Trigger mechanisms of gravity flow deposits in the Lower Cretaceous lacustrine rift basin of Lingshan Island, Eastern China

Abstract

Gravity flows were commonly developed in subaqueous, steep slopes in lacustrine basins, which show distinct sedimentary records due to different flow behaviors and trigger mechanisms. Outcrops of the Lower Cretaceous deposits on the Lingshan Island are well exposed to illustrate flow transformations between turbidity currents and debris flows in shallow water and basin floor in a lacustrine basin. Outcrop sections were carefully measured to characterize sedimentary records of the gravity flows. Facies associations include laminated mudstone (FA1), deformed silty mudstone (FA2), massive very-fine sandstone with rip-up mud clasts (FA3), graded very-fine sandstone with (partial) Bouma sequences (FA4), massive gravelly sandstone with rip-up mud clasts (FA5), gravelly sandstones with partial Bouma sequences (FA6), and cross-stratified pebbly coarse-grained sandstones (FA7). Trigger mechanisms for the gravity flow include mainly plunging of hyperpycnal river discharges and earthquakes, which both induced failure of delta front/slope and the consequent debris flows/turbidity currents. Flow transformations occurred both at the shallow water and basin floor, which produced distinct sedimentary structures, stacking patterns, and architectural units. The high-density hybrid event beds (HEBs) with both gravels and mud clasts overlaid by sandy turbidites were mainly induced by floodwater from the active river mouths of deltas, generating fast-moving debris flows and deceleration of turbidity currents. Damping of turbidity currents and deceleration of muddy debris flows due to high cohesion induced low-density turbidites were overlaid by HEBs, showing thin-bedded, lateral extended sandy beds on the basin floor.