EARLY-MIOCENE CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY FROM LOW-LATITUDE, PISCO BASIN, PERU

Few onshore or offshore localities for early Neogene calcareous nannofossils have been studied from the eastern equatorial Pacific. This study presents preliminary results of calcareous nannofossil biostratigraphy from the Pisco Basin, a low latitude (-14°) forearc basin in central, coastal Peru.

The Pisco Basin is one of the few locations that provide the opportunity to study Miocene calcareous nannofossils onshore for the eastern equatorial Pacific. Most units in the Pisco Basin are dated using mollusks or siliceous microfossils, but some important sequence boundaries do not contain these taxa. We are using calcareous nannofossils in one of these sequences. Study locations are on the southeastern side of Cerro Yesera de Amara and in hills to the southeast. Sections were measured from a basal dolomite overlying a coarse clastic unit thought to be Oligocene. These sections consist of very fine sandstones, siltstones, calcareous mudstones, and dolomite cemented mudstones. We have a single related Ar-Ar age of 17.70±0.24 Ma from biotite in a tuff about 20m stratigraphically above the fossiliferous units. Thirty samples of calcareous siltstone were processed as smear slides (Bown and Young, 1998) and calcareous nannofossils identified using the petrographic microscope. Abundance of species was evaluated semi-quantitatively by the cascading counts method (Styzen, 1997) and the preservation noted (Farrell et al., 1995). Nannofossil assemblages are moderately-rich (37 species) with rare to common abundances. Preservation is poor to moderate showing dissolution and/or overgrowth in almost all samples. Cyclicargolithus abisectus, Cyclicargolithus floridanus, Discoaster deflandrei, Discoaster adamanteus, Sphenolithus moriformis, Sphenolithus conicus, Reticulofenestra lockeri, Reticulofenestra bisecta, Helicosphaera granulata, Helicosphaera carteri, Helicosphaera euphratis and Coccolithus pelagicus are the dominant taxa recovered. Considering the co-occurrence of C. abisectus, Sphenolithus dissimilis, S. conicus, and Reticulofenestra bisecta, this study suggests an age of earliest Miocene, NN1 (Martini, 1971) or CN1b (Okada and Bukry, 1980) for the base of the onlapping section.