Abundant soft-sediment deformation structures in the Upper Cretaceous sequence of the Chelopech sub-basin, Central Bulgaria

Soft-sediment deformation structures (SSDS) in the Upper Cretaceous sequence of the Chelopech sub-basin (Central Bulgaria) provide a high-resolution record of the regional geodynamic evolution, from syn-rift to orogenic stages. Detailed analysis identifies a high-density distribution of synsedimentary slump folds, convolute bedding, diapiric structures, and peperites. The stratigraphic distribution of these features reveals three distinct evolutionary phases: (1) Lower Turonian basal units characterized by storm-induced reworking during initial subsidence; (2) Middle Turonian (Chelopech Formation) peperites and clastic diapirs triggered by coupled seismic activity and sub-aqueous magmatism during the late syn-rift phase; and (3) Santonian–Maastrichtian (Mirkovo and Chugovitsa Formations) large-scale gravitational slumps and mass-gravity flows. This terminal phase reflects a shift toward regional tectonic inversion and steepened basin margins during orogenic closure. These results demonstrate that SSDS serve as a critical diagnostic tool for reconstructing the rapid transitions in the Srednogorie Zone tectonic regime.