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The Record of Sediment Transfer on Deep-Water Slopes: Characterization of an Aggradational Channel Complex 25 km from the Shelf Edge, Tres Pasos Formation, Chile


Insights into sediment transfer across deep-water slopes from the outcrop record relies, in part, on the ability to correlate deposits to a coeval shelf edge. Many outstanding exposures of deep-water strata lack this directly measurable depositional context. In the Cretaceous Tres Pasos Formation of Chile, a depositional dip-oriented cross-section of a progradational shelf margin sequence is preserved, featuring clinoforms with >1000 m relief and tens of kilometers in length. One of these surfaces, the Puma Clinoform, represents a sequence boundary characterized by river-dominated shelf-edge deposits, and shelf-edge incisions up to 60 m deep. 25 km downslope of the shelf-edge, the deposits of a slope channel complex are exposed at Arroyo Picana, and offer an opportunity to characterize the record of channelized erosion, sediment bypass and deposition on the slope. The objectives of this work are to deduce the record of sediment transfer (erosion, bypass and deposition) through documentation of slope channel stacking patterns on the Puma Surface, as well as intra-channel facies distribution trends and stratal geometries. Data is collected via sedimentological sections and traditional field mapping, as well as differential GPS surveying. These data are used to construct a 3-D architectural model of the channel outcrop, which fosters characterization and quantification of stratigraphic architecture and facies distribution patterns. The outcrop consists of at least six distinctive channel elements, with the earliest phase of channelization dominated by gravel and sand transport, within a conduit at least 800 m wide. Overall, this element (17 m thick), displays disorganized internal architecture with high amalgamation, and represents deposition following establishment of the slope conduit. Subsequent channels are narrower (180-240 m), and exhibit more vertical aggradation and systematic lateral stacking. These channel bodies are 7 to 17 m thick, asymmetric, and commonly characterized by basal erosion surfaces overlain by mudstone-clast conglomerate in their axes, transitioning laterally on the inner margins to thinly interbedded sandstone and mudstone. The composite channel complex, including all six channel elements, is 55 m thick. These strata contain a complex record of erosion, sediment bypass and deposition, which contrasts channelform bodies on the coeval shelf-edge and upper slope, through which coarse-grained sediment largely bypassed.