Testing the Limits of Provenance Analysis From Basaltic Fluvial Sediment Near Sandvatn, Iceland, as a Mars Analog

Abstract Detrital sediments that accumulate downstream and are preserved in sedimentary rocks can allow characterization of geologic formations that are inaccessible for spatial or temporal reasons. However, mixing, sorting, and alteration of sediment during transport may complicate reconstruction of protolith characteristics. We test the preservation of three key provenance signals in coarse fluvial sand at a Mars analog watershed in Iceland to determine whether detrital sediments capture (a) watershed magmatic chemical variation, (b) textural indicators of lava‐water interaction during eruption and cooling, and (c) hydrothermal alteration. Specifically, we tested whether diagnostic variations in rock mineralogy, chemistry, and texture can be recovered in the bulk mineralogy and chemistry of sand‐sized sediments. In first‐order basaltic fluvial sediments transported <10 km, sediment sorting segregated grains with distinct inherited cooling textures and phenocrysts, and rendered some altered materials undetectable in bulk measurements of sand. This sorting also obscured signals of magmatic variability across the watershed and reduced the range of recorded lava cooling rates. The input of exogenous tephra to the Sandvatn watershed could only be distinguished in depositional environments with minimal active deposition or erosion and was most evident in reworked lakeshore deposits far from the delta. Bulk chemistry and mineralogy recorded different aspects of sediment sorting at Sandvatn, highlighting that integrating paired chemical and mineralogic data is key to disentangling the records of provenance composition and texture from sorted volcanic sediment.