Origin and Development of Interbedded Clays and Sulfates in Equatorial Layered Deposits of Meridiani Planum, Mars

Equatorial Layered Deposits (ELDs) are sedimentary landforms found at equatorial latitudes of Mars showing repetitive bedding and commonly associated with hydrous minerals. These deposits are important archives of Mars' past aqueous conditions yet in most cases their formation mechanisms are still debated and elusive. Here we provide a detailed characterization of the mineralogy and stratigraphy of three different exposures of ELDs in Meridiani Planum, where such formations are found within small impact craters and are dated back to the Noachian-Hesperian boundary. Our analysis highlights a varying degree of mixture between polyhydrated Mg sulfates and Fe/Mg phyllosilicates within the beds of all three chosen targets, with several alternating strata of sulfate-rich and phyllosilicate-rich materials. In one case, Al-phyllosilicates are also found along with their Fe/Mg counterpart. This is a much more varied mineral assemblage than previously reported for ELDs in this area. We propose a formation mechanism which explains the observed interbedding as a result of surface ponding of groundwaters, aqueous alteration of atmospherically-sourced basaltic materials and water table/pH oscillations (wet/dry cycles) leading to recurrent sulfate precipitation. Our findings show how local environments play a substantial role in recording small-scale variations of complex aqueous processes, generally not observed at regional and global scales on Mars.