THE DEPOSITION AND ALTERATION HISTORY OF THE NORTHEAST SYRTIS LAYERED SULFATES

The stratigraphy at northeast Syrtis Major, emplaced after the Isidis Basin-forming impact and capped by Syrtis Major lavas (Hiesinger & Head, 2004), provides a key Martian sequence spanning the Noachian–Hesperian transition. The stratigraphy superposes a ~300-m thick sulfate-bearing deposit, layered at m-scale, atop mafic basement altered to carbonates and clays. This relationship records a transition from alkaline to acidic aqueous environments that tracks changing global hydrology on early Mars (Ehlmann & Mustard, 2012). Here we use high resolution MRO images and digital elevation models coupled with CRISM data to show the unit was deposited as sediment and underwent burial, diagenesis, fracture mineralization, and differential erosion.

The layered sulfate is exposed over ~1000 km² beneath the erosional margin of the Syrtis Major lavas. Roughly 40% of the unit contains a network of ridges forming 500 m-wide polygons. These ridges rise up to 30 m above the surrounding terrain and inhibit erosion of the mostly recessive sulfates. Parallel mineralized zones visible in HiRISE imagery are consistent with isopachous cementation of pre-existing fractures, and jarosite fill shows precipitation from acidic waters.

The fractures lack a preferred orientation and penetrate the full thickness of the unit, indicating volume-loss fracturing. A likely model is layer-bound polygonal faulting (Goulty, 2008), which forms similar features on Earth during the shallow burial and dewatering of fine-grained sediments.

Bedding-orientation measurements from HiRISE stereo elevation models show dominantly shallow (~0-3º) dips regionally, consistent with equipotential deposition of the layered sulfates. However, dips of 5-8º in some areas may result from draping sedimentation (e.g. by ash fall) or localized post-emplacement tilting. Also, volume loss may have occurred during initial burial (e.g. dewatering of lacustrine sediments) or due to wetting and drying of sediments by groundwater.

The layered sulfates were not necessarily deposited subaqueously, but they show interaction with abundant water in the shallow subsurface. Ongoing work with CTX elevation data will provide regional context for local variations in bedding orientation, further constraining the depositional environment of the layered sulfates.