The late Valanginian Pucusana Formation in central Peru consists of more than 800 m of pyroclastic breccias, lapillistones, tuffs, and interbedded flows of intermediate to basic composition. Minor amounts of limestone, marl, and laminated gypsum occur toward the top of the formation. This unit records the construction and lateral facies relations of a single arc vent and its stratovolcano cone.

Field evidence indicates volcanism under subaerial to subaqueous conditions. Four facies in the formation represent different processes and environments of deposition. Facies A is non-welded structureless pyroclastic debris up to 50 cm in diameter. The largest size of the clasts was controlled not only by the distance from the volcanic vent but also by differences in the eruption style. The lack of welding is indicative of emplacement of these breccias in a cool state, probably by air fall and debris flowage very close to volcanic vent. Facies B consists of massive pyroclastic breccias in units showing reverse grading and matrix support. The material was deposited by debris flows originating on the flanks of the volcano. Facies C includes well-stratified lapilli and pyroclastic debris generally less than 6 cm in diameter, deposited subaqueously around the volcano by high-density turbidity currents. Facies D consists of well-stratified fine-grained tuff and granule to pebble breccia deposited by relatively diluted turbidity currents. Near the top of the section, there are some interbedded fossiliferous and oolitic limestones and laminated gypsum representing a more distal marine facies. These facies document lateral changes from proximal subaerial to distal subaqueous conditions and a secular decrease in the rate of volcanic activity.

Modal analysis indicates a relatively homogeneous composition of an intermediate calc-alkaline magma. The volcanic source was apparently related to Cretaceous subduction beneath western South America. This sequence provides an excellent example of vent-to-basin facies in an arc setting.

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