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The Stratigraphic Expression of Slope Channel Evolution: Insights From Qualitative and Quantitative Assessment of Channel Fills From the Cretaceous Tres Pasos Formation, Southern Chile


Submarine slope-channel fills are the composite product of an intricate history of sedimentary processes (e.g., erosion, bypass, and deposition). Proportionally, channel fills are dominated by thick-bedded sandstones, which provide a biased record of channel infilling via deposition from high-density turbidity currents. The record of erosion and bypass processes, as well as deposition from more dilute currents, is less evident as the features that record them are proportionally subordinate or less readily preserved (e.g., mud-clast-rich beds or mudstone-prone channel-base drapes). As such, it can be harder to deduce the role of these processes during channel evolution, and their influence upon preserved channel-fill architecture. Exposures of ancient slope-channel fills from the Cretaceous Tres Pasos Formation, southern Chile, provide a unique opportunity to document the varied stratigraphic expression of channel evolution. Over 5000 m of section is measured within the context of a well-constrained stratigraphic framework, featuring > 25 stacked channel elements (each typically ~20-25 m thick, and 250-300 m wide). Quantification of sedimentological characteristics (e.g., net-to-gross, amalgamation ratio, bed thicknesses, bed grain sizes, and bed types) from > 100 individual channel-fill measured sections, multivariate analysis and observations of channel-fill architecture have provided a number of insights: 1) Strata from the edges, or margins, of channel fills exhibit the greatest variability amongst sedimentological characteristics in comparison to more high-energy thalweg-related units preserved near axes of channel fills. 2) At least two distinct styles of channel-margin architecture are identified and quantitatively distinguished from one another. 3) Discrete styles of vertical variation in the proportion of grain size, facies and bed type are recognized in axis, off-axis and margin channel fill. Our observations highlight variations in the degree of focusing of gravity flows during the channel evolution; focusing is influenced both by the geometry of the basal-channel incision surface, and variation in the sequence and relative magnitudes of successive gravity flows as they transect the slope. This robust data provides insights into channel fill development and preservation in the stratigraphic record, and permits more informed modeling studies of fluid storage and connectivity in subsurface reservoirs.