Impacts of water volume and sediment characteristics on deposit morphology and stratigraphy as records of Amazonian flows on Mars

Introduction: Several recent studies have investigated how laboratory analog, Mars sediment flow experiments would be influenced by the extreme surface conditions on Mars, with low pressure and temperature affecting morphology and sedimentology in ways not seen on Earth [1,2,3,4,5,6,7,8]. With low pressures, greater length-to-width ratios (L:W), vug quantity, levee height, and roughness are observed in the morphology, and more convoluted bedding is observed in the sedimentology. These differences are mainly due to vaporization (boiling) at low pressures, which leads to several key processes: levitation via the Leidenfrost effect, grain sputtering, and evaporative cooling. While these studies explored pressure effects on sediment flows, one question that has not been addressed is the magnitude of these effects for basaltic sediment flows with lower sediment-to-water ratios and varying grain size, which is the focus of this study. In this study, we demonstrate how water content and grain size affect deposit morphology and stratigraphy under present-day, Mars-relevant pressures (~ 7 mbar). 

Methods: We performed sediment transport experiments of basaltic sediments at present-day Mars average pressures (7-mbar) in a pressure chamber at the Open University, U.K. The sediment-water mixtures (blended inside the chamber) were poured onto a 10° inclined test bed of warm (293 K), unconsolidated sand to simulate gravity-driven flows over loose regolith. We systematically varied parameters across 10 experiments, each consisting of basaltic sediments of two grain size regimes mixed with different amounts of deionized water to create a range of sediment:water mixture ratios (30:70, 40:60, 50:50, 60:40, 65:35, and 70:30). We used two basaltic grain size types: a fine-grain basalt sediment sourced from volcanic deposits in British Columbia and a coarse-grain basalt sediment sourced from the Colombia Plateau River Flood Basalt in Washington. For each run, videos were recorded from top-down, front-facing, and 45° side-view angles to document the morphological evolution of the flows along the test bed. Final deposit morphologies were imaged with a DSLR camera, and digital elevation models (DEMs) were created using a standard Structure-from-Motion photogrammetry workflow in Agisoft Metashape. We also collected cross-sectional images for stratigraphy using a cell phone camera to measure longitudinal flow thickness and bedding geometry, and used a Mastersizer 3000 to measure grain size and sorting trends with grain size distribution data.

Results: At 7 mbar, water content had a greater effect on deposit morphology and stratigraphic characteristics. As water content increases, trends differ in the two grain-size regimes. Fine-grained flow deposits show more constant increases. In contrast, coarse-grained deposits show less constant, step-wise increases. Higher water content causes L:W values to increase for fine-grained sediments and slightly decrease for coarse-grained sediments. The number of lobes generally decreases as water content increases for both sediment types. While vug quantity and levee height increase for both sample types at higher water content. These variations result from different magnitudes of low-pressure processes. We observed bubbling and vaporization in both basaltic sediments for all sediment-to-water ratios. Levitation (the Leidenfrost effect) decreased as water content dropped in both sample types. Grain sputtering also decreased as water content decreased. In general, as water content decreases, the magnitude of low-pressure processes declines. This decline yields flow deposits with shorter runout distances, more lobes, fewer vugs (pits), smaller levees, and lower roughness values.

We also observe stratigraphic characteristics trends, such as bedding thickness and geometry, median grain size, and sorting, across increasing water content for two grain-size regimes. We did not observe any systematic trends in bedding thickness and geometry with water content and grain size. However, we did observe trends in the grain-size distribution of the flows. As water content increases, the median grain size and sorting level increase for both grain-size types. This reflects greater vaporization with higher water content. Downslope trends occur only for low-water-content flows, for which we observe an increase in median grain size and sorting further downslope. The increase in median grain-size and sorting is greater for CGB flows than for FGB flows.

Conclusion: Results from this study provide a framework for characterizing martian sedimentary deposits and landforms that formed under varying water and sediment availability conditions at present Mars-relevant pressures. Results indicate that we can distinguish between flows with low and high water volumes. With higher water content, we expect more pronounced processes, such as vaporization, that impact surface morphology (i.e.,higher L:W, vug quantity, and levee height) and vice versa for lower water content. Additionally, sediment grain size can influence some morphological characteristics; for example, vug quantity is consistently higher in fine-grained deposits than in coarse-grained deposits. However, for some stratigraphic characteristics, we also observe no prevailing trend in bedding geometry and thickness across the various water contents and sediment types tested, whereas smaller-scale changes are evident in grain-size distributions (i.e., median and sorting values).  Ultimately, this work establishes usable constraints on how we can detect current water availability on Mars. While we expect flows to be affected by low-pressure processes regardless of other factors, the extent to which these pressures affect flow deposits will vary with regolith characteristics, such as water content and grain size.

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