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An extensive distribution of water-altered equatorial layered deposits (ELDs) characterizes the densely cratered terrain of Arabia Terra (AT), Mars. The majority of these deposits reside within craters and are easily identified by laterally continuous layering. The processes that led to their formation have been widely investigated, but remain unresolved. Furthermore, their precise spatial distribution as a whole, as well as their relationship to one another individually, has yet to be fully appreciated. This work examines 1,013 craters and emphasizes 45 that were observed to contain ELDs within the eastern half of AT. We present the statistical relationships between crater characteristics (e.g., location, diameter, depth), as well as evidence supporting a southeast-northwest facies change. The 30-2,000-m range of measured deposit thicknesses, accompanied with individual layer thicknesses, correlate with crater elevation either due to water level differences within craters, or a proximal-distal relationship to the source. Air fall or fluid expulsion appear to stand out among all the prevailing depositional hypotheses, however the volume required to fill these craters in an ash fall scenario is in opposition with the locations of known volcanic provinces and the volume of ash that volcanic eruptions produce. This new evidence of a regional facies change provides a unique opportunity to better understand past climate and sedimentary processes on Mars, as well as the putative groundwater level in ancient AT. Ultimately, our results do not agree well with a unified depositional method for these deposits and the possibility of mixed origins should be taken seriously.

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.

The depositional processes proposed to explain the formation of the Equatorial Layered Deposits (ELDs) on Mars invoke very different systems such as sub-glacial volcanism, aeolian/airfall, lacustrine, lacustrine/volcanic and spring-fed deposition. We performed a detailed geological analysis of Firsoff crater and surrounding areas, where ELDs are present within and outside craters, using all the available datasets. ELDs stay on top of the Noachian Plateau Sequence and are covered by the Hesperian Ridged Plains Material. Within Firsoff crater, ELDs form a central bulge - estimated more than one kilometer thick - while, outside the craters, ELDs form flat-lying deposits probably not thicker than few tens of meters. Inside craters, terraced-like morphologies made of ELDs are found to mark the topographically highest deposits toward the rim. Although locally heavily eroded by wind, ELDs within Firsoff crater and within the other craters of the area show several morphologies that appear to be depositional, which would exclude that the craters were originally filled by ELDs. Within craters, ELDs consist of two facies: a roughly meter thick high albedo layered unit which drape and onlap the Plateau Sequence and hundreds meters large / tens of meters thick cone-shaped mounds made of layered breccia embedded in a matrix. The layered unit is disrupted in a post-depositional polygonal pattern and show no evidence of sedimentary structures. Mounds sometimes display an orifice at the top of their structure. Both facies appear to be linked to and possibly sourced from tectonically controlled fissure ridges. The two facies appear to be stratigraphically in heteropy thus suggesting a temporal as well as a genetic association. Polyhydrated sulfates have been detected in correspondence of ELDs. Outside the craters, ELDs show large-scale cross stratification associated to duneforms. On the base of these morphological, sedimentological and stratigraphic elements, we distinguish between fluid expulsion ELDs forming inside craters and aeolian ELDs outside and the craters. Fluid expulsion ELDs would result from groundwater upwelling and subsequent evaporitic precipitation, whereas aeolian ELDs from reworking and redeposition of the spring ELDs. While the climate supporting such a cycle was likely arid by terrestrial standards, these surface conditions in a terrestrial environment would have been conducive for microbial colonization. The potential for habitable conditions of evaporite settings as shown on analogues on Earth coupled with the high preservation potential within sulfates, make these deposits good candidate to understand the potential past habitability of Mars.