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Description: The ancient stratigraphy on the western margin of the Isidis basin records the history of water on early Mars. Noachian units are overlain by layered, basaltic-composition sedimentary rocks that are enriched in polyhydrated sulfates and capped by more resistant units. The layered sulfates – uniquely exposed at northeast Syrtis Major – comprise a sedimentary sequence up to 600-m thick that has undergone a multi-stage history of deposition, alteration, and erosion. Siliciclastic sediments enriched in polyhydrated sulfates are bedded at m-scale and were deposited on slopes up to 10º, embaying and thinning against pre-existing Noachian highlands around the Isidis basin rim. The layered sulfates were then modified by volume-loss fracturing during diagenesis, and the fractures hosted channelized flow and jarosite mineral precipitation to form resistant ridges upon erosion. The depositional form and diagenetic volume-loss recorded by the layered sulfates suggest deposition in a deepwater basin. After their formation, the layered sulfates were first capped by a “smooth capping unit” and then eroded to form paleovalleys. Hesperian Syrtis Major lavas were channelized by this paleotopography, capping it in some places and filling it in others. Later fluvial features and phyllosilicate-bearing lacustrine deposits, which share a consistent regional base level (~-2300 m), were superimposed on the sulfate-lava stratigraphy. The layered sulfates at northeast Syrtis Major suggest surface bodies of water and active groundwater upwelling during the Noachian–Hesperian transition. The continued regional fluvio-lacustrine activity demonstrates a long and varied interaction with water over Mars history with phases of activity from Noachian to early Amazonian time.