Cenozoic marine sedimentation in the Sechura and Pisco basins, Peru

Abstract

The central and northern Peruvian margin consists of a series of 8 paired forearc basins which may be separated into an inner set of shelf basins and a seaward set of slope basins. We have examined the Cenozoic stratigraphy of the onshore portions of the Sechura Basin (5–7°S) and Pisco Basin (13–16°S), two shelf basins which have accumulated marine sediment discontinuously since the mid to late Eocene. Cenozoic sediments in the Pisco Basin were deposited during at least three major transgressive cycles. Each sequence is preserved as a similar vertical progression of facies including coarse nearshore bioclastic conglomerates and sandstones grading upwards into sandy siltstones and mudstones, and capped by biogenic deposits including diatomites, diatomaceous mudstones, dolomitic horizons, and phosphate deposits. Stratigraphic nomenclature for the Pisco Basin has recently evolved; a stratigraphy presented here includes the Eocene Caballas Fm., upper Eocene Los Choros fm., upper Eocene to lowermost Oligocene Yumaque fm., uppermost Oligocene to middle Miocene Chilcatay fm., and upper Miocene to Pliocene Pisco Fm. Major hiatuses in the Pisco Basin span the Late Cretaceous to middle Eocene, early to late Oligocene, middle Miocene, and late Pliocene/Pleistocene to Recent. Cenozoic sediments of the Sechura Basin were deposited within at least 4 major transgressive cycles with hiatuses during the Paleocene to middle Eocene, Oligocene, early to middle Miocene, and late Miocene. Based on recent biostratigraphic studies, sediments enriched in biogenic components accumulated between about 40-36 Ma, 24-16 Ma, and 11-3 Ma in the Pisco Basin and between 40-37 Ma and 8.5-4.5 Ma in the Sechura Basin. In both basins, the most diatomaceous sediments are restricted to the Late Eocene and Late Miocene through Pliocene. The temporal distribution of biogenic sediments suggests that high productivity conditions linked to coastal upwelling have occurred episodically since at least the Late Eocene. The occurrence of diatomites and phosphorites is diachronous between the Pisco and Sechura Basins and between the Peruvian forearc and other circum-Pacific Monterey Formation analogs, a reflection of the strong influence of local tectonism on sedimentation patterns. The volume of Neogene sediments along the Peruvian forearc is nearly twice that of the Monterey Fm.; despite basin-to-basin facies diachroneity, these deposits very likely contributed to fluctuations of the late Miocene carbon/CO 2 system by acting as large carbon sinks.