Deciphering Ancient Tidal Depositional Systems from Sedimentary Structures and Paleogeographic Controls: A Late Permian Example from the Lower Yangtze Region, South China

Ancient tidal deposits provide important records of coastal hydrodynamics and basin evolution, yet their recognition remains challenging because tidal processes commonly interact with fluvial and wave processes in transitional environments. This study integrates petrography, grain-size characteristics, sedimentary structures, and regional paleogeography to decipher the Late Permian tidal depositional system of the Longtan Formation in the Lower Yangtze region, South China. The sandstones are mainly fine- to medium-grained and display multiple grain-size transport populations, indicating variable hydrodynamic conditions. An assemblage of bidirectional cross-bedding, double mud drapes, rhythmic sand–mud laminae, heterolithic bedding, and thick–thin sandstone bundles provides evidence of current reversals, slack-water deposition, and fluctuating hydrodynamic conditions. The facies-dependent distribution of these structures indicates spatial variations in the expression of tidal processes. Paleogeographic reconstruction suggests that the NE-opening, semi-enclosed basin configuration may have favored tidal circulation and the development and preservation of tidal deposits. Porosity and permeability measurements show differences among tidal sandstone types. Muddy matrix content is negatively correlated with both porosity and permeability, with the matrix–porosity relationship yielding R2 = 0.791, indicating that muddy matrix is an important factor associated with reservoir heterogeneity. These findings demonstrate the value of integrating sedimentological evidence with paleogeographic constraints for interpreting ancient tidal depositional systems and provide a basis for evaluating reservoir heterogeneity in comparable tide-influenced successions.