Correlation and Paleoenvironments above West T9.3 Tuff, Pisco Formation, Peru

The Pisco Basin is a Cenozoic forearc basin in west central Peru. Though outcrops are well exposed, contain little diagenetic overprint, or complex structural deformation, only rudimentary correlations between outcrops are published and no attempts have been made to define time-bounded subunits. The goal of this study is to develop high resolution-stratigraphy and paleoenvironmental interpretations for a time-bounded unit in the lower part of the Miocene Pisco Formation.;In this study, we correlated, mapped, and dated a tuff couplet and associated beds over 25 km2. We measured eight sections that include the tuffs and conducted detailed facies analysis. Sections were broken down into eight facies including: hummocky cross-stratified diatomaceous siltstone, muddy siltstone, tuff, dolomitized siltstone, phosphate pebble conglomerate, and four distinct very-fine sandstone facies. Sub-parallel laminated diatomaceous siltstones comprise the majority of the lower portion in every section. This depositional interval includes some tuffaceous and dolomitized beds. A shift in lithology and the types of sedimentary structures occurs above a phosphate pebble conglomerate in a sandstone unit. Bedforms within this very fine sandstone include climbing-ripple, wave-ripple, massive, hummocky, swaley and trough cross-bedding. Bioturbation, channels, and erosional contacts are also present in the sandstone unit.;Depositional environments show a progression through the studied interval. Diatomaceous siltstones were deposited on a marine shelf below normal wave base with associated diatom or clay-rich zones. Occasional volcanic events flooded the system with tuffaceous material. Phosphate pebbles formed during a local sea-level maximum. These phosphate-rich beds were later winnowed during regression and condensed into an erosion-resistant phosphate pebble/fossil conglomerate. Units above the phosphate pebble conglomerate show a transition from offshore to wave and storm dominated middle to upper shoreface. The sandstone is interpreted as a shoreface bar through lateral comparisons of unit thickness. Muddy siltstones and very fine-grained sandstones were reworked by waves and currents and hosted abundant burrowing organisms. Siltstone above the sandstone represents the transitional zone between shoreface and offshore deposits. This study provides the first framework for developing sequence stratigraphic models for the Pisco Formation.