Shelf Paleoenvironments During Regression in the Lower Pisco Formation, Peru

Coronado, Jankel L.; Stanton, Caleb W.; Nick, Kevin E.
[email protected]

Currently, depositional models for the Miocene Pisco Formation are general in nature and unsupported by detailed stratigraphy or correlation. The Pisco Basin is a Cenozoic fore-arc basin about 200km in length along the Peruvian coast. Previous authors state that the Pisco Formation is characterized by low energy, shallow water deposits on a restricted shelf. The objective of our study is to develop the first depositional models in the Pisco based on high-resolution correlation.

We correlated, mapped, and dated a 9.28 ± 0.08Ma tuff couplet and sandstone lens over 25km². To date, we have identified 13 lithofacies around the tuff. Two lithologies dominate the sections; diatomaceous mudstone and very fine-grained sandstone. Below the base of the sandstone is a phosphate pebble layer with large U-shaped horizontal burrows. Carbonate-rich beds, tuffs, and a phosphate layer were used as marker beds. The median grain sizes of sandstone samples range from 70-90µm and the maximum size is 140µm. Detrital grains include clay pellets and feldspar, ash, trace biotite and some quartz with variable amounts of clay matrix. The sandstone shows massively burrowed units, channels draped with diatomite layers, gutter casts, trough cross bedding, burrows, and marine mammal fossils. The sandstone is 2.3m thick in the NNW and thins to the SSW to only 1.5m. A fence diagram summarizes lateral changes. Some mudstone units consist of subequal amounts of mud and clay and about 25% diatoms; while other mudstones are almost pure diatomites. Sedimentary structures include subplanar lamination, hummocky cross stratification, and swaley-cross stratification.

Mudstones were deposited during highstands on a storm-dominated shelf. This culminated in a phosphate bed that represents a local maximum in sea level followed by deposition of a prograding sand bed during regression. Fence diagrams correlating the tuffs, pebble layer, and the sandstone body on the shelf show thinning to the south and southwest. The data suggests that the sandstone was deposited by multiple events, as there is evidence of reworking by burrowing and draping of channels by diatomites. Trace and body fossils confirm marine environments for all units. Our interpretation changes the environment to a much more energetic shelf model in the lower Pisco Formation.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013