The present research was conducted within the scope of study of Lower Carboniferous sediments of southeastern slope of North Tatar Arch in the Republic of Tatarstan. The study area covers the Nizhnekamsk Uncompensated Trough as a part of Kamsko-Kinel Troughs System. In the course of this study, a comprehensive analysis of well logs, seismic and core data was performed. An object of special interest is the interval with uncharacteristic changes in lithology and thickness of Tournaisian sediments, the so-called Saraylin series, identified by well logging and core data. The Saraylin series is a peculiar lithologic interval in the Lower Carboniferous with increased organic matter content, which is confined to sediments from the lower boundary of the Tournaisian Stage to the bottom of overlying Tournaisian carbonate rocks or terrigenous sediments of another (post-Tournaisian) age. Seismic interpretation revealed clinoform complexes in the northern flank structural-facies part of the Trough, while no similar sediments were observed within the southern flank of Nizhnekamsk Uncompensated Trough (within the study area). Sedimentation pattern is proposed to explain the structural differences between two flanks. Then sequential analysis was used to identify cyclites, sequences, and tracts which enabled to reproduce the sedimentation history of Nizhnekamsk Uncompensated Trough flanks and refine the stratigraphic model of this part of the Trough.
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