Mineralogy of a Sulfate-rich Inverted Channel in the Atacama Desert, Chile: Clues to its Formation and Preservation

Abstract

We have performed a stratigraphic and mineralogical analysis of a vertical transect across a ridge located at the distal end of a system of eroded alluvial deposits in the northern Atacama Desert of Chile. The ridge, which is interpreted to be an inverted channel, exhibits a history of sedimentary, evaporitic, and diagenetic origin that includes groundwater mobilization and precipitation of anhydrite cements throughout the volume of the ridge. The ridge consists of two units: a lower one exhibiting a sedimentary and diagenetic history, and an upper one exhibiting an evaporitic history. Interbedded in the section are also anhydritic and gypsic paleosols. Two mechanisms that contribute to channel preservation and inversion are identified in this case. The first mechanism is the cementation of the volume by anhydrite cements during early diagenesis, and the second newly identified mechanism is the armoring of the lateral slopes of the ridge by halite-rich cement. The slope-conforming armor formed by this second mechanism developed subsequent to the formation of the ridge as a consequence of the remobilization of soluble salts. Finally, we identify a series of Ca-sulfate-rich plates on the surface of the ridge, which we interpret here to form by fracturing and subsequent erosion of an evaporitic deposit. The plates exhibit a reticulated surface texture, which we interpret as the result of periodic deliquescence and reprecipitation of a thin surface film of the evaporite deposits in response to thick morning fogs that occur in this part of the Atacama. The cross section of the plates exhibits a thin portion of biological material, which we ascribe to bacterial mats that take advantage of the deliquescence of the substrate to obtain their water. This later has important implications in the search for extant or extinct life on Mars.