TEPHROCHRONOLOGY AND HIGH RESOLUTION CORRELATION IN THE UPPER PISCO FORMATION, MIOCENE, PERU

The Pisco Formation, located in central, coastal Peru, is well known for its diverse and abundant assemblage of Miocene marine vertebrate fossils. Because exposures in uplifted blocks inland from the coastal batholith are isolated and the lithologies are monotonous, it is difficult to correlate between outcrops. Because of a lack of correlation between outcrops across the basin, to date environmental and paleontological interpretations are general in nature. We have improved correlation in the Pisco through tephrochronology. More than 40 tuff layers, thicker than 5 cm, have been located so far. Pisco sediments are very well preserved and most tuffs are nearly unaltered and retain high porosity. Twenty four tuffs have been dated so far by ⁴⁰Ar/³⁹Ar. Most dates are from biotite separates that produced excellent results with dates from 9.31 to 5.96 Ma. These dates are generally older as you approach the coast even though elevations of the hills are similar. Three of the stratigraphically lowest samples are poor quality and their results are not being used. Microprobe analyses of glass and biotites from dated and additional tuffs show SiO₂ compositions from 62 to 79 wt%. Principle component analysis of compositional results isolates MgO as the most discriminating component. SiO₂ plotted with compatible elements, shows a typical negative slope. Glass compositions suggest at least two volcanic sources for the tuffs. A few tuffs contain monazite, apatite, and magnetite inclusions in biotite crystals. So far, attempts to uniquely correlate elemental analyses to radioisotopic dates have not been successful. We are using these results to correlate between outcrops and subdivide the Pisco Formation into time bounded units that will greatly improve paleoenvironmental studies.