Subsidence mechanisms and sedimentation in alluvial sinkholes inferred from trenching and ground penetrating radar (GPR). Implications for subsidence and flooding hazard assessment

Abstract Sinkholes function as small sediment traps that may host valuable stratigraphic records for paleoenvironmental reconstructions and hazard assessments (e.g., subsidence, floods, hurricanes, tsunamis, tephra fall-out). The sedimentological features and completeness of such archives are influenced by the sedimentation and subsidence patterns and rates. However, karst depressions are frequently treated as static basins unaffected by settlement. This work illustrates the practicality of integrated studies combining trenching, numerical dating and shallow geophysical techniques (GPR) for characterizing the subsurface subsidence structure associated with sinkholes and reconstructing their deformational and sedimentary history. The approach is applied in two collapse sinkholes located in contrasting geomorphic settings (relict terrace and floodplain) related to deep-seated interstratal karstification of evaporites. The analysis of the sinkholes, particularly the trenching technique, provides practical information for assessing the associated subsidence hazard, including the presence of larger cavities at depth, the kinematic regime (episodic versus progressive), evidence of catastrophic displacement (fluidization structures) and the magnitude and timing of collapse events, especially the most recent one. The sinkhole located in the floodplain offers the opportunity of analyzing the possibilities and limitations of subsidence sinkholes as recorders of past floods in alluvial environments. This depression shows a largely incomplete record attributable to the high frequency of flood events compared to that of the collapse events, which create the accommodation space for sediment deposition. These limitations could be partially overcome by selecting old sinkholes situated in low terraces and/or affected by rapid subsidence.