Lacustrine and fan-delta sediments in syn-rift lake basins

Lakes are sensitive indicators of the balance between accommodation and sediment supply, recording high-resolution changes in palaeoenvironmental conditions. Long-lived rift lake basins, however, are predominantly controlled by episodic accommodation changes and pronounced basinward facies shifts, complicating the generalisation of tectonic and climatic controls on rift lake successions. This study proposes a sequence framework and depositional pattern for asymmetric half-grabens in syn-rift lake basins by characterising the lacustrine fan-delta deposits of the Lower Cretaceous Shahezi Formation in the Songliao Basin. Detailed sedimentologic and petrographic analyses identified 24 lithofacies categorised into seven facies associations. A sequence stratigraphic framework was constructed to outline the tectono-stratigraphic evolution during the syn-rift phase. The results indicate that the syn-rift Lishu palaeo-lake is characterised by its relatively small size, steep slopes, poorly developed and siliciclastic-dominant shoreline strata and significant input of allochthonous biodetritus. The syn-rift deposits show a distinct threefold conglomerate-sandstone-mudstone motif, with a complete cycle comprising a prolonged retrogradational phase (LST and TST) and a brief progradational phase (HST). Basin-bounding faults accelerated hinterland erosion and increased sediment feeder system slopes by rotating hangingwall blocks; consequently, rapid sediment transport and localised gravitational collapse caused the common occurrence of soft-sediment deformation structures and sublacustrine fan conglomerates. The substantial increase in accommodation space, resulting from fault-generated subsidence, triggered lake expansion and further contributed to the development of transgressive system tracts and continuous mudstone deposition. These mudstones, rich in terrigenous organic matter and allochthonous fossils, correlate with carbonaceous mudstones, coals and conglomeratic sandstones in proximal overfilled sections, indicating a dynamic interplay between fan delta progradation and Lake Shoreline transgressions. This study proposes a depositional model within a sequence stratigraphic framework for non-marine sediment accumulation in asymmetric half-grabens bounded by active faults. The findings offer insights that complement existing models developed for marine rift systems.