Deposits of the Socosani Formation in the Pucayacu and Pumani sections (Ayacucho Department, Peru), along several kilometres, have yielded Upper Bajocian ammonoid fossil-assemblages characterized by the occurrence of juvenile individuals belonging to endemic or pandemic genera, such as Megasphaeroceras and Spiroceras respectively. In addition, certain Bajocian genera relatively common in the Mediterranean-Caucasian Subrealm, but very scarce in the Eastern Pacific Subrealm, such as the strigoceratid Cadomoceras and the phylloceratid Adabofoloceras, occur in this area. According to the taphonomic, palaeoecological and palaeobiogeographical evidence from the Pumani River area, the maximum deepening, relative sea-level rise and oceanic accessibility of a Bajocian-Bathonian, second-order, transgressive/regressive facies cycle in the marine Arequipa Basin were reached during the Late Bajocian Niortense Biochron. However, synsedimentary regional tectonics in the Pumani River area disturbed this general deepening/shallowing cycle of the Arequipa Basin, particularly during the Late Bajocian post-Niortense time-interval of the Garantiana and Parkinsoni biochrons. © 2013 Elsevier Ltd.