“water-laid Tuff” of the Utah Desert and Similar Surge Deposit Misinterpretations: a Possible Lesson for Mars?

Introduction: The cross-bedded, finely layered, altered, salty sediments observed at the Opportunity landing site on Mars were first interpreted as having been deposited by flowing and standing water in an evaporating acid lake environment [1] and, more recently [2], as having been deposited by the wind, in a near-playa setting similar to that at White Sands, NM, but complicated by an episode of flowing (but not standing) water [3], and multiple episodes of rising and falling near-surface groundwater with a remarkably variable chemistry [4]. This second interpretation, like the first, appears complex and contradictory and could, like the first, represent a misinterpretation [5], because a much simpler explanation, impact surge followed by simple weathering, seems to account for all features observed to date by the Opportunity rover [6,7]. Layered, cross-bedded, altered rocks somewhat similar to those at the Opportunity landing site on Mars occur in the Spor Mountain mining district, west central Utah. These altered rocks host the largest beryl-lium deposit in the USA, actively mined since the 1960's. They also are genetically related to older uranium and fluorite mines. At Meridiani on Mars, hema-tite mineralization is contained in disseminated perfectly spherical nodules initially called " blueberries " [1]; their origin (and the origin of the hematite itself) is enigmatic. At Spor Mt., beryllium (in the mineral ber-trandite) is dissemined in scattered purple nodules of fluorite and silica replacing dolomite clasts [e.g., 8]. The origin of this mineralization likewise is enigmatic [8]. For nearly 15 years these bedded, mineralized rocks of Spor Mt., and similar unmineralized rocks in the nearby Thomas Range and adjacent ranges, were interpreted as alluvial " water-laid tuff " in numerous publications by U.S. Geological Survey (USGS) authors. In fact, they mainly appear to be base surge deposits resulting from explosive rhyolitic volcanism [e.g., 8]. Of various ages, they underlie associated lava flows of F-rich topaz rhyolite. It might appear that the geologists who initially misinterpreted them as water-laid lacked adequate training and preparation in volca-nology. Nevertheless, their mistake was natural and common, because the differences between surge deposits and normal sedimentary deposits can indeed be subtle, and the unique characteristics of surge deposits