A nonmarine stratigraphic boundary where major facies shifts occur indicates drastic changes in environmental conditions, but may have highly variable magnitude of facies shifts to rapid lateral facies changes, common autogenic processes, and irregular topography. This hypothesis is tested for the boundary separating fluvial-lacustrine Lower-Permian Lucaogou and Hongyanchi formations, deposited in a continental rift setting. The boundary is identified on 5 stratigraphic sections 0.2-5 km apart over ~80 km² area, NW China. Regionally, the boundary indicates a drastic shift from a widespread, NE-deepening lake during uppermost Lucaogou time to lake contraction, fluvial incision and regression in the north, and transgressive beach and deltaic systems in the south in early Hongyanchi time. Lake contraction may be caused by climatic change, tectonic processes including volcanism, and/or sediment infilling of the lake basin.