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High Resolution Correlation and Paleoenvironments of Diatomaceous Siltstone and Sandy Intervals, Miocene Pisco Fm, Peru

Caleb W. Stanton, Jankel P. Coronado, and Kevin E. Nick
Loma Linda University

The Pisco Basin is a Cenozoic forearc basin with stratigraphy limited to lithotypes at the epoch level. Though outcrops are well exposed, contain little diagenetic overprint, or structural deformation, there are only rudimentary correlations between outcrops and no attempts have been made to define time- bounded subunits. 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 siltstone 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. Four tuff markers allow detailed comparison of sediment accumulation across the area.

Diatomaceous siltstones were deposited on a marine shelf near or below wave base with periodic dilution of diatoms and mud. Phosphates represent a local sea-level maximum followed by regression. Sandy mudstones and very fine-grained sandstones were reworked by waves and currents and hosted abundant burrowing organisms. The sandstone interval appears to be a part of sand tongues formed by coastal upwelling.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013