High Resolution Correlation and Paleoenvironmental Transects of Fine-Grained Shallow Marine Bay Deposits of the Miocene Pisco Formation, West-Central Peru

The East Pisco Basin is a 50x200 km foreland basin containing Eocene to Miocene marine sediments and well-known vertebrate faunas. The goal of this research was to determine what distance lithostratigraphic correlation can be extended in this fine-grained, tuffaceous and diatomaceous shallow-marine bay setting and to examine paleoenvironmental trends in a 30 m thick unit bounded by tuff beds. The Pisco Fm is analogous to the Monterey Fm, in that both were deposited in Miocene, east Pacific basins and contain abundant diatomites, fine-grained clastics, and isolated dolomite beds. Unlike the Monterey, the Pisco exhibits well exposed outcrops, simpler structure, little diagenesis, and a shallower depositional setting that was protected from the open ocean by islands. Pisco outcrops occur as isolated hills in the Ica River valley. Previous studies established a general chronostratigraphy for the basin using siliceous microfossils and vertebrate fossils, but lack of correlation between outcrops has stymied progress in sedimentological research. This study correlated a series of outcrops across the valley and interpreted lateral paleoenvironmental changes along that transect.

Ar-Ar dates guided correlations. We mapped a distinctive group of three tuffs over 30 km and located all outcrops containing these beds in the valley. We measured six detailed sections from a 6.9 Ma Ar-Ar dated tuff at the base to a pair of tuffs at the top. Basal units are dominated by fine-grained, arkosic, sandy, tuffaceous siltstones, silty tuffs, tuffs, and phosphatic pebble lags. Nearshore sections contain coquinas, conglomerates, and megarippled sandstones that thin and pinch out within 2 km of Jurassic volcanic island sources. Upper parts of all sections are dominated by silty and muddy diatomites with interbedded tuffs and Fe-dolomite or gypsum cemented horizons. Vertical changes are reflected in magnetic susceptibility profiles. Sections show little variation in a direction parallel to the present coast and basin axis. Perpendicular to that trend, the total number and thicknesses of silty and sandy clastic units decrease to the west and southwest, and diatomites increase, indicating paleogradients. A distinct transition from siltstones to diatomites suggests a major water depth increase just after 6.9 Ma. Storm, wave reworking, and channel structures indicate water depths above storm wave base, although these effects were likely enhanced by the low density of the sediments.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California