HIGH RESOLUTION CORRELATION AND PALEOENVIRONMENTS OF DIATOMACEOUS MUDSTONE AND SANDY INTERVALS, MIOCENE PISCO FM, PERU

The Pisco Basin is a Cenozoic fore-arc basin with stratigraphy limited to the epoch level. This has limited interpretations based on abundant marine vertebrates. Though outcrops are well exposed, contain little diagenetic overprint, or structural deformation, there are only rudimentary correlations between outcrops. Previously, our group has identified and correlated an interval from 6.9 to 6.4 Ma over 25 km in a region north of this study. Our goal is to develop high resolution-stratigraphy and paleoenvironmental interpretations in the Miocene Pisco Formation.

This study has correlated and mapped a 9.26 Ma tuff couplet and associated beds over at least 10 km. We measured eight sections that included the tuffs. A typical section is dominated by diatomaceous mudstone above and below very fine-grained sandstone. The mudstone has sub-parallel parting and opal-A diatoms. Sandstones to the south are nearly homogenized by burrowing and to the north contain wave ripple lamination and small trough-cross beds. Sandstones are dominated by feldspar with significant amounts of ash and biotite and minor quartz and thin to the south and southwest. Fossil whales, seals, large bivalves, gastropods, and burrows are all present in this unit. Below the sandy interval exists a prominent phosphate pebble deposit with shark teeth, bones and fossil fragments and 2 cm diameter sub-horizontal burrows. Near the top of the sandy interval are diatomaceous lenses in eroded channels. Between the phosphate pebble layer and the sandstone is heavily burrowed muddy sandstone. Four tuff markers allow detailed comparison of sediment accumulation across the area.

Diatomaceous mudstones were deposited on a marine shelf near or below wave base with periodic dilution of the diatoms by mud. Phosphates represent a local maximum in sea level followed by regression. Sandy mudstones and very fine-grained sandstones were reworked by waves and some currents and hosted abundant burrowing organisms. It appears that all environments were closer to shore and shallower in the north with increased clastic input and biogenic activity. The presence of channeled diatomaceous lenses in the sandy interval suggests diatom deposition during breaks in sand deposition. Correlation of tuff bounded intervals has allowed detailed interpretations in the Pisco.