Rapid Lacustrine Transgression and Mixed Siliciclastic-Carbonate Shoal-Bar Deposition in Rift Basins: Insights From the Palaeogene Baxian Sag, Bohai Bay Basin

Abstract

Although mixed siliciclastic-carbonate deposits in lacustrine rift basins are recognized as sensitive recorders of tectonic-climatic-sedimentary interactions, their high-resolution spatial and temporal evolution within a parasequence framework has rarely been documented. This study integrates cores, well logs, 3D seismic data and geochemical proxies from the lower Shahejie Formation (Es₁) in the Baxian Sag to define four genetic shoal-bar types and to reconstruct their response to rapid lacustrine transgression during early rift subsidence. Four genetic types of shoal-bar deposits are identified based on sediment provenance and composition: terrigenous clastic shoal-bars, endogenic carbonate shoal-bars, mixed siliciclastic-carbonate shoal-bars and reworked carbonate shoal-bars. Seismic stratigraphic slices and well correlations reveal that these shoal-bars are preferentially developed in three palaeogeomorphic settings: the terminations of strike-slip faults, the footwalls of weakly active normal faults and submerged low-relief uplifts. Vertically, shoal-bar facies exhibit a symmetric stacking pattern around the maximum flooding surface, transitioning from terrigenous clastic, through mixed siliciclastic-carbonate, to endogenic carbonate and back to mixed and terrigenous clastic facies. This stacking pattern defines a complete parasequence set (retrogradational to aggradational to progradational) that records high-frequency lake-level fluctuations. The initial stage of rift subsidence coincided with rapid lacustrine transgression and significant reduction in terrigenous sediment supply, such that wave reworking of sediment-starved deltas provided the primary material for shoal-bar construction, whereas coeval arid conditions promoted endogenic carbonate precipitation. The combination of these processes thus facilitated the mixing of terrigenous clastics and endogenic carbonates. Consequently, the types and spatial distribution of shoal-bars provide robust proxies for reconstructing the intricate interplay among sediment supply, palaeoclimate and early rift tectonic dynamics.