Giant submarine collapse of a carbonate platform at the Turonian-Coniacian transition: The Ayabacas Formation, southern Peru

The Ayabacas Formation of southern Peru is an impressive unit formed by the giant submarine collapse of the mid-Cretaceous carbonate platform of the western Peru back-arc basin (WPBAB), near the Turonian-Coniacian transition (similar to 90-89 Ma). It extends along the southwestern edge of the Cordillera Oriental and throughout the Altiplano and Cordillera Occidental over > 80 000 km(2) in map view, and represents a volume of displaced sediments of > 10 000 km(3). The collapse occurred down the basin slope, i.e. toward the SW. Six zones are characterised on the basis of deformational facies, and a seventh corresponds to the northeastern 'stable' area (Zone 0). Zones 1-3 display increasing fragmentation from NE to SW, and are composed of limestone rafts and sheets embedded in a matrix of mainly red, partly calcareous and locally sandy, mudstones to siltstones. In contrast, in Zones 4 and 5 the unit consists only of displaced and stacked limestone masses forming a 'sedimentary thrust and fold system', with sizes increasing to the southwest. In Zone 6, the upper part of the limestone succession consists of rafts and sheets stacked over the regularly bedded lower part. The triggering of this extremely large mass wasting clearly ensued from slope creation, oversteepening and seismicity produced by extensional tectonic activity, as demonstrated by the observation of synsedimentary normal faults and related thickness variations. Other factors, such as pore pressure increases or lithification contrasts probably facilitated sliding. The key role of tectonics is strengthened by the specific relationships between the basin and collapse histories and two major fault systems that cross the study area. The Ayabacas collapse occurred at a turning point in the Central Andean evolution. Before the event, the back-arc basin had been essentially marine and deepened to the west, with little volcanic activity taking place at the arc. After the event, the back-arc was occupied by continental to near-continental environments, and was bounded to the southwest by a massive volcanic arc shedding debris and tuffs into the basin.