphy, and regional seismic reflection data, the Carboniferous succession is subdivided into two third-order sequences (SQ1 and SQ2). Sequence boundaries are identified mainly by abrupt lithologic and log-response changes, local erosion, and depositional hiatuses, whereas maximum flooding surfaces commonly correspond to regionally traceable high-gamma mudstone intervals. Conodont and fusulinid assemblages from the Xiaohaizi Formation indicate an overall Late Carboniferous age and provide key biostratigraphic constraints for regional correlation. SQ1 corresponds to the Bachu Formation and is dominated by a laterally persistent carbonate highstand systems tract. SQ2 comprises the Kalashayi and Xiaohaizi formations, in which the clastic interval of the Kalashayi Formation represents the transgressive systems tract and the carbonate succession of the Xiaohaizi Formation forms the highstand systems tract. Together, they record an environmental evolution from a relatively open intraplatform sea to shallow-water, locally restricted, and ultimately subaerially exposed settings. The highstand systems tract of SQ2 is further subdivided into three fourth-order sequences, and is characterized overall by high-frequency shallowing-upward cycles. Within the Xiaohaizi Formation, 16 fifth-order sequence units are further recognized, including PS1-PS6 in Member 1, PS7-PS11 in Member 2, and PS12-PS16 in Member 3. Under this framework, the Xiaohaizi Formation shows differential preservation, local erosion, and an overall migration of depositional distribution toward the central part of the study area. This study refines the internal stratigraphic subdivision of the Xiaohaizi Formation and provides a reliable basis for regional correlation and stratigraphic framework analysis of the Carboniferous in southwestern Tarim.