Equatorial layered deposits in Arabia Terra, Mars: Facies and process variability

Light-toned layered deposits are widespread on Mars in different geographical and geological settings and many different processes have been suggested to explain their formation, including lacustrine, sub-ice volcanism, weathering of basalts, pyroclastic, impact base surge and dust airfall. Interplay between airfall and aeolian processes with groundwater fluctuations leading to episodes of evaporite precipitation, interaction between lacustrine deposition and hydrothermal vents or large mud volcanoes, groundwater upwelling leading to spring deposits precipitation and/or mud volcanoes deposition have been also proposed. In order to contribute to this discussion, we present the recently published results on the Equatorial Layered Deposits (ELDs) of Arabia Terra in Firsoff crater and on the adjacent plateau. We produced a detailed geological map that included a survey of the relative stratigraphic relations and crater count dating. We reconstructed the geometry of the layered deposits and inferred some compositional constraints. We analyzed the sedimentary structures and textures observable at the available scale both inside and outside the craters. ELDs drape and onlap the plateau materials of Late Noachian age while they are unconformably covered by Early and Middle Amazonian units. ELDs show the presence of polyhydrated sulfates. Layer morphologies, albedo, structure, composition and stratigraphic setting enable us to include the ELDs in Firsoff crater and the surrounding plateau in the wider frame of the sulfate-bearing layered deposits seen elsewhere on Mars at the Noachian-Hesperian transition. ELDs found in craters and in the plateau can be distinguished from each other on the basis of basin-scale morphologies (kilometers-thick crater bulges vs 100s meter-thick sheet drape deposits) and local-scale morphologies (rounded shape, sub-circular depressions, raised rims, bowl-shaped appearance and serrated layer vs flat-topped bodies or dune fields with cross stratification). These morphologic differences suggest that genetic processes might at least partly be different, too. In the crater ELDs consist of mounds made of breccia sometimes displaying an apical pit laterally grading into a light-toned layered unit disrupted in a meter-scale polygonal pattern. These units are frequently associated with fissure ridges suggestive of subsurface sources. We interpret the ELDs inside the craters as spring deposits, originated by fluid upwelling through the pathways likely provided by the fractures related to the crater formations, and debouching at the surface through the fissure ridges and the mounds leading to evaporite precipitation. On the plateau, ELDs consist of rare mounds, flat-lying deposits and cross-bedded dune-fields. We interpret these mounds as possible smaller spring deposits, the flat-lying deposits as playa deposits, and the cross-bedded dune-fields as aeolian deposits. Groundwater fluctuations appear to be the major factor controlling ELDs deposition, leading to different depositional environments depending on the physiography of the basin.