Process variability and depositional architecture of the ELDs in the Firsoff south crater (Arabia Terra, Mars)

Abstract

The genesis of the Equatorial light-toned sulfate bearing layered deposits (ELDs) in Arabia Terra has long been debated with many processes/environments proposed (including aeolian, airfall, lacustrine, volcanic, playa, spring deposits and combinations among these, stratigraphic relations with the clay bearing possible fluvial deposits) and different depositional geometries inferred (with implications of different degrees of erosional overprint). Because of the position of Arabia Terra as a sort of low-dipping connection between Highlands and Lowlands, the depositional environment(s) of the ELDs might have implications at a supra-regional scale either in term of lateral transitions and controls on deposition. Moreover, western Arabia Terra is also an area of interest for ExoMars Landing Site selection. We are continuing our work aimed at unraveling the depositional processes, stratigraphy, geometry and composition of the ELDs in the area surrounding Firsoff crater. Here ELDs are well exposed both in the craters, including the 80 to 100 km large ones such as Crommelin and Firsoff to some smaller ones roughly 20 km across, and in the plateau. In particular, here we focus on the ~60 km large Firsoff south crater as mounds, light-toned and dark-toned layers are exceptionally well-exposed at this site and a HiRISE stereo pair enables investigating their mutual stratigraphic relations throughout the whole crater. The thickness of the sedimentary infill can be roughly estimated as 1 to 1.3 km, with maximum values around the crater rim. These deposits gently drape (dip generally less than few degrees) and onlap the Noachian Plateau Sequence and are disconformably covered by a cap rock. The sedimentary infill consists of three Units which are interlayered both vertically and laterally. The layers appear to be few meters thick at the scale of the available resolution. They consist of light-toned and darktoned layers where the light-toned ones appear to be more resistant to weathering and erosion even if the entire succession is heavily affected by aeolian erosion as shown by numerous yardangs. The light-toned layers are quite irregular, disrupted in a meter-scale polygonal pattern and dish-shaped depressions, with sinuous locally rimmed margins and do not show any evidence of cross-bedding while the dark-toned layers are frequently covered by a recent aeolian dark mantle either showing dunes or not. Where visible, these layers appear smooth and fill the irregularities of the lighter ones. Locally, hundreds of meters large and tens to hundreds meter high mounds form clusters, sometimes aligned along possible fissure ridges. The mounds sometimes display an apical pit. Even if not in this crater, the ELDs in the nearby craters and plateau have been shown to contain polyhydrated sulfates. We suggest that the ELDs primary deposition was controlled by groundwater upwelling sourcing from mounds and fissure ridges and depositing in spring deposits and playa settings. ELDs lightertoned layers might represent evaporites (polygonal pattern, draping geometry, syn-to syn-post depositional morphologies) while darker-toned layers might reflect a clastic fluid-expulsion (smooth, filling irregularities of the lighter-toned layers).