Slurry-Flow Deposits of the Punta Barrosa Formation, Southernmost Andes, Chile

Fildani, Andrea¹ (1) Chevron Energy Technology Company, San Ramon, CA

The Magallanes foreland basin of southern Chile is marked by the influx of the sand-rich deep-marine turbidites of the Turonian Punta Barrosa Formation. At least three key Punta Barrosa Formation outcrops contain a significant number of mud-rich sandstone beds deposited by flows transitional between fully turbulent currents, with sediment supported by turbulence, and cohesive debris flows, with sediment supported by flow strength and characterized by cohesive rheology. These deposits are here described as ‘slurry beds'. By virtue of their lateral extent and quality of exposure of sedimentary structures, these outcrops present a unique opportunity to study slurry beds in detail. The beds commonly show a vertical sequence that includes, from base to top: heterogeneously structured mud-rich sandstone organized in variable thickness banding, unstructured mud-rich sandstone, and a mudstone cap. The mudstone caps are commonly sheared suggesting plastic behavior during subsequent beds emplacement. Selected slurry flow deposits were sampled for compositional and textural trends. Alternating light and dark bands of different scales comprise a primary sedimentary structure type within mud-rich sandstones very useful in field recognition. Micro-, meso-, and macrobanding are all present at different intervals together with a variety of water escape features. Preliminary petrographic and compositional analyses of bands show that dark bands are clay-rich and finer-grained than light bands. This work integrates detailed measured sections, lateral correlations, and petrographic observations with conceptual dynamic models of flow evolution to explain the evolution of a typical slurry flow and deposition of its associated mud-rich sandstone.