Evidence for Sedimentary Hiatuses on Early Mars

The ancient Martian sedimentary cycle remains poorly constrained because sedimentary deposits older than ~3.7 Gy are rare and sparsely exposed. In this study, we investigate rare ancient sedimentary exposures, where “sediments” are defined as accumulations of material formed by depositional processes, including volcanoclastic deposits.

We focus on deposits dated between ~4.0 and 3.7 Gy, specifically the Oxia Planum stratigraphic sequence (selected as the future landing site of the ExoMars 2028 Rover mission) and the basal sequence of Mawrth Vallis. Both sites are characterized by Fe/Mg-rich clay-bearing deposits, but exhibit distinct spectral types (vermiculite/saponite-bearing at Oxia Planum vs nontronite-bearing at Mawrth Vallis). Access to these stratigraphic records provides key insights into sedimentary processes during the Noachian period.

At both locations, we identified paleosurfaces, defined as remnants of ancient surfaces that were buried by younger deposits and later re-exposed by erosion. These paleosurfaces are recognized by flat-lying, cratered surfaces in which craters are infilled by overlying, younger, material. Some of these paleosurfaces extend over several thousand square kilometers and expose hundreds of preserved paleocraters, indicating prolonged sedimentary hiatuses.

We identified two major paleosurfaces. The older one, likely dated at ~4.0 Gy, is located between two sets of strata within the Oxia Planum sequence. The younger one, dated between ~4.0 and 3.7 Gy, occurs at the boundary between the Oxia Planum and Mawrth Vallis sequences. These paleosurfaces indicate time intervals during which the Noachian Martian sedimentary cycle was effectively halted: sedimentation ceased, as evidenced by crater accumulation, and erosion was minimal, allowing the preservation of paleocraters.

Using statistical analysis of preserved paleocraters observed at stratigraphic boundaries, we estimate the duration of these sedimentary hiatuses as a function of surface age. These results have significant implications for our understanding of the early Martian sedimentary cycle and planetary habitability, as they indicate very ancient periods of major climatic and environmental change embedded within this stratigraphic record, during which sedimentation ceased.