A facies analysis of the Pucara Group (Norian to Toarcian carbonates,organic-rich shale and phosphate) of Central and Northern Peru

The Pucará Group contains an excellent record of marine uppermost Triassic and basal Jurassic sedimentation. New biostratigraphic and lithostratigraphic studies have led to a reassessment of the regional internal correlation of the group and thus to a new understanding of the facies relationships. There is a relatively complete record of sedimentation from late Norian to late early Jurassic, the only notable omissions apparently occurring in the latest Norian and at the Hettangian-Sinemurian boundary. The oldest constituent formation, the Chambará, which is locally of great thickness, consists largely of dark cherty limestones representing basinal conditions of sedimentation. The succeeding Aramachay Formation marks widespread recommencement of sedimentation in the Semicostatum Zone of the Sinemurian, with organic-rich shales passing up into phosphatic deposits. It is proposed that this event is the consequence of a significant eustatic rise of sea level. The record of sedimentation of the youngest, Condorsinga, formation is restricted to Central Peru and no other middle-upper Lias units are identified. Alternating marginal marine and shallow neritic conditions are inferred with high carbonate production rates and dolomitizing environments. The character of the sediments, particularly the phosphatic deposits, suggests a tectonically quiescent continental margin with unrestricted access to the palaeo-Pacific Ocean, in contrast to earlier palaeogeographic interpretations. As such the Pucará Group represents unusual conditions in the development of the Andean regime.