Tectonic and climate control on the stratigraphic cycles in a Late Paleozoic evaporitic basin in NW China

Abstract

The Fengcheng Formation in the Junggar Basin, northwestern China, represents a suite of deposits formed in a hydrologically closed, alkaline saline lake during the Late Paleozoic. This formation hosts high-quality source rocks (oil shales) and significant trona resources. Closed-basin lacustrine systems exhibit stratigraphic architectures that are fundamentally distinct from those of marine or open-lake settings; consequently, conventional sequence stratigraphic models are often inadequate for interpreting the distribution of trona and oil shale resources within such confined depositional systems. Based on integrated analyses of drilling sequences, well logs, and seismic profiles, we identify four key findings: (1) Three third-order sequences can be delineated using lithostratigraphic patterns and seismic-stratigraphic relationships; (2) The Mahu Sag remained predominantly underfilled throughout most of the depositional history, evolving into a balanced-fill basin only at the end of Sequence 3 (SQ3); (3) Trona deposition occurred during arid phases within repeated climatic cycles, with the Fengcheng Formation preserving four long-term cycles, each comprising multiple intermediate- and short-term climatic fluctuations; (4) The highest-quality oil shales accumulated during the transgressive systems tracts (TST) of SQ1 and SQ2, coinciding with periods of rising lake levels. Additional oil shale intervals formed at the onset of individual cycles when lake levels were elevated. These findings offer a comprehensive stratigraphic framework for understanding the spatial and temporal distribution of trona and oil shale resources in a Late Paleozoic alkaline saline lake system characterized by asymmetric basin geometry.