MIOCENE NON-MARINE DIATOMS FROM THE WESTERN CORDILLERA BASINS OF NORTHERN PERU

Abstract

Diatom assemblages are documented from diatomite layers of two Miocene fluvio-lacustrine units from the basins of the western Cordillera of northern Peru: the Namora formation and the Cajabamba formation. Emphasis is given to taxa of particular stratigraphic interest such as Mesodictyon sp., Cyclotella stelligera Cleve & Grunow, and Aulacoseira agassizii (Ostenfeld) Simonsen and their ultra- structure is investigated with the SEM. The diatom assemblages indicate for the Namora formation the occurrence of swampy conditions with very dilute, low alkalinity water. The diatom assemblages of the Cajabamba formation reflect the occurrence of fresh, slightly alkaline, eutrophic lakes with deep water in some samples, and swampy conditions with relatively high salt content in other samples. The Namora formation is late Miocene in age based on the diatom assemblages and radiometric analyses (7.2 ± 0.6 Ma). The diatom layers of the Cajabamba formation are dated as late middle to early late Miocene (ca. 11–8 Ma) based on the occurrence of Mesodictyon sp. and Cyclotella stelligera.