Sedimentary texture similarities in deep-marine channel sandstones: Insights from petrographic characterization of Cretaceous Tres Pasos Formation, Chile

Deep-marine channels are conduits for turbidity currents that transport sediment from the continental shelf to the deep ocean floor. Along the way, some of the sediment suspended in the current is deposited within the channel whereas the rest of the sediment makes its way to deeper water to construct basin-floor lobes. Slope channel deposits provide information about past climates and other perturbations to Earth systems; the transfer of nutrients, pollutants, and organic carbon; and can be important subsurface reservoirs (hydrocarbon, groundwater, etc.). However, studying the processes associated with deep-marine channels is challenging and difficult to monitor directly. The Cretaceous Tres Pasos Formation in southern Chile is exposed in an extensive and well-documented outcrop belt where preserved slope channels and their sedimentary fill can be studied. Previous work has focused on these channels at a macroscopic scale (meters to kilometers) but very few studies have been conducted on the microscopic scale. For example, little is known about how the sedimentary texture, specifically grain size and sorting, of channel-fill sandstones vary across the width and along the length of a channel. Here, we quantify grain size and sorting by hand using petrographic analysis of 57 sandstone samples from eight channel-fill transects. Our results show that there are no systematic relationships of deep-water sandstone size and sorting within channel architectural transects (e.g., from axis to margin). We also show that there are no textural trends throughout the entire study area of the Figueroa unit (~25 km length of the slope system). We also find that these sandstones are poorly to very poorly sorted, which has implications to understanding the depositional processes. Finally, we also identify that these rocks should be classified as silty sandstones rather than sandstones according to Folk (1954). Based on our results we can predict that these flows were probably well fit to the channels as well as having similar velocities, concentrations, carrying capacity for grain size in suspension, etc. across multiple flows. We can also make predictions about these samples having a detrital vs. diagenetic matrix based on their volcanic lithic rich composition and looking at hybrid event beds (HEBs) to see if similar processes could be going on at this location. We hope that this human measured dataset can be used as a baseline and for the improvement of automative measuring processes that have started to develop.