Rift basin-fill architecture of fluvial-lacustrine Lower Permian Lucaogou and Hongyanchi low-order cycles, Bogda mountains, NW China
Rapid lateral facies and thickness changes, autogenic processes, and irregular topography of nonmarine rift basins challenge the application of traditional marine sequence stratigraphic techniques. Stratigraphic architecture may be reconstructed with a process-based approach, using interpreted climatic and tectonic controlling processes on sedimentation in addition to observable attributes to correlate sedimentary cycles. This hypothesis is tested for Lower Permian Lucaogou and Hongyanchi low-order cycles (LCs) in the Tarlong-Taodonggou half graben, using outcrop and petrographic data. The exposed graben fill covers 88 km². Depositional environments and controlling sedimentary processes were interpreted on five measured sections, 0.2 – 5 km apart. Microscopic and X-ray Diffraction data on grain composition and texture substantiate field interpretations and minimize stratigraphic miscorrelation. Humid to arid climatic conditions were interpreted using climate-sensitive lithologies, such as paleosols; tectonic movements were interpreted in terms of source area uplift, basin subsidence, and spill-point movement. The Lucaogou-Hongyanchi LC boundary (LCB) separates uppermost Lucaogou fluctuating profundal lacustrine high-order cycles (HC) from basal Hongyanchi fluvial-deltaic HCs, indicating drastic environmental change. The LCB is an erosional unconformity across which the type and magnitude of facies shifts varies greatly across the half-graben. A process based sequence-stratigraphic reconstruction across the LCB enables reconstruction of the three dimensional distribution of lithofacies across the boundary, and provides insights to the potential causes for drastic environmental change. A better understanding of the nature and origins of the LCB may be applicable to other similar nonmarine rift basins.