Late Miocene stratigraphy, palaeoclimate and evolution of the Sandanski Basin (Bulgaria) and the chronology of the Pikermian faunal changes

We study the sedimentology, palaeontology and palaeomagnetism of the 500 m thick sedimentary sequence in the Gorna Sushitsa gorge in the Sandanski Basin (southwest Bulgaria), which is exceptionally rich in fossil mammals of Pikermian type. Magnetostratigraphy indicates that this section was deposited within 2 myrs, between 8.5 Ma and 6.5 Ma (late Tortonian to early Messinian). Our investigations reveal that the Sandanski Basin infill represents two stacking synrift phases (Delcevo and Kalimantsi formations), with an intervening tectonic quiescence package (Sandanski Formation). Kalimantsi Formation deposition ends with the cessation of slip on basin-bounding faults at 6.9 Ma, probably caused by a switch from regional E-W to N–S extension between 7 and 6.5 Ma. High slip rates compared to hanging wall sedimentation rates induced the development of mega-breccia dominated substantial fault scarps during the younger synrift phase (8.2 to 6.9 Ma). Tectonic changes are superimposed by several climatic changes. Moderately humid conditions prevail from 8.5–8.2 Ma, followed by relatively dry period from 8.2 to 7.8 Ma. A second humid interval from 7.74–7.56 Ma, which may be responsible for the Maeotian transgression in the Eastern Paratethys lake, is followed by a significant trend in local and supra-regional aridization, indicated by the onset of orbital driven aeolian dust deposition at 7.42 Ma. Our new data expand the Northern Mediterranean Sahara-born dust record at the Tortonian-Messinian transition to at least 600 kyrs (7.42 to 6.85–6.8 Ma). Coeval with the onset of dust deposition we observed evolutionary changes in the large mammal record, leading to the classical Pikermian association dated to between 7.42 and 7.27 Ma. We interpret faunal changes at the Tortonian-Messinian transition in the Eastern Mediterranean as driven by global cooling and mid-latitudinal desertification.