Preceding studies divided the Late Cretaceous deposits in the Cyrenaica Basin into two parts: the first part was deposited on the coast (Al Hilal and Al Athrun formations) and the second part in the inland (Qasr Al Abid, Al Baniyah, Al Majahir and Wadi Dukhan formations). These studies also showed that the Paleocene rocks are represented by Al Uwayliayah Formation, but the Middle Paleocene is missing. New 87Sr/86Sr data of crystalline calcites demonstrate that the Late Cretaceous formations from oldest to youngest are as follows: Qasr Al Abid Formation (Late Cenomanian (94.06–95.15 Ma)), Al Baniyah Formation (Late Turonian–Late Coniacian (86.51–90.82 Ma)), Al Hilal Formation (Late Santonian (84.35- 84.50 Ma)), Al Majahir Formation (Middle Campanian (76.25–79.65 Ma)), Al Athrun Formation (Early Maastrichtian (70.20–71.70 Ma)) and Wadi Dukhan Formation (Late Maastrichtian (67.70–67.85 Ma)), which indicate that the deposition in the Late Cretaceous on the coast is identical to that in the inland, but as a result of the sedimentological and structural processes there is an absence of some deposits. Moreover, the isotope analysis gives an age of Middle Danian–Middle Thanetian (57.13–64.41 Ma) for the Al Uwayliayah Formation, which point to the presence of the Middle Paleocene. Therefore, we concluded that the K-Pg boundary is between the Wadi Dukhan and Al Uwayliayah formations. The new age specifies the existence of six unconformity surfaces.