Depositional sequences in northern Peru: new insights on the palaeogeographic and palaeotectonic reconstruction of western Gondwana during late Permian and Triassic

Abstract

Late Permian to Early Jurassic strata in northern Peru allow us to carry out a seismostratigraphic, lithotectonic and chemostratigraphic analysis connecting the Andean–Amazonian foreland basins of Huallaga, Ucayali, southern Marañón and the Eastern Cordillera. This analysis and data integration from Ecuador to western Brazil and southern Peru and Bolivia allow us to redefine the timing of the major documented tectonic phases and corresponding palaeogeographies of western Gondwana from the late Permian to the Triassic. Three lithotectonic sequences and four associated deformation stages are recognized: (1) A sequence, tectonic relaxation, during the late Permian; (2) A–B intra-sequence, folding-and-thrusting attributed to a continuation in time of the Gondwanide Orogeny, during the Early to Middle Triassic; (3) B sequence, rifting, attributed to Gondwana breakup during the Middle and late Triassic; (4) C sequence, thermal sag, during the Late Triassic. Evaporites and carbonates (A sequence) dominated a low-subsidence basin with southern restricted marine inflow at the Permian–Triassic boundary. A novel palaeogeographic model for these evaporites suggests that this saline basin extended up to 50 000 km2 in a restricted environment area with a potential bullseye pattern. The last pulse of the Gondwanide Orogeny and associated fold and thrust belt (A–B intra-sequence) exhumed prior to the sequence, generating emerged areas with little to no sedimentation. Red beds (B sequence) characterize the rifting stage, representing the syn-depositional infill of continental grabens, likely extending to the Acre Basin in Brazil. Finally, during the thermal sag, a marine inflow likely from the northwestern part of Peru generated sedimentation of carbonates and evaporites (C Sequence) to the west and east of the Peruvian margin. This sediment differentiation was, in part, controlled by the existence of pre-existing grabens associated with the previous rifting stage. This interpretation, together with other evaporitic occurrences, attributed here to a Late Triassic epoch in south and north Peru and west Brazil, suggest the existence of an evaporitic basin filling an undeformed area of probably c. 170 000 km2. It is therefore suggestive of the existence of a Late Triassic (Norian to Rhaetian; 217 to 204 Ma) salt giant controlled by thermal sag in western Gondwana. Our results are of great relevance for any future interpretation related to mass extinctions, palaeoclimatic analysis and ocean dynamics during the Permian and Triassic as well as natural resource distribution between Ecuador and Bolivia.