Stratigraphy and chemical sedimentology of Cenozoic biogenic sediments from the Pisco and Sechura Basins, Peru

Neogene sediments rich in siliceous microfossils, phosphate, and organic carbon formed simultaneously in many basins around the Pacific. This work focuses on these (and similar older) sediments from two basins of western South America (East Pisco and Sechura Basins) and studies the effects of tectonism, climate, and oceanic circulation on their formation. Transgressions inundated the East Pisco Basin during the late Eocene, late early Miocene, and late Miocene. Diatomaceous sediments formed during each transgression. Late Eocene diatomaceous sediments suggest that: upwelling off Peru dates from the late Eocene, a proto-Humboldt current existed during the Eocene, Antarctic cooling began before the latest Eocene, and the terminal Eocene event was the culmination of oceanic-climatic change. The late Eocene diatomites differ from overlying late Miocene sediments. Eocene diatomites are restricted to the western (offshore) margin of the basin, are separated from paralic sediments by a mud blanket, and were deposited well below wave base. Upper Miocene-Pliocene diatomites occur throughout the basin, grade into paralic deposits, and were deposited, in part, above storm wave base. The Sechura Basin experienced four transgressions between the Eocene and late Miocene. These transgressions formed four sedimentary sequences. Diatomaceous sediments are found in each sequence except the second (upper Oligocene-lower Miocene). In the third sequence (lower Zapallal Formation) diatomite-phosphorite became important between 14 and 8.1 Ma, and apparently reflects increased biogenic and decreased (?) terrigenous sedimentation rates. The increased biogenic accumulation rate reflects increased primary productivity or preservation. Sediments of the third sequence are separated from the fourth sequence by an angular unconformity (which correlates with subduction of the Nazca Ridge under the basin). Sediments of the fourth sequence differ from those of the third, and apparently record cooling and strengthened currents during the latest Miocene