Surge deposit misidentification at Spor Mountain, Utah and elsewhere: A cautionary message for Mars

Before base surges were described in association with nuclear blasts and explosive volcanic eruptions (especially, the 1980 eruption of Mount St. Helens, Washington), laminar and cross-bedded volcanogenic surge deposits were commonly misinterpreted as being of fluvial or aeolian origin. One well-documented example involves the “water-laid tuffs” in and near the Spor Mountain beryllium mine, Utah; other examples abound. In light of how frequently volcanogenic surge deposits have been misinterpreted on Earth, extreme caution is urged for Mars studies. Contrary to what has been claimed, the markedly cross-bedded, salty deposits at Meridiani Planum on Mars need not have been formed by a combination of aeolian and aqueous processes, and their contained hematitic spherules need not have formed as aqueous concretions. Given the lack of indications of volcanism in the vicinity, and the planet-wide abundance of impact craters, deposition by surges associated with distant impact targets consisting of brine-soaked, locally sulfidic regolith is a reasonable explanation for all features observed, especially if diagenesis and weathering are considered. The uniformly sized and shaped, Ni-enriched blue-gray hematitic spherules would then be some type of vapor condensation spherules (including accretionary lapilli). A similar interpretation is possible for deposits in the Home Plate area, Gusev Crater. Unlike on the dry and atmosphereless Moon, salty impact surge deposits containing spherules should be common, and well-preserved, on Mars.