Evolution from shallow-water deltas to fluvial fans in lacustrine basins: A case study from the Middle Jurassic Shaximiao Formation in the central Sichuan Basin, China

Distinguishing between shallow-water delta and fluvial fan deposits in the subsurface of lacustrine basins is challenging due to their similar depositional characteristics and distribution patterns. This study focuses on the Middle Jurassic Shaximiao Formation in the central Sichuan Basin using core observations, seismic analyses, petrology analyses, zircon analyses, palaeoclimate indicators and palaeocurrent analyses to address this issue. Fifteen sedimentary lithofacies and eight lithofacies associations were established, corresponding to channelized fluvial deposits, non-channelized fluvial deposits and shallow-water delta deposits. Shallow-water deltas are dominated by channels and mouth-bar complexes with grey, red and green interbedded mudstone. Shingled seismic reflection, green mud clasts, small burrows and wave ripples are common with occasional palaeosols. Fluvial fans are dominated by channels and crevasse-splays with pink colour and accompanied by red coloured floodplain deposits. Bright spot seismic reflection, red mud clasts, big burrows, current ripples and palaeosols are common. The increased uplift of the Dabashan Mountains controls the palaeocurrent direction, promoting the evolution from a shallow-water delta in relatively humid environments to a fluvial fan in relatively arid environments. The channel widths in shallow-water deltas are wider than those in fluvial fans, whereas the opposite applies for the channel amalgamation rate. Highly frequent channel-width variations are controlled by short-cycle climate fluctuation, corresponding to chemical index of alteration value fluctuations in different sandstone groups. The channel width in the relatively humid environments is wider than those in arid environments for both shallow-water deltas and fluvial fans. It is likely that the sedimentary evolution from shallow-water deltas to fluvial fans is relatively common in lacustrine basins in relatively arid environments with wide and gentle slope landforms.