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Slope Evolution Revealed by Analysis of Sandstone Body Architecture, Tres Pasos Formation at Cerro Mirador, Chile


High-relief slope systems are characterized by a breadth of sediment conduits dominated by varying degrees of erosion, sediment bypass and deposition from shelf edge to basin floor. These conduits are variably controlled by inherited topography, slope instability and mass wasting, amongst other factors. The Upper Cretaceous Tres Pasos Formation, exposed along the Rio Zamora at Cerro Mirador, southern Chile, offers the opportunity for multi-scale documentation of depositional relationships and stratigraphic evolution of a deep-water slope system. Laterally continuous seismic-scale outcrop exposures along a 15 km transect preserve complex stratigraphic architectures and sandstone package geometries that vary both laterally and along paleoslope. The exposure of slope strata is >800 m thick, providing stratigraphic context and elucidating architectural changes throughout the evolution of the slope system. Turbiditic sandstone-dominated packages that range from 5-35 m thick are characterized by internal facies variability and scour, locally amalgamated bedsets, and a high degree of lenticularity. Thinner sandstone-prone units (1-5 m) consist of interbedded sandstone, siltstone, and mudstone with a vertical increase in lateral continuity. The development and distribution of these units were likely influenced by the voluminous (25 – >150 m thick) fine-grained mass transport deposits (MTDs) that commonly underlie the sandstone-rich packages. Within the larger vertical stratigraphic context (100s of m), intervals of sandstone deposition bracketed by MTDs evolve from thick, dominantly depositional amalgamated sandstone packages (15-35 m) with architectures influenced by inherited MTD topography to thinner sandstone packages (5-15 m) dominated by channelform architectures that include internal erosional surfaces with up to 15 m of relief. Results show a vertical evolution from MTD-influenced architectures with laterally variable sandstone thicknesses to increasingly slope-channel-influenced architectures characteristic of sediment bypass and lateral channel migration. This vertical trend links stratigraphic architectures to relative positions along the slope profile and a distinct suite of sedimentary processes. These results have important implications for quantifying system dimensions, developing system-scale correlations and stratigraphic context, and developing improved metrics for reservoir characterization and prediction in deep-water depositional systems.