Cretaceous facies in Peru commonly fall into a lower, mainly clastic, group of formations and an upper group consisting of limestones, dolomites and shales. The clastic formations are largely non-marine. Over much of the country they range in age from Valanginian to Late Aptian or Albian, though in eastern Peru they extend into the Upper Cretaceous. The basal part of the limestone and shale group of formations is commonly Albian in age, though it is younger in the east. Tuffs and flows form an important part of the Cretaceous sequence of coastal Peru, but are not common elsewhere in the country. These Cretaceous units are overlain by redbed formations, some of which may be as old as the Campanian. The relationships between the redbeds and the underlying units range from conformity to slight angular unconformity.
The Cretaceous formations range in thickness from about 3,000 meters on the western flank of the Andes and 2,000 m. in eastern Peru to 1,000 m. or less in the intervening region. On this basis the Andean belt is believed to have consisted of two troughs (the West Peruvian trough and East Peruvian trough of this report) separated by a relatively positive area called the Maranon geanticline.
The clastic sequence in the West Peruvian trough was probably derived from the geanticline and from tectonic lands on the southwest. The clastics in the East Peruvian trough were contributed by the Brazilian shield, and probably by the geanticline.
Although there were temporary marine advances into the troughs during the Neocomian and Aptian the main transgression did not begin until the Albian. The West Peruvian trough and the Marañon geanticline were submerged by the Medial Albian, and marine conditions began to spread into the northern part of the East Peruvian trough. The latter was not completely submerged, however, until the sea reached its greatest extent during the Coniacian.
Although there was Late Albian tectonism in parts of the Andean belt, widespread emergence did not begin until the Santonian or Campanian, when the West Peruvian trough was uplifted. Subsequently the whole of the belt was folded and uplifted by orogenic phases which took place possibly in the Miocene and Pliocene.
The Andean belt in central Peru may be divided into structural provinces, which are from southwest to northeast: Paleozoic massifs; gently folded and block-faulted Mesozoic sediments and volcanics; batholith; strongly folded Cretaceous formations; folded and metamorphosed Paleozoic formations overlain by gently folded Mesozoic and Cenozoic formations; and moderately folded Mesozoic and Cenozoic formations.